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otifers are microscopic pseudocoelo-mates that have attracted scientific interest for many years because they are conspicuous in aquatic environments, swim slowly and can be easily cultured. Early on, many rotifer workers were ama-teurs, who were interested in cataloging different morphologies found in benthic and planktonic samples. As academic sci-entists became interested in rotifers, they focused on more general and less descriptive issues. Peculiarities of the group, such as eutely, parthenogenetic reproduction, heterogony and dormancy have been exploited in experimental investigations. Thus, rotifers have be-come useful biological models for testing ecological and evolutionary theories.

Rotifer research has been signifi-cantly enhanced by a series of inter-national symposia where rotiferologists can meet, discuss, plan collaborations, and establish scientific and personal inter-actions. The themes of the latest meeting in the series*reflected the heterogeneity of interests. However, because there were no concurrent sessions, all partici-pants were exposed to a variety of topics and different modes of interpretation. The topics ranged from evolution and phylogeny, to genetics, ecotoxicology, trophic interactions, zoogeography and biochemistry.

Rotifer evolution

Much attention focused on ancient asex-uality of bdelloid rotifers, which repre-sents an evolutionary challenge because bdelloids are obligate parthenogens. They seem to have escaped Muller’s ratchet and still retain considerable evolutionary potential. David and Jessica Mark-Welch

(Harvard University, Cambridge, MA, USA) presented work on molecular evo-lution in bdelloids. Intron structure of bdelloids is particularly informative and fluorescent in situ hybridization showed that, although diploid, bdelloids do not possess homologous chromosomes. This reinforces the hypothesis that recombin-ation has been suppressed for a long time and that bdelloids are indeed ancient asexuals.

The molecular phylogeny of rotifers was reviewed by David Mark-Welch, who warned against the use of a single species as a representative of entire taxa because some groups possess a high GC content biasing any comparisons. He concluded that Acanthocephala, long considered as a separate taxon, is a highly derived class of Rotifera, and that there is molecular support for joining the two classes Mono-gononta and Bdelloidea into a single monophyletic clade, Eurotatoria, in agree-ment with morphological cladistics. A pu-tative relative of rotifers was suggested by Martin Sørensen (Copenhagen University, Denmark), who compared the Gnamoth-ostomulida jaws with the rotifer trophi.

Rotifers inhabit environments with high temporal heterogeneity and diverse life cycles have evolved to cope with this, thus providing opportunities to under-stand the adaptive significance and consequences of dormancy. Claudia Ricci (Milan University, Italy) compared the two forms of dormancy in rotifers, dia-pause and quiescence, focusing on condi-tions that elicit dormancy. She concluded that these are alternative, mutually exclu-sive responses to disturbance. Bdelloids face unpredictable, fine-grained environ-ments and respond deterministically by entering a fast, short-lasting dormancy (anhydrobiosis). Monogononts produce a long-lasting dormant stage, in response to a more predictable coarse-grained

environment. Substantial stochasticity exists in the timing of the initiation of sex-ual reproduction and resting egg produc-tion. This was illustrated independently by Thomas Schroeder (Berlin Free Univer-sity, Germany), who examined the plank-tonic rotifers of the Oder River floodplains, and by Eduardo Aparici et al.(Valencia University, Spain), who studied clones iso-lated from a single rotifer population. Both showed that rotifers vary the start of the sexual phase and thus dormancy, fol-lowing bet-hedging strategies to cope with temporally unpredictable environments.

Dormant forms are viewed as a bank of biodiversity – genetic variability is introduced upon hatching. Tomonari Kotani et al.(Nagasaki University, Japan) hatched resting eggs of Brachionus rotundiformis, collected from sediment cores of different depths from a Japanese lake, and performed mating assays to test for reproductive isolation among the tem-porally separated populations. The oldest resting eggs recovered were 65-years-old and were still viable. Mating tests and binding assays, using an antibody for a mate-recognition pheromone, showed that the four temporally isolated popu-lations were reproductively compatible. Africa Gómez (University of Hull, UK) applied microsatellite analysis to the study of polymorphic loci in rotifers. She found evidence of discrepancies between clonal diversity in the resting egg bank and that of the early planktonic popu-lation of Brachionus plicatilis. Her results suggest that clonal diversity decreases during parthenogenetic reproduction, that mating produces a resting egg bank in Hardy–Weinberg equilibrium and that hatching is not synchronized among the different genotypes. In contrast with the extensive genetic variation at microsatel-lite markers, strong clonal selection was suggested by the theoretical analysis of Charles King (Oregon State University, Corvallis, USA), who had previously re-ported low allozyme variation among clones at the same sampling date.

Rotifer ecology

Some contributions tested the theory of ecological interactions at individual,

Small, beautiful and sexy: what rotifers

tell us about ecology and evolution

*The IX International Rotifer Symposium, Khon Kaen, Thailand, 16–23 January 2000.

17 Lele, S. and Richtsmeier, J.T. (1991) Euclidean distance matrix analysis: a coordinate free approach for comparing biological shapes using landmark data. Am. J. Phys. Anthropol. 86, 415–427

18 Lele, S. and Cole, T.M. (1996) A new test for shape differences when variance–covariance matrices are unequal. J. Hum. Evol. 31, 193–212

19 Rao, C.R. and Suryawanshi, S. (1996) Statistical analysis of shape of objects based on landmark

data. Proc. Natl. Acad. Sci. U. S. A. 93, 12132–12136

20 Rao, C.R. and Suryawanshi, S. (1998) Statistical analysis of shape through triangulation of landmarks: a study of sexual dimorphism in hominids. Proc. Natl. Acad. Sci. U. S. A. 95, 4121–4125

21 Rohlf, F.J. (2000) On the use of shape spaces to compare morphometric methods. Hystrix Int. J. Mamm. 11, 8–24

22 Richtsmeier, J.T. et al. (1993) The role of postnatal growth in the production of facial morphology. Syst. Biol. 42, 307–330

23 Coward, W.M. and Conathy, D.M. (1996) A Monte Carlo study of the inferential properties of three methods of shape comparison. Am. J. Phys. Anthropol. 99, 369–377

24 Rohlf, F.J. (2000) Statistical power comparisons among alternative morphometric methods. Am. J. Phys. Anthropol. 111, 463–478

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NEWS & COMMENT

population and community levels. John Gilbert (Dartmouth College, Hanover, USA) found both genetic variance and environmental (inducible) effects for the length of posterior spines used for preda-tor defense in Brachionus quadridentatus, the 11th rotifer species found with predator-induced spine development. His results illustrate how selection works to fine-tune phenotypic plasticity in rotifers; this was also shown by the effects on offspring fitness of differential allocation of resources to eggs (Nadia Santo et al., Milan University). Control of rotifer population density by predators was addressed by Norbert Walz et al.

(Freshwater Ecology and Inland Fish-eries, Berlin, Germany), who reported an unexpected impact of freshwater mussel predation. Another contribution demon-strated density-dependent effects and regulation in natural rotifer populations using time-series analysis and complex phenomenological models (Terry Snell

et al., Georgia Institute of Technology, Atlanta, USA). The impact of size-depend-ent predation by copepods and aquatic insects on rotifer populations and assem-blages was analysed in several contribu-tions (e.g. Stephanie Hampton, Dartmouth College). This work can help us to under-stand the maintenance of ecological diversity at low taxonomic levels (e.g. Rainer Deneke, Brandeburg Technical University, Bad Saarov, Germany). With

similar focus, Jorge Ciros et al.(Valencia University) analysed competitive dynam-ics between sympatric sibling species. Using Tilman’s experimental design1_,

seldom applied to animals, they showed that coexistence is possible based on differential use of algae prey.

Two reviews helped revise the inter-pretation of ecological interactions at community and ecosystem levels. Peter Starkweather (Nevada University, Las Vegas, USA) stressed the dual role of rotifers as predators and prey, and showed, in agreement with other partici-pants (e.g. Ian Duggan, Waikato Univer-sity, Hamilton, New Zealand), that rotifers are prominent components of aquatic food webs, in contrast with the prevailing crustacean-biased view. The second review by Ramakrishna Rao (Delhi University, India) compared the impact of predation and competition on tropical and temperate rotifer commu-nities, concluding that top-down control is relatively more important in the trop-ics, with high predation pressure select-ing for smaller body size. The concomi-tant disadvantage of small body size in competition is not so important because of the relative abundance of food in trop-ical aquatic ecosystems.

Prospects

Knowledge of the biology of the rotifers is approaching a mature stage. As the

biology of this group becomes better understood, the role of rotifers as a key group influencing the dynamics of aqua-tic ecosystems becomes better appreci-ated. Dormancy, parthenogenesis and short life cycles give rotifers a central role in aquatic ecosystems. Their utility as experimental models for testing contemporary ecological and evolution-ary theory is documented by the recent literature, and by this, and previous, symposia.

Acknowledgements

This meeting was organized by La_orsi Sanoamuang, Khon Kaen University, Thailand.

Claudia Ricci

Dipartimento di Biologia, Università di Milano, via Celoria 26, 20133 Milano, Italy (claudia.ricci@unimi.it)

Manuel Serra

Institut Cavanilles de Biodiversitat i Biologia Evolutiva, Universitat de València, Apartado Oficial 2085, 46071 València, Spain

(manuel.serra@uv.es)

Terry W. Snell

School of Biology, Georgia Institute of Technology, Atlanta, GA 30332-0230, USA (ts41@prism.gatech.edu)

References

1 _{Tilman, D. (1982) Resource Competition and}

Community Structure, Princeton University Press

I

n most fields within biology, a small number of taxa receive disproportion-ate scientific attention. This phenom-enon is evident at all phylogenetic levels (e.g. ‘model species’ within genera and ‘model genera’ within families, etc.) and operates differently within different fields of study. Particular taxa can serve as ‘model organisms’ for some types of question, whereas other parts of their biology attract little attention. Why does this pattern exist and what factors some-times cause it to change? Here, we exam-ine the growing popularity of a previ-ously neglected lineage – the snakes – as study organisms for ecological research.

With a few notable exceptions (es-pecially the Hymenoptera), most of the popular model organisms in behavioural

ecology are vertebrates – particularly endothermic vertebrates (birds and mammals). This research emphasis is massively disproportionate to species richness, even if we restrict comparisons to within the terrestrial vertebrates. For example, there are more species of amphibians than mammals1_{, but almost}

ten times as many papers on behaviour and ecology of mammals than of amphib-ians (over the period 1979 to 1998, Zoo-logical Recordshad _.32 000 entries for behaviour and ecology of mammals, and about 4000 for amphibians).

There are both costs and benefits of concentrating a high proportion of re-search upon a small number of taxa. The scientific benefits are perhaps most obvi-ous. Some sorts of animal have features

that expedite particular types of re-search (e.g. birds are generally much easier to observe than mammals). Other benefits of such taxonomic concen-tration of research include historical fac-tors: there are more giants upon whose shoulders one can stand. More cynically, there might be other logistical advan-tages also. Those same giants (or their academic offspring) might well review your papers and be inclined to look upon them more favourably. Nonetheless, there is also a cost in generality. We cannot hope to understand the diversity of organismal tactics unless we study a diversity of organisms.

The system is not static, because the relative intensity of effort devoted to par-ticular types of research within particu-lar taxa varies substantially through time. Analysis of Zoological Records en-tries reveals some interesting patterns in this respect. We counted the number of entries for each major group in specific fields (Fig. 1). The relative numbers of publications per group remained fairly constant through time, with an increasing