Phylogenetics

         Stichopus chloronotus belongs to the phylum Echinodermata. This phylum is generally characterised by the spiny "skin" or skeleton for which it is named (Edgar 2008). In holothurians however this is reduced to small ossicles (Edgar 2008). Although echinoderms belong to the Bilateria, meaning that they have bilateral symmetry, echinoderms have generally reverted to a state of pentaradial symmetry (Ruppert et al. 2004). This is also true for holothurians, although when looking at a sea cucumber, it appears to have the classic bilaterally symmetrical form (Ruppert et al. 2004). The pentaradial symmetry can be observed when looking at certain anatomical features, such as the five longitudinal muscle bands (Ruppert et al. 2004). 

          Within the phylum of Echinodermata, Stichopus chloronotus belongs to the class Holothuroidea, giving it the name holothurian (Ruppert et al. 2004). Holothurians are most closely related to the class Echinoidea, the sea urchins (Janies 2001). These two classes share a calcified ring around the pharynx, elongation of the body along the oral-aboral axis, and ossicles in their tube feet (Ruppert et al. 2004). This sea cucumber also belongs to the order Aspidochirotida. This order is characterised by bilateral symmetry, a creeping sole, and their shield-shaped oral tentacles (Ruppert et al. 2004). 
        
        Aspidochirotida is made up of several different families. Stichopus chloronotus belongs to the family Stichopopidae. This family is distinguished by having gonads in two tufts on either side of the dorsal mesentary, C or S shaped ossicles, leaf-shaped tentacles, well developed respiratory trees, and pedicels in the body surface (Rowe and Gates 1995; Edgar 2008). Although several names have previously been used for S. chloronotus, its current name is generally accepted.



Figure 1. The phylogenetic relationships of Echinodermata (adapted from Ruppert et al. 2004).

          The phylogenetic relationships within Echinodermata are shown in Figure 1. It shows how Echinoidea and Holothuroidea are grouped together within the Echinozoa, joined by several characteristics as listed above. It is very likely that the Crinoidea split from other echinoderms quite early, as they have several distinct characteristics including being attached to substrate and having the oral end facing up, rather than down (Ruppert et al. 2004).