CLADOCERA AS A RELICT

Korovchinsky, N. M., 2006, The Cladocera (Crustacea: Branchiopoda) as a relict group, Zoological Journal of the Linnean Society 147 (1), pp. 109-124 : 115-117

CLADOCERA AS A RELICT GROUP

The evidence for cladoceran antiquity and traits of their zoogeographical distribution coincide with those of archaic insects Megaloptera, Mecoptera, Plecoptera, Peloridiidae and Trichoptera, spiders, molluscs and primitive vertebrates used in the modern version of the ‘theory of ejected relicts’ ( Razumovsky, 1971; Zherikhin, 1978; Eskov, 1984, 2004). All these have been well studied neontologically and palaeontologically. This also provides the possibility to reconstruct cladoceran faunal formation. Thus, I hypothesize that starting from the Late Palaeozoic – Early Mesozoic until the Early–Mid Tertiary (Eocene–Oligocene), probably all supraspecific taxa of Cladocera had a wide distribution throughout Pangea and later Laurasia and Gondwanaland. It is suggested that the cladoceran fauna of those epochs was probably richer than the extant fauna. I assume that its mass extinction was similar to that in ‘Conchostraca’ and Ostracoda. These changes resulted in a reduction of adaptive zones and ranges, especially in the present tropical and boreal regions, the climates of which became hotter and colder, respectively. The initial range reduction may have been variable ( Eskov, 1984), and some species survived in a few localities. The remotely isolated species and populations ( Table 2) testify to a wider distribution of these taxa in the past. Many cladocerans were ejected into the neighbouring regions retaining moderately high temperatures (warm temperate and subtropical regions of the five continents and New Zealand), which are inhabited now by most of the rare endemic taxa. Fewer taxa were able to adapt to the severe new conditions of the northern and southern hemispheres and true tropics.

Within the new climatic zones speciation and changing patterns of distribution appeared. Thus, the origin of Holopedium amazonicum with an age of 4–20 Myr ( Hebert, 1998; Rowe, 2000), and the wide distribution of tropical forms such as Daphnia lumholtzi Sars ( Benzie, 1987; Havel, Colbourne & Hebert, 2000), Diaphanosoma excisum Sars , Moina micrura s.l. and the Ceriodaphnia cornuta Sars species complex might reflect the Tertiary perturbations in the recent tropical zone. The northern temperate and cold regions were populated by complexes of evolutionarily young taxa of Daphnia ( longispina , pulex , obtusa , retrocurva ) and Bosmina with an estimated speciation time within the last 3 Myr ( Lieder, 1983; DeMelo & Hebert, 1994b; Colbourne et al., 1997, 1998; Dufresne & Hebert, 1997; Little et al., 1997; Haney & Taylor, 2003).

The Cladocera qualify as relicts. Many extant representatives are morphologically primitive, highly specialized ( Fryer, 1968, 1974, 1995; Korovchinsky, 1986; Mordukhai-Boltovskoi & Rivier, 1987), with restricted ranges and ‘marginal’ biotopes (temporal, acidic, saline, mountain, cave waters and even terrestrial environments). For example, 23 members of the family Sididae , comprising about a half the known species, may be considered rare. The same is true of anomopod genera such as Simocephalus , Chydorus and Alona . Many sidids are highly specialized ( Sida , Latona , Pseudosida , some Diaphanosoma ) and narrowly restricted to specific localities (e.g. D. australiensis Korovchinsky , Pseudosida australiensis Korovchinsky in Australian coastal acid dune lakes, D. senegal Gauthier , D. celebensis Stingelin ). Some Australian endemics occur exclusively in acid lakes ( Smirnov & Timms, 1983, 1984), others only in granite pools ( Plurispina ) or in saline waters ( Celsinotum , Daphniopsis ). Acantholeberis curvirostris (O. F. Müller) , Alona rustica Scott and some other anomopods are also specific to acid waters ( Fryer, 1980). Species of Daphniopsis and Daphnia may be endemics of ephemeral spring pools, temporary waters of arid regions and high-altitude lakes ( Hann, 1986; Schwartz & Hebert, 1987).

Species differ with respect to relict status: most of them probably have a considerable age and may be considered relicts, whereas others, common and widely distributed within their extensive primary ranges, possibly are not [e.g. Sida crystallina , Diaphanosoma brachyurum s.s., D. mongolianum , D. excisum , D. sarsi Richard , D. dubium , D. birgei Kořinek , D. spinulosum Herbst , D. fluviatile Hansen , D. brevireme Sars , Holopedium gibberum s.s., Simocephalus vetulus (O. F. Müller) , S. exspinosus , S. serrulatus , Scapholeberis mucronata (O. F. Müller) ]. This non-relict status is certainly also true of some northern Daphnia ( D. longispina , D. pulex Leydig , D. obtusa Kurz , D. retrocurva Forbes species groups) and Bosmina ( Lieder, 1983; Colbourne et al., 1997, 1998; Dufresne & Hebert, 1997; Little et al., 1997; Haney & Taylor, 2003).

The distribution of Cladocera has been explained in terms of vicariant biogeography (e.g. Bayly & Morton, 1978; Hebert, 1978; Lieder, 1982, 1983; Smirnov & Timms, 1983; Korovchinsky, 1986; Benzie, 1987; Fernando et al., 1987; Bayly, 1993, 1995; Dumont & Negrea, 2002) with the origin of different groups treated as ‘Gondwanian’, ‘Laurasian’ or connected with particular parts of these supercontinents. Here, this idea is challenged (see also Hartmann, 1982; Briggs, 1987, 1995). Plate tectonics played only a secondary role, possibly in the diversification of taxa of species level. But extinction and the unique evolutionary histories of regional cladoceran faunas were of primary significance.

Suggestions regarding the relict nature of cladoceran species and populations ( Dumont, 1980; Dumont & Verheye, 1984; Hann, 1986; Schwartz & Hebert, 1987; Benzie, 1988; Benzie & Bayly, 1996) or bipolar patterns of distribution ( Smirnov & Timms, 1983, 1984) have been made, yet were mostly connected with the periods of glaciation and aridity in the Pleistocene. Certainly, these events affected distribution, but they only followed great previous perturbations. Berg (1962) explained bipolarity as resulting from the glacial period, but assumed that in supraspecies taxa this phenomenon may be of older origin. Cladoceran bipolarity precisely concerns the latter taxonomic level (see above). Even for the species that are involved, their age is frequently pre-Pleistocene, as shown by DNA studies on Daphnia and Holopedium ( Colbourne & Hebert, 1996; Taylor et al., 1996, 1998; Hebert, 1998; Rowe, 2000).

High vagility may also account for bipolarity. However, the idea of successful long-distance dispersal has come under fire. Cladoceran resting stages may be well dispersed by different agents (e.g. Maguire, 1963; Proctor, 1964, 1967; Vil d ek, 1978; Jarnagin, Swan & Kerfoot, 2000), but that does not mean that colonization of particular habitats will be successful ( Fryer, 1972; Frey, 1986; Hebert & Finston, 1993; Hebert & Wilson, 1994; Jenkins & Underwood, 1998; De Meester et al., 2002). Ecological barriers may often be more effective than physical barriers ( Dumont, 1980; Eskov, 1984; Frey, 1986)! Evidence of isolation is also provided by those species with extremely restricted distributions.

True Holarctic species in the southern hemisphere have not been substantiated well enough. Frey (1982a) suggested species rank for the isolated South American and South African populations of Eurycercus . South African representatives of ‘ Daphnia magna ’ and ‘ Alona affinis ’ differ from palearctic forms (A. A. Kotov & A. Y. Sinev, pers. comm.). Upon close examination, more and more populations of ‘widely distributed species’ appeared to be new, sometimes sibling species, for instance Eurycercus longirostris Hann and E. vernalis Hann ( Hann, 1982) , Daphnia villosa Kořinek et Hebert and D. latispina Kořinek et Hebert ( Kořinek & Hebert, 1996) . The Australian ‘ Moina mongolica ’ ( Smirnov & Timms, 1983, 1984) proved to be a true endemic, M. baylyi Forró ( Forró, 1985) , as were species of Alona and Pleuroxus on subantarctic islands ( Frey, 1988, 1993).