Cladoceran studies: where do we go from here? - Springer Link

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But will it help? Sometimes, when I hear that some alumni of the course have started a course of their own,. I feel encouraged. Sometimes, when I receive manu-.
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Hydrobiologia 360: 301–303, 1997.

A. Brancelj, L. De Meester & P. Spaak (eds), Cladocera: The Biology of Model Organisms. c 1997 Kluwer Academic Publishers. Printed in Belgium.

Cladoceran studies: where do we go from here? Henri J. Dumont Institute of Animal Ecology, University of Ghent, Ledegansckstraat 35, B-9000 Gent, Belgium

Abstract For ecology and genetics of the Cladocera to progress more rapidly, it would be beneficial if efforts were concentrated on a single taxon. Daphnia magna is probably the best candidate. But what are the Cladocera? Upon inspection, confusion is found to rage at any level. We do not know whether the group as a whole is monophyletic, the number of families and genera is uncertain, and we have only a faint idea of the total number of species. Neither do we know much about the geographical distribution of most species, which may have been blurred by human dispersal since the appearance of large-scale navigation. Currently, introductions and invasions continue to occur. These are well monitored, but events in the past could also be reconstructed, using the (sub)fossil record. There is thus a good future for the use of cladoceran remains in paleolimnology. Model organism Others in this section discuss aspects of ecology and genetics, so I will be brief on these topics. Suffice it to say that I strongly advocate a conduct similar to that of our colleagues in classical genetics and in molecular biology, which is that we should select and concentrate on one or few taxa about which we could try to learn everything there is to know, instead of going in all directions at once. A candidate for this role of model organism is to be looked for in the genus Daphnia. I would recommend Daphnia magna as a candidate to become the Drosophila melanogaster of aquatic ecology but realise that, like D. melanogaster, magna might not be everybody’s choice. It is not known to hybridise with other Daphnia, which is a disadvantage, but it also has a number of distinct advantages: it occurs over a broad geographic range, in a wide variety of water types, and it is easy to culture. In particular, as nucleotide sequencing facilities become widely available, more rapid, and less expensive, it might soon be realistic to think of compiling a full record of Daphnia magna’s genome. Apart from revealing how a crustacean is to be assembled, organisms like Daphnia (we could use a few others, from other major branches of the zoological tree) could be used to tell us how robust (or weak) single-locus (or even multi-locus) phylogenies really are. With yeast

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and the human genome already partly available, Daphnia should rank high among the candidates next in line. It strikes me as odd that, as far as physical gene mapping is concerned, Daphnia should lag so far behind that other branchiopod, Artemia. I can see no reason for this other than that Artemia workers have benefited from about two decades of concerted action, which translates into a number of sizeable monographs. In contrast, it took until 1987 for a book, boldly entitled ‘Daphnia’, to see the light (Peters & De Bernardi, 1987). I’d like to believe that nobody will regard this as the definitive work; rather, it is a useful start. About every chapter merits to be worked out to a full-sized volume in its own right.

Morphology and systematics Which brings me to morphology and systematics (or classification). Is there reason to continue working on these traditional disciplines, and if so, what subjects should be prioritised? In working with Stefan Negrea from Bucharest on the introductory volume ‘Cladocera’ for my series of zooplankton identification guides, it struck me that we are currently in no position to even define what cladocerans really are. Geoffrey Fryer (1987, 1995) started this off, and recently new voices were raised (e.g. Martin & Cash-Clark, 1995) in what

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302 I expect to become a debate. The problem is not so much to work out whether the aggregate known as the Cladocera is a monophyletic group, than at what level the cut-off points between branches are situated, hence what are the ranks of the four constituent groups. Clearly, they are not at the same level. A Diaphanosoma is closer to a true anomopod like Daphnia than to a Bythotrephes or to Leptodora. But that is a crude evaluation, in need of much refinement. Progress can be expected from explicit comparisons between one or more homologous genes. Current favourites are 18s and 28s RNA genes, tubulin genes, cytochrome b genes, and the list is growing continually. The techniques for this are available and straightforward, yet work on the cladocerans lags behind that in other groups. Even if it might require only few years of work to close this gap, I am not aware of anyone having embarked on the subject. Of course, such phylogenies can also be established on morphological grounds, given that a sufficient number of character states can be evaluated. Here, too, cladocerans sadly lag behind. Trunk limb structure, for example, rich in interesting traits, is not much better known today than in the pioneer days of Behning, Eriksson and others. A notable exception is the recent book by Alonso (1996), where the study of trunk limb morphology was finally taken seriously, as well as some recent papers by Alexei Kotov on the Bosminidae (e.g. in this volume). Much to my dismay, I recently discovered that within the anomopods, family boundaries are unclear too. Thus, Smirnov (1992) correctly took the Ilyocryptidae out of the Macrothricidae. Ilyocryptids are in fact so different from all other anomopods in trunk limb structure that they assume an isolated position within the order. But numerous other problems at the family level await a solution: neither the chydorids nor the macrothricids are in fact homogeneous (or, in the current jargon, monophyletic). Down one more step then, to genus level. Let me first state that the recent avalanche of papers on natural hybridisation within Daphnia lends support to the controversial view of the genus as an objective taxon by A. Dubois. Dubois (1988) claims that a genus is the collection of all species capable of interbreeding. This definition (what can’t interbreed belongs to different genera), however intellectually pleasing, unfortunately is of limited practical use in cyclical parthenogens, most of which have only been seen in pickled samples. Yet, it should free us of the fear that ‘new species’, described on morphological grounds only (a practice that is likely to be with us for a while), need to be ‘well

demarcated’ from all their congeners. I rather consider that minute morphological characters may be quite significant, if it can be shown that they are consistent. With this in mind, I would not be surprised if future work would show that some of the more specious anomopod genera need to be split. Imagine indeed some six co-existing species of Alona or Pleuroxus to go sexual and all interbreed. Could so much reproductive waste be absorbed without introgression? And, finally, at the bottom line there is the classical question: how many species of Cladocera are there? Korovchinsky (1996) recently attempted to answer it, but was frustrated by the number of ill-described species. With that, I agree. My own estimate, based more on a gut-feeling than on anything else, is ‘around 500’, but that refers to known species, not to the world’s total. Part of Korovchinsky’s and my own uncertainty stems from the flood of synonyms – both old and new – that bedevils all such estimates. This is not the place to judge the taxonomic work of others, but it seems that the cripple descriptions of our forefathers are currently replaced by cripple descriptions by poorly trained workers from underequipped and underfunded laboratories in the developing world. In my own limited way, I have attempted to do something about this situation, by organising an in-depth training course on zooplankton taxonomy (which I had to sell to my sponsor, the Belgian minister of cooperation, as a course in lake management). For the past eight years, we yearly trained about ten people from Africa, Asia, and Latin America per session. We send them home with a wagon load of literature, and often even with a plankton net. But will it help? Sometimes, when I hear that some alumni of the course have started a course of their own, I feel encouraged. Sometimes, when I receive manuscripts for Hydrobiologia from those same countries, written in incredibly poor language, and with unspeakable figures, I feel discouraged. In our days of a renewed interest in the world’s biodiversity, taxonomists around the world hope that more money will start flowing their way. However, for this hope to be fulfilled, much will depend on the quality of their work. In the case of cladoceran taxonomy, uplifting its caliber is a major priority for the future. To get there, we may, inter alia, need to specify what the minimum requirements for a species description are. Some standard description format should be developed in the near future. But what of the ultimate number of cladoceran species? Possibly no quest is more academic in nature than this, and surely no ecosystem will collapse for

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303 the loss of a cladoceran species. Yet, it is undeniable that there is something in human curiosity that simply demands to get an answer to this sort of question. Let us therefore continue to spend modest amounts of money and energy on it. I would dare to speculate that we currently know about half of the species, but where are the others? Exploring some type of environment that was left untouched by our predecessors is one way of extending the list. Thus, various types of underground waters have been successfully explored of late (Brancelj, 1990; Dumont & Brancelj, 1994), but temporary waters in the dry tropics and small waterbodies in equatorial forests are high on my list of priorities too. If we don’t know how many species there are, we often don’t know where they are either. It is probably no longer true that the distribution of cladocerans reflects that of cladocerologists (there are in fact too few cladocerologists for that), but clearly, now that we no longer believe in cladoceran cosmopolitanism (Frey, 1987), we need to substantiate non-cosmopolitanism. What are the ranges of all these species, and can we ever understand why they live where they live? In the light of our bad habit of dispersing living things all over this planet, we have doubtlessly created numerous artefacts of distribution already. We are well aware of some of the more recent ones, and they get adequately documented, but what of the earlier ones? Possibly we will never know, and possibly some of the residual cases of cosmopolitanism reflect involuntary human phoresis (like by the thousands of ships that circled the earth between the fifteenth and the nineteenth century, and were dependent for their drinking water on wooden casks, that were rinsed and refilled at each stop, a perfect way of dispersing cladoceran resting stages). But there is one way of approaching at least some aspects of this question: many cladocerans, contrary to

copepods, fossilize well, and their faunas leave a rich signature in the sediments of the waters they inhabit. Cladoceran microfossils thus have a story to tell, which may enlighten us about past environmental change, but also, in a variety of cases, about the date of their arrival at a given site. Browsing through the pages of e.g. the Journal of Paleolimnology, I find little evidence of this approach being used to potential. It definitely merits more attention.

References Alonso, M., 1996. Crustacea, Branchiopoda. Fauna Iberica 7: 486 pp. Brancelj, A., 1990. Alona hercegovinae n.sp,. (Cladocera: Chydoridae), a blind, cave-inhabiting cladoceran from Hercegovina. Hydrobiologia 199: 7–16. Dubois, A., 1988. The genus in zoology: a contribution to the theory of evolutionary systematics. Mem. Mus. natn. Hist. nat. Paris (A) 140, 124 pp. Dumont, H. J. & A. Brancelj, 1994. Alona alsafadii n.sp. from Yemen, a primitive groundwater-dwelling member of the A. karua-group. Hydrobiologia 145: 5–7. Frey, D. G., 1987. The taxonomy and biogeography of the Cladocera. Hydrobiologia 145: 5–17. Fryer, G., 1987. A new classification of the branchiopod Crustacea. Zool. J. linn. Soc. 91: 357–383. Fryer, G., 1995. Phylogeny and adaptive radiation within the Anomopoda: a preliminary exploration. Hydrobiologia 307: 57– 68. Korovchinsky, N. M., 1996. How many species of Cladocera are there? Hydrobiologia 321: 191–204. Kotov., A. A., 1997. Structure of thoracic limbs in Bosminopsis deitersi Richard, 1895. (Anomopoda, Branchiopoda). Hydrobiologia 360: 25–32. Martin, J. W. & C. E. Cash-Clark, 1995. External morphology of the onychopod ‘Cladoceran’ genus Bythotrephes. Zool. Scr. 24: 61–90. Peters, R. H. & R. de Bernardi (eds), 1987. Daphnia. Mem. ist. Ital. Idrobiol. 45, 502 pp. Smirnov, N. N., 1992. The Macrothricidae of the world. Guides to the Identification of the Microinvertebrates of the Continental Waters of the World 1. SPB Academic Publishing, The Hague, 143 pp.

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