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.
