GENETICS: The family Stichopodidae contains 32 species in 9 genera, genetics has helped to further define the 5 genera that could be separated based on morphology (Byrne et al., 2010). The complete mitochondrial DNA of Stichopus horrens has been sequenced and can be accessed through GenBank (Fan et al., 2012). It consists of 16 257 base pairs and genetic barcoding has been developed so that differentiation can easily occur between commercial species, when it is so difficult based on morphological characteristics alone (Uthicke et al., 2010). S. horrens and the closest sister group (S. monotuberculatus) diverged less than 1mya (Byrne et al., 2010).
EVOLUTIONARY HISTORY: The closest sister group to the Holothurians are the Echinoids (Smith, 1988), with support from genetics and the fossil record (Smith, 1988). Sea cucumbers do not fossilize well, however two fossils have been found, one from the Upper Carboniferous, the second from the Lower Devonian (Smith, 1988). It is thought that two genera may have been the ancestors to the extant groups of Holothurians, these are Rotasaccus and Palaeocucumaria (Smith, 1988).
Rotasaccus had a hard body wall (made of spicules which are similar to those found today) and tube feet (although plated) like a sea cucumber, but also an Aristotle’s lantern like modern echinoids (Smith, 1988). Palaeocucumaria also had a hard body wall made of spicules, plated tube feet, and an oral feeding tube (Smith, 1988). It seems that this paleontological evidence would support the relationship between Ophiuroids and Holothurians.