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INTEG.
AND
COMP. BIOL., 42:660–667 (2002)
Rotifers: Exquisite Metazoans1
ROBERT LEE WALLACE2
Department of Biology, Ripon College, Ripon, Wisconsin 54971
SYNOPSIS. Rotifers comprise a modestly sized phylum (ø1,850 species) of tiny (ca. 50–2,000 mm), bilaterally
symmetrical, eutelic metazoans, traditionally grouped within the pseudocoelomates or Aschelminthes. These
saccate to cylindrically shaped protostomes possess three prominent regions (corona, trunk, foot). They are
distinguished by a ciliated, anterior corona (used in locomotion and food gathering) and a pharynx equipped
with a complex set of jaws. Unfortunately, these generalizations grossly oversimplify a rich and fascinating
diversity. Chief among the charms of the study of rotifers are their ecological importance, ease of culture
(including chemostat technology), and the fact that much remains unknown about this exquisite phylum.
ROTIFERAN BAUPLAN
Rotifera is a moderately sized phylum of some
1,850 species (Segers, 2002) of tiny (ca. 50–2,000
mm), bilaterally symmetrical, eutelic (ca. 1,000 cells),
gonochoristic protostomes. They are saccate to cylindrically shaped animals with three prominent regions
(corona, trunk, foot) (Fig. 1). Unfortunately, this simple description oversimplifies a rich and fascinating
diversity.
The defining rotiferan feature is an anterior ciliated
field, the corona (Fig. 1). In many species, the corona
is developed as two concentric rings of cilia (trochus
and cingulum) that beat in a metachronous pattern.
This action, resulting in an illusion of a rotating wheel,
informs the allusion used to conceive the phylum’s etymon (L., rota 1 ferre, wheel-bearers). A second critical feature is a muscular pharynx (mastax) with chitinous jaws (trophi). Trophi are composed of seven
articulating pieces which process food in a variety of
ways (e.g., grinding, piercing-pumping, grasping) (Fig.
2). In some forms, classification to species may be
ascertained based on trophi alone. Challenges—There
appears to be homology in the jaws of Rotifera and
Gnathostomulida (Rieger and Tyler, 1995), which extends to Micrognathozoa (Ahlrichs, 1997; Kristensen
and Funch, 2000). Can molecular analysis confirm this
relationship? (See also the section Phylogenetic quandaries, below.)
The syncytial integument of rotifers is notable in
that it contains an intracytoplasmic lamina (ICL) composed of two filamentous, keratin-like proteins (39K,
47K Daltons), cross-linked by disulfide bonds (Bender
and Kleinow, 1988; Clément and Wurdak, 1991).
When major portions of the ICL are thickened the
bodywall is somewhat less flexible; such species are
called loricate. Species lacking such thickening are
called illoricate. Although the ICL has little taxonomic
significance, it is synapomorphic with phylum Acanthocephala, indicating a close phylogenetic relationship. Challenges—Rotifers appear to be very different
from acanthocephalans. What is the phylogenetic relationship between these taxa? (See also Phylogenetic
quandaries.)
‘‘The rotifers are the most important soft-bodied invertebrates in the fresh-water plankton.’’
G. Evelyn Hutchinson (1967)
A Treatise on Limnology, Vol. II
INTRODUCTION
Rotifers ought to be well known. Being among the
first microorganisms seen by children exploring pond
life with their new microscope or high schoolers in
biology class, rotifers have fascinated people for more
than 300 yr, ever since Leeuwenhoek began describing
them late in the 17th century (Dobell, 1958). They are
available from biological supply houses or are easily
obtained from a variety of freshwater and terrestrial
habitats, including soils and the water films on mosses.
These factors alone privilege their use in introductory
biology courses as a representative of the groups traditionally termed pseudocoelomates or Aschelminthes.
Textbooks on invertebrate zoology offer good summaries of the phylum, while advanced treatises provide
intriguing details (e.g., Clément and Wurdak, 1991;
Nogrady et al., 1993; Wallace and Snell, 2001). Every
three years beginning in 1976 the community of rotiferologists meets to exchange ideas; to date nine symposia have been published, mainly in Hydrobiologia.
Rotifers have even entered our cultural ethos, being
the subject for children’s literature (Bayliss, 1912), a
cartoon in the news magazine Discover, and artists
working with glass (Miner, 1931). However, while
some species are well known and easy to culture, our
efforts have lagged behind those of many other invertebrate zoologists. In fact, much of what we know
about rotifers is based on only a few genera, especially
Brachionus, called the ‘‘white mouse’’ of rotifers by
Charles King (personal communication) (see also Yúfera, 2001). Here I review some recent trends in rotifer
research, offering questions that provide distinct challenges for future work.
1 From the Symposium Lesser-Known Protostome Taxa: Evolution, Development, and Ecology presented at the Annual Meeting of
the Society for Integrative and Comparative Biology, 3–7 January
2001, at Chicago, Illinois.
2 E-mail: [email protected]
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ROTIFERA
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FIG. 2. Scanning electron photomicrograph of the trophi of Encentrum villosum (left 5 ventral view; right 5 dorsal view). Bar 5 10
mm. (Courtesy of Hendrik Segers.)
FIG. 1. Schematic representation of the rotiferan bauplan. (NB: not
illustrated here are the protonephridial, reproductive, and internal
muscular systems.) (With permission from: Barnes et al., 1993.)
REPRODUCTION
Reproductive strategies vary among the classes
(Wallace, 1998; Ricci, 2001). Class Seisonidea is dioecious with gametogenesis taking place by ordinary
meiosis. Class Bdelloidea (diploid) reproduces exclusively by ameiotic parthenogenesis. Class Monogononta (haplodiploid) exhibits a cyclical parthenogenesis with the asexual cycle (amictic females) dominating (Fig. 3). Sexual reproduction in monogononts
(mixis) occurs only after being triggered by specific
environmental signals (e.g., high population density).
Mixis results in the production of mictic females
which produce haploid eggs or, if fertilized, diploid
embryos (resting eggs or cysts). Resting eggs undergo
obligatory diapause, eventually hatching as amictic females. Thus, the ecological and reproductive roles of
amictic and mictic females differ, and it is of interest
to understand the strategies that allot resources to each
(Aparici et al., 1998; Serra and King, 1999). Challenges—(1) What is the optimal sex allocation of amicitic vs. mictic females and can published models be
tested in natural or laboratory populations (Aparici et
al., 1998, 2002; Serra and King, 1999; Ciros-Pérez et
al., 2002)? (2) Although sexuality is still believed to
be plesiomorphic, for many monogononts sex has nev-
er been observed. Has this character been lost in these
species, are local environmental signals unable to trigger mixis, or have sexual periods been missed due to
infrequent sampling?
Other challenges—Three other interesting features
of the monogonont life cycle await additional study
(Fig. 3). (1) Females in some species of Asplanchna,
Conochilus, and Sinantherina are capable of producing
both diploid (mitosis) and haploid (meiosis) eggs;
these so-called amphoteric females can produce males
and resting eggs or females and resting eggs. What is
the mechanism of this switch in nature? Pseudosexual
eggs have been reported in a planktonic species of
Keratella, but this may simply be due to the fact that
inadequate sampling has missed the brief period of
sexuality (Nogrady et al., 1993). (2) In culture Synchaeta pectinata is capable of producing diapausing
amictic eggs along with non-diapausing eggs, with the
proportions being controlled by food limitation (Gilbert and Schreiber, 1998). The significance of this phenomenon appears to be a risk-spreading strategy under
conditions of low food. Does amictic diapause occur
in other species? (3) Males of most species are structurally reduced (dwarfism, progenesis), within a continuum ranging from slightly reduced in Rhinoglena
to highly reduced in Asplanchna (Ricci and Melone,
1998; Serra and Snell, 1998). Selection for male dwarfism may be a strategy that maximizes male production
with a minimum reproductive cost. Is there a pattern
to distribution of male dwarfism in rotifers and is it
reflected in their habitats?
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ROBERT LEE WALLACE
However, although individual biomass is minute, large
population size, coupled with high turn-over rates
make rotifers an important component of food webs
(Herzig, 1987; Starkweather, 1987; Walz, 1997). Adding to their importance is the fact that rotifers are eaten
by invertebrate predators and are also the first food of
fish fry, thereby making their energy available to higher trophic levels. While recent work indicates that rotifers are significant components of microplankton
community structure, the magnitude of that importance
has not been completely explored (Arndt, 1993; Berninger et al., 1993; Rublee, 1998). Challenges—(1)
How important are rotifers to microplankton food
webs in a variety of habitats? (2) How important are
rotifers to the ecology of springs and soils (Pourriot,
1979), interstitial (Wallace and Ricci, 2002), and periphytic habitats (Duggan, 2001)?
FIG. 3. Generalized life cycle of rotifers. Thickness of the arrows
illustrates the relative frequency of life cycle components (asexual
and sexual) in monogonont rotifers. The life cycle of bdelloid rotifers comprises only the asexual component. Symbols 5 dv, development; mt, mitosis; h, hatching; hs, hatching stimulus; ms, mixis
stimulus; mi, meiosis; mr, mate recognition; mb, mating behavior;
f, fertilization; –f, not fertilized; dp, diapause.
TAXONOMY
Simply put, when compared to other freshwater micrometazoans (i.e., cladocerans, copepods) rotifers suffer from a serious lack of taxonomists. In Europe,
where the situation has always been better, taxonomic
training appears to be healthy, while in North America
the number of taxonomists has fallen to a critically low
level. In all, what we know about rotifer taxonomy,
biogeography, and records of invasions are limited to
those places were rotifer workers live or where they
have had the opportunity to sample. In practical terms,
much of the world, including the U.S., remains aqua
incognita for detailed rotifer work. Challenge—How
can we increase taxonomic training and study?
ECOLOGY
Rotifers are important components in freshwater
ecosystems
Until the 1980s contributions of rotifers to the trophic dynamics of lakes were frequently overlooked.
Population biology
Examination of annual population cycles of rotifers
from a variety of habitats indicates that species abundance can differ markedly from year to year (Herzig,
1987). Unfortunately, even short-term sampling schedules (weekly) can miss the details of population peaks
(Berner-Fankhauser, 1987). Thus variability, coupled
with difficulties involved in sustaining intensive sampling schedules, makes studying population dynamics
difficult. Use of chemostats along with particle-counting technology, however, permits systematic manipulation of experimental conditions, thereby allowing
tests of population growth models (Walz, 1993). For
example, Boraas et al. (1998) and McNair et al. (1998)
offer a model with separate components for eggs
(based on age) and free-swimming individuals (based
on age-specific biomass). Challenges—(1) How can
sampling techniques be improved to assess spatial and
temporal variation within populations? (2) Can chemostat models be employed to test interpretations of
the dynamics of natural populations?
A miner’s canary?
Rotifers have been used in pollution monitoring either as bioindicator species or as part of the saprobic
assessment system for some time. The former simply
relies on a list of species known to indicate pollution
(Mäemets, 1983), while the latter integrates a large
number of abiotic and biotic characteristics into a single variable describing the relative level of eutrophication (Sládecek, 1983; Marneffe et al., 1998). In addition, because rotifers are relatively easy to culture
and sensitive to pollutants they have become important
tools in ecotoxicological testing (ET). In ET, rotifers
are exposed according to standardized protocols to
compounds with results being reported as LC50s,
EC50s, or NOEs (no observed effects) for reproductive and/or behavioral endpoints (ASTM, 1991; Snell
and Janssen, 1995). Other work in this field has focused on cellular and enzymatic biomarkers as indicators of sublethal effects (Snell and Janssen, 1995).
Recent examination of UV radiation as a factor influ-
ROTIFERA
encing survival and reproduction of aquatic organisms
has provided valuable information on the effects of
UV on rotifers (e.g., Leech and Williamson, 2000).
This work indicates that there is a wide range in tolerance, with Keratella spp. being among the most tolerant and Asplanchna priodonta among the least. Similarly, Perez-Legaspi and Rico-Martinez (2001) have
shown that LC50s varied up to 22 times for three species of Lecane. Challenges—(1) How robust are these
testing protocols when only a few species have been
used and when LC50s vary widely even among members of the same genus? (2) Is it feasible to use rotifers
as models for toxicity screening given the numbers of
chemicals that need to be tested? (3) Can rotifers be
of practical use as biosensors of environmental degradation or contamination? (4) Is it possible to determine the fate of toxicants and the presence and efficiencies of detoxification systems in rotifers?
SYSTEM
AND
ORGANISMAL LEVEL STUDIES
Knowledge of neurobiology is meager
Comprising about 25% of the total number of cells
in the body, the rotiferan nervous system is simple,
consisting of a concentration of ganglia at the anterior
end (brain), several additional ganglia serving the mastax, body, and foot, paired ventral neurons, and three
sensory organs (mechano-, chemo-, photo-receptors).
In addition, many species possess a retrocerebral organ, comprising paired subcerebral glands, an unpaired
retrocerebral sac, and ducts leading to the coronal surface. Challenges—(1) What is the function of the retrocerebral organ? (2) Can neuroanatomy provide additional phenetic traits for phylogenetic studies (see
below)?
A few neuro-pharmacological investigations have
shown that cholinergic systems function in at least 12
species representing six families and that adrenergic
and catecholaminergic systems have been observed in
other species, including both bdelloids and monogononts (e.g., Nogrady et al., 1993; Kotikova, 1995,
1998). These studies indicate that (1) distribution of
neurons in rotifer brains are bilateral with distinct species-specific patterns, (2) bipolar neurons are typical,
but that uni- and multipolar neurons also are present
in small numbers, and (3) catecholaminergic neurons
comprise ø5% of the total number of brain cells.
Challenge—Are the details of rotifer neuroanatomy
and neuophysiology uniform across a wide array of
taxa?
Several researchers have studied the effects of environmental stimuli on rotifer behavior including
movement (Charoy et al., 1995), feeding (Starkweather, 1987, 1995; Bevington et al., 1995), mating (Snell,
1998), and substratum selection (Wallace, 1980). However, efforts by Joanidopoulos and Marwan (1998)
have yielded some extraordinary insights to the relationship between neuroreceptors and behavior. While
holding male Asplanchna sieboldi on the tips of microcapillary tubes, these workers stimulated their cor-
663
onal sensory receptors using the chemical and mechanical stimuli associated with male-female encounters. These stimulations triggered specific male mating
responses. Challenge—Will we employ these and other procedures such as microelectrode technology, dye
injection, cell labeling, photometry, and video imaging
to better characterize rotifer neurobiology?
Aging studies
Use of rotifers as models for aging makes sense as
they are eutelic, have short lifespans, and are inexpensive to culture under conditions suitable for life history
analysis. However, although the earliest aging work
dates from the 1880s, there has never been a consistent
effort to examine a variety of species or to systematically test all aging theories using rotifers. Some of the
studies that have been done have examined effects of
maternal age, pattern of reproduction, photoperiod,
and various antioxidants on aging and life history.
However, of the dozen or so theories of aging, only
four have been examined using rotifers (Enesco,
1993). According to Enesco, the rate of living theory
is not substantiated, but there is support for the free
radical damage and calcium theories (cf. King, 1983).
Based on the fact that chemical signals are known to
evoke a variety of life history events in rotifers, Enesco also proposed programmed aging as a plausible
model for rotifer aging. In this model, the withdrawal
of a reproductive hormone serves as the programmed
signal for termination of life. Challenges—(1) Can
other theories of aging be examined using rotifers? (2)
Is it possible to link aging studies to those of ET (see
above)? (2) Given that rotifers are eutelic, have they
retained DNA repair enzymes?
Coloniality in rotifers is not understood
Unlike some colonial invertebrates, rotifers do not
share energy resources among colony members.
Therefore, the fact that colony formation should be
found at all is remarkable; intuitively one would expect that their only interactions would be as mates,
competitors, or in predator-prey relationships (Wallace
and Snell, 2001). Nevertheless, although not a widespread phenomenon, ø25 species in all eight genera
of family Flosculariidae (microphagous monogononts)
form permanent colonies. Further, there appears to be
a connection between coloniality and being sessile, for
about 70% of colonial taxa (18 species) are sessile
(Wallace, 1987). Challenge—How is coloniality linked
to the sessile life-style?
Colony size varies considerably from a minimum of
two members (Floscularia ringens) to truly gargantuan
colonies of .1,000 in certain Lacinularia species. Regardless of size, colonies form by one of three different methods termed allorecruitive, autorecruitive, and
geminative (Fig. 4). Coloniality may have three important implications (Wallace and Snell, 2001). (1) Coloniality may improve individual energetics. (2) It may
decrease predatory success of tactile and even certain
visual predators. (3) Colony formation method implies
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ROBERT LEE WALLACE
ing tactile predators. Challenge—What is the mechanism of embryonic induction of spines in B. calyciflorus and other species which form spines by induction?
Not all appendages in rotifers function by directly
interfering with predatory attack. Some species possess movable extremities that swing, making wide, arclike movements (i.e., Filinia, bristles; Hexarthra, setous arms; Polyarthra, paddles). For example, when
Polyarthra detects shear disturbances, as might be produced by a predator (copepods) or large suspension
feeder (daphnids), it flicks its paddles very rapidly.
The results of this behavior are a series of swift jumping (escape) movements which achieve remarkable
speeds: ca. 20 mm/sec or .150 body lengths/sec (Gilbert, 1987). Challenge—The mechanics of these jumps
appear to be more akin to rowing (i.e., of water striders). What is the biophysics of these remarkable movements?
FIG. 4. Modes of colony formation in rotifers. Upper panel: Allorecruitive—juveniles settle on tubes of conspecifics that had previously attached to another substrate; these colonies begin with larval
attachment to a previously settled adult and end when recruitment
ceases (e.g., sessile: Floscularia). Middle panel: Autorecruitive—
young remain within their parental colony; these colonies develop
continuously increasing in size as new individuals are added and
diminishing when the colony fragments or dies off (e.g., planktonic:
Conochilidae, Lacinularia, Sinantherina; sessile: Octotrocha). Lower panel: Geminative—all young hatching within a few hours of
each other leave the parent colony together as a planktonic juvenile
colony; as an aggregate, these juveniles explore and attach to a new
substratum together, thus maintaining their colony (e.g., sessile: Lacinularia, Sinantherina).
differences in the genetic relatedness of colony mates.
Challenge—The methodologies appear to be available
to test these hypotheses, but is it possible to do so?
Spines are used as foils and in making rapid
jumping movements
Spine formation in rotifers has been recognized as
a defensive mechanism that reduces predation by tactile predators (Gilbert, 1999). The classic example is
seen in Brachionus calyciflorus. When amictic females
of this planktonic monogonont are exposed to a watersoluble factor from the predatory rotifer Asplanchna,
the next generation is born with posterolateral outgrowths of the body wall (spines). These spines are
extended slightly away from B. calyciflorus when it is
disturbed, thus making it more difficult to manipulate
and ingest by Asplanchna. While there are other consequences to spine production, it is clear that some
spines, whether moveable or not, act as foils frustrat-
GENETICS AND PHYLOGENY
Progress in genetics
For many years progress in understanding rotifer genetics was hampered by the fact that no morphological
trait had been linked to a specific gene (King, 1977).
However, our knowledge has been greatly advanced
through application of several techniques, including
cross-mating tests (Rico-Martı́nez, 1998), combination
of allozyme and morphometric techniques (Serra et al.,
1998; Gómez, 1998), DNA hybridization (D. Mark
Welch and Meselson, 1998), karyotyping (J. Mark
Welch and Meselson, 1998), and gene sequencing
(Garey et al., 1998; Mark Welch, 2000; Mark Welch
and Meselson, 2001). One specific aspect of rotifer
genetics that is being untangled is the determination
that bdelloids have evolved without sex for tens of
millions of years (Mark Welch and Meselson, 2000).
Another fundamental question of rotifer phylogeny is
whether there are cryptic species present within what
otherwise appears to be a solid species. For example,
using a region of the mitochondrial cytochrome oxidase subunit I gene, Gómez et al. (2000) have concluded that populations of a brackish water species,
Brachionus plicatilis, from the Iberian Peninsula show
deep phylogeographic structure. Gómez and her coworkers have concluded that two main lineages probably began to separate early in the Pleistocene. This
molecular work has been augmented by classical taxonomy with the description of three species within the
B. plicatilis complex (Ciros-Pérez et al. 2001). Challenges—(1) Regardless of progress made in the past
25 yr it is interesting to note that many significant
questions articulated ø25 yr ago are still largely unanswered: i.e., compare King (1977) to King and Serra
(1998). (2) Are cryptic species common within the
phylum? (3) What is the genetic system responsible
for the switch from asexual to sexual reproduction in
monogononts?
Phylogenetic quandaries
Rotifer phylogenetic studies began to catch up to
the rest of zoology with application of modern tech-
ROTIFERA
665
fine points of the way the analyses have been worked.
For example, in some morphologically based studies
the issue is whether the lemnisci and proboscis of
acanthocephalans are homologous to the hypodermic
cushions and apical rostrum of bdelloids (Lorenzen,
1985; Ricci, 1998; Nielsen, 2001). Without detailed
ultrastructural and development support, it seems premature to proclaim these features to be homologous.
Molecularly based studies also differ in several
ways, and that appears to depend on which genes are
analyzed and whether the euryhaline monogonont,
Brachionus plicatilis is included. The study of Garey
et al. (1998), which used two genes (18S rRNA and
mt 16S rRNA) and B. plicatilis, yielded a tree in which
acanthocephalans cluster as modified bdelloids. However, a study by Mark Welch (2000), which did not
include B. plicatilis, and used the hsp82 gene, yielded
a tree that placed acanthocephalans as a sister-group
to the Eurotatoria. Challenges—(1) Future work needs
to include (a) additional taxa (especially from within
Rotifera), (b) fresh morphological characters, and (c)
more gene sequences. (2) Also unresolved are the phylogenetic positions of Cycloneuralia, Chaetognatha,
Gnathostomulida, Cycliophora, and Micrognathozoa
(Funch and Kristensen, 1995; Winnepenninckx et al.,
1998; Kristensen and Funch, 2000).
FIG. 5. Phylogenetic controversy over placement of acanthocephalans and friends (1–3, mophphological; 4–6, molecular). 1, Ahlrichs (1997); 2, Melone et al. (1998); 3, Sørensen et al. (2000); 4,
Winnnepenninckx et al. (1998); 5, Garey et al. (1998); 6, Mark
Welch (2000, 2001). A, Acanthocephala; B, Bdelloidea; Cy, Cycliophora; G, Gnathostomulida; Mg, Micrognathozoa; M, Monogononta; S, Seisonidea.
niques, but this does not mean there is unanimity of
thinking on rotifer evolution. Nevertheless, some progress has been made. For example, taxon Digononta
has been rejected by both morphologically and molecularly based analyses (Ahlrichs, 1997; Melone et al.,
1998; Mark Welch, 2000, 2001). However, the relationship between Rotifera and Acanthocephala has not
been resolved. Moreover, the hypotheses regarding the
evolutionary relationships of these and other taxa are
somewhat chaotic (Fig. 5). These ideas may be
summed up as follows: acanthocephalans are a sistergroup of Rotifera, sensu stricto (Melone et al., 1998;
Nielsen, 2001), Bdelloidea (Lorenzen, 1985; Garey et
al., 1998; Garey and Schmidt-Rhaesa, 1998; Winnepenninckx et al., 1998), Seisonidea (Ahlrichs, 1997),
or Eurotatoria (Bdelloidea 1 Monogononta) (Mark
Welch, 2000, 2001). In part, this debate rests on some
OTHER INTERESTING TOPICS
Space does not permit an evaluation of other inviting topics, including physiology, development, morphology, dormancy patterns, biogeography and biodiversity, chemical ecology, and host-parasite relationships. As a group, the bdelloids offer numerous interesting problems, but except for a scant handful of
workers, this taxon does not receive the research it
deserves (e.g., Ricci, 2001).
PRACTICAL STUDIES AND SOME SPECULATION
Wedding knowledge of culture methods and the fact
that rotifers are highly nutritious foods for the larvae
of marine crustaceans and fish, aquacultural technologies have been producing enormous qualities of rotifers in very large culture systems for more than 30
yr. In this regard, rotifers may be seen as living nutrients (i.e., tiny, free-swimming, food capsules) that may
be artificially augmented with highly unsaturated fatty
acids and antibiotics, both of which are important to
improve survival and growth of the larvae. As a result
of these intensive efforts, rotifers provide an important
link in the food supply in many countries including
China, India, Israel, Japan, Spain and Thailand (e.g.,
Lubzens et al., 1989, 2001). While much of this work
has concentrated on B. plicatilis, recent efforts with
freshwater species in this genus have been undertaken
and are expected to grow. Challenge—An important
aspect of this practicum is to refine the technology
making it more economically feasible for use in developing countries.
One exciting extension of this aquacultural labor is
that knowledge gained from commercial enterprises
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ROBERT LEE WALLACE
may be adapted to space travel. So far the work is
very preliminary, involving only the feasibility of rotifer culture in low- or zero-g conditions (Ricci et al.,
1998). The next steps will require development of fully functional micro- or mesocosms and then practical
aquaculture systems modeled after extant systems.
Challenge—Might rotifers become part of artificial
ecosystems onboard spacecraft which are then readapted into an intensive aquaculture system for an
offworld colony?
Mining the biological world for useful biomolecules
currently engages a large research effort. Although this
is pure speculation, I suggest that rotifers are a target
organism for a similar effort. For example, as a group,
rotifers possess trophi adapted to capture and process
a wide variety of foods in diverse ways (De Smet,
1998; Kleinow, 1998). Some of these trophi share a
striking resemblance to complex tools (forceps) as
might be used in surgery (Fig. 1). Challenge—Could
certain rotifer trophi serve as models for a new microtechnology?
CONCLUSIONS
This review considers research trends in rotifers and
suggests what might be accomplished with additional
effort. With that in mind, I offer the same sagacity
presented to me over 30 yr ago (Donald Zinn, personal
communication). Should you wish to study a group of
invertebrates about which little is known, consider rotifers.
ACKNOWLEDGMENTS
I thank Alan Kohn, Doug Light, Tom Nogrady, Elizabeth Walsh, and an anonymous reviewer who offered
valuable comments that significantly improved this
manuscript. I also offer a special thanks to the Family
of Rotiferologists, who are too numerous to mention.
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