Geoffrey Fryer

Diapause, a potent force in the evolution of freshwater crustaceans

After a brief historical review of the discovery of diapause in freshwater crustaceans, its dramatic nature in certain cyclopoid copepods, in which diapausing individuals may occur at densities of > 106 per m2, is used to illustrate the enormous ecological significance of the phenomenon. Some of the problems presented by dispause in cyclopoid copepods are noted, including the different behaviour in different lakes of what appears to be a single species. Different physiological cues or different genetic endowments are clearly involved.The wider incidence of diapause in freshwater copepods and ostracods is noted.Among freshwater crustaceans it it the Branchiopoda that have universally adopted diapause, always at the egg stage. Even such an ancient order as the Anostraca, perhaps the most primitive of all crustaceans, produces elaborately constructed resting eggs that are capable of cryptobiosis, can remain viable in a dry state for long periods, and can tolerate extreme conditions. The nature of branchiopod resting eggs is briefly reviewed. Of these, only those of the Anomopoda are protected by containers derived from the parental carapace. These are mechanically complex in the most advanced species but, as shown by fossils, are extremely ancient structures.Factors initiating the onset and termination of diapause in branchiopods are briefly noted, and the process of hatching of resting eggs is outlined.

Morphology and the classification of the so-called Cladocera

The validity as a monophyletic taxon of the group of branchiopod crustaceans long regarded as constituting the Cladocera is questioned. This seems in fact to be a heterogeneous assemblage whose members probably merit assignment to four separate orders. The Onychopoda and Haplopoda (the so-called Gymnomera) clearly stand apart from the Ctenopoda and Anomopoda (the so-called Calyptomera) and differ in important respects from each other. While sharing several characters, some doubtless indicative of distant ancestral similarities, others probably convergent, the Ctenopoda and Anomopoda differ in many respects, have clearly evolved along different lines, and are probably much less closely related than is generally assumed.

Comparative aspects of adaptive radiation and speciation in Lake Baikal and the great rift lakes of Africa

Lakes Baikal, Tanganyika and Malawi have similar origins, are physiographically similar, and of similar size. The hydrological regime of Baikal is, however, very different from that which prevails in its African sisters. Apart from being much cooler, it differs fundamentally in being oxygenated to all depths while the two great African rift lakes possess only a relatively thin oxygenated surface layer and have vast oxygenless, and therefore azoic, abyssal regions. Nevertheless, like Baikal, they have rich endemic faunas.That these faunas originated largely by intralacustrine speciation and not by multiple invasion is now well established. They provide some of the worlds most spectacular examples of species flocks, and some groups display what has been aptly described as explosive speciation. Certain features, and especially the adaptive radiation, of some of the groups involved, are noted. Comparisons between lakes are illuminating. Some species flocks, such as those of amphipods, sponges and turbellarians of Baikal and the atyid prawns and potamid crabs of Tanganyika, have no counterparts in the other lakes. Other groups, such as the prosobranch gastropods, ostracods and harpacticoid copepods of Baikal and Tanganyika, and the fishes of all three, involve representatives of the same major group, though often of different families or even higher taxonomic categories.That allopatric speciation has been involved is universally acknowledged but the problems posed by species multiplication in deep water in L. Baikal have led to suggestions that sympatric speciation could have played a part. Notwithstanding the difficulties, it is suggested that the process can be explained without invoking the assistance of the sympatric model.The faunas of these lakes provide immense fields for investigation and enormous intellectual challenges. While each is an entity in itself, comparative studies may be particularly enlightening.

Phylogeny and adaptive radiation within the Anomopoda: a preliminary exploration

The distinctness of the Anomopoda and the polyphyletic nature of the so-called Cladocera are emphasized. An attempt is made to reconstruct the ancestral anomopod, which probably lived in Palaeozoic times. This task is facilitated by the availability of detailed information on extant forms, which includes functional as well as purely morphological considerations and enables us to understand the means whereby complex mechanisms were transformed during evolution. Comparative studies on the ecology and habits of extant forms also throw light on the probable way of life of the ancestral anomopod. Adaptive radiation within the Anomopoda is briefly surveyed and an outline of the suggested phylogeny of the order is indicated.

Conservation of the Great Lakes of East Africa: A lesson and a warning

Abstract The Great Lakes of East Africa are not only the repositories of the worlds three richest lacustrine fish faunas but also harbour unique assemblages of invertebrates. A few of these animals are illustrated. These lakes are of immerse scientific interest, not merely by virtue of their richness in species, nor even because so many species are endemic, but as field laboratories of evolution whose operations we have scarcely begun to understand. They are also natural resources of great value, being enormous reservoirs of pure water, and the sites of fisheries producing many thousands of tons of fish per annum. Evidence is presented which unequivocally indicates that, by virtue of their tropical location and hydrological regimes, these lakes are much more susceptible to the damaging influences of oxygen-demanding pollution than their temperate-zone counterparts, and that, should they become contaminated, their renewal times are so great that a virtually irreversible catastrophe is possible. Evidence of the mismanagement of the fisheries of Lake Victoria is also presented and the danger of further uninformed exploitation pointed out. Certain apparent remedies are shown to be highly dangerous. The technology of fishing has advanced more rapidly than our scientific understanding of the extremely complex ecosystems involved.

Endemism, speciation and adaptive radiation in great lakes

SynopsisEvolution in great lakes has often been both quantitative (many endemic species of distantly related taxa often being present) and qualitative (outstanding levels of adaptive radiation having sometimes been achieved). These situations pose many questions, such as why there are so many endemics and so many superspecialists (and at the same time often many sibling species), as well as presenting problems relating to such matters as convergent evolution in different lakes, the possible role of key innovations, the nature of isolating mechanisms, competition and co-existence in complex communities, the roles of diverse mutualistic associations, and many others. These rich faunas also provide particularly favourable opportunities for studying patterns of speciation, while attempts to elucidate phylogenies in groups such as African cichlid fishes, that have radiated in several lakes, can be pursued on both a broad scale and at the intralacustrine level using both recently developed techniques and time-honoured methods. Rates of evolution, which differ widely between ecologically equivalent taxa in different lakes, have sometimes been extremely rapid, as attested by both molecular data and evidence from field studies. Notwithstanding their evolutionary exuberance, these rich faunas are fragile as demonstrated dramatically by the appalling tragedy that has befallen the haplochromine cichlid flock of Lake Victoria.

Contributions to our knowledge of the biology and systematics of the freshwater copepoda

ZusammenfassungDie vorliegende Arbeit enthält vier im folgenden beschriebene Beiträge zur Kenntnis der Süsswasserkopepoden:1.Die eng verwandten KalanoidenDiaptomus laticeps Sars undD. gracilis Sars, welche nebeneinander im Plankton des Lake Windermere vorkommen, zeigen bestimmte Grössenunterschiede, welche auf Verschiedenheiten in der Ernährung zurückzuführen sind.D. laticeps frisst hauptsächlichMelosira, während sichD. gracilis von winzigen Algenpartikeln crnährt. Eine Nahrungskonkurrenz besteht demnach nicht, wie auchHutchinson vermutete, und zeigt gleichzeitig den selektiven Einfluss der Ernährung bei den Kalanoiden.2.Es wird auf die verschiedene Empfindlichkeit der planktischen Kalanoiden und Zykloiden gegenüber ungünstigen Lebensbedingungen, insbesondere zu grosser Bestandesdichte, hingewiesen, und es wird gezeigt, dass die Zyklopiden als die erfolgreichere der beiden Gruppen solche Bedinguen besser zu überstehen imstande sind als die Kalanoiden.3.Die planktonische Form vonCyclops strenuus aus dem Englischen Seendistrikt wird von systematischen Gesichtspunkt aus betrachtet, wobei die AnsichtLindbergs, dass es sich dabei umC. lacustris Sars handeln könne, widerlegt wird. Im weitern werden vonSars undKozminski beschriebene Abweichungen von der «typischen Form» angegeben, die jedoch noch als in den Variabilitätsbereich der Spezies hineingehörend betrachtet werden. In diesem Zusammenhang werden Populationen von Windermere und Coniston Water erwähnt, welche durch verschiedenheiten in der Grösse ihrer Eisäcke während bestimmter Zeiten des Jahres gewisse Rassenmerkmale erkennen lassen.4.Mit Hinsicht auf den taxonomischen Wert vonCyclops vernalis americanus Marsh. in England wird der «vernalis-Komplex» der ZyklopidenuntergattungAcantbocyclops diskutiert, wobeiLindbergsAnsicht, es handle sich dabei um nichts anderes als eine «robustus-Modifikation» vonC. vernalis, als nicht zutreffend aufgezeigt wird. Obschon eine überschneidung von Merkmalen vonC. vernalis americanus Marsh. undC. vernalis s. str. festgestellt wird, können doch die Extrema, wie die graphische Darstellung zeigt, ohne Schwierigkeiten getrennt werden, womit die beiden Formen eindeutig unterschieden sind. Auch ökologische Faktoren stehen mit diesen Ergebnissen nicht im Widerspruch. Bis zur Ausführung weiterer Arbeiten wird deshalb für die besprochene Form eine subspezifische Stellung vorgeschlagen. Beobachtungen in der Natur ergeben, dass der Einfluss der Temperatur auf die Variation der Körperform-sofern taxonomisch wichtige Merkmale betrachtet werden-ohne Belang ist.

Biological implications of a suggested Late Pleistocene desiccation of Lake Victoria

If, as recently suggested, Lake Victoria dried up completely in theLate Pleistocene and refilled c 12,400 BP, not only must therate of speciation of its flock of cichlid fishes have been extremelyrapid but, more significantly, so too must the rate of morphologicaldifferentiation. Such desiccation also implies that fishes belongingto seven other families achieved endemic status since the lakerefilled, and in one case became generically distinct, acquiredstriking morphological/physiological adaptations to life in deepwater, and split into two species. Such rapid evolution within thesefamilies appears to have no parallel in Africa. This suggests thatprudence be applied in the interpretation of what appears to beunambiguous evidence, especially as this seems to be at variance withwhat also appears to be convincing geophysical evidence of adifferent kind.

Structure and habits of living branchiopod crustaceans and their bearing on the interpretation of fossil forms

Branchiopods are primitive crustaceans whose history extends back at least to the Devonian, yet some are highly successful today. Most fossil branchiopods do not show many features that lead to an understanding of function, but the Devonian lipostracan Lepidocaris is an exception. Recent work on living branchiopods, especially anostracans and anomopod cladocerans, enables Lepidocaris to be considered as a living animal. Work on extant anostracan nauplii also makes it possible to deduce how its larval stages swam and collected food. Other early crustaceans are briefly considered and comments are made on the concept of “living fossils”. While rates of morphological and genomic evolution are often discordant, the concept is still useful. Some branchiopods, such as the Notostraca, display remarkable morphological stasis: on the basis of morphology, a Triassic and a present-day form appear to be conspecific. The latter may be referred to as a “living fossil”. The outstanding morphological stasis of the crustacean nauplius is noted. Its existence in Cambrian times, deducible from its distribution among modern taxa, has been confirmed by Mullers finds. Such stasis, involving part of the genome of organisms whose adults display enormous adaptive radiation, has persisted with relatively small modifications since before the commencement of the entire radiation of vertebrates.

The ecology and distribution of the genus Daphnia (Crustacea: Cladocera) in restricted areas: the pattern in Yorkshire

The old county of Yorkshire provides a diversity of habitats for freshwater crustaceans. Eight species of Daphnia (Cladocera: Anomopoda) are recorded. Taxonomic problems relating to some of these are discussed where appropriate. The ecological requirements and limitations of the various species are described and the patterns of geographical distribution to which these give rise within the area are reported. Most species are restricted to low altitudes. This is not related to altitude as such, all elevations being relatively modest, but reflects the rather sharp distinction in chemistry of upland and lowland waters in Yorkshire. Upland waters are generally acidic, markedly so on moorlands, and of low ionic content, often even in limestone areas which are frequently plastered with glacial drift: lowland waters are generally alkaline and relatively rich in inorganic ions, though small areas of lowland health are exceptions. Most species are restricted to waters of the latter type. Experiments using D. magna ...