Aspidochirotid holothurians of some stations of algerian coastal area: systematic revision and phylogenetics relationships

Abstract

The aspidochirotid holothurians, commonly named “sea cucumbers”, are a major component of Posidonia oceanicameadows. They play an important role in recycling organic matter and in the oxygenation of the bottom sediment. The systematic study of Mediterranean aspidochirotid holothurians was esta-blished in the 19th century, when all species were described at the morphological and anatomical levels. The identity, validity and limits of these species has been the subject of considerable debates through the 20 th century, mostly because there are substantial intra-specific variations, and limited interspecific differential characters. Until now, spe-cies limits and relationships in this group have rarely been explored with molecular tech-niques. The present study contributes to the phylogeny and systematics of this class of Echinodermata. The systematic study was carried out using both molecular and morphological methods (clustering). This allowed us to differentiate six forms of holothurians: Holothuria (Holothuria) tubulosa, Holothuria (Roweothuria) poli, Holothuria (Holothuria) stellati, Holothuria (Panningothuria) forskali and both morphotypes of Holothuria (Platyperona) sanctori.This provided a definitive test of species boundaries in the Mediterranean species and showed that: (1) The two colour morphs of Holothuria (P.) sanctorithat have been debated in the literature are genetically identical and thus represent the same species; (2) Holothuria (H.) stellati,which has long been confused with Holothuria (H.) tubulosa, is a distinct and well defined species, characterized at both the morphological and genetic levels; (3) Holothuria (H.) tubulosa, the most com-mon species and the one “best known” in the Mediterranean Sea, is not one species, but two cryptic species that have not been previously recognized or even suspected. We determined their distinctiveness in terms of genetic, morphological and endoskeletal characters.