Cumacea (Crustacea; Peracarida)of the Antarctic shelf - diversity,biogeography, and phylogeny

Abstract

The crustacean order Cumacea belongs to the Peracarida and comprises anevolutionary old group with conservative morphology. Predominantly bound to softbottom habitats in benthic marine environments they show a cosmopolitan distribution.As other Peracarida they display brood protection; juvenile stages are carried in themarsupium. It is supposed that the marsupium plays a major role in the success of thisabundant and specious group of Crustacea.The Peracarida are a dominant group in Southern Ocean benthic communities.Quantitative investigations of the Ross Sea shelf fauna demonstrated that thePeracarida contribute 63% to abundance and 50% to biomass. Amphipods dominatedclearly, while different sample sites yielded high dominances by Cumacea, Isopoda,and Tanaidacea. The recorded number of peracarid species from the Ross Sea is lowerthan in other high-Antarctic regions. The present study could show, that cumaceandiversity with respect to species richness resembles that of the Weddell Sea or the EastAntarctic. Species number has now increased from 13 to 34 for the Ross Sea, whichhighlights the requirement for choosing the appropriate sampling gear, and continued'classical' taxonomical as well as biogeographical work. With the present study equaldistribution of cumacean species with an affinity to the Magellan region in all high-Antarctic regions could be demonstrated.A new species Leucon rossi (see front page) and the subspecies Diastylisenigmatica rossensis was described from the Ross Sea. Further species from the Ross Seashowed slight morphological differences to literature. In the context of the discussionabout cryptic speciation these differences might indicate that diversity of Antarcticcumaceans is likely much higher as currently known. In the present study geneticdifferences in the 16S rRNA gene of populations of Leucon antarcticus from the RossSea and the Weddell Sea make clear that these have genetically separated for anextended period of time. According to the analysis of 16S rRNA data, populations of thespecies Leucon intermedius from the Ross Sea and the Weddell Sea belong to thesame species. Genetic diversity of the cytochrome oxidase I (COI) gene of twocaridean decapods supports the concept of circumantarctic species distribution inmarine broadcasters. A broadcasting mode in reproduction seems to favour high geneflow and homogeneous populations around Antarctica. Contrarily, brooders withlimited capability to disperse over long distances are more likely exposed togeographic isolation on the Antarctic continental shelf, i.e. in glacial periods, whichfavours cryptic speciation patterns and high diversity in these taxa.The phylogenetic history of cumaceans is obscure as there is almost no fossilrecord and derived and primitive characters, which vary within and between families,distinguish families. Though assumptions about the succession of cumacean familiesexist, details are still ambiguous. The present molecular study of mitochondrial 16S rDNA confirmed the Cumacea as a monophylum with respect to Tanaidacea and Isopodawith the monophyletic Diastylidae as a basal family. The hypothesis of a derived groupof Cumacea bearing a fused pleotelson was confirmed as well. Furthermore this studydemonstrated that within the family Leuconidae the genus Leucon is paraphyletic,whereas the subgenus Crymoleucon resolved monophyletic.