Towards an understanding of the shallow-water holothuroid fauna (Echinodermata: Holothuroidea) of the western Indian Ocean
Samyn, Y. (2003). Towards an understanding of the shallow-water holothuroid fauna (Echinodermata: Holothuroidea) of the western Indian Ocean. PhD Thesis. Vrije Universiteit Brussel: Brussel. III, 384 + 1 cd-rom pp.
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Document type: Dissertation
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| Keywords |
Aquatic organisms > Marine organisms
Biodiversity
Biogeny > Phylogeny
Classification > Taxonomy
Geography > Biogeography
Water > Shallow water
Echinodermata [WoRMS]; Holothuroidea [WoRMS]
ISW, West Indian Ocean [Marine Regions]
Marine/Coastal |
| Abstract |
The study of the Holothuroidea, commonly known as sea cucumbers, started some 23 centuries ago when Aristotle defined them as a kind of motionless marine organisms. Only in the mid sixteenth century were holothuroids recognized as animals per se. Nowadays, Holothuroidea is firmly recognized as one of the five extant classes of echinoderms. Currently some 1600 species are described; these occur from the intertidal to the deep ocean trenches and from the polar to the tropical regions.The ultimate aim of this dissertation is to understand the shallow-water holothuroid biodiversity of the western Indian Ocean, the area stretching from Suez to Cape Town and from the East African coast (Red Sea and Persian Gulf included) to 65 degrees East. To attain this goal, several expeditions to two contrasting regions of the western Indian Ocean (the tropical coast of Kenya together with Pemba Island in northern Tanzania and the subtropical coast of KwaZulu-Natal in the northeast of the Republic of South Africa) were undertaken. The purpose of these was to assemble a representative collection of species.An extensive part of this dissertation is concerned with the construction of a reliable and up-to date faunistical list of the holothuroid fauna of these two case areas. The faunistics of the rest of the western Indian Ocean was tilled in with the aid of important collections from the Seychelles and Inhaca (which were deposited as largely unidentified material in the collection of the Royal Africa Museum, Tervuren, Belgium), with detailed study of virtually all the available literature as well as with numerous loans of specimens from museums worldwide. In the course of the construction of this species inventory, my colleagues and I discovered several species and one genus new to science. We, however, did not blindly follow the standing biological classifications when describing our findings. Au contraire, while constructing the species lists we took great care to critically question the employed classifications. This attitude resulted in the taxonomic revision of the Holothuria subgenus MertensiothuriaLabidodemas. Interestingly, the type species of the latter genus revealed itself as a cryptic species. We were rewarded with the discovery of two additional new species.Throughout this process, the historical opinions towards holothuroid taxonomy and systematics were never denied. This approach enabled us to revive methodically ignored characters such as the ossicles from the musculature. These characters proved not only diagnostic in the recognition of taxa, but also were informative in terms of recovery of phylogenies. By using our rejuvenated insights into such (and other) characters we were able to construct a large, morphology-based dataset, which allowed the recovery of the phylogeny of the Holothuriidae, the family best represented in this work. This cladistic analysis not only revealed that Labidodemas was indeed monophyletic {as suggested by our earlier systematic revision), but also allowed us to state that it has arisen fiom within the (now paraphyletic) genus Holothuria. As such, Labidodemas is evolutionary much younger than generally assumed. Our phylogenetic studies further suggest a close relationship between the genera Actinopyga, Bohadschia and Pearsonothuria, but unfortunately the recovered support proved low. Future studies (see also annex on CD Rom) will have to decide whether a new classification of the Holothuriidae is desired.Taking all these caveats into account, we finally succeeded to construct the wanted faunistical list of the Holothuroidea from the western Indian Ocean. This list was then further used to analyze the patterns of biodiversity by means of cluster analysis on several bèta-diversity coefficients and parsimony analyses of endemicity. These analyses showed that the investigated holothuroid fauna of the |
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