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Sexual dimorphism of external morphological characters in some deepwater skates (Rajidae, Rajiformes, Chondrichthyes) of the North Atlantic

Authors:

Abstract

Our studies of the external morphology of five species of deepwater skates (Amblyraja jenseni, Bathyraja pallida, B. richardsoni, Rajella bigelowi, and R. kukujevi) showed that their males and females differ significantly in 9 characters in the first species, 14 characters in the second, Our studies of the external morphology of five species of deepwater skates (Amblyraja jenseni, Bathyraja pallida, B. richardsoni, Rajella bigelowi, and R. kukujevi) showed that their males and females differ significantly in 9 characters in the first species, 14 characters in the second, 11 characters in the third and fourth, and 10 characters in the fifth. Sexual differences occur most frequently in the length 11 characters in the third and fourth, and 10 characters in the fifth. Sexual differences occur most frequently in the length of the first gill slit (in four species out of five), the length of the nasal curtain, the mouth width, and the length of of the first gill slit (in four species out of five), the length of the nasal curtain, the mouth width, and the length of the third gill slit (in three species out of five), the width and length of the disc, the length of the first and second dorsal the third gill slit (in three species out of five), the width and length of the disc, the length of the first and second dorsal fin bases, the tail height at the tip of the ventral fin, the length of the fifth gill slit, the distance between the first fin bases, the tail height at the tip of the ventral fin, the length of the fifth gill slit, the distance between the first and fifth gill slits, the interspiracular distance, the preanal length, distance from the tip of the snout to the maximum and fifth gill slits, the interspiracular distance, the preanal length, distance from the tip of the snout to the maximum disc width, and the interorbital distance (in two species out of five). disc width, and the interorbital distance (in two species out of five).
ISSN 00963925, Moscow University Biological Sciences Bulletin, 2010, Vol. 65, No. 1, pp. 40–44. © Allerton Press, Inc., 2010.
Original Russian Text © A.M. Orlov, C.F. Cotton, D.A. Shevernitsky, 2010, published in Vestnik Moskovskogo Universiteta. Biologiya, 2010, No. 1, pp. 46–48.
40
INTRODUCTION
The representatives of the skate superorder (Bato
morpha) are an important component of the bottom
shelf ichthyocoene and the continental slope of the
world ocean. They feed on fish, cephalopods, and
decapod crustaceans, and they also utilize the same
food resources as the predators of the higher trophic
level: sea birds, sea mammals, and sharks.
Skates, like all cartilaginous fish, are characterized
by sexual dimorphism, which is expressed in different
structure of ventral fins, the lower part of which turns
into a copulative organ in males. In addition, sexually
mature males have alar and malar hooks which help to
hold the female during copulation [1 and others].
Many species have sexual differences in body length
and mass, size, and the time of sexual maturity, some
external morphological traits, and diet [1 and others].
Mature skate males and females differ in the form and
length of teeth [1 and others]. Some species have dif
ferences in the form and width of the pectoral and pel
vic girdle [2]. Still others have differences in the size
and form of the olfactory and electric organs [1 and
others]. However, despite numerous signs of sexual
differences in skates, this question has not been suffi
ciently studied, especially regarding deepwater skates.
The aim of this paper is to cite new data on sexual
differences in external characters in five species of
deepwater skates (
Amblyraja jenseni, Bathyraja pal
lida, B. richardsoni, Rajella bigelowi
and
R. kukujevi
)
of the North Atlantic.
EXPERIMENTAL
To conduct a comparative analysis of some external
morphological characters in different species of
skates, 45–47 morphometric and 2–15 meristic char
acters were collected according to the scheme used in
the last years [3, 4]. Specimens of five species of deep
water skates from ichthyological collections of various
museums were studied by A.M. Orlov and C.F. Cotton
in 2005–2006. All measurements were conducted
according to generally accepted methods [5]. The fol
lowing skate species were studied (the numbers in
parantheses represent the ratio of females and males of
each species, the abbreviation is the name of the
museum, and the numbers are the collection number
of the specimen):
Amblyraja jenseni
(7/5): MCZ 37899, 38354, 55011,
1380201, 1380202, 155628; ME 11666, 16098, 16112.
Bathyraja pallida
(3/6): ME 16322, 17161; NMCZ
2000.130.462, 2000.130.80; BMNH 1967.2.13.2,
1985.11.14.1, 1985.11.14.2, 1985.11.14.3, 1985.11.14.4.
Bathyraja richardsoni
(13/13): BMNH 1999.2.2.1,
1999.2.2.2, 1999.2.2.3, 1999.2.2.4, 1999.10.1, 1999.10.2;
NMHN 19991156; NMCZ 2000.130.260; ME 157511,
157512, 153031, 153032, 153033, 153034, 1061,
1643, 16321, 16336, 16307, 16335, 11652.
Rajella bigelowi
(4/8): NMHN 1988361, 1987
482, 19991162; ME 12265, 14025; MCZ 55316(A),
55316(B), 55316(C), 58444, 55314, 158964, 57327.
Sexual Dimorphism of External Morphological Characters
in Some Deepwater Skates (Rajidae, Rajiformes, Chondrichthyes)
of the North Atlantic
A. M. Orlov
a
, C. F. Cotton
b
, and D. A. Shevernitsky
c
a
Russian Federal Research Institute of Fisheries and Oceanography, Verkhnyaya Krasnosel’skaya 17, Moscow, 107140 Russia
email: orlov@vniro.ru
b
Virginia Institute of Marine Science, Virginia, United States
c
Ichthyology Department, Moscow State University, Moscow, 119991 Russia
Received August 20, 2007
Abstract
—Our studies of the external morphology of five species of deepwater skates (
Amblyraja jenseni,
Bathyraja pallida, B. richardsoni, Rajella bigelowi, and R. kukujevi
) showed that their males and females differ
significantly in 9 characters in the first species, 14 characters in the second, 11 characters in the third and
fourth, and 10 characters in the fifth. Sexual differences occur most frequently in the length of the first gill slit
(in four species out of five), the length of the nasal curtain, the mouth width, and the length of the third gill
slit (in three species out of five), the width and length of the disc, the length of the first and second dorsal fin
bases, the tail height at the tip of the ventral fin, the length of the fifth gill slit, the distance between the first
and fifth gill slits, the interspiracular distance, the preanal length, distance from the tip of the snout to the
maximum disc width, and the interorbital distance (in two species out of five).
DOI:
10.3103/S0096392510010086
MOSCOW UNIVERSITY BIOLOGICAL SCIENCES BULLETIN
Vol. 65
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SEXUAL DIMORPHISM OF EXTERNAL MORPHOLOGICAL CHARACTERS 41
Rajella kukujevi
(2/6): ZISP 46195; ZMUB 15709;
NMHN 19980727; 19991165, 19961157, 1996
1153, 1996235, 19961154.
In addition, published data was used when analyz
ing the external morphological characters of
B. rich
ardsoni
and
A. jenseni
[6–10].
Commonly accepted sources were used when cit
ing the museums’ abbreviations [11] with the excep
tion of the material from the Zoological museum of
the Bergen University (ZMUB), which was collected
during the research voyage of the Norwegian Scientific
Research Vessel
G.O. Sars
in 2004, under the interna
tional project MAR–ECO (www.mareco.no) and
given the abbreviation ME.
Because of the insufficiency of the comparative
material (12 specimens of
A. jenseni
, 8 specimens of
B. pallida
, 26 specimens of
B. richardsoni
, 12 speci
mens of
R. bigelowi
, and 8 specimens of
R. kukujevi
),
the statistical validity of the compared characters was
not estimated. To evaluate the validity of the obtained
data, an average value (M) and the error in mean (m)
were calculated for each characteristic. If the ranges of
mean values including the errors in mean (M
±
m) did
not overlap in males and females, such values were
considered valid.
RESULTS AND DISCUSSION
Despite the fact that the
Amblyraja jenseni
species
was described in the middle of the last century [7] and
a number of captures of this skate was registered in the
North Atlantic since then, until recently, the published
data on its external morphology had been limited by
the description of three specimens [7–9]. And
although the knowledge about the peculiarities of the
A. jenseni
morphology has been significantly expanded
in the last years through the study of the MCZ collec
tions [4], the information on sexual dimorphism in
this species had been absent up to now.
Our research showed that the biggest differences in
A. jenseni
males and females can be observed in the
disc length, the length of the second dorsal fin base,
the tail height at the tip of the ventral fin, the nasal cur
tain length, the length of first and third gill slits, the
distance between the fifth gill slits, and the number of
nuchal and interdorsal thorns (table). If the sexual dif
ferences in morphometric characters may be consid
ered sufficiently valid, the differences found in meris
tic characters (the number of nuchal and interdorsal
thorns) are most likely accidental. This is due to the
small number of studied specimens (12) and to the
fact that the differences in the numbers are not signif
icant (0–2).
Bathyraja pallida
was described in the second half
of the last century [12]. And although eight specimens
have been caught since then, until now, the published
data on its external morphology had been limited by
just two specimens [12]. A recent publication [4]
added somewhat to the knowledge of the morphology
of this species but did not illuminate the details of its
sexual differences. Besides, it was based on the results
of studying just three specimens.
The maximum differences between males and
females of the
B. pallida
species were found in the fol
lowing characters (table): the interspiracular distance,
the length of the first dorsal fin base, the mouth width,
the nasal curtain length and width, the lengths of the
first and third gill slits, the distance between the first
and the fifth gill slits, the preanal length, the distance
from the center of the anus to the first dorsal fin and
to the tip of the tail, and the number of teeth on the
lower jaw.
The external morphological characters of the
Bathyraja richardsoni
are researched comparatively
well. The data on the morphology of 16 mature indi
viduals of the species were published in several works
[9, 10, 13, 14]. However, they did not contain the
information on a number of morphometric and meris
tic characters used in later publications. The measure
ment scheme adopted in the last years was used in a
recent article for four embryos of the studied species
[3], and also in the studies of the specimens caught at
the MidAtlantic ridge during the abovementioned
MAR–ECO expedition and in the studies of the
BMNH and MNHN collections [4]. However, not
one of the abovementioned works gives any data on
the absence or presence of sexual differences in
Bathyraja richardsoni
in external morphological char
acters.
Our studies showed that the sexual dimorphism
manifests itself in
B. richardsoni
in the following char
acters (table): the disc width and length, the horizontal
diameter of the orbit, the tail width at the tip of ventral
fins, the mouth width, the lengths of the first, third,
and fifth gill slits, the length of the rear lobe of ventral
fins, the preanal length, and the distance from the tip
of the snout to the maximum disc width.
The morphology of the
Rajella bigelowi
was
researched in more detail in comparison with the
other species under consideration (there exist pub
lished data on 30 specimens) [15, 16]. A recent publi
cation [4] includes additional data on external mor
phological characters of
R. bigelowi
from the North
Atlantic (the collection material is MCZ and MNHN)
and on two juvenile individuals from the MidAtlantic
ridge area (ME). Nonetheless, the data on the sexual
dimorphism of the external morphology in this skate
species had been absent until now.
An analysis of 12 specimens of
R. bigelowi
showed
that males and females of this species differ signifi
cantly in the following characters (table): the interor
bital distance, the interspiracular, the length of the first
dorsal fin base, the mouth width, the length of the
nasal curtain, the width of the lobes of the nasal cur
tain, the lengths of the first and third gill slits, and the
number of preorbital and scapular thorns. As in the
A. jenseni
, the sexual differences in meristic characters
found in the species under study are hardly valid
42
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ORLOV
et al.
Differences in external morphological characters in males and females of five species of North Atlantic deepwater skates
Characteristic
Amblyraja
jenseni Bathyraja
pallida Bathyraja
richardsoni Rajella
bigelowi Rajella
kukujevi
Morphometric characters
Disc width + +
Disc length + +
Snout length, preorbital –––––
preoral –––––
prenasal –––––
Horizontal orbit diameter +
Interorbital distance + +
Spiracle length –––––
Interspiracular distance + +
Orbit + spiracle –––––
D1, height –––––
D1, base length + +
D2, height –––––
D2, base length + +
Distance between D1 and D2
C base length
Tail
postdorsal length
height at the tip of V + +
width at the tip of V
height at the origin of D1 +
width at the origin of D1 –––––
lateral fold length +
Head length –––––
Mouth width –+++–
Internasal width –––––
Nasal curtain
length + + – + –
width nd + nd nd –
width of each lobe +
distance between lobes nd nd
Gill slit length
1st ++++–
3rd +–++–
5th –++––
Distance between the first gill slits + +
between the fifth gill slits + +
V length, anterior lobe +
posterior lobe
Distance from the tip of the snout
to the center of the anus + +
to the maximum disc length + +
Distance from the center of the anus
to D1 –+–––
to D2 –––––
to the tip of the tail +
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SEXUAL DIMORPHISM OF EXTERNAL MORPHOLOGICAL CHARACTERS 43
because of insignificant difference in numbers and a
small number of samples.
The data on the morphology of
Rajella kukujevi
obtained for the holotype caught in the MidAtlantic
ridge area were reported in just one paper [17]. How
ever, the first description of the species was done on a
limited number of morphological characters. Since
then, three specimens of
R. kukujevi
have been caught
in the North Atlantic but their description did not
include any morphological characters. Thus, the exter
nal morphology of this species is almost unstudied, and
until now, nothing had been known about its sexual
dimorphism in external morphological characters.
Differences between males and females were found
in the specimens under study in the following charac
ters (table): the disc width, the interorbital distance,
the length of the second dorsal fin base, the lateral fold
length, the distance between the first gill slits, the dis
tance from the tip of the snout to the maximum width
of the disc, the number of middorsal thorns, the
number of lateral tail spines on the left and right sides,
and the number of body vertebrae. Although the dif
ferences in the numbers of meristic characters was
sometimes significant, the validity of the data can be
questionable because of the small number of studied
females (two specimens).
CONCLUSIONS
Studies on the sexual dimorphism in the external
morphology of skates are very few. Various species of
skates can have sexual differences in different charac
ters. For example, in the South American species
Dis
copyge tschudii
and
Bathyraja malcoviana,
males and
females have different disc length [18]. The males and
females of
Psammobatis extenta
from the Argentine
waters differ in the snout length (prenasal), distance
from the anus to the tip of the tail, and the preanal
length [19]. In
Raja clavata
from the Adriatic Sea, the
sexual dimorphism is found in such characters as the
disc length and a maximum eye diameter [20]. In
North Pacific skates from the Japanese waters, males
and females of most species differ in the snout length,
the length of the ventral fins, and the tail length [1].
Our studies showed that in the deepwater skates of
the
Amblyraja, Bathyraja
, and
Rajella
genera from the
North Atlantic, the sexual differences are observed
most often in the length of the first gill slit (in four spe
cies out of five), the nasal curtain length, the mouth
width, and the length of the third gill slit (in three spe
cies out of five), the disc width and length, the length
of the first and second dorsal fin bases, the length of
the fifth gill slit, the interspiracular distance, the prea
nal length, the distance from the tip of the snout to the
maximum disc width, and the interorbital distance (in
two species out of five). The sexual dimorphism in the
disc length and the preanal length was registered ear
lier for other skate species as well [18–20]. Most likely,
it is associated with the fact that females of most skate
species surpass males in size. As our studies show,
females of some skate species have a wider disc in
comparison with males, and this, probably, determines
Table
(Contd.)
Characteristic
Amblyraja
jenseni Bathyraja
pallida Bathyraja
richardsoni Rajella
bigelowi Rajella
kukujevi
Meristic characters
Snout angle nd–––
Number of preorbital thorns * * + nd
Number of postorbital thorns * * nd
Number of orbital thorns * * * * nd
Number of interspiracular thorns * *
Number of scapular thorns * * + nd
Number of nuchal thorns + * * nd
Number of interscapular thorns * * nd
Number of median thorns +
Number of interdorsal thorns + * *
Number of lateral spines on the right * * * * +
Number of lateral spines on the left * * * * +
Number of body vertebrae nd nd nd nd +
Number of caudal vertebrae ndndndnd –
Number of teeth on the upper jaw nd
Number of teeth on the lower jaw + nd
Note: (+) is valid differences, (–) is invalid differences, (*) means the characteristic is absent, and (nd) means no data or the validity of
differences could not be estimated because of insufficient data.
44
MOSCOW UNIVERSITY BIOLOGICAL SCIENCES BULLETIN
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No. 1
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ORLOV
et al.
the corresponding differences in the comparative sizes
of the interorbital distance, the interspiracular dis
tance, and the distance between the gill slits. Different
mouth width in males and females of some of the stud
ied species is most likely determined by sexual dimor
phism in mature individuals in the form and number
of teeth and the character of their diet.
ACKNOWLEDGMENTS
This study would have been impossible without the
help of many of our colleagues. The help of Matthias
Stehmann (ICHTHYS, Hamburg, Germany) in
determining the skate species proved invaluable.
James Orr (Alaska Fisheries Science Center, Seattle,
USA) and Hajime Ishihara (W and I Associates Cor
poration, Fujisawa, Japan) explained the specifics of
some measurements. Romain Causse (Museum
National d’Histoire Naturelle, Paris, France) took the
Xrays of the specimens of
Rajella kukujevi
and
R. big
elowi
from the museum’s collection. Ingvar Byrkjedal
(Zoological Museum, University of Bergen, Bergen,
Norway), Bernard Séret (Museum National d’His
toire Naturelle, Paris, France), Karsten Hartel and
Andy Williston (Museum of Comparative Zoology,
Harvard, USA), Patrick Campbell and Roberto
Miguez (British Museum of Natural History, London,
Great Britain), Geoff Swinney (National Museum of
Scotland, Edinburgh, Scotland), and Monty Priede
(Oceanlab, University of Aberdeen, Newburgh, Scot
land) made it possible for us to study the collection
materials and provided the equipment and accommo
dation. Boris Sheiko (Zoological Institute, Russian
Academy of Sciences, St. Petersburg, Russia) took the
measurements of the
Rajella kukujevi
holotype. Nata
lia Popova (Russian Federal Research Institute of
Fisheries and Oceanography) calculated some meris
tic characters from the Xrays. We are extremely grate
ful to all listed colleagues for their invaluable help.
This work was supported in part by the Institute of
Marine Research, Bergen, Norway, project no. 102842.
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... Teresa,FL,USA e mail: orlov@vniro.ru extremely limited (Bigelow and Schroeder, 1927, 1953Leim and Scott, 1966;Orlov et al., 2006Orlov et al., , 2010Sulak et al., 2009;Séret, 2010;Orlov and Cot ton, 2011;Ebert and Stehmann, 2013), this type of information is badly needed for this species. ...
... A previous study of sexual dimorphism in external morphological characters of the Jensen's skate (Orlov et al., 2010;Orlov and Cotton, 2011) was based on 13 specimens (7 females and 6 males). In that study, the following characters were found to be sexually dimorphic: disc length, DII base length, tail height at the V tips, nasal curtain length, 1st and 3rd gill slit lengths, distance between the 5th gill slits, and the number of nuchal and middorsal thorns. ...
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... Differences in morpho 1 The article is published in the original. metric and meristic characters of nine male and female pale rays are generally presented in the paper of Orlov et al. (2010). Changes of the external morphol ogy and coloration of this species with size increasing were never previously analyzed. ...
... Data on the external morphology of pale ray was limited until present by description of characters of holotype, paratype and single specimen from MAR (Forster, 1967a;Orlov et al., 2006) and also by com parison of morphometrics and meristics of nine males and females of the species under consideration (Orlov et al., 2010). ...
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The most comprehensive description of variations of morphometric and meristic characters and coloration of rare deepwater pale ray Bathyraja pallida (Forster, 1967) from the North Atlantic based on all available materials is given. For the first time morphometrics are presented as percent of disc width that might be used in taxonomic studies of specimens with damaged snout and tail. Relationships between total length and disc width and the rest morphometrics have linear character except for clasper length and snout angle. Comparison of external morphological characters of juveniles and adults revealed some notable differences between them. There were almost no considerable distinctions in external morphological characters between skates from the northeastern Atlantic and Mid-Atlantic ridge. It is shown that coloration of pale ray has changed during life span. Juveniles overall have darker coloration as compared to adults. At the same time, both juveniles and mature skates keep common species-specific features of coloration of dorsal and ventral surfaces.
... Несмотря на то, что данный вид является довольно многочисленным в сравнении с другими глубоководными скатами северной Атлантики, до сих пор его биология остается изученной недостаточно (Tempelman, 1973b;Orlov et al., 2006), а в большинстве опубликованных работ описываются преимущественно особенности морфологии данного вида (Garrick, 1961;Forster, 1965;Tempelman, 1973aTempelman, , 1973bGarrick, Paul, 1974;Stehmann, Merrett, 2001;Orlov et al., 2006Orlov et al., , 2010Orlov, 2011;Orlov, Cotton, 2011). Между тем, опубликованные сведения об окраске ската Ричардсона разрознены и ограниченны. ...
... Amblyraja jenseni (Bigelow & Schroeder, 1950) Jensen's skate Also in northern MAR, off Iceland and to the northwest of the British Isles Comments: Species studied by Orlov et al. (2006Orlov et al. ( , 2010 and Orlov & Cotton (2011. Three of the specimens (ZMUB 19462, 19463, 19529), for which DNA samples were available from ethanol-preserved tissue, were subsequently sequenced for the mitochondrial gene COI. ...
... Amblyraja jenseni (Bigelow & Schroeder, 1950) Jensen's skate Also in northern MAR, off Iceland and to the northwest of the British Isles Comments: Species studied by Orlov et al. (2006Orlov et al. ( , 2010 and Orlov & Cotton (2011. Three of the specimens (ZMUB 19462, 19463, 19529), for which DNA samples were available from ethanol-preserved tissue, were subsequently sequenced for the mitochondrial gene COI. ...
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... Initial scatterplots of TL and DW indicated a linear relationship. Since skates are sexually dimorphic (Ebert et al., 2008b;Orlov et al., 2010), this relationship may differ between sexes. Therefore, a linear regression model was applied, with TL as the response variable, DW as a linear explanatory variable and sex as a factor. ...
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