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The biology and ecology of lotic rotifers and gastrotrichs
CLAUDIA RICCI* AND MARIA BALSAMOy
*Dipartimento di Biologia, Universita
Ádegli Studi di Milano, via Celoria 26, I-20133 Milano, Italy
yIstituto di Scienze Morfologiche, Universita
Ádegli Studi di Urbino, via Oddi 21, I-61029 Urbino, Italy
SUMMARY
1. The occurrence of Rotifera and Gastrotricha in the meiobenthos of lotic habitats is
reviewed. About 150 rotifer and 30 gastrotrich species are reported in such habitats
worldwide.
2. The two phyla share some morphological and biological features that might account for
their presence in the meiofauna. Small-size, a soft and elongate body, adhesive glands on
the posterior body end, movement through cilia, relatively short life cycles,
parthenogenesis and dormant stages are common characteristics.
3. Most species of both taxa inhabiting the superficial sediments in streams and rivers may
move downward into the hyporheos in response to both biotic (predation) and abiotic
(spates, erosion, desiccation) disturbances.
Keywords: microinvertebrates, ecology, morphology, parthenogenesis, dormancy
Introduction
Both gastrotrichs and rotifers are included in the
meiofauna because they are multicellular organisms
but are often smaller in size than protists. Rotifers and
gastrotrichs can be abundant in both sandy and coarse
sediments of lotic habitats (Palmer, 1990a; Schmid-
Araya, 1997; Fregni, Balsamo & Tongiorgi, 1998) and
are important in food webs (Schmid-Araya & Schmid,
1995a). However, their small size and difficulties with
sampling, fixing and identifying the animals has led
to an underestimation of their abundance, diversity
and ecological role. Here we focus on the occurrence
of Gastrotricha and Rotifera in the sediments of
streams and rivers and describe their biology and
diversity in such environments.
Gastrotricha is a phylum of pseudocoelomates,
closely related to acoelomates, divided into two
orders, Chaetonotida and Macrodasyida. The order
Chaetonotida has about 350 species (mainly fresh-
water) while the order Macrodasyida has about 200
species, most of which are marine or estuarine. The
250 gastrotrich species commonly found in freshwater
inhabit the sediment surface and the submerged
vegetation in eutrophic waters. Of these, about 70
species have been reported in the interstitial, and less
than one half of them have been found in running
waters as well (a species list is given in Table 1).
Gastrotrichs can occur in both lotic and lentic
sediments, but their abundance is usually higher in
lentic habitats (up to 100 000 individuals m
±2
) (Strayer
& Hummon, 1991).
Rotifera are also pseudocoelomates and comprise
more than 2000 species of which the great majority
(about 1800) belongs to the order Ploima (class
Monogononta). Many species are planktonic and
may contribute up to 30% of the total freshwater
plankton biomass (Nogrady, Wallace & Snell, 1993).
As a consequence, our knowledge of rotifer biology is
chiefly based on such planktonic species. Rotifers are
also an important component of the fauna of fresh-
water sediments and one class, the Bdelloidea, and
many species in the Monogononta are entirely
benthic. Three families, the Notommatidae and
Dicranophoridae (both Monogononta) and the Philo-
dinidae (Bdelloida), account for most of the species in
streams and rivers. Nevertheless other rotifer families
are consistently present in the lotic meiobenthos: these
include the Colurellidae, Lecanidae and Proalidae
among the Monogononta, and all bdelloid families
(e.g. Evans, 1984; Schmid-Araya & Schmid, 1995b).
Freshwater Biology (2000) 44, 15±28
ã2000 Blackwell Science Ltd. 15
Correspondence: Claudia Ricci, Dipartimento di Biologia, via
Celoria 26, I-20133 Milano, Italy.
E-mail: rotiferi@mailserver.unimi.it
About 150 rotifer species are reported from the lotic
benthos (Higgins & Thiel, 1988; Schmid-Araya, 1999)
(a species list is given in Table 2), a higher rotifer
species diversity than is found in lake or marine
benthos.
Palmer (1990a,b, 1992) and Schmid-Araya (1995)
showed that both rotifers and gastrotrichs are abun-
dant in the sediments of lotic habitats, where
individual rotifers may constitute up to 75±80% of
the whole meiofaunal assemblage. Reports on hor-
izontal density distribution in various studies of
freshwater psammolittoral habitats show values in
the order of hundreds or even thousands of rotifers
per 5 cm
2
of sand (see Higgins & Thiel, 1988; Giere,
1993). Schmid-Araya (1993) recorded densities as high
as 1782 per 10 cm
2
of one bdelloid rotifer (Embata
laticeps Murray) from interstitial hyporheic water. On
the other hand, Turner & Distler (1995) reported
densities of two rotifers per cm
2
and a total of 24
species over a 9-month period in sandy sediments.
Taxonomy and origin
Gastrotricha
Chaetonotid gastrotrichs are characterised by a
tenpin-shaped, transparent body, and three regions
are easily recognizable: a distinct head, an ovate trunk
and a caudal furca (Fig. 1a,b). The body is very small,
80±500 mm, wholly covered with a cuticle usually
sculptured with scales and spines, and is provided
ventrally with an extensive ciliation allowing a gliding
locomotion. The rounded head bears tufts of sensory
cilia, cuticular plates with protective function and a
subapical mouth often strengthened by a ring of
protrusible hook-like structures. A thick, muscular
pharynx extends to the trunk region. Two protone-
phridia and two lateral ovaries lie in the trunk, which
appears enlarged when one or two full-grown oocytes
are present. Two long, duo-gland adhesive tubes form
the caudal furca (Ruppert, 1991).
Hyman (1951), Schwank (1990) and Ruppert (1991)
provide detailed accounts of the morphology and
anatomy of Gastrotricha.
Order Chaetonotida
Family Chaetonotidae: body length ranging from 80 to
500 mm; caudal furca; cuticular scales and spines
Table 1 List of species of chaetonotid gastrotrichs reported from
interstitial lotic habitat
Gastrotricha
Chaetonotida
fam. Dichaeturidae
Marinellina flagellata Ruttner-Kolisko, 1955 x
Dichaetura capricornia (Metschnikoff, 1865) y
fam. Chaetonotidae
Chaetonotus acanthocephalus Valkanov, 1937 yy
C. aemilianus Balsamo, 1978 xx
C. brevispinosus Zelinka, 1889 xx
C. daphnes Balsamo & Todaro, 1995 xx
C. disjunctus Greuter 1917 xx
C. fluviatilis Balsamo & Kisielewski, 1986 {xx
C. heideri Brehm, 1917 yy
C. heterospinosus Balsamo, 1978 z
C. hystrix Metschnikoff, 1865 zxx
C. laroides Marcolongo, 1910 xx
C. larus Mu
Èller, 1786 xx
C. macrochaetus Zelinka, 1889 *
C. maximus Ehrenberg, 1830 z{** xx
C. multispinosus Gru
Ènspan, 1908 xx
C. mutinensis Balsamo, 1978 xx
C. oculifer Kisielewski, 1981 ** xx
C. oplites Balsamo, Fregni & Tongiorgi, 1994 z
C. persetosus Zelinka, 1889 zxx
C. polyspinosusBalsamo, 1983 xx
C. ventrochaetus Kisielewski, 1991 yy
Heterolepidoderma ocellatum, Metschnikoff, 1855 xx
Ichthydium podura (Mu
Èller, 1773) * zxx
I. squamigerum Balsamo & Fregni, 1995 zxx
I. tanytrichum Balsamo, 1983 xx
Lepidochaetus brasilense Kisielewski, 1991 yy
L. zelinkai (Gru
Ènspan, 1908) zxx
Lepidodermella amazonica Kisielewski, 1991 yy
L. squamata (Dujardin, 1841) zxx
*Preobrajenskaia (1926); Cecera River, Kossino, Russia, sand
with mud.
yMola (1932); Mogoro Creek, Sardinia, Italy, sand.
zConinck (1939); Oxara
ÁRiver, Iceland, sand.
xRuttner-Kolisko (1955b); Ybbs River, Austria, sand.
{Balsamo & Kisielewski (1986); Magra River, Liguria, Italy,
coarse sand and gravel.
**Kisielewska & Kisielewski (1986); streams of Tatra Moun-
tains, Poland, sand.
yyKisielewski (1991); Guarau
ÁStream, S. Paulo; Amazonic
Estuary, Brazil, sand.
zBalsamo et al. (1994); streams of Montecristo Island, Tuscany,
Italy, sand with mud.
xxFregni et al. (1998); streams and rivers of the Tuscan-Emilian
Apennines and Apuan Alps; River, Sardinia, Italy, medium to
very coarse sand.
16 C. Ricci and M. Balsamo
ã2000 Blackwell Science Ltd, Freshwater Biology, 44, 15±28
greatly different among genera and species. Five out
of nine genera have been reported from lotic
sediments (Fig. 2a-e) (Table 1).
Two other genera (Dichaetura and Marinellina)
(Fig. 2f) were reported from the lotic interstitial,
both with a single species described on the basis of
only one specimen, but their inclusion in the order
Chaetonotida remains questionable (Mola, 1932; Rutt-
ner-Kolisko, 1955a).
Rotifera
Characterised by an apical ciliated region, the corona,
and by a specialized pharynx (the mastax) with hard
jaws (the trophi) rotifers are unsegmented, bilaterally
symmetrical pseudocoelomates commonly found in
freshwater. They are small, usually less than 1 mm in
length, and their body is elongate and transparent.
Three body regions may be distiguished: the head that
can be either broad or narrowed or lobed, the
elongated trunk and the terminal foot (Fig. 1c,d).
Apically on the head is the corona, made up of an
unciliated central region, the apical field, encircled by
one or more lobated ciliary zones that are elevated on
retractile pedicels in some rotifer taxa (Philodinida,
Bdelloidea). The apical field usually bears the outlets
of the ducts of the retrocerebral organ, a peculiar
apparatus present in many rotifers, and whose
function is still unclear. Several rotifers (all bdelloids
and some monogononts) have a mid-dorsal projec-
tion, the rostrum, equipped with cilia dorsally lined
by a cuticular plate, the lamella. Bdelloids use the
rostrum in locomotion by looping, with alternating
adhesion of the body ends to the substratum. The
trunk may be cylindrical or flattened and broadened,
lined by a syncytial body wall, thickened by an
intracytoplasmic skeletal lamina, that may constitute a
lorica. The trunk contains the digestive, excretory and
reproductive organs. The foot is commonly present,
but is reduced or absent in taxa leading a wholly
pelagic life. The foot may terminate in an adhesive
disk or in movable projections, the toes, that bear the
outlets of pedal glands secreting a sticky cement used
for temporary attachment to the substratum while
creeping. More details on rotifer morphology and
anatomy can be found in Hyman (1951), de Beau-
champ (1965), and Nogrady, Wallace & Snell (1993).
The phylum Rotifera comprises three classes:
Seisonidea (two species), Bdelloidea (about 360
species) and Monogononta (about 1800 species).
Fig. 1 Scheme of the anatomy of a chaetonotid gastrotrich from dorsal (a) and ventral (b) views and of a rotifer from dorsal (c) and
lateral (d) views.a, anus; ag, adhesive gland; b, brain; bl, bladder; c, cilia; cl, cloaca; co, corona; cp, cuticular plates; da, dorsal antenna;
e, egg; f, foot; gg, gastric gland; gv, germo-vitellarium; h, head; i, intestine; m, mastax; n, nephridium; np, nephridial pore; o, ovaries; p,
pharynx; ra, retrocerebral apparatus; sb, sensory bristle; sc, sensory cilia; sg, salivary gland; t, toe; tr, trunk; xo, x-organ.
Lotic rotifera and gastrotricha 17
ã2000 Blackwell Science Ltd, Freshwater Biology, 44, 15±28
Table 2 List of species of rotifers reported from interstitial lotic
habitat
Rotifera
Monogononta
o. Flosculariaceae
fam. Testudinellidae
Pompholyx complanata Gosse, 1851 {
o. Ploima
fam. Brachionidae
Brachionus nilsoni Ahlstrom, 1940 y
B. calyciflorus Pallas, 1766 y
Keratella cochlearis (Gosse, 1851) y**
Notholca foliacea (Ehrenberg, 1838) *
N. labis Gosse, 1887 *
N. squamula (Mu
Èller, 1786) * **
Plationus patulus (Mu
Èller, 1786) {
fam. Colurellidae
Colurella adriatica Ehrenberg 1831 {
C. anodonta Carlin, 1939 {
C. colurus (Ehrenberg ,1830) * y
C. gastracantha (Hauer, 1924) {
C. hindeburgi Steinecke, 1917 {
C. obtusa (Gosse, 1886) {
C. sinistra Carlin, 1939 z
C. uncinata (Mu
Èller, 1773) **
Lepadella acuminata (Ehrenberg, 1834) * z{
L. cf. eurysterna Myers, 1942 {
L. ovalis (Mu
Èller, 1786) * **
L. patella (Mu
Èller, 1826) z
L. cf. patella (Mu
Èller, 1826) y
L. triptera Ehrenberg, 1830 *
fam. Dicranophoridae
Aspelta angusta Harring & Myers, 1928 {
A. circinator (Gosse, 1886) {
Dicranophorus artamus Harring & Myers, 1928 {
D. difflugiarum (Penard, 1914) *
D. forcipatus (Mu
Èller, 1786) *
D. hercules Wiszniewski, 1932 yy
D. liepolti Donner, 1964 *
D. lutkeni-sigmoides (Bergendal, 1892) *
D. remanei Wulfert, 1936 **
D. uncinatus (Milne, 1886) *
Encentrum gulo Wulfert, 1936 *
E. incisum Wulfert, 1936 * **
E. longipes (Wulfert, 1960) *
E. cf. lupus Wulfert, 1936 *
E. mucronatum Wulfert, 1936 *
E. mustela (Milne, 1885) *
E. putorius Wulfert, 1936 *
Erignatha clastopus (Gosse, 1886) {
Myersinella tetraglena (Wiszniewski, 1934) *
Wierzejskiella velox (Wiszniewski, 1932) * yyy
fam. Euchlanidae
Euchlanis alata Voronkov, 1912 **
E. deflexa (Gosse 1851) * **
E. dilatata Ehrenberg, 1832 {**
E. incisa Carlin, 1936 {
fam. Gastropodidae
Ascomorpha ecaudis (Perty, 1850) *
fam. Ituridae
Itura aurita (Ehrenberg, 1830) y
I. aurita intermedia (Wulfert, 1935) * **
fam. Lecanidae
Lecane bulla (Gosse, 1851) y{
L. clara (Bryce, 1892) z
L. closterocerca (Schmarda, 1859) y{**
L. cf. copeis (Harring & Myers, 1926) y
L. cornuta (Mu
Èller, 1786) {
L. curvicornis (Murray, 1913) **
L. flexilis (Gosse, 1886) {
L. furcata (Murray, 1913) z
L. cf. furcata (Murray, 1913) y
L. hamata (Stokes, 1896) {
L. hastata (Murray, 1913) y
L. inermis (Bryce, 1892) y
L. lauterborni Hauer, 1924 z
L. cf. levistyla (Olofsson, 1917) y
L. luna (Mu
Èller, 1776) * y
L. lunaris (Ehrenberg, 1832) * z{
L. obtusa (Murray, 1913) {
L. papuana (Murray, 1913) y
L. psammophila (Wiszniewski, 1932) yy
L. pyriformis (Daday, 1905) z{
L. quadridentata (Ehrenberg, 1832) {
L. tenuiseta Harring, 1914 {
fam. Lindiidae
Lindia torulosa Dujardin, 1841 * {**
fam. Mytilidae
Lophocharis salpina Ehrenberg, 1834 *
Mytilina ventralis (Ehrenberg, 1832) * {
fam. Notommatidae
Cephalodella apocolea Myers, 1924 z
C. auriculata (Mu
Èller, 1773) {
C. catellina (Mu
Èller, 1786) * y
C. delicata Wulfert, 1937 {
C. forceps Donner, 1950 *
C. forficula (Ehrenberg, 1832) * {
C. gibba (Ehrenberg, 1832) *
C. gibba microdactyla Koch-Althaus, 1963 {
C. gigantea Remane, 1933 **
C. cf. gobio Wulfert, 1937 *
C. cf. gracilis (Ehrenberg, 1832) *
C. hoodii (Gosse, 1886) {
C. cf. incila Wulfert, 1937 *
C. megalocephala (Glasscott, 1893) * {**
C. oxydactyla Wulfert, 1937 *
C. reimanni Donner, 1950 *
C. cf. rigida Donner, 1950 *
18 C. Ricci and M. Balsamo
ã2000 Blackwell Science Ltd, Freshwater Biology, 44, 15±28
Many species of the order Ploima (Monogononta) live
in lotic sediments.
Family Brachionidae: loricate rotifers that are
commonly found in the plankton include only one
genus (Notholca) that can live in lotic sediments
(Higgins & Thiel, 1988; Schmid-Araya, 1995), while
other genera are occasional. Notholca's roundish
shape, and the absence of a foot, contrasts with the
presumed morphological adaptations to benthic life-
style. Nevertheless, it possesses pedal glands,
although rudimentary (Ruttner-Kolisko, 1974).
Family Colurellidae: benthic loricate rotifers that
browse on microorganisms. The lorica may be dorso-
C. sabulosa Myers, 1942 {
C. sterea mutata Donner, 1950 {
C. tenuior (Gosse, 1886) *
C. ventripes (Dixon-Nuttall, 1901) {
Drilophaga bucephalus Vejdovsky, 1883 *
Notommata glyphura Wulfert, 1935 {
N. pachiura (Gosse, 1886) {
N. thopica Harring & Myers, 1924 *
Pleurotrocha petromyzon Ehrenberg, 1830 *
Resticula gelida (H.&M., 1922) **
R. nyssa H.&M., 1924 *
R. vermisculus Wulfert, 1935 *
fam. Proalidae
Bryceella tenella (Bryce, 1897) yy
Proales globulifera (Hauer, 1921) *
P. fallaciosa Wulfert, 1937 * {
P. minima (Montet, 1915) z{yy
P. similis De Beauchamp, 1907 *
P. theodora (Gosse, 1887) * **
Proalinopsis caudatus (Collins, 1873) *
fam. Scaridiidae
Scaridium longicaudum (Mu
Èller, 1786) {
fam. Synchaetidae
Synchaeta tremula (Mu
Èller, 1786) * **
fam. Trichocercidae
Trichocerca porcellus (Gosse, 1886) *
T. taurocephala (Hauer, 1931) * {
T. tenuior (Gosse, 1886) {
T. tigris (Mu
Èller, 1786) *
fam. Trichotriidae
Trichotria pocillum (Mu
Èller, 1776) **
T. tetractis (Ehrenberg, 1830) z{**
Bdelloidea
o. Philodinida
fam. Habrotrochidae
Habrotrocha collaris (Ehrenberg, 1832) *
H. constricta (Dujardin, 1841) x
H. elusa vegeta Milne, 1916 x
H. gracilis Montet, 1915 x
H. ligula Bryce, 1913 x
H. proxima Donner, 1953 *
H. cf. pusilla (Bryce, 1893) *
Scepanotrocha corniculata Bryce, 1910 x
fam. Philodinidae
Dissotrocha aculeata (Ehrenberg, 1832) * y**
D. macrostyla (Ehrenberg, 1838) * {
Embata laticeps (Murray, 1905) *
E. hamata (Murray, 1906) *
Macrotrachela ehrenbergii (Janson, 1893) x
M. cf. habita (Bryce, 1894) *
M. papillosa (Thompson, 1892) * x
M. plicata (Bryce, 1892) * x
M. quadricornifera Milne, 1886 *
M. timida Milne, 1916 *
M. vesicularis (Murray, 1906) *
Mniobia obtusicornis Murray, 1911 *
M. scarlatina (Ehrenberg, 1853) *
Philodina acuticornis Milne ,1916 * {
P. flaviceps Bryce, 1906 * x{
P. cf. inopinata Milne, 1916 x
P. megalotrocha Ehrenberg, 1832 x
P. nemoralis Bryce, 1903 *
P. plena (Bryce, 1894) *
P. roseola Ehrenberg, 1832 x
P. rugosa Bryce, 1903 {
P. vorax (Janson, 1893) *
Rotaria macroceros (Gosse, 1851) *
R. rotatoria (Pallas, 1766) * {
R. sordida (Western 1893) * x
R. socialis Kellicott, 1888 *
R. tardigrada (Ehrenberg, 1832) x
o. Adinetida
fam. Adinetidae
Adineta barbata (Janson, 1893) * x{
A. steineri Barto, 1951 *
A. vaga (Davis, 1873) *
o. Philodinavida
fam. Philodinavidae
Henoceros falcatus (Milne, 1916) *
Philodinavus paradoxus (Murray, 1905) *
*Schmid-Araya & Schmid (1995); Oberer Seebach, Austria,
gravel coarse bottom.
yTurner & Distler (1995); Ninnescah River, Kansas, sand
bottom.
zEvans (1984); Raccoon Creek, Ohio, sand bottom.
xZullini & Ricci (1980); Carrega Wood Stream, Italy, coarse
bottom.
{Turner & Palmer (1996); Goose Creek, Virginia, coarse sand
interstitial.
**Braioni & Gottardi (1979); Adige river, Italy, coarse sand
interstitial.
yyNeistwestnowa-Shadina (1935); Oka River, Russia, coarse
sand interstitial.
Lotic rotifera and gastrotricha 19
ã2000 Blackwell Science Ltd, Freshwater Biology, 44, 15±28
Fig. 2 Some morphotypes of chaetonotid gastrotrichs (a±f) and rotifers (g±l) that inhabit lotic sediments. Gastrotricha: a,
Heterolepidoderma;b,Lepidodermella;c,Ichthydium; d,e, Chaetonotus;f,Marinellina. Rotifera: g, Lepadella;h,Encentrum;i,Lecane;
j, Cephalodella;k,Proales;l,Trichocerca.
20 C. Ricci and M. Balsamo
ã2000 Blackwell Science Ltd, Freshwater Biology, 44, 15±28
ventrally flattened with a lateral notch (e.g. Lepadella,
Fig. 2g), or laterally constricted with a mid-ventral
furrow (e.g. Colurella), where the foot lies. All have a
segmented telescopic foot with two toes.
Family Dicranophoridae: illoricate or semiloricate
rotifers with protrusible trophi, they are omnivorous,
or, more commonly, carnivorous. Several benthic
species seem to feed on dying organisms that are
settling on the bottom, but some lotic meiobenthic
forms (e.g. Dicranophorous,Encentrum, Fig. 2h) prey
actively on testate amoebae, nematodes, or other
rotifers. Their corona is ventral and has lateral ciliary
tufts like auricles. The rostrum is commonly well
developed and hook-shaped. Most species possess a
segmented foot with two movable toes.
Family Lecanidae: dorso-ventrally flattened, lori-
cate rotifers that have a foot with two movable toes. A
rostrum is lacking. All species belong to a single
genus, Lecane (Fig. 2i),and are bottom-dwellers able
to swim for short distances only. They are micro-
phagous.
Family Notommatidae: as many as 19 genera belong
to this family, and only some (e.g. Resticula,Cephalo-
della, Fig. 2j) inhabit lotic sediments. Mostly loricate,
their body is cylindrical with three to five lorica plates.
The foot is short with sharply pointed toes.
Family Proalidae: soft-bodied rotifers that may
have lateral nonretractable auricles and a rostrum-like
process (e.g. Proales, Fig. 2k). Most species are micro-
phagous. The foot is more or less long and ends in two
toes. Foot and toes are often retractable. The trunk of
many species is pseudosegmented.
Family Trichocercidae: mostly cylindrical, and
loricate with a well-developed foot with asymmetrical
rigid toes (e.g. Trichocerca, Fig. 2l).
Class Bdelloidea comprises rotifers that are all
characterised by (a) an elongate body with pseudo-
segments, (b) ramate trophi, (c) a developed foot with
a variable number of toes (0±4), and (d) parthenoge-
netic reproduction. Except for a few species, all
bdelloids are benthic dwellers.
Order Philodinida: characterized by a corona
divided into two ciliated parts on trochal pedicels.
These rotifers filter-feed by creating water currents
with their corona (Fig. 3a). The same corona may be
used for swimming but their locomotion is commonly
by looping.
Order Adinetida: the corona consists of a ciliated
ventral surface lined caudally by a cuticular specia-
lization, the rake (Fig. 3B). These rotifers are unable to
swim, but move very actively by crawling with their
cilia.
Order Philodinavida: only three genera are known,
of these two (Henoceros and Philodinavus; Fig. 3c) live
as meiofauna in lotic habitats (Schmid-Araya, 1997;
Ricci & Melone, 1998). Their distribution is patchy.
The corona is poorly developed, if present at all. They
live in the bottom of the upper course of streams,
attached to hard substrata, where periphyton or
filamentous algae are present.
The structure of both rotifers and gastrotrichs may
be advantageous to the meiobenthic lifestyle, as all are
minute, with a more or less slender, flat and soft body
(Giere, 1993). Both rotifers and gastrotrichs glide by
means of cilia and some rotifers have pseudosegments
to facilitate flexibility in narrow spaces. The foot of
benthic rotifers, as well as the caudal furca of
gastrotrichs, may act as an anchor preventing dis-
placement. Adhesion to the bottom is necessary in
flowing water, and both rotifers and gastrotrichs are
able to adhere through the secretion of glands present
at the posterior body end. These are a duo-gland
system and allow the animal to stick firmly to a
surface at one time and to detach at the following
moment. The psammobiotic rotifers are characterised
Fig. 3 Some morphotypes of bdelloid rotifers that inhabit lotic
sediments. a, Philodina;b,Adineta;c,Philodinavus.
Lotic rotifera and gastrotricha 21
ã2000 Blackwell Science Ltd, Freshwater Biology, 44, 15±28
by a flattened body shape and well-developed foot
with toes and adhesive glands (Table 4). Psammic
gastrotrichs have cuticular scales and spines com-
monly smaller than those of the epibenthic species.
The spines, particularly, may work as do annelid
chaetae to allow burrowing into the sediment.
Collection, examination and identification
Most studies have been qualitative compilations of
rotifer and gastrotrich species (e.g. Donner, 1970;
Schwank, 1990). More recently, quantitative studies of
gastrotrichs and rotifers have been carried out (i.e.
Strayer, 1985; Kisielewski & Kisielewska, 1986; Hum-
mon, 1987; Palmer, 1990a; Nesteruk, 1991, 1993;
Schmid-Araya, 1995, 1998). Hess samplers can be
used to collect these animals from the uppermost
layers of the sediment, and also stand-pipe traps or
sediment corers to collect individuals from the deeper
interstitial habitats.
The sediment should be processed directly to
extract the animals (see also Gallagher, 1956). Some
rotifers may be extracted by stirring the sediment into
a suspension and decanting the water through a fine
mesh net (Turner, 1988). However, this method
cannot be profitably applied to extract soft-bodied
rotifers or gastrotrichs. To detach the animals from the
sediment particles small amounts of substratum (2±
3cm
3
) can be repeatedly washed in a 4% aqueous
solution of MgCl
2
that acts as relaxant for gastrotrichs
(Higgins & Thiel, 1988). However, most bdelloid
rotifers are not sensitive to MgCl
2
and stick firmly to
the sediment particles. The loss of specimens that
adhere to sediment particles can be avoided only by
careful scanning of the sediment material and manual
extraction with a pipette under a stereomicroscope.
Rotifers and gastrotrichs must be observed and
processed when freshly collected, because most do
not survive laboratory conditions. Some monogonont
rotifers are loricate and can be fixed with neutralized
4% formalin; most animals are soft-bodied and must
be identified alive, because their shape and general
morphology are entirely distorted by fixatives. Gas-
trotrichs and several monogonont rotifers can be
anaesthetized by MgCl
2
or by bupivacaine and once
relaxed, can be fixed with formalin and mounted on a
slide. Other rotifers (bdelloids in particular) are
mostly insensitive to any anaesthetic (Melone, 1999).
In addition, they cannot be fixed and observed for
taxonomic studies because several diagnostic char-
acters can be seen on active animals only. Even if the
specimens can be preserved, the observation must be
carried out as soon as possible because certain
diagnostic characters, such as the pigments of eye
spots, deteriorate rapidly.
Micrographs and videotapes may be useful for
morphological comparisons of specimens from differ-
ent localities, since some freshwater species can show
a great intraspecific variability. Gastrotrichs and
several rotifers may be fixed and stored in neutralized
4% formalin, or mounted on slides in formalin or in a
mixture of formalin±glycerol (3 : 1), but this method
does not allow a complete taxonomic study, because
of the deterioration of most diagnostic features.
Taxonomic keys for identification of genera and
species of freshwater Gastrotricha are presented by
Balsamo (1983), Schwank (1990) and Strayer &
Hummon (1991).
Identification keys for Rotifera Monogononta are :
Koste (1978), Ruttner-Kolisko (1974) and the serial
volumes `Guides to the identification of the micro-
invertebrates of the continental Waters of the World'
edited by Nogrady (1995±1997). The only available
key for the identification of Bdelloidea is that of
Donner (1965).
Biology and life history characteristics
Both phyla are characterised by direct development,
relatively short life cycles and parthenogenetic repro-
duction (Table 3).
In all Rotifera, three life stages can be recognised
after hatching: a prereproductive phase, when most
somatic growth occurs, a reproductive phase (com-
monly the longest), when the rotifer produces a
succession of offspring parthenogenetically, and a
postreproductive phase (Nogrady, Wallace & Snell,
1993). The latter phase is often present in laboratory
cultures, but is likely to be rare in natural populations.
In chaetonotid gastrotrichs three life phases are also
described, but the prereproductive and the partheno-
genetically reproductive phases are followed by a
third, in which the animal becomes hermaphroditic
(Levy, 1984; Balsamo & Todaro, 1988). The male
fertility of this last phase is still an open question.
Parthenogenesis is the only reproductive modality
of bdelloids and of young gastrotrichs. In monogo-
nonts it is often cyclical, punctuated by male produc-
22 C. Ricci and M. Balsamo
ã2000 Blackwell Science Ltd, Freshwater Biology, 44, 15±28
tion through arrhenotoky (development of haploid
eggs that hatch into males) and subsequent fertiliza-
tion of haploid eggs (mixis). This leads to the
production of resting eggs, which are able to survive
harsh conditions. However, the occurrence and timing
of mixis in interstitial monogonont species has not yet
been explored in detail. If the life-history traits of the
planktonic species hold true for the lotic species, then
their mean life span would be about 7±10 days and
their mean fecundity about 15±20 offspring per female
at 20 °C (Wallace & Snell, 1991). The life cycle of
bdelloid rotifers is much longer, about 30 days, and
their life-time fertility is about 30±40 offspring under
laboratory conditions (22±24 °C with no food limita-
tion) (Ricci, 1983). Of course these data are only
indicative, since there is a large variation among
species and culturing conditions and results obtained
in the laboratory hardly fit natural circumstances. For
example, the doubling time of a population of a
benthic bdelloid species, Embata laticeps, has been
estimated at between 20 and 60 times longer in the
field than in the laboratory (Schmid-Araya, 1993).
Chaetonotid gastrotrichs lay four parthenogenetic
eggs that develop immediately. At the end of the
gastrotrich life cycle one resting egg is commonly laid,
but more might be produced in response to some
environmental stress. Factors influencing the rate of
resting egg production, and causing their hatching,
are still unknown. At 20 °C, the lifespan of chaetono-
tids usually ranges from 15 to 23 days, with no more
than four eggs laid during the life time (Hummon,
1986; Balsamo & Todaro, 1988).
The life history of chaetonotid gastrotrichs possibly
combines a parthenogenetic phase with a hermaph-
roditic one in the same generation, which seems to be
unique in the animal kingdom. Such a life history
might allow a fast growth of the population through
parthenogenesis, then introducing genetic variation
through cross-fertilization (Balsamo, 1992) (Table 3).
Similarly, the alternating reproduction of mono-
gonont rotifers combines the advantage of fast growth
due to ameiotic parthenogenesis with a short mictic
phase when genetic recombination occurs through
random assortment of chromosomes during meiosis
and through cross fertilization. Planktonic rotifers are
known to be r-strategists, with short development
time, short life span and high reproduction rate, but
nothing is known about the life history of lotic
monogononts. In contrast, bdelloid rotifers have
longer life spans and a comparatively low reproduc-
tive activity, and remarkable differences in life-history
traits have been reported for species living in different
habitats (Ricci, 1983).
The life cycles of rotifers and chaetonotid gastro-
trichs are characterised by the formation of dormant
stages. These often consist of resting eggs that are
produced after mictic reproduction in monogononts,
and likely at the end of the parthenogenetic reproduc-
tion in chaetonotids. Bdelloids do not produce resting
eggs, but are capable of dormancy at any stage during
Table 3 Life-history traits and biological characteristics of rotifers and gastrotrichs
Rotifera Gastrotricha
Monogononta Bdelloidea Chaetonotida
Reproduction Cyclic parthenogenesis*
arrhenotoky, thelitoky, mixis
Obligate parthenogenesis*
thelytoky
Obligate parthenogenesis (late
hermaphroditism??)y
thelytoky, ? mixis ?
Sexual dimorphism Dwarf males ± ±
Egg number 15±20 per female* 20±40 per femaley3±4 per femaley
Egg timing Continuous during maturity Continuous during maturity Continuous during maturity
Egg development Hours DayszHours
Resting eggs Present Absent Present
Growth Continuous Continuous Continuous
Life span 7±10 days* 30±40 daysz15±20 daysy
Locomotion Gliding, swimming Inching, creeping Gliding
Food requirement Small preys, algae, bacteria, fungi Algae, bacteria, yeasts Algae, bacteria, yeasts
*Nogrady, Wallace & Snell (1993).
yBalsamo (1992).
zRicci (1983).
Lotic rotifera and gastrotricha 23
ã2000 Blackwell Science Ltd, Freshwater Biology, 44, 15±28
their lives. Through dormant stages the population
can survive harsh conditions in a sort of suspended
animation, and resume activity (i.e. development of
the egg, or `resurrection' of the animal) when the
conditions become suitable again. The dormant stages
of all taxa represent also the means for passive
dispersal between different habitats (Pourriot &
Snell, 1983; Levy, 1984) (Tables 3 and 4).
Ecology
Food requirement and feeding mechanisms
Because of their small size, most rotifers and gastro-
trichs may enhance energy transfer from bacteria and
algae to higher trophic levels by feeding on particles
of a size not efficiently grazed by larger invertebrates,
and in turn serve as prey items for larger individuals
(Schmid-Araya & Schmid, 2000). Some rotifers are
filter-feeders, and several benthic rotifers, such as
Notommatidae, feed by piercing algal filaments and
sucking the cytoplasm out of the cells. Most benthic
rotifers scrape the substratum or browse by seizing
food particles. Although some species are highly
selective in collecting their food items, most, and in
particular the filter-feeders, can consume a wide
variety of food items, and the size of food appears
to be the most important discriminating factor
(Table 4). Thus herbivorous species can ingest small
ciliates and predatory ones `prey' on algae larger than
about 15 mm (Nogrady, Wallace & Snell, 1993). Filter-
feeding species feed on yeasts, bacteria and flagellates
and therefore can affect microbial web composition
and density (Arndt, 1993).
Gastrotrichs, too, are part of the microphagous,
detritivorous benthic community; they mainly feed on
bacteria, but also on unicellular algae, fungi, particu-
late organic matter, and occasionally on very small
invertebrates (Table 4). Like nematodes, they ingest
food by means of the powerful sucking action of a
muscular pharynx. The chemo-tactile ability of a
marine species to discriminate among different
bacterial strains (Gray & Johnson, 1970) is also likely
to be present in freshwater gastrotrichs.
Physical and chemical requirements
Although both rotifers and gastrotrichs are among the
most abundant freshwater benthic animals, the factors
controlling their population dynamics and their
ecological importance have not yet been examined.
Their spatial and temporal distribution is often patchy
and it is likely to follow the presence of particulate
organic matter and biofilm as food sources (Swan &
Palmer, 2000). Oxygen, that affects animal distribution
in lentic sediments, is likely to be abundant in lotic
substrata even in the sediment, thus allowing animals
to colonise deeper layers. The vertical distribution of
Table 4 Major adaptations and ecological requirements of rotifers and gastrotrichs
Rotifera Gastrotricha
Monogononta Bdelloidea Chaetonotida
Adaptations to benthic life Minute body size, elongate soft body with foot, ciliary gliding, adhesive posterior glands*
Substratum preference Coarse-grained,
biofilm, periphytony
Coarse-grained, biofilm,
periphytony
Fine-to coarse-grained,
biofilm, periphytonz
Physico-chemical preferences
Temperature, oxygen, pH, trophic
degrees
Insufficient reports
Resistance, resilience and dispersion Resting eggsxCryptobiosis{Resting eggs **
Sensitivity to flow Decreasing species richness with increasing flowyy
Species common to
benthos and hyporheos
Not sufficiently documented, vertical migrations are strongly suspectedyy
*Giere (1993).
ySchmid-Araya (1998).
zFregni et al. (1998).
xNogrady, Wallace & Snell (1993).
{Ricci (1998).
**Levy (1984).
yySchmid-Araya (1998).
24 C. Ricci and M. Balsamo
ã2000 Blackwell Science Ltd, Freshwater Biology, 44, 15±28
the meiofauna is known to be related to oxygen, as
well as to flow (Palmer, 1990a), but also the grain size
and the detritus content of the sediment are expected
to affect the psammon community since they influ-
ence the water circulation in the interstitial spaces
(Ruttner-Kolisko, 1955b, 1961). The relationship
between the presence and abundance of rotifers and
gastrotrichs and sediment granulometry has not been
explored in detail, but the interaction between bottom
characteristics and fluvial dynamics probably influ-
ences rotifers and gastrotrichs as other taxa (see
Higgins & Thiel, 1988; Giere, 1993; Ward et al., 1998).
Modes of displacement
In hyporheic communities, detritus is probably an
important trophic source, because of the bacteria and
algae attached to it that serve as the food of small-
sized animals, such as rotifers and gastrotrichs. The
passive and active movements through the surface
and deeper sediment layers involve exchange of
organic matter and of energy between different
ecosystem compartments (Ward et al., 1998). Sedi-
ments derived from crystalline primary rocks with an
open pore system are traps for detritus, while sand
tends to become clogged with fine silt, reducing
permeability and making colonisation by animals
difficult. Size of the pores and body size of the
animals should be related (Williams, 1972), but also
the mode of animal mobility changes with granulo-
metry: animals that move by `sliding' prefer coarse-
grained sediments, while those that burrow tend to
inhabit fine-grain substrata (Wieser, 1959). Meio-
benthic rotifers and gastrotrichs move by crawling
or sliding or creeping on sand grains, but are not
capable of burrowing into muddy sediments. There-
fore lotic as well as lentic sediments are richer in
species and individuals if the silt fraction is slight, so
that sandy to coarse substrata are preferentially
colonized; morphological, lithological and geographi-
cal features of the substratum appear of little
importance. Laboratory studies suggest that physical
and chemical characteristics of water and anthropo-
genic pollutants can affect gastrotrich populations
profoundly (Hummon & Hummon, 1979).
Substratum type
Differences in the density of meiobenthic rotifers
between rivers is remarkable: from a rather depaupe-
rate fauna in streams with a bed of fine sand (Evans,
1984; Turner & Distler, 1995) to a rich one, in terms of
diversity and abundance, in rivers with medium to
coarse sediment (Palmer, 1990a; Schmid-Araya, 1998).
On the basis of the rotifer species list (Table 2), it seems
that all genera, except Lepadella and Lecane, `prefer'
coarse or gravel bottoms. However, the variety of
sampling methods used makes the results of different
studies hard to compare quantitatively. The hyporheic
habitat is known to vary widely in space and time as
well as from one system to another (Brunke & Gonser,
1997). The vertical distribution of rotifers in the
hyporheos has been documented in only a very few
studies. The animals are commonly most abundant in
the top 10 cm (Evans, 1984), but some have been found
in sediments as deep as 50±60 cm (Palmer, 1990a). In
contrast, rotifer densities in the sediment of an alpine
stream were positively related to depth (Schmid-
Araya, 1997): higher numbers of monogononts were
reported for the hyporheic interstitial than at the
surface of the stream bed (Schmid-Araya, 1998).
Most gastrotrich species live in the upper 5 cm of
the sediment, even if in the littoral sand of water
bodies they can reach 15±20 cm depth. In lotic gravel
sediments, the abundance of gastrotrichs is related to
a variety of combined parameters, such as sediment
depth, water depth, water temperature and flow, and
decreases in particular with increasing discharge
(Schmid-Araya, 1997). Highest density was reported
deeper in the sediment, possibly due to heterogeneous
grain size that facilitates oxygen penetration into
interstitial habitat (Schmid-Araya, 1997).
It seems therefore that the hyporheos may act as a
temporal refugium for benthic animals in response to
disturbances and spates, as species commonly inha-
biting the substratum surface might move downward
if the flood erodes the surface sediments (Palmer,
1990b; Schmid-Araya & Schmid, 1995a; Brunke &
Gonser, 1997; Ward et al., 1998).
Global distribution patterns
Rotifers and chaetonotid gastrotrichs are believed to
be cosmopolitan, mainly because of passive dispersal
of resting stages, but this presumption may simply
reflect inadequate study (Dumont, 1983). In general,
data about geographical distribution are scanty and
scattered, because (1) animal descriptions are poor
Lotic rotifera and gastrotricha 25
ã2000 Blackwell Science Ltd, Freshwater Biology, 44, 15±28
and identification may be uncertain, and (2) most
studies have focused on few regions only and the
sampling efforts are not comparable. Actually, current
knowledge does not allow us to outline any biogeo-
graphic distribution of the gastrotrichs, as most
genera are assumed cosmopolitan (except Marinellina
and Dichaetura which have been reported from
Europe only, Mola, 1932; Strayer & Hummon, 1991).
The current view about rotifers is that, while some
species are cosmopolitan, many others are not
(Nogrady, Wallace & Snell, 1993). Most research has
dealt with planktonic species, and the benthic groups
have generally been neglected. The best case is the
distribution of the littoral, benthic genus Lecane
(Segers, 1996). About 40% of the 167 species of the
genus are truly cosmopolitan, and endemic species
are in the range of 10±20%. On the whole, a narrower
distribution is suspected in the benthic rotifers than in
the pelagic ones possibly because the higher instabil-
ity and unpredictability of the benthic habitat requires
more specific adaptations (Segers, 1996).
Conclusions
Interstitial gastrotrichs and rotifers are still poorly
documented and their biology is poorly known. There
is growing evidence that most benthic species of both
taxa are not exclusive to the interstitial habitat but are
ubiquitous, since they can migrate between the
epibenthos, periphyton and, eventually, plankton.
Resting stages, parthenogenetic reproduction and
relatively short life cycles are probably key features
that enable gastrotrichs and rotifers to survive in lotic
sediments where they are diverse and abundant. In
addition, both gastrotrichs and rotifers are capable of
resistance and resilience after all kinds of disturbances
(Palmer, Bely & Berg, 1992; Palmer et al., 1997) and
can thus inhabit disturbed habitats such as lotic
sediments. Both taxa are capable of being transported
passively between different water bodies (through
dormant stages) or between different places in the
same water body (through drift in the water column).
This accounts for their wide distribution.
Species are assigned to functional groups on the
basis of the role they play in their ecosystem by linking
ecosystem functioning and the diversity of the organ-
isms in a given habitat (see Freckman et al., 1997; Ward
et al., 1998). Different species may contribute differ-
ently to a process, and thus the loss, or absence, of a
species may be partly compensated for by functionally
similar species: the higher the functional redundancy,
the more resistant is the ecosystem to disturbance. The
diversity of species and function among the small
invertebrates, such as rotifers and gastrotrichs, in
freshwater sediments is largely unknown and needs to
be documented (Palmer et al., 1997). Possessing highly
resistant resting stages, rotifers and gastrotrichs may
be prominent in the fauna of `recovering' streams, and
their role in benthic ecosystems, while probably
relevant, is still poorly known.
Acknowledgments
The authors wish to acknowledge the constructive
criticism and help of Anne Robertson, Jenny Schmid-
Araya, John Green and of two anonymous referees.
Giulio Melone gave invaluable help with the
illustrations.
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