Aspects of Biogeography of Stygobiontic Isopoda

Bijdragen tot de Dierkunde, 60 (3/4) 145-150 (1990) SPB Academie Publishingbv, The Hague Aspects of the evolution and biogeography of stygobiontic Isopoda (Crustacea: Peracarida) Johann-Wolfgang Wägele Fachbereich 7, Universität Oldenburg, D-2900 Oldenburg, W. Germany Keywords: Crustacea, Isopoda, evolution, stygofauna, biogeography Abstract Introduction The the and The evolution of the stygobiontic isopods is discussed corre- increasing knowledge on taxonomy lating the phylogenetic with biogeography. All stygobi- system distribution of stygobiontic isopods allows some ontic isopods probably are derived from remote marine ances- general observations on their phylogeny and bio- tors. The colonization of subterranean aquatic biotopes geography. The "Amsterdam Expeditions to the occurred in two ways: (1)via the coastal groundwaterin the case West Indian Islands" and other expeditions of the of the Microparasellidae, Gnathostenetroididae, Stenetriidae, and in Cyathura (Stygocyathura), (2) in most families, however, group of Prof. Dr. Jan H. Stock proved that many via epigean freshwater ancestors. Ancient freshwater isopods hypogean genera have a wide distribution, not only that must have existed before the Cretaceous and whose already in the both sides peri-Caribbean area, but also on stygobiontic phylogenetic lines partly must have existed before of the Atlantic, some even reaching into the Indo- the opening of the Atlantic are the Aselloidea, Calabozoida, Pacific Botosaneanu et 1986; Botosanea- Phreatoicidea, and probably also the Protojaniridae. In the (e.g. al., course of Cretaceous and Tertiary regressions other, more apo- nu, 1987; Notenboom, 1984; Stock, 1977; 1985). morphic taxa “stranded” and adapted to stygobionticbiotopes, These stygobionts are of high scientific interest, as such as the “Monolistrini” (Sphaeromatidae). some of them obviously are relicts, without close relatives in marine biotas (e.g. Calabozoa, Curas- Zusammenfassung santhura), stimulating research and speculation be- cause of their distribution and phylogenetic age. Die Evolution der stygobiontischen Isopoden wird in einem Ver- Evolutionand biogeography cannot be separated in gleich des phylogenetischen Systems mit der geographischen any effort to understand the origin of the aquatic Verbreitung der Taxa diskutiert. Alle stygobionte Isopoden haben wahrscheinlich marine Vorfahren gehabt. Die Besiedlung hypogean species. limnischer,unterirdischer Biotopeerfolgt auf zwei Wegen: Über das Küstengrundwasser im Fall der Microparaselliden, Gna- thostenetroididae, Stenetriidae, und bei Cyathura (Stygo- Monophyly and biogeography cyathura), bei den meisten Familien jedoch über epigäische Süßwasserformen. Alte Süßwasserisopoden, die schon vor der Kreide gelebt haben müssen und deren hypogäische Linien z.T. A summary of hitherto accumulated knowledge on schon zum Zeitpunkt der Öffnung des Atlantiks existierten, sind the evolution of the has been Isopoda recently pre- die Aselloidea, Calabozoida,Phreatoicidea und wahrscheinlich pared (Wägele, 1989). A simplified scheme of the auch die Protojaniridae.Im Verlauf von Regressionen während results all those taxa and containing groups of Kreide und Tertiär sind andere, “modernere” Taxa “gestran- that is in 1. det” und ins Süßwasser und Grundwasser gelangt, wie die genera are stygobiontic represented Fig. “Monolistrini” (Sphaeromatidae). Aquatic hypogean biotopes were conquered several 146 J.-W. Wagele - Aspects of the evolution and biogeography of stygobiontic Isopoda Fig. 1. Dendrogramfor the phylogenetic system of the Isopoda (after Wägele, 1989), representing the taxa that belong to the stygofauna. Note that most groups evolved via epigean freshwater ancestors. Question marks indicate that it is not known if such epigean ancestors could have existed. Black circles without names represent single taxa or groups of taxa. if took times by members of nearly all isopod suborders, expected or several parallel invasions place with the of Oniscidea within In the first exception the (which have a genus or a family. case a genus some cave-dwelling species) and the Valvifera. occurring for example at both sides of the Atlantic Nearly always some marine sister-group is known must have a common stygobiont ancestor and be at and there is no doubt about the marine origin of least as old as the central AtlanticOcean. In the sec- ond of marine most families. To understand the mechanisms lead- case populations a ancestor species it is find of wide distribution could in ing to present-day distribution necessary to adapt independently out whether within a phylogenetic line crossing the remote areas to the coastal or estuarine interstitial border marine benthos/aquatic stygofauna only a or other hypogean biotas. Monophyly of the adapt- vicari- single event (adaptation to hypogean biotas) is to be ed group of species can be an indication of Bijdragen tot de Dierkunde 60 (3/4) - 1990 147 continental other mechanisms known how the of this are related anee by drift, being not genera family less probable. to each other; some may belong to monophyletic subterranean groups. Neither is known, if hypo- Eurasia have Phreatoicidea gean species of North America and a common stygobiontic ancestor. The Phreatoicidea is the oldest known isopod The remaining families of the aselloid line suborder (300 million years: Schram, 1970). Pres- evolved in a different way: arising from an line ent-day species live exclusively in fresh water of unknown common aselloid ancestor this Gondwana fragments (New Zealand, Australia, evolved completely in subterranean fresh water. Tasmania, India, South Africa). The Nichollsidae, The Stenasellidaestill have the size of epigean asel- a subterranean family known only from India, lids and have a wide distribution in southern Eu- most probably evolved on this subcontinent in- rope, Africa, Central America, and Asia (summary in The last ancestor dependently of other subterranean groups. Other Henry et al., 1986). common have lived in Laurasia and will not closely related hypogean species are known for must descendents have this continent before the forma- the Amphisopidae (Hypsimetopinae), obviously spread over representing several separate invasions of the un- tion of the Atlantic. Gondwana was probably also less derived of the sister- derground (Knott, 1986); the species in question oc- populated. The species dwarfish Atlantasellidae and Microcer- cur in Australia and Tasmania. In the third family group, the in southern Africa (Phreatoicidae) hypogean species are known from beridae, occur partly (Protocer- Bermuda New Zealand and Tasmania. The Nichollsidae are berus, Afrocerberus), partly on (Atlan- the only monophyletic hypogean family; the tasellus). Likewise, this line of dwarfish forms was in existence before the Atlantic phylogeny of the other hypogean groupsof species already was is not known. formed. Atlantasellus has an intermediate morphology between stenasellids and microcerberids, Calabozoida sharing some synapomorphies with the microcerberids (Wägele, 1983), suggesting that some con- According to Wägele (1989) the Calabozoida are nection between the present-day Bermuda Island the sister-group of the Asellota, the close relation- and older continental fragments must have existed. ship having been noted already by Van Lieshout While the most primitive Microcerberidae live in is for (1983). Only one species known (from Venezue- fresh water, the more apomorphic species occur la): Calabozoa pellucida (Van Lieshout, 1983). It the most part in the brackish or marine coastal seems to be a relict; no close marine relatives are mesopsammal. They must have means for dispersal have found known. over ocean basins, as they been e.g. on the Maldives and Laccadives, and on many Carib- Asellota bean islands (summary in Coineau, 1986a). Some freshwater species with the more apomorphic be of marine Most species of asellotes are marine. The more "coastal" morphology may aproduct familes live in shallow coastal while Mexicerberus primitive areas, regressions (e.g. troglodytes Schultz, a large number of specialized genera and families 1979, Microcerberus plesai Chappuis & Delamare, radiated in the deep-sea. A separate phylogenetic 1958, and M. remyi Chappuis, 1954). line evolved in fresh water, namely the Aselloidea. Further stygobiontic species are known in those Withinthis line the less specialized family Asellidae of the remaining asellote families, which probably has epigean as well as hypogean species. It is very evolved in shallow marine habitats. Interestingly, line obvious that hypogean species of this family the more primitive genera in the janiroid (Wil- evolved not have to interstitialconditions very often in independent, closely re- son, 1987) an affinity lated groups. In Asellus aquaticus (Linné, 1758) (Caecostenetroides, Neostenetroides, Anneckella, epigean as well as hypogean populations exist. It is Enckella, Protojanira). Species of these genera are 148 J.-W. Wdgele - Aspects of the evolution and biogeography of stygobiontic Isopoda rare and must be survivors derived from ancient tions that "stranded" during Miocene regression. stocks which were displaced by the more advanced The Australian freshwater species ofAngeliera is of Stock Janiroidea. Africa (Gondwana) and the areas adja- enigmatic origin (Stock, 1985); suggests that be the where could have stranded late Ter- cent to the Tethys Sea seem to regions, populations during Gnathostenetroidoidea in Australia. The these genera radiated. The tiary uplift

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