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Twenty years " sperm drinking " by female catfi shes – and still nothing new?

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In 1995 KHODA et al. published a study, in which they described a " unique reproductive behaviour and a new mode of egg insemination " (p. 1) of the armoured catfi sh Corydoras aeneus. From their observations and experiments the authors claimed to have demonstrated that the females swallow the sperm of males during spawning (" sperm drinking ") and that after the passage through the gut these sperm inseminate and fertilize the eggs in the fi n pocket formed by the female pelvic fi ns. Since then sperm drinking seems to be a commonly accepted fact in the primary and secondary scientifi c literature and was never seriously questioned herein. However, the way ejaculated sperm take to inseminate and fertilize egg was vigourously discussed already at the beginning and middle of the 20 th century in the so-called " gray " literature by German and later North American aquarists on the basis of numerous observations, hypotheses and even experiments. For example, SCHUBERT (1907) suggested the oral uptake of sperm by the female and LIEBIG (1912) considered the possibility of " sperm drinking " and the intestinal passage of sperm, but also other fertilization scenarios were taken into consideration. Especially the aquaristic literature from USA favoured the view that eggs are inseminated and fertilized during egg deposition. Later KNAACK (1955 ff.), who intensively studied the reproductive behaviour of several Corydoras spp., confi rmed not only the earliest, but largely forgotten observations made by CARBONNIER (1881), but also plausibly rejected the intestinal passage of sperm performing similar experiments as KHODA et al. We give a brief overview of the lively debate on this subject in the fi rst half of the last century, acknowledge the experiments performed by KNAACK as well as by KHODA et al., and discuss some problems that arise from these studies. Generally, our essay tells about an interesting episode of aquaristics and illustrates how much useful information and ideas are hidden in the so-called " gray " literature. Zusammenfassung: KHODA et al. veröffentlichten im Jahr 1995 eine Studie, in der sie ein " unique reproductive behaviour and a new mode of egg insemination " (p. 1) bei dem Panzerwels Corydoras aeneus beschreiben. Die Autoren glaubten aufgrund von Beobachtungen und einer Reihe von Experimenten nachgewiesen zu haben, dass die Weibchen während des Laichaktes die Spermien der Männchen verschlucken (" sperm drinking ") und die Spermien die Eier in einer Flossentasche, die von den Bauchfl ossen des Weibchens gebildet wird, erst nach einer Passage durch den Darm besamen und befruchten. Dieses " Spermatrinken " wird seitdem bis heute in der wissenschaftlichen Primär-und Sekundärliteratur als Fakt angesehen und ist dort unseres Wissens auch niemals ernsthaft in Frage gestellt worden. Demgegenüber stehen zahlreiche Veröffentlichungen in der sogenannten " grauen " Literatur, vor allem Anfang bis Mitte des 20. Jahrhunderts, in denen aufgrund von Beobachtungen, Hypothesen und sogar Versuchen zunächst deutscher, dann aber 10 auch nordamerikanischer Aquarianer heftig diskutiert wurde, wie die Spermien zu den Eiern gelangen und diese befruchten. So vermutete beispielsweise SCHUBERT (1907) bereits ein Aufsaugen der Spermien durch das Weibchen und LIEBIG (1912) erwog die Möglichkeit des " Spermatrinkens " und der Darmpassage der Spermien, doch wurden darüber hinaus noch andere Befruchtungsszenarien vermutet, unter anderem, vor allem in der insgesamt relativ spärlichen nordamerikanischen aquaristischen Literatur, dass die Eier während der Eiablage befruchtet würden. KNAACK (1955 ff.) hat dann nach intensivem Studium des Ablaichverhaltens mehrerer Corydoras-Arten nicht nur die ersten diesbezüglichen, aber zum Teil in Vergessenheit geratenen Beobachtungen von CARBONNIER (1881) bestätigt, sondern bereits 1955 mit Versuchen, die im Wesentlichen denen von KHODA et al. entsprachen, eine Darmpassage der Spermien plausibel zurückgewiesen. Wir geben einen kurzen Überblick über die lebhaften Diskussionen zu diesem Thema aus der ersten Hälfte des vori-gen Jahrhunderts, würdigen die Experimente von KNAACK und von KHODA et al. und erörtern einige sich daraus ergebende Probleme. Generell behandelt unser Essay eine interessante Episode der Aquaristik und zeigt auf, wie viele Anregungen und Informationen in der so genannten grauen Literatur verborgen sind.
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9
Bull. Fish Biol. 15 (1/2)
Bulletin of Fish Biology Volume 15 Nos 1/2 31.07.2015 9-31
Twenty years “sperm drinking” by female catfi shes
– and still nothing new?
An essay about the different perception
of amateurs and professionals
Zwanzig Jahre „Spermatrinken“ weiblicher Welse – und immer noch nichts Neues?
Ein Essay über die unterschiedliche Wahrnehmung von Amateuren und Profi s
Axel Zarske1 & Hartmut Greven2
1Senckenberg Naturhistorische Sammlungen Dresden, Museum für Tierkunde, A.-B.-Meyer-Bau,
Königsbrücker Landstraße 159, D-01109 Dresden, Germany; axel.zarske@senckenberg.de
2Institut für Zoomorphologie und Zellbiologie der Universität Düsseldorf, Universitätsstr. 1,
D-40225 Düsseldorf, Germany; grevenh@uni-duesseldorf.de
Summary: In 1995 KHODA et al. published a study, in which they described a “unique reproductive
behaviour and a new mode of egg insemination” (p. 1) of the armoured cat sh Corydoras aeneus. From
their observations and experiments the authors claimed to have demonstrated that the females swallow
the sperm of males during spawning (“sperm drinking”) and that after the passage through the gut these
sperm inseminate and fertilize the eggs in the n pocket formed by the female pelvic ns. Since then sperm
drinking seems to be a commonly accepted fact in the primary and secondary scienti c literature and was
never seriously questioned herein. However, the way ejaculated sperm take to inseminate and fertilize egg
was vigourously discussed already at the beginning and middle of the 20th century in the so-called “gray”
literature by German and later North American aquarists on the basis of numerous observations, hypotheses
and even experiments. For example, SCHUBERT (1907) suggested the oral uptake of sperm by the female and
LIEBIG (1912) considered the possibility of „sperm drinking“ and the intestinal passage of sperm, but also
other fertilization scenarios were taken into consideration. Especially the aquaristic literature from USA
favoured the view that eggs are inseminated and fertilized during egg deposition. Later KNAACK (1955 ff.), who
intensively studied the reproductive behaviour of several Corydoras spp., con rmed not only the earliest, but
largely forgotten observations made by CARBONNIER (1881), but also plausibly rejected the intestinal passage
of sperm performing similar experiments as KHODA et al. We give a brief overview of the lively debate on
this subject in the rst half of the last century, acknowledge the experiments performed by KNAACK as well
as by KHODA et al., and discuss some problems that arise from these studies. Generally, our essay tells about
an interesting episode of aquaristics and illustrates how much useful information and ideas are hidden in
the so-called “gray” literature.
Keywords: Corydoras spp., reproductive behaviour, T-position, „sperm drinking“, insemination and fertili-
zation, n pocket, aquaristics, gray literature.
Zusammenfassung: KHODA et al. veröffentlichten im Jahr 1995 eine Studie, in der sie ein „unique reproduc-
tive behaviour and a new mode of egg insemination“ (p. 1) bei dem Panzerwels Corydoras aeneus beschreiben.
Die Autoren glaubten aufgrund von Beobachtungen und einer Reihe von Experimenten nachgewiesen
zu haben, dass die Weibchen während des Laichaktes die Spermien der Männchen verschlucken („sperm
drinking“) und die Spermien die Eier in einer Flossentasche, die von den Bauch ossen des Weibchens
gebildet wird, erst nach einer Passage durch den Darm besamen und befruchten. Dieses „Spermatrinken“
wird seitdem bis heute in der wissenschaftlichen Primär- und Sekundärliteratur als Fakt angesehen und ist
dort unseres Wissens auch niemals ernsthaft in Frage gestellt worden. Demgegenüber stehen zahlreiche
Veröffentlichungen in der sogenannten „grauen” Literatur, vor allem Anfang bis Mitte des 20. Jahrhunderts,
in denen aufgrund von Beobachtungen, Hypothesen und sogar Versuchen zunächst deutscher, dann aber
10
auch nordamerikanischer Aquarianer heftig diskutiert wurde, wie die Spermien zu den Eiern gelangen und
diese befruchten. So vermutete beispielsweise SCHUBERT (1907) bereits ein Aufsaugen der Spermien durch
das Weibchen und LIEBIG (1912) erwog die Möglichkeit des „Spermatrinkens“ und der Darmpassage der
Spermien, doch wurden darüber hinaus noch andere Befruchtungsszenarien vermutet, unter anderem, vor
allem in der insgesamt relativ spärlichen nordamerikanischen aquaristischen Literatur, dass die Eier während
der Eiablage befruchtet würden. KNAACK (1955 ff.) hat dann nach intensivem Studium des Ablaichverhaltens
mehrerer Corydoras-Arten nicht nur die ersten diesbezüglichen, aber zum Teil in Vergessenheit geratenen
Beobachtungen von CARBONNIER (1881) bestätigt, sondern bereits 1955 mit Versuchen, die im Wesentlichen
denen von KHODA et al. entsprachen, eine Darmpassage der Spermien plausibel zurückgewiesen. Wir geben
einen kurzen Überblick über die lebhaften Diskussionen zu diesem Thema aus der ersten Hälfte des vori-
gen Jahrhunderts, würdigen die Experimente von KNAACK und von KHODA et al. und erörtern einige sich
daraus ergebende Probleme. Generell behandelt unser Essay eine interessante Episode der Aquaristik und
zeigt auf, wie viele Anregungen und Informationen in der so genannten grauen Literatur verborgen sind.
Schlüsselwörter: Corydoras spp. Fortp anzungsverhalten, T-Stellung, „Spermatrinken“, Besamung und
Befruchtung, Bauch ossentasche, Aquaristik, „graue“ Literatur.
1. Introduction
About 20 years ago the journal „Environmental
Biology“ published a study entitled „Sperm
drinking by female cat shes: a novel mode of
insemination“ (KHODA et al. 1995). The authors
of this study claimed to have (indirectly) found
evidence that during mating females of Corydoras
aeneus (and probably also females of other Co-
rydoras species) take up sperm with the mouth,
which then pass through the intestine within
seconds reaching a “pocket“ formed by the
female‘s pelvic ns, in which the just deposited
eggs were fertilized.
Understandably, this „novel mode of insemi-
nation“ has attracted considerable attention in
the subsequent literature. However, it is remark-
able that immediately after publication of this
phenomenon critical discussions were primarily
initiated by aquarium enthusiasts. The majority
of aquarists was not convinced either accord-
ing to the saying “nothing can happen that isn’t
allowed to happen“, referring to own observa-
tions and ideas or to reports in the so-called
“gray” literature, which the Japanese authors
and many professional scientists apparently were
not aware of, or more or less carefully weigh the
differing views down to the present time (e.g.
FINLEY 1995; PINTER 1996; STEINLE 1996; AN-
DER 1997; SCHMID 2004; VAN DER JEUGHT 2013).
According to the various notes published in the
“gray” literature, this issue appeared to be settled
at the latest since 1955 by KNAACK (1955) – but
not in the sense of KHODA et al. (1995) – , and
KNAACKs interpretation was accepted by several
authors of manuals and encyclopedias (see FREY
1957, 1974; STERBA 1963; CURRIER & SMITH 1969;
VOGT 1970; KLEE 1970; FRANKE 1983, 1985).
Professional scientists, however, seemed and
seem to take “sperm drinking” for granted
in both original articles (e.g. HUYSENTRUYT &
ADRIAENS 2005; MAZZOLDI et al. 2007; FRANCE-
SCHINI-VICENTINI et al. 2007) as well as in text
books of ichthyology and reviews (e.g. HELFMAN
et al. 2007; NELSON & DEHN 2011; ARMBRUSTER
2011; MOL 2012; PANDIAN 2012; WOTTON &
SMITH 2014).
The idea behind our present essay, in which we
reopen the debate on whether sperm drinking
in cat shes is likely or not, was
- to summarize the most important data on
reproduction, in particular on insemination
and fertilization of the eggs in callichthyid
cat shes published since 1891 and not or
insuf ciently considered by scienti c studies;
- to acknowledge the “gray” literature, which
clearly shows that the hypothesis of sperm
drinking in cat shes has been formulated as
early as 1912 by LIEBIG, but was rejected later
due to reliable experiments and their plausible
interpretations, and last but not least
- to call attention to an interesting and obvi-
ously still acute episode of ichthyology and
to acknowledge the informational content of
11
Bull. Fish Biol. 15 (1/2)
some so called “gray” literature.
We are aware of the problems of our essay,
because the “gray” literature is hardly to be
mastered, i.e. in the present case the numerous
observations published in magazines for aquari-
um enthusiasts. However, the early phase in the
discussion of this phenomenon seems to have
been largely a German matter. Therefore, our
selection, especially of non-German articles,
remains subjective. Contrary to convention, we
add an appendix with detailed textual passages
mainly of the concerned “historic” authors
(both original and in an English translation).
2. Observations on the reproductive beha-
vior of
Corydoras
spp. (1881-1955)
2.1. The early work of nature-lovers and
pioneers of aquaristics
Already since the end of the 19th century and
especially in the rst half of the 20th century
armoured cat shes were bred successfully and
their reproductive behaviour was observed ca-
refully by many aquarists. At that time, however,
only Corydoras paleatus and somewhat later Co-
rydoras aeneus were available, but were described
under different names (tab. 1). However, the
interpretation of what the authors had seen dur-
ing courtship and mating differed considerably.
Certainly, this was due (among others) to their
different background, such as experience in
breeding exotic shes, talent for observation,
imagination and general scienti c training. The
problematic elds heavily discussed were (1)
the sex-speci c activity of the mates, (2) the
signi cance of the so-called T-position, namely
at what site and how the female adheres to the
male, and (3) how and where sperm fertilize the
eggs (see tab. 2).
The French ichthyologist PIERRE CAR-
BONNIER (1828–1883) was the rst to describe
the reproductive behaviour of the peppered
cat sh Corydoras paleatus (CARBONNIER 1880,
1881). Especially his observations published
in the “Bulletin de la Société d‘Acclimatation”,
a former journal of the French “Société na-
tionale de protection de la nature (SNPN)”
are pretty good and the interpretation of what
he has seen is essentially correct (CARBONNIER
1881). CARBONNIER (l. c.) considered the male
as the active partner during courtship, men-
tions an attachment of the male in transverse
position (“se posa en travers”, p. 106), later
called T-position. Further, he recognized that
the male blocks the female by clasping her
barbels with the rst spine of his pectoral n
and suspected that the eggs become fertilized
in a pocket formed by the ventral ns of the
female.1 The data concerning courtship and
mating were translated into German and pub-
lished twelve years later (1893) in the “Blätter
Tab . 1 : List of the species names used for Corydoras paleatus and Corydoras aeneus by the various authors cited
in this essay. Note that in some articles the same species was mentioned with different names.
Tab . 1 : Liste der Artnamen, unter denen Corydoras paleatus und Corydoras aeneus von den in diesem Essay
zitierten Autoren erwähnt wurden. Man beachte, dass in einigen Artikeln ein und dieselbe Art unter ver-
schiedenen Namen erwähnt wurde.
12
für Aquarien- und Terrarienkunde”, a popular
magazine for aquarists and herpers founded
1890. GILL included CARBONNIERs description
in his review of “Parental care among fresh-
water shes” (GILL 1906).
In spite of these careful descriptions, in the
following period, especially in the rst half of
the 20th century, the above mentioned issues
were discussed intensively and controversially,
especially in magazines. The literature primarily
intended for scientists remained reserved (if this
issue was concerned at all) considering the ques-
tion of insemination and fertilisation unsolved
(e.g. KOSSWIG 1936).
In German magazines, reports on this matter
were published in rapid succession, sometimes
even simultaneously. However, respecting the
interests of the principal target group of those
magazines, reports were often con rmatory,
redundant mostly without any debate on infor-
mation available from previous reports of other
aquarists, much less the scienti c literature. A
Tab . 2 : Key elements of the reproductive behaviour of Corydoras spp. and ways of interpreting events sug-
gested by aquarists until 1955.
Tab . 2 : Elemente des Fortp anzungsverhaltens von Cor ydoras spp. und verschiedene Interpretationsvor-
schläge von Aquarianern bis 1955.
13
Bull. Fish Biol. 15 (1/2)
“culture of citation” did not exist, although it
was urged by Willy WOLTERSTORFF (1910), at that
time editor of the “Blätter für Aquarien- und
Terrarienkunde”. Therefore, it is often dif cult
to distinguish between independent observa-
tions and foreign in uences, e.g. from previous
publications. We have selected only those articles
that illustrate in our opinion the “struggle for
truth” most impressively.
From these early observations ve rather
self-con dent notes on Callichthys punctatus (now
Corydoras paleatus, see tab. 1) by JÜRGENS (1900,
1906, 1909, 1923, 1939) are noticeable, from
which those from 1900 and 1939 are the most
important. In 1900 JÜRGENS considers the female
as the more active partner, clearly describes the
T-position, says that the female rmly adheres
to the male by suction, while the male moves
convulsively, and hears growling sounds of the
latter (studied 1998 in detail by PRUZSINSZKY &
LADICH 1998). He assumes, as CARBONNIER did,
that eggs become inseminated and fertilized in
the pocket the female forms with her ventral
ns.
JÜRGENS does not explicitly mention discharge
of sperm into the water, but obviously assumes
that this has happened during the convulsive
movements of the male.2 The note of 1906
con rms earlier statements3, but JÜRGENS thinks
that the bending of the male’s body guides sper-
matozoa towards the belly of the female, which
then were captured by folding of the ventral ns,
and shows for the rst time a drawing of the
T-position ( g. 1). By the way, he quotes (for the
rst time in his articles) CARBONNIER (1881), but
obviously expects his readers to know French,
because he cites a paragraph from the original
without a translation, and REITZ (1905 b), who
assumed that the male merely presses against
the female to discharge his sperm in the n
pocket. In 1909 JÜRGENS criticized the article
from BOEKER (1909), who wrote that the females
suck off sperm from the male’s genital pore,
spits them on the site of nal egg deposition
and then glues eggs to this site.
In 1923 and 1939 JÜRGENS defends his opini-
on against other views, which he believes to be
wrong. Especially in the article published in 1939,
a kind of review, he rejects the views of SCHU-
BERT (1907), REITZ (1910 a, b), MERTENS (1917),
ROELICKE (1933), and ROHRBACH (1939) – which
are brie y described below – ending with the
note „all has been resolved, there is only the need
to answer the question how sperm come from
the male’s genital pore in the n pocket, but the
story of sucking in the sperm and in salivating
the eggs must be henceforth dismissed in the
“boundless area of fantasy” and must disappear
from our aquarium literature („alles ist geklärt,
nur die Frage, wie gelangen die Spermatozoen
vom Geschlechtsporus des Männchens in die
Flossentasche, muß noch gelöst werden, aber das
Märchen von dem Absaugen des Spermas und
der „Einspeichelung“ der Eier muß von jetzt ab
in das „uferlose Phanatasie Gebiet der Dichtung”
verwiesen werden und aus unserer Aquarienli-
teratur verschwinden.“ (JÜRGENS 1939, p. 356)).
However, it may be of interest to take some
notice of the observations and their interpre-
tation in the articles published between 1900
and 1939 in particular, because some of them
contained some new aspects and some others
were totally ignored by JÜRGENS.
Fig. 1: The rst drawing of the T-position in Callich-
thys punctatus (today Corydoras paleatus) (from JÜRGENS
1906).
Abb. 1: Erste zeichnerische Darstellung der T-Stel-
lung bei Callichthys punctatus (heute Corydoras paleatus)
(aus JÜRGENS 1906).
14
SCHUBERT (1907) doubts fertilization of the
eggs in the n pocket. Rather he thinks that
the female sucks the sperm with her mouth,
insalivates them and spit them onto the sub-
strate before deposition of the eggs.4 Curiously,
in an article of THUMM (1909) there is a bit of
everything. The author describes that the male
grasps the female as described by JÜRGENS, but
after that she grasps his anus to suck there.
REITZ (1910) emphasizes that the female at-
taches to the centre of the males’ body and not
to his genital opening and thinks that sperm
move along the belly of the female to reach the
n pocket, that she heavily breathes after separa-
tion from the male, and – the new result – he
veri es that eggs become inseminated already in
the n pocket by raising larvae from eggs that
were removed from this pocket.5
An author, who really enriched the debate
on the way sperm may take after ejaculation,
was LIEBIG (1912, 1924 a-e). Strikingly, he was
totally ignored by other aquarists, namely by
JÜRGENS. In his notes – in reality two longer
articles published in serial form – LIEBIG
con rmed the known facts (T-position, fe-
male adheres to the male near the anus, male
slightly tilts on his side and shivers, etc.), but
considered for the rst time a passage of sperm
through the intestine (LIEBIG 1912 a), stress-
ing, however, that this is only a wild guess.6
In 1924 some notes followed with own ideas
and speculations providing altogether a good
and precise analysis of the hitherto existing
aquaristic literature concerning this issue. He
summarized taxonomy, keeping and breeding
Corydoras spp. (LIEBIG 1924 a, b, c) und asks
the right questions, e.g. questions concerning
the reasons for the strange release of sperm
(among others the body plan of armoured
cat shes), the release of sperm (the speci c
contact of mates), when (exact time unclear)
and how the n pocket is formed (he mainly
discusses inconsistent observations of sev-
eral authors, the disadvantages of this pocket,
which may even impede spawning on a at
substrate), and the way sperm may take to reach
the pocket (LIEBIG 1924 d). The latter issue is
dealt with in LIEBIG (1924 e).
Here, he discussed in great detail und criti-
cally the following options: (1) a portion of the
water containing spermatozoa is collected by
folding up the ventral ns (objection: collection
of too little sperm, loss of too much sperm);
(2) sperm directly swim from the male’s genital
pore towards the ventral ns directed by the
male through bending his body or moving his
caudal n (objection: unfavourable position of
the mates; a position, in which the male’s ventral
body surface is perpendicular to the ventral body
surface of the female would be better); (3) the
female takes up sperm with her mouth and spit
them out before depositing the eggs (objection:
the uptake of sperm by the mouth is possible,
but according to the most observations not
directly from the male’s genital opening; spit-
ting does not occur during egg deposition; eggs
must be placed in a way that the micropyle can
be reached by sperm); and again (4) the female
drinks sperm and sperm are channelled via the
accessory respiratory system to the n pocket
(objection: mates often eat during spawning,
signi cant gill breathing, but no air breathing
after spawning). Nevertheless, he continued to
assume that sperm reach the n pocket via the
female’s body.7
All in all, however, the author did not come
to a nal conclusion. He states: „After all these
... comments I conclude that nearly all what
belongs to the reproduction of Corydoras paleatus
... remains undetermined and dark“ („Nach allen
diesen … Ausführungen bleibt somit das Fazit,
dass so ziemlich alles, was zum Fortp anzungs-
geschäft von Corydoras paleatus gehört … noch
ungeklärt und dunkel ist“; LIEBIG 1924, 231).
Between the rst note of LIEBIG and his
last summarizing remarks in 1924 also other
aquarists have published their observations and
conjectures. WITTIG (1913) again described the
female’s attachment by suction near the ventral
ns of the male and the deposition of eggs
in the n pocket. After a little rest, the female
spit on a leaf and then she glued an egg on it.
WITTIG (l. c.) assumed that insemination and
fertilization takes place on the leaf and eggs were
glued on by secretions of the mouth. Further,
he accurately described tone pitches produced
15
Bull. Fish Biol. 15 (1/2)
by the male, which obviously change depending
on the degree of excitement.8
MERTENS (1917) followed the view of JÜR-
GENS (see above) and acknowledges the merits
of aquarists concerning the reproduction of
Corydoras spp.9 Meanwhile DUNGAN (1917) had
bred Corydoras paleatus successfully in the USA.
The author says nothing about courtship and
mating, but in an appendix entitled “Notes on
the Panzerwels” attached to this note the editor
of the journal reports on the above mentioned
article of WITTIG, which is not easy to identify
at rst glance on the basis of the given bib-
liographic information. FRIEBE (1924) favours
insemination and fertilization after the nal
deposition of eggs. DÖRSCHEL (1925) believes
that the female is surrounded by a cloud of
sperm that moves along her body up to her
n pocket due to the quivering male. ROELICKE
(1933) describes the spawning behaviour very
accurately, excludes an internal insemination and
favours the oral uptake of sperm and their pas-
sage through the intestine, arguing that, because
the air easily and rapidly passes the intestine, the
same may apply for sperm. SELEUTHNER (1938)
does not know how insemination and fertiliza-
tion may take place, but excludes oral uptake of
sperm as he never has seen the female’s mouth
near the male genital opening. However, later,
he favours the intestinal passage of sperm
(SELEUTHNER 1950). ROHRBACH (1939) describes
the spawning behaviour of Corydoras hastatus,
recognizes heavy movements of the gill covers
of the female during the putative oral uptake of
sperm and discusses whether sperm leave the
gill openings or whether they pass the intestine
to reach the n pocket.
Finally, the strange and fully deviating specu-
lation of Ada LATHAM (1935) should be men-
tioned herein for the sake of completeness. She
describes the clasping (=interlocked position),
but otherwise cannot con rm previous studies
suggesting that the female‘s relatively long pec-
toral n may have a tube that pick up sperm to
channel them into the n pocket.
From around 1939 there are no real new obser-
vations; and the discussion – only conjectures and
speculations – goes round in circles. Principally,
the scenarios listed in table 2 are checked again
and again often without acknowledging previous
providers of ideas. However, some of them
should be mentioned brie y (see also tab. 2).
BREITLING (1940) argues against the oral uptake
of sperm and the intestinal passage in Corydoras
hastatus, because the female did not adhere to the
male‘s anus and because she has eaten between
deposition of the eggs in the n pocket and the
nal deposition of eggs. CARD (1949) mentions
a broad spectrum of possibilities how to insemi-
nate and fertilize the eggs, e.g. immediately when
the spawn is deposited on the glass by sperm
released onto the female‘s ventral ns, when the
male chases the female (his own suggestion), the
oral uptake of sperm by the female and their
discharge before or after deposition of eggs, and
that the male releases sperm in certain areas and
then guides the female there. STETTLER (1950)
speculates that the (invisible) cloud of ejaculated
sperm is sucked in by the sudden opening of the
n pocket, which process he considered more
probable than the oral uptake of the ejaculate.
FEIGS (1954) again rejects the intestinal sperm
passage, thinks to have observed oral uptake of
sperm and postulates their discharge from the gill
opening and their transport along the female‘s
body to the n pocket. NIEUWENHUIZEN (1955)
referring only to SEELEUTHNER (l.c.) and FEIGS
(l.c.) agrees, with FEIGS‘ view and considers the
intestinal way of sperm unlikely. Last not least
PINTER (1955) assigns active roles to both mates,
observes that the female adheres (by suction) to
various parts of his body, occasionally also to
the genital opening, but thinks the oral uptake
of sperm wrong. Further, he believes that sperm
might be highly mobile in the water and brings
chemoattractants into play.
The secondary literature before KNAACK‘s
articles recapitulates previous views, e.g. of
LIEBSCHER (l. c.) and CARBONNIER (l. c.) (e.g. BADE
1931) or favoured insalivation and deposition of
sperm before deposition of eggs (INNES 1935,
ARNOLD & AHL 1936). INNES (l .c.) also mentions
clasping the female‘s barbels by the pectoral n
of the male.
Really new insights in the reproductive behavi-
our of Corydoras spp. based on pure observations
16
could not longer be expected. This changed in
1955, when KNAACK started a series of articles,
of which especially the rst one takes an experi-
mental approach to the subject (KNAACK 1955 ff.).
2.2. The publications of KNAACK (1955-1964)
From 1955 to 1964, KNAACK published four ar-
ticles on the reproductive behaviour of various
Corydoras spp. (C. paleatus, C. hastatus, C. aeneus,
C. elegans, and C. caudimaculatus). Of course, also
the publications of KNAACK do not meet current
scienti c standards as he says for instance too
little about the methods he used10, probably
not to overtax his readers. However, his ar ticles
differ substantially from previous (popular)
publications in so far as he observed carefully
conducting inventive experiments. In addition,
the photos he added to his articles were really
excellent for that time.
The most important article dates from 1955
(KNAACK 1955). Here KNAACK described very
detailed courting and spawning of Corydoras
paleatus (without touching, however, previous
observers) as a sequence of relative stereotyped
activities. He distinguishes:
(1) restless swimming of mates and “cleaning”
of putative egg laying places by the female;
(2) males tickle the female with their barbels and
pectoral ns starting from her tail and then
going to her head;
(3) a kind of wedding dance, in which the male
performs a variety of movements;
(4) the male blocks the female in T-position
by clasping her barbels with his pectoral n
(mostly near the substrate) (see g. 2) and
mates sink to the bottom and remain here for
50 bis 60 s until separation;
(5) discharge of sperm and their distribution
into the water (see below);
(6) deposition of eggs into the n pocket;
(7) insemination and fertilisation of eggs in
the pocket;
(8) egg deposition on the substrate.
This process may be repeated several times.
Then KNAACK pays special attention to the
way sperm take to reach the n pocket. In his
survey of what has been previously suggested,
he confines himself to secondary aquaristic
literature. KNAACK dissected a female of Cory-
doras paleatus ready to spawn to exclude internal
fertilization and did not found developing egg in
the ovary. Further, when he separated the mates
during courtship or during spawning, females
continued to lay eggs that, however, were unfer-
tilized, but eggs removed from the pocket were
fertilized (see REITZ 1910 b). Using a pipette with
a nely drawn-out tip he applied a solution of
concentrated sodium uorescein (the amount
was comparable to that of the respiratory water)
directly in front of the female´s mouth, which
rapidly spread, but did not reach the n pocket.
The procedure was repeated at the time the male
discharges his sperm, i.e. when he had caught
the female with the spine of the pectoral n. In
this case a “cloud” of the uorescein solution
(and in that very probably sperm) spreads in the
water through the movements of the mates ( g.
3). In addition, during different stages of mating
he removed water at different body sites (genital
opening of the male, mouth and gill slit of the
female) to check it for sperm. Detection of sperm
seemed to have succeeded, but he does not give
any further details formulating this result some-
what cryptically.11 He noticed that spermatozoa
in freshwater remained viable for 15 min.
In the following years KNAACK (1956, 1961,
1964) extended his studies to various Corydoras
species. Apart from minor probably species-
speci c differences the observation of their
Fig. 2: Reproductive behaviour. A Courtship (Corydoras napoensis), male in front (Photo: H.-J. RICHTER). T-
Positon. B Corydoras paleatus. C Corydoras aeneus. Note clasping of the female’s barbels by the male’s pectoral
n (B drawing: Ramona ALLSTADT-TORRAS after a photo by R. ZUKAL (1973 b). C Photo: H.-J. RICHTER).
Abb. 2: Fortp anzungsverhalten. A Balz (Cor ydoras napoensis), Männchen vorne (Foto: H.-J. RICHTER). T-
Stellung. B Corydoras paleatus. C Corydoras aeneus. Man beachte die Brust osse des Männchens, die die Barteln
des Weibchens einklemmt (B Zeichnung: Ramona ALLSTADT-TORRAS nach einem Foto von R. ZUKAL (1973
b). C Foto: H.-J. RICHTER).
17
Bull. Fish Biol. 15 (1/2)
18
reproductive behaviour con rmed the general
sequence of the events given above.
In the 1964 article, in which he also intro duces
Corydoras aeneus he gives a kind of summary and
deals with selected articles from the aquaristic
literature.
In conclusion, KNAACK, partially con rming
and partially disproving previous views, showed
on the basis of photos, lm sequences and some
experiments
(1) that the reproductive behaviour of several
Corydoras spp. is broadly the same,
(2) that the male is the more active partner,
(3) that the male blocks the female during the
T-position by clasping her barbels with its
pectoral n,
(4) that sperm are released into the free water, and
(5) that eggs become fertilized in the n pocket.
His articles have brought an endless debate
to a temporary conclusion.
Fig. 3: Distribution of the breathing water of Corydoras paleatus after application of a uorescein solution.
On the top: a single female; below: in a couple in T-position during spawning. Note the different distribution
of the uorescein. For further explanations see text (from KNAACK 1955).
Abb. 3: Verteilung des Atemwassers bei Corydoras paleatus nach Applikation einer Fluorescein-Lösung. Oben:
bei einem einzelnen Weibchen. Unten: bei einem Paar in T-Stellung. Man beachte die unterschiedliche Ver-
teilung des Fluoresceins. Weitere Erklärungen s. Text (aus KNAACK 1955).
19
Bull. Fish Biol. 15 (1/2)
KNAACKs observations have been appreciated
relatively early also in non-German magazines
(e.g. VA N RAMSHORST 1957) and later FINLEY
(1995) acknowledges his studies, when com-
menting the study of KHODA et al. (1995) in
the magazine „Tropical Fish Hobbyist“. FINLEY
quotes a statement of KLEE from a 1963 ar-
ticle, in which he does not want to consider
“the ridiculous suggestion that she (the female
Corydoras) swallows the sperm and later passes
it through her vent” (FINLEY 1995, p. 110).
However, his note “American aquarist Albert
Klee later reported repeating the experiments
in the early 1960s with similar results” (FINLEY
1995, p. 109) is not quite correct. Albert KLEE
reported on KNAACK’s experiments in Tropicals
Magazine, September-October 1963, and did
not add anything new, but stated at the end of
the article: “As a postscript, it should be men-
tioned that I have tried the pipette-dye technique
myself (also experimenting with other dyes)
and have found it most effective. Patience is
needed (as well as the proper equipment) but
water currents and ows can be pinpointed quite
nicely.” (KLEE 2015, pers. comm.) In 1970, KLEE
published a Corydoras-booklet for beginners, in
which he brie y touches “sperm drinking” and
transport of sperm by the female’s barbels to
fertilize eggs after deposition, but then he goes
on to say: “The true story is much simpler one.
The T-formation (and this is not to be taken too
literally … the T, at times, may even degenerate
almost into two parallel lines) serves to trigger
the release of the male’s sperm at the exact
time the eggs are cupped in the female’s pelvic
ns. The close proximity of male with female
during this process virtually insures proper fer-
tilization.” (KLEE 1970, p. 7/8) This is entirely
consistent with KNAACKs experiments, but KLEE
regrettably does not name any source of this
information.
VOGT (1970) summarized courtship and mat-
ing of armoured cat shes in „Grzimeks Tierle-
ben” (at that time a famous multi-volume work
with several editions and translations in many
languages, also in English) notably appreciating
the ndings of KNAACK.12 His ndings appeared
to be largely accepted until the publication of
the study of KHODA et al. (1995). It was put in
several manuals for cat sh keepers and breeders
(see especially the contributions of H.-J. FRANKE
in Sterba 1978, 1983; FRANKE 1985; MATSCHKE &
MATSCHKE 1986; HIERONIMUS 1985). Especially
FRANKE (in STERBA 1983, p. 175-176) expresses
this issue brie y and precisely: “When mating,
the male clamps the barbels of the females fast
with the P and releases sperm at the same time.
At the same moment the female produces 3–5
eggs and puts them in the Vs which have folded
to make a bag. Then she swims though the cloud
of sperm to fertilize the spawn and sticks this
rmly after a short search.” CURRIER & SMITH
(1969, p. 70) stated: “The other long standing
myth concerns the manner in which Corydoras
eggs are fertilized. Although scienti c proofs
have been available since 1955 the discussion
still ranges in many quarters.”
BREDER & ROSEN (1966) summarizing the
reproductive behaviour of Callichthyidae in
their well-known synopsis “Modes of repro-
duction in shes” considered most of the above
mentioned aquaristic literature until 1957, but
obviously were not aware of KNAACK‘s studies.
However, authors describe the sperm transfer
through the female‘s intestine as “surely an
untenable conclusion”, but do not give reasons
for this statement (p. 268). BURGESS (1987,
1989) describes the clasping of the female’s
barbels with the male‘s pectoral spines, favours
the idea of a cloud of sperm surrounding the
mates, and considers other theories “to be less
plausible” (p. 348), but does not mention either
the sug gested intestinal transfer of sperm or any
relevant literature concerning this issue.
2.3. The study by KHODA et al. (1995) and
its consequences
From the above results and discussions noth-
ing can be found in KHODA et al. (1995). This
is strange, as the authors quote the synopsis
of BREDER & ROSEN (1966), which summa-
rizes most of the previous aquaristic literature
concerning this issue (see above). Rather they
extracted three hypotheses from popular books
(among others BURGESS 1989) how eggs are
20
inseminated “Insemination occurs (a) by sperm
spread in the aquarium at various times after
spawning; (b) on the egg attachment site by
sperm, and there discharged from the mouth
of the female held from the time of T-position,
and (c) inside her pouch by sperm coming from
a male in the T-position” (p. 3) or to formulate
the relevant questions (1) Which male does fer-
tilize the eggs? (2) Where fertilization does take
place? (3) How sperm enter the pocket formed
by the female’s ns? Hypotheses 1 and 2 were
disproved in favour of hypothesis 3 for a long
time (see above).
In the abstract author’s state: “We report on
unique reproductive behaviour and a new mode
of egg insemination in a small cat sh Corydoras
aeneus (Callichthyidae). A male courts a female
by presenting his abdomen to her. Before releas-
ing eggs, the female attaches her mouth to the
male’s genital opening and directly drinks his
sperm. The sperm pass through her intestine
and are discharged together with eggs into the
“pouch” formed by her pelvic ns. Thus eggs
are mixed with fresh non-dispersed sperm in an
enclosed space, ensuring effective insemination.
This mode of insemination is novel to shes,
but likely not restricted to cat shes of the genus
Corydoras.” (p. 1).
The brief description of courtship and spaw-
ning of Corydoras aeneus corresponds largely to
that what is known from previous descriptions
including ejaculation of sperm that according
to general opinion takes place when mates are
in the T-position. However, their description
differs in two crucial details from previous ob-
servations: (1) Authors describe that the female
attach her mouth to the male’s anal region or
genital opening instead of clasping her with
his pectoral n (interestingly, gure 1 in their
article illustrating the sequence of behaviour
during reproductive of C. aeneus shows in section
gure d that the male clasps the barbels of the
female with his pectoral n), and (2) that the
female closed her gill covers tightly for approx.
7 s, i.e. approximately the time the attachment
of mates takes.
To clarify the way of sperm authors perform-
ed three experiments. In experiment I they
used different strains of mates (albino and
normal type) what shows that all eggs of one
clutch were inseminated by sperm of the given
mate. Results disproved the above mentioned
hypothesis 1. In experiment II they collected
eggs at various times after release 1 and showed
that eggs are inseminated inside the n pocket
rejecting hypothesis 2. These results con rmed
experimentally what was already expressed in
earlier opinions (see above). In view of the
above mentioned observations, experiment III
is of speci c interest.
Authors pipetted an aqueous solution of
methylene-blue at the mouth of a female in
T-position and describe their observations as
follows: “After 4.2 s (± 0.8 SD, n=22) of the
release of the solution, blue water appeared
from her anus and was funnelled into her ventral
pouch. During the time that the gill covers were
closed (about 7 sec), the blue solution did not
ow from her gills. The blue solution remaining
around the mouth of the female did not reach
the abdominal area (…). The ow of the blue
water clearly indicates that the female drinks
sperm together with some water from around
the anal region of the male and discharges this
into her pouch” (p. 4). With this experiment,
authors obviously excluded a “sperm ow or
swim along the outside of the body of the
female” (p. 4). Unfortunately, authors did not
state how and by what means they obtained such
exact measurements of time.
Conclusions are as follows: “The reproductive
behaviour of Corydoras aeneus includes a female’s
mouth attachment to a male’s anal region in the
T-position and releasing eggs into her pouch
formed by ventral ns as in congeners (…). The
ow of the blue water strongly indicates that
while the gill covers of the female of C. aeneus
are closed during the T-position, she directly
drinks the sperm with some water, which is
transported quickly through her intestine and
discharged into her pouch. Eggs are released just
after the discharge of sperm and will be insemi-
nated inside her pouch, while she remains still on
the bottom. Although shes exhibit a variety of
reproductive behaviours (…), this mode of egg
insemination in C. aeneus is novel among shes,
21
Bull. Fish Biol. 15 (1/2)
and perhaps in the animal kingdom (KHODA et
al. 1995, p. 4-5).
Presence of a short gut length, frequent gulp-
ing air and its rapid passage through the gut,
a n pocket near the anus and the T-position
during mating, all these features typical for
many Callichthyidae prompted the authors to
assume a wider distribution of sperm drinking
among cat shes. Generally, “sperm drinking”
has been considered as highly adaptive as this
strategy may ensure a high insemination rate in
turbulent habitats.
Critical responses to this study were primarily
published in aquarist magazines, whereas reac-
tion of the scienti c community was reserved or
scientists simply accepted the study (see above
and below). Here we refer only to two articles.
One study focusing on sound production of
Corydoras paleatus, brie y describes the courtship
sequences based on video sequences, and gives a
drawing that shows a male clasping the female’s
barbels (PRUZSINSZKY & LADICH 1998). Authors
cite KHODA et al. (1995), but did not take any
further notice of clasping.
The second study, a review on the reproduc-
tive biology of the cat sh Hoplosternum littorale
(HOSTACHE & MO 1988) describes the contact
of the female‘s mouth with the male‘s genital
opening, while mates are in the T-position.
During this time the female obviously collects
the sperm in her mouth. After that she rests up
to 60 sec on the bottom. According to GAUTIER
et al. (1998; see also BREDER & ROSEN 1966),
the female of H. littorale simultaneously release
sperm (collected in her mouth) and eggs (depo-
sited in her n pocket) in the foam nest. In view
of the study of KHODA et al. (1995), HOSTACHE
& MO (1998) think that “sperm drinking” in H.
littorale is possible.
3. Are there arguments against “sperm
drinking”?
Results and their interpretation presented by
KHODA et al. (1995) are, indeed, highly sugges-
tive. Nobody doubts today that only one male
fertilizes the eggs per clutch and that eggs are
fertilized in the female‘s n pocket. Regarding
“sperm drinking”, two observations by KHODA
et al. (1995) cannot be con rmed or denied at
present. These are (1) the release of the methy-
lene blue solution applied to the female during
T-position through the female‘s anus, and (2) the
tight closure of the gill covers during the period
the female may take up the sperm. By the way
many of the above mentioned observers noted
heavy movements of the gill covers during this
time. Concerning the distribution of colour
solutions a contrary study exists (KNAACK 1955),
whereas the closure of the gill cover has been
never mentioned before to our knowledge.
Provided that sperm do really pass the in-
testine as described by KHODA et al (1995), it
is reasonable to interpret the results of other
studies on the biology of armoured cat sh
within the meaning of this phenomenon. Thus,
KHODA et al. (2002) argued in a follow-up study
that the strikingly small gonadosomatic index of
Corydoras aeneus, an obviously missing contest
competition and, thus, sperm competition cor-
respond with the small testis (see also STOCKLEY
et al. 1997).
A similar approach is evident in two studies
on the seminal vesicles published in the same
year. Seminal vesicles are accessory glands of the
male genital tract of a variety of non-related te-
leost taxa, whose secretions containing a variety
of substances contribute to the seminal uid.
Secretions contain a variety of substances, of
which various glycoproteins (“mucins”) consti-
tute the largest part, which are assumed to have
a variety of functions such as ensuring sperm
viability, immobilization of sperm, nutrition of
sperm etc., but strictly speaking functions are
not clear (for further readings see MAZZOLDI et
al. 2007). FRANCESCHINI et al. (2007) studying
the seminal vesicles of C. aeneus by conventional
histochemistry speculate that their secretions
might protect the sperm cells during the pas-
sage through the gut “forming a gelatinous
environment” (p. 114) that also keeps sperm
aggregated and immobile. In a broader study
MAZZOLDI et al. (2007) found that within the
Callichthyidae Corydoras spp. and Hoplosternum
littorale, both assigned to the “sperm drinking”-
type, possess seminal vesicles, whereas Callichthys
22
callichthys ascribed to the classic type of external
spawners did not have this structure. Again they
assume that the viscous mucus secreted by the
seminal vesicles may reduce sperm dispersal (of
signi cance for males with a low investment in
gonads, see above) and protect the sperm in the
gut. To our knowledge there is no experimental
evidence for the suspected protective role of
seminal vesicle secretion.
Aside from references to previous studies and
a general feeling of suspicion (see introduction)
some inconsistencies arising from the article
of KHODA et al. (1995) should be commented
more detailed.
Since 1881 courtship and mating of cat shes
of Corydoras spp. have been observed countless
times. Therefore the general sequence of beha-
vioural events as described above is suf ciently
proven. As a result it is unquestionable that the
male blocks the female by clasping her barbels
during the T-position, in which, and this is the
overall opinion, sperm are discharged by the
male. Clasping has been documented repeatedly
by photos (s. KNAACK 1955, 1964; FREY 1957;
BURGESS 1987; ZUKAL 1973 b) and drawings (e.g.
PRUDZINSKI & LADICH 1998) and even KHODA
et al. (1995) depict this event (see their g. 1 d),
using, however, the ambiguous word “attaches”
to the genital opening (p. 1) or “attachment”
to the anal region (p. 5) to describe the contact
between female and male. Therefore it is ques-
tionable, whether females do place her mouth
over the genital opening of the male to suck
in sperm. If so, drinking of discharged sperm
would be extremely ineffective, if at all possible.
The time (approx. 4 s) for passage of sperm
through the gut has caused surprise. Corydoras
spp. are benthivores (KHODA et al. 2002) special-
ized primarily on aquatic invertebrates (IBANEZ
et al. 2007). Intestines of Corydoras spp. (mean
standard length 51 mm) measures 28 mm on
average (distance from the beginning of the
esophagus to the anus) (IBANEZ et al. 2007) and
of C. aeneus 1.3-1.6 times the standard length
(KRAMER & MCCLURE 1980) or 5.4 cm (KHODA et
al. 1995; measured distance and standard length
of specimens are not given). The mean forward
velocity spermatozoa of freshwater teleost’s
may reach is 135-164 μm/s and longevity is
within the range of a few seconds up to several
minutes depending on the species (ALAVI et al.
2005; see also BROWNE 2015). The maximum
travel distances of sh sperm is relatively short
(2.3–14.0 mm), because their high velocity rang-
ing from 65 to 250 μm/s failed to compensate
for their motility period of seconds to minutes
(for review see BROWNE et al. 2015). Although
relatively short, it is clear that spermatozoa are
not able to cover this distance on their own.
The digestive tract of Corydoras spp. consists
of the pharynx, a short, straight esophagus, a
small muscular stomach, a thick anterior intes-
tine with a highly folded mucosa, a thin-walled
translucent posterior intestine with a at and
highly vascularized mucosa ( g. 4 A-F), and the
rectum. Anterior and posterior intestine form
some loops in the body cavity. The posterior
intestine, approximately 21-69% of the entire
intestine, serves as an accessory respiratory or-
gan. It is unsuitable for digestive functions and is
generally found to be devoid of digesta (KRAMER
& MCCLURE 1980; PODKOWA & GONIAKOWSKA-
WITALINSKA 2002; PERSAUD et al. 2006; NELSON
& DEHN 2011). The speci c organisation of
Fig. 4: The various parts of the digestive tract of Corydoras aeneus. A Transition from the pharynx (ph) to the
esophagus (es). Note pharyngeal teeth (arrow). B Transition from the esophagus (es) to the stomach (st);
muscles (m). C Transition from the stomach (st) to the digestive intestine (di); note digesta in the lumen (ar-
rowhead), tubular glands (tg). D Digestive intestine with high folding of the mucosa (di). E Transition from
the digestive intestine (di) to the respiratory intestine (ri). Digesta (arrow). F Flat epithelium of the respira-
tory intestine with several goblet cells (arrows); muscle layer (m). Staining: AB-PAS for the demonstration
of various glycoconjugates that are dyed in various red-blue shades. Scale bar: A-E = 1 mm, F = 100 μm.
Abb. 4: Die verschiedenen Abschnitte des Verdauungstraktes von Corydoras aeneus. A Übergang von Pha-
rynx (ph) zu Ösopagus (es), man beachte die Pharynxzähne (Pfeil). B Übergang vom Ösophagus (es) zum
Magen (st); Muskulatur (m). C Übergang vom Magen (st) zum verdauenden Darmabschnitt; man beachte
23
Bull. Fish Biol. 15 (1/2)
die Nahrungsreste (Pfeilspitze), tubuläre Drüsen (tg). D Verdauender Darmabschnitt mit starker Auffaltung
der Mukosa (di). E Übergang vom verdauenden (di) zum respiratorischen Darm (ri). F Flaches Epithel des
respiratorischen Darms mit Becherzellen (Pfeile); Muskellage (m). AB-PAS-Färbung zum Nachweis verschie-
dener Glykokonjugate, die in blauroten Farbschattierungen gefärbt sind. Maßstab: A-E = 1 mm, F = 100 μm.
24
the posterior intestine, which allows to digest
and breath simultaneously by channelling (see
below), may serve as tool for explaining the
rapid passage of sperm through the gut (see
HELFMAN et al. 2007). KHODA et al. (1995) even
suggested that intestinal breathing may be one
of the preconditions enabling the evolution of
“sperm drinking”.
As gut air breathers callichthyid catfishes
frequently gulp air, which then passes quickly
through the gut leaving the anus as an air bub-
ble. To ensure ef cient gas exchange, digesta in
the posterior intestine are quickly removed by
the unidirectional transport of air rather than
by peristaltic movements. However, to create a
more or less permanent air channel a transi tional
muscular zone between the digestive and respira-
tory portion compacts the digesta and possibly
encases it with mucus, which results in a string of
compressed boluses (PERSAUD et al. 2006). Obvi-
ously, formation of compressed boluses does not
occur in the stomach and the digestive (anterior)
intestine, which are often lled to some extent
as Corydoras spp. are opportunistic feeders eating
more or less continuously also between the single
batches (see also g. 4 C, E). This means that
according to the view of KHODA et al. (1995) the
water (with sperm) sucked in by the female must
nd its way through the esophagus, the stomach,
the anterior intestine ( lled at least in part) and the
column of air present in the posterior intestine
within seconds. These issues have not yet been
suf ciently considered.
4. Conclusions
Comparing the observations and experiments
of amateurs done in the last century on repro-
duction of callichthyid cat shes with that of
the only scienti c publication on this matter
some questions and problems have arisen that
need to be clari ed before “sperm drinking” in
cat sh can count as a “proven” fact. However,
before it makes sense to approach this problem
again, novel experimental strategies and designs
should be developed. In this context, it is notice-
able that within the last 20 years apparently no
one has made a serious effort to experimentally
reprocess this “novel” and “unique” mode of
egg insemination.
Another issue we only touch here is the fact
that the most important data summarized by us
have been published at the beginning and the
middle of the last century in magazines, which
we would today call at best „gray“ literature.
However, at that time aquarium shes appeared
to swim more easily across the boundaries
between amateurs and professionals bringing
considerable knowledge in their train.
According to today’s usage, the term “gray”
literature denotes not only conference proceed-
ings, institutional reports not peer-reviewed, but
generally “unconventional” (scienti c) literature
outside of the book trade, but also conventional
informational material, e.g. magazines for hobby-
ists that usually do not contain original scienti c
contributions. Especially the latter are ignored
to the largest extent by the scienti c community.
This is quite understandable considering the
almost unmanageable amount of publications
issued annually, and in view of the fact that these
contributions mostly do not meet scienti c stand-
ards (e.g. precise description of the methods used,
plausible interpretation supported by the existing
(scienti c) literature, etc.), that “gray” literature is
generally harder to access (for many people the
inaccessibility is enough reason why it should
not be cited), that those articles are published in
national or even local magazines and, thus, are
entirely unknown, or/and that they are written
in languages not the easiest to understand. These
disadvantages are not compensated by the often
high circulation, good print quality and the low
subscription rate. However, although mostly pub-
lished in this kind of literature, experience and
careful observations made by numerous aquarists
signi cantly contributed and still contribute to the
knowledge of the biology of (ornamental) shes
just as it was illustrated above. However, we have
to ask ourselves whether these contributions are
always acknowledged properly.
Acknowledgements
We would like to thank Mr. Kees DE JONG,
Mr. Harro HIERONIMUS, Dr. Dieter HOHL, Mr.
25
Bull. Fish Biol. 15 (1/2)
Michael KEMPKES, and Mr. Marc VAN DRIESSEN
for providing some articles from a literature,
which we did not always nd easy to obtain, to
Mr. H.-J. RICHTER and Dr. H.-J. HERRMANN for
providing and approving the photos of Corydoras
napoensis and Corydoras aeneus ( g. 2 A, C) for
publication, and to Ms. R. ALLSTADT-TORRAS
for the drawing of Corydoras paleatus ( g. 2 B).
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Received: 25. 04.2015
Accepted: 26. 05. 2015
Appendix
In the following we literally quote some passages
from articles of aquarium magazines that to our
opinion are particularly informative and origi-
nal. Further, we add an translation into English
maintainig all inconsistencies such as imprecise
information on anatomical details etc.
1 „Séduits et encouragés sans doute par la déclaration
qui venait de leur être adressée par la femelle, deux
ou trois mâles des plus entreprenants se précipitèrent
vers celle-ci; ils s’agitaient sur ses côtés tout le long
de l´abdomen, un autre se plaça sur son dos, un plus
audacieux encore se posa en travers, au-dessus de la
tête et à l’aide du premier rayon osseux de sa nageoire
pectorale, comme avec une main, il étreignit avec
force la femelle par ses barbillons. Accroché ainsi et
posé dans le sens transversal, il se laissa glisser jusque
sous la tête de la femelle, en émittant avec force
dans la direction de l’abdomen de cette dernière ses
principes fécondants… Au moment même où elle se
sentait étreinte par le mâle, elle rapprochait l’une de
l’autre ses deux nageoires ventrales à la façon de deux
éventails ouverts qu’on réunirait par leurs bords, for-
mant ainsí une sorte de cul-de-sac dont son abdomen
et les membranes des nageoires formaient les parois,
et au fond duquel se trouvait l’ouverture des ovaires.
Les principes fécondants du mâle se trouvent ainsi
emprisonnés, sans pouvoir s’échapper, dans cette
sorte de sac membraneux, et lorsqu’une minute après
les œufs vont y arriver expulsés par les contractions
abdominales de la femelle, ils se trouveront en contact
immédiat avec un liquide riche en spermatozoaires et
par suite fécondés.“ (CARBONNIER 1881, p. 106/107)
[Seduced and encouraged by the female’s declaration
of love, two or three more enterprising males approa-
ched her moving to her abdominal side; another male
positioned himself over the female’s back and an even
more bolder male placed himself transversely directly
over her head vigorously clasping (like a hand) the fe-
male’s barbels with the rst bony spine of his pectoral
n. Attached in this way and in the transversal posi-
tion he slipped under the head of the female, where
he released his fertile principles (= sperm) towards
her abdomen with great power … At the same mo-
ment she felt clasped by the male, she brought close
29
Bull. Fish Biol. 15 (1/2)
to each other her two pelvic ns like two open fans
touching one another at the edges. Thus, she formed
a kind of pocket, whose walls consisted of the the
abdomen and the membranous ns, while the ovaries
opened on its oor. Sperm of the male are stowed
in this kind of membranous sac, without being able
to run out, and if a minute later the eggs, expelled
through the female´s abdominal contractions, arrive
there they come into direct contact with a uid rich
ine sperm and become thereby fertilized ...]
2 „Jetzt packte das Weibchen das quer vor ihm
liegende Männchen mit dem Maule dicht über der
Bauch osse in die Seite und sog sich an ihm fest. Das
Männchen bog sich halbkreisförmig unter fortwäh-
rend zitternden Bewegungen, so daß die konkave (S.
217) Seite dem Weibchen zugekehrt war, und zuckte
konvulsivisch zusammen. Während dem hatte das
Weibchen die gespreizten Bauch ossen zusammen-
gefaltet und daraus eine Tasche gebildet, die den
After, welcher zugleich als Geschlechtsöffnung dient,
vollständig umschloss. Gleich darauf sah ich kurz
hintereinander etwa ein halbes Dutzend weiße Eier
in die glashelle Flossentasche fallen. Während dieser
Vorgänge ließ das Männchen öfter ein knurrendes
Geräusch hören,…Trotz größter Aufmerksamkeit
habe ich nicht wahrnehmen können, daß das Männ-
chen die Eier erst nach dem Ablegen befruchtet. Die
Befruchtung derselben wird vielleicht schon in dem
Moment erfolgen, wo das Männchen vom Weibchen
angesogen wird und in konvulsivische Zuckungen
gerät. Wahrscheinlich wird das mit den männlichen
Spermatozoen geschwängerte Wasser, welches von
der Flossentasche des Weibchens aufgefangen wird,
im Innern der Tasche die Befruchtung der Eier be-
wirken“.(JÜRGENS, 1900, S. 218)
[Now the female grabbed the male that lies
across her with her mouth just above the pelvic n and
rmly adhered to him by suction. The males formed a
semicircle while constantly shivering his body, the cur-
ved side facing the females, and twitching convulsively.
Meanwhile the female has folded up the pelvic ns to
form a pocket that completely enclosed the anus, which
serves also as a genital porus. Shortly after this I saw
how about half a dozen white eggs dropping in quick
succession into the bright n pocket. During these
operations, the male make a croaking noise ... Despite
the utmost attention I could an not observe fertilsaten
by the male after dropping the eggs. Their fertilization
probably takes place at the moment the male is sucked
in by the female and twitches convulsively. Perhaps the
water lled with sperm and captured by the female‘s n
pocket, will fertilize the eggs in the pocket.]
3 Die Eier werden aber nicht erst nach dem Absetzen
der Eier befruchtet, sondern dieser Akt ist schon
vorher vollzogen. Der Befruchtungsvorgang ist nur
so zu erklären, daß durch den Reiz des Ansaugens
beim Männchen ein geringer Teil Sperma austritt
und das Weibchen unmittelbar darauf durch Zu-
sammenklappen der Bauch ossen ein Teil des mit
Spermatozoen geschwängerten Wassers abfängt.
Die Befruchtung geschieht demnach schon in der
Flossentasche. Es scheint, daß durch die in einem
Ruck erfolgende halbkreisförmige, seitliche Biegung,
die der Körper des Männchens im Momente des
Ansaugens einnimmt, wobei die konkave Seite dem
Weibchen zugekehrt ist, die Spermatozoen nach der
Mitte des Bauches des Weichens dirigiert und mit
den zusammenklappenden Bauch ossen aufgefangen
werden“. (JÜRGENS, 1906, S. 368-369)
[The eggs become fertilized not after their dropping
(on the substrate), but fertilization has taken place
already previously. The fertilization process can only
be explained by the fact that the females triggers the
release of a small amount of sperm by sucking on
the male and that the female catches portions of
water loaded with sperms by folding up her ventral
ns. Thus, fertilization takes place already in the n
pocket. It seems that the sperm is directed towards
the middle of the female‘s belly and captured with the
folded up pelvic ns, while the male abruptly bends
his body laterally in a semicircle during the sucking,
his curved side directed against the female.]
4 „Die Befestigung des Laiches erfolgte in der Weise,
daß das Weibchen zunächst eine Stelle mit dem Samen
des Männchens, den es von ihm angesaugt hatte,
einspeichelte, sodann je ein Ei darauf klebte … Die
Befruchtung des Laiches erfolgt demnach nicht, wie
bisher angenommen wurde, in der Flossentasche,
sondern außerhalb derselben dadurch, daß das Weib-
chen die Eier mit dem vom Männchen ausgesaugten
Samen einspeichelt“ (SCHUBERT, 1907, S. 546).
[Fixing the spawn was carried out in such a way
that the female rst insalivated a site with the male’s
sperm sucked in before and then attached a single
egg there…Thus, fertilization of the spawn does not
take place in, as previously thought, but outside the
n pocket by the fact that the female insalivates the
eggs with male sperm sucked in before.]
5 „Direkt nach einem Paarungsakt ng ich das be-
treffende Weibchen mit voller Flossentasche aus dem
Zuchtbehälter und bemächtigte mich mittels eines
P anzenstengels der Eier, die ich in einem gesonder-
ten Behälter unterbrachte. Dieselben waren mit dem
30
Maule des Weibchens absolut nicht in Berührung
gekommen und fielen trotzdem ordnungsgemäß
aus, … Es darf also als einwandfrei gelten, dass die
soeben ausgestoßenen Eier schon in der Flossen-
tasche des Weibchens befruchtet werden.“ (REITZ,
1910 b, S. 375)
[Immediately after mating I removed the respective
female, whose n pocket was full, from the breeding
tank, seized the eggs by means of a plant stem and
brought them in a separate container. They de nitely
did not come in contact with the mouth of the female,
but developed properly ... It is certain that the just
ejected eggs are fertilized already in the n pocket
of the female.].
6 „Ich überlegte mir nun, weshalb eigentlich das
Weibchen die Eier in der Bauchflossentasche
festhält, da dies zum Zwecke des Anheftens doch
nicht erforderlich ist, wie zum Beispiel die Cichliden
zeigen. Ich kam deshalb auf den Gedanken, daß es
vielleicht doch nur zum Zwecke der Eibefruchtung
geschieht und daß möglicherweise das Sperma den-
selben Weg nimmt wie die akzessorische Atmung.
Es ist dies natürlich nur eine Vermutung.“ (LIEBIG,
1912 a, S. 427)
[Now I was wondering why the female holds the eggs
in the bag formed by the pelvic ns, because this is
not required for the purpose of adhering them, as,
for example, is shown in cichlids. I therefore had the
idea that this might happen only for the purpose of
fertilization of eggs and that sperms possibly take
the same route as the accessory breathing. This is of
course just a guess.]
7 „Die von mir 1912 ausgesprochene Vermutung, daß
das Sperma den gleichen Weg durch den Körper des
Weibchens nimmt wie die der akzessorischen Atmung
dienende Luft, beruhte auf dem Glauben, daß neben
dem Verdauungsdarm noch ein Atmungsdarm her-
liefe. Heute bin ich in solchen Sachen vorsichtiger,
leider über den Bau des akzessorischen Atmungsor-
gans auch nur insoweit unterrichtet, daß ein Teil der
Darmschleimhaut hierzu dient. Trotzdem komme ich
aber auch jetzt wieder zu einem ähnlichen Schluß.“
(LIEBIG, 1924, S. 230)
[My assumption from 1912 that sperms take the
same route through the female’s body as the air for
accessory respiration was based on the idea of a
“respiratory gut” paralleling the digestive gut. Today
I am more cautious in such matters; unfortunately
I know only that a portion of the intestinal mucosa
serves for this purpose. However, again I come to
a similar conclusion.]
8 „Plötzlich drehte das Weibchen den Spieß um. Mit
einer schnellen Bewegung hatte es das Männchen an
der Seite gepackt und heftete sich unter kräftigem
Saugen an das Männchen in der Gegend der Bauch-
ossen, wo neben dem After die Geschlechtsöffnung
mündet. Mit zitternden Bewegungen hafteten so die
Tiere eine Weile, 10 bis 20 Sekunden, aneinander, wo-
bei das Männchen sich etwas auf die Seite legte und
dem Weibchen den Bauch zukehrte. Während dieses
Vorganges traten 3-4 Eier aus der Genitalöffnung des
Weibchens in eine Tasche, die durch Zusammenlegen
der Bauch ossensäume gebildet wurden. Auf diesen
Akt folgte ein kurzer Zustand der Schwäche besonders
für das Weibchen … Nachdem es sich dann erholt hat-
te, schwamm es zwischen den P anzen umher, suchte
sich ein geeignetes Blatt der Ludwigia aus und berührte
es auf der Unterseite mit speienden Bewegungen des
Maules, wobei es jedenfalls einen klebenden Stoff
absonderte und an die Blätter heftete. Dann legte sich
das Weibchen … auf den Rücken, … drückte schnell
die Eier an das Blatt, wo sie gleich festhafteten …
Wann die Befruchtung statt ndet, … habe ich nicht
genau feststellen können. Entweder erfolgt sie in der
Flossentasche oder dann, wenn die Eier an die Blätter
geheftet werden. Das letztere ist mir wahrscheinlicher.
Daß die Samen üssigkeit in die Flossentasche aufge-
nommen wird, ist bei der zu einander senkrechten
Lage der Tiere während dieses Aktes vollständig
ausgeschlossen. Dagegen wäre es möglich, daß durch
die saugenden Bewegungen des Weibchens die Sa-
menfäden zwischen den Kiemen hindurch gegen die
Bauch ossen geschleudert würden und hier die Eier
befruchteten. Allein dagegen spricht wieder der feste
Zusammenschluß der Flossensäume. Es hat wohl we-
nig Wahrscheinlichkeit für sich, daß das Saugen allein
ein Reizmittel für den Austritt der Samen üssigkeit ist,
denn dann wären gerade die Eier möglichst weit von
der männlichen Geschlechtsöffnung entfernt und da
gelegen, wo sie die geringe Aussicht auf Befruchtung
hätten. So bleibt nur noch der Fall übrig, daß das
Weibchen die Spermatozoen aufsaugt und sie dann
gegen das Blatt speit, wo nun erst die Eier befruchtet
werden (WITTIG 1913, S. 286-287).
[Suddenly, the female turned the tables. With a quick
movement she grasped the male‘s side whilst vigorously
sucking adhering to the area of his pelvic ns near the
anus, where the genital opens. With continuous quive-
ring mates adhered to each other for a while, 10 to 20
seconds; during this event … the male slightly lay on
his side turning his belly towards the femael. Meanwhile
3-4 eggs were expelled from the female‘s into a pocket
formed by folding the edges of the pelvic ns … After
that followed a short condition of weakness, especially
31
Bull. Fish Biol. 15 (1/2)
for the females … When she had recovered, she swam
among the plants searching for a special leaf of Ludwigia,
which she touched on the underside with a spitting mo-
tion of the mouth, whereby a adhesive substance was
exuded and fastened to the leaves. Then, the female
turned on her back and quickly pressed the eggs on the
leaf, where they adhere immediately. When fertilization
takes place … I could not exactly see. Either it takes place
in the n pocket or when the eggs are fastened on the
leaves. The latter is more probable. That the sperm is
admitted into the n-pocket is completely excluded due
to the animal’s perpendicular position to each other dur-
ing this act. On the other hand, it could be that through
the sucking operation of the female the spermatozoa are
drawn through the gills and thrown back on the ventral
ns to fertilize here the eggs. Against this speaks the fact
that the n edges are tightly closed. It appears unlikely
that the sucking alone stimulates ejaculation for at that
time the eggs are farthest away from the male’s genital
opening and placed where they would have the least
chance of being fertilized. So the theory only remains
that the female sucks out the sperm and spits it against
the leaf, where the eggs are fastened, and here fertiliza-
tion takes place.]
9 „Das Interessanteste, was Corydoras paleatus dem
Beobachter zu bieten vermag, ist ohne Zweifel
seine Fortp anzung. … Das Verdienst, diesen
Vorgang entdeckt zu haben, kommt der Aquari-
enkunde zu.“ ( MERTENS, 1917, S. 211).
[The most interesting thing Corydoras paleatus offers
the observer is undoubtedly its reproduction. The
spawning of this sh is in fact very peculiar and
unique among shes. The merit of having discov-
ered this matter goes to aquarists.]
10 „Um den Weg des Spermas und damit die Be-
fruchtung kennenzulernen, habe ich eine Reihe von
Versuchen durchgeführt, die ich im Rahmen dieses
Aufsatzes nicht ausführlich besprechen kann. Ledig-
lich einige wichtige Beobachtungen und Ergebnisse
sollen mitgeteilt werden“ (KNAACK 1955, S. 165).
[To get to know the way of sperm and with that
fertilization, I performed some experiments that
I can not discuss in detail within this article. Only
some important observations and results shall be
communicated here.]
11 „Als Ergebnis kann gesagt werden, daß, nachdem
das Weibchen vom Männchen festgeklemmt ist
und gegen die Geschlechtsorgane des Männchens
gedrückt wird, die Spermatozoen aus der Genital-
öffnung ins Wasser treten.“ (KNAACK, 1955, S. 166)
[To conclude, spermatozoa are discharged from the
genital opening into the water, after the female is
clamped by the male and pressed against the repro-
ductive organs of the male.]
12 „Bis vor wenigen Jahren sind über die Fortp an-
zung der Panzerwelse falsche Angaben gemacht
worden. Seit die erste Zucht dieser Tiere im Aquarium
gelang, hieß es über Jahrzehnte hinweg immer wieder,
daß sich das Weibchen in der Geschlechtsgegend oder
gar an der Genitalpapille des Männchens festsauge
und dabei den Samen absauge und verschlucke, so
dass es zu einer Art von „innerer Befruchtung“
komme. Es gab schon immer eine Reihe anatomische
Beweisgründe, die gegen diese Annahme sprachen;
aber erst 1955 hat KNAACK eine Richtigstellung vor-
genommen. Er beobachtete und fotogra erte die
Panzerwelse bei der Paarung und fand schnell heraus,
daß die Weibchen sich nicht ansaugen. Die Männchen
legen sich zur Paarung vor das Weibchen, umfassen
dann die gegen ihre Körperseiten stoßende Partnerin
mit den kräftigen Strahlen der Brust ossen unter den
Bartfäden und klemmen sie am eigenen Körper fest.
Dann gibt das Männchen seinen Samen einfach in das
Wasser ab, während das Weibchen ein oder zwei Eier
austreten läßt und mit den Bauch ossen auffängt.
Nun löst das Männchen seine Klammerung; und das
Weibchen schwimmt mit der aus den Bauch ossen
gebildeten, nach vorn offenen Tasche, in der die Eier
kleben, durch die Wolke von Spermien, um den Laich
zu befruchten.“ (VOGT, 1970, S. 405/406)
[Incorrect statements about reproduction of the
armored cat sh have been made until a few years
ago. Since the rst successful breeding of these
animals in the aquarium it was published again and
again over decades that the female adheres to the
male’s genital area or even to his genital papilla suck-
ing in and swallowing the spermlieding to a kind of
“inner fertilization”. There have always been several
anatomical reasons against this suggestion, but only
KNAACK has corrected this view in 1955. He observed
and photographed armoured cat shes during mat-
ing and soon he become aware that the females do
not adhere by suction. During mating the males lie
before the female, then he clasps the barbels of the
female that pushes against the side of his body with
his strong rays of the pectoral ns and anchors her
rmly to his body. Then, the male discharges sperm
into the water, while the female lays one or two eggs,
which she collects with her pelvic ns. Now the male
releases the female that swims through the cloud of
sperm to fertilize the eggs stacked in the pocket that
is formed by the pelvic n and open anteriorly.]
32
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