ABSTRACT
Cuvierian tubules are specialized defense organs occurring exclusively in some holothuroid species from the family Holothuriidae. Within the family, these organs differ greatly in terms of their morphology and their mode of functioning. The goal of this work was to determine the evolutionary path of Cuvierian tubules by the character mapping method and by ultrastructural analyses. A fragment of the mitochondrial genome corresponding to two genes was first sequenced for 20 species of Holothuriidae (3 Actinopyga, 3 Bohadschia, 12 Holothuria, Labidodemas semperianum and Pearsonothuria graeffei) and the relationships between these species were estimated from the molecular data obtained. The methods used to reconstruct those relationships were the neighbour joining, the maximum parsimony and the maximum likelihood. The consensus phylogenetic tree indicates that: (1) the genus Actinopyga is monophyletic and was the first to diverge from the rest of the family, (2) the second diverging group was a clade comprising the 3 Bohadschia, P. graeffei and 4 Holothuria, (3) within this clade the genus Bohadschia is monophyletic, (4) the remaining clade comprises the other species of Holothuria and L. semperianum, (5) the genus Holothuria is paraphyletic. The analysis of the different characteristics of Cuvierian tubules from the viewpoint of this phylogenetic tree strongly suggests that the common ancestor of the Holothuriidae had Cuvierian tubules and that those tubules were ramified, non-adhesive, non-expellable and non-stretchable; that those tubules have evolved to give the non-ramified, adhesive, expellable and stretchable tubules; and that the loss of Cuvieran tubules has occurred several times independently during evolution.
