Actinopyga caerulea
Actinopyga caerulea, the blue sea cucumber, is a species of sea cucumber in the family Holothuriidae. Named for its unique blue coloration, this species can be found along the continental shelf of the tropical Western Indo-Pacific region, at depths between. It is a commercially important species, and is harvested for food along its range.
Etymology
The specific epithet, caerulea, is derived from the Latin caeruleus, meaning "blue or greenish-blue; cerulean, azure." This is in reference to its distinctive blue coloration, which is also the origin of its vernacular name, the blue sea cucumber.Taxonomy
While Actinopyga caerulea was first discovered and photographed off the coast of New Caledonia in 1984, it was initially misidentified as Actinopyga crassa, and none were collected for further study. The species was photographed at a number of Pacific localities throughout the next decade, including Sulawesi, Bali, and Thailand, but it would not be accurately recognized as a undescribed species until 1998, when Australian zoologist F.W.E. Rowe identified a specimen photographed in the Philippines by Erhardt & Baensch as Actinopyga ''bannwarthi. In 2003, several specimens were collected in Comoros and Papua New Guinea by Yves Samyn, Didier VandenSpiegel, and Claude Massin, who formally described the new species as Actinopyga caerulea'' in 2006.The holotype specimen of this species was collected by Samyn and VandenSpiegel on November 22, 2003, and is deposited in the zoology collections of the Royal Museum for Central Africa, Tervuren, Belgium. The type locality is off the coast of Iconi, Grande Comore, Comoros, at a depth of. The four paratype specimens were also collected off the coast of Grande Comore, at depths ranging from.
Phylogeny and evolutionary history
Actinopyga caerulea was initially identified by Samyn et. al, 2006 to be a member of the echinites group, a paraphyletic group within the genus Actinopyga which consisted of A. echinites, A. agassizii, A. bannwarthi, A. crassa, A. flammea, A. serratidens, and the now-obsolete A. plebeja.However, a DNA barcoding study published in 2010 revealed that the closest known relative of Actinopyga caerulea is actually Actinopyga lecanora, and that the two species comprise a sister group within the genus with a genetic distance of just 0.9%. In fact, the two species are so closely related that their taxonomic validity has been called into question: there is a possibility that one of them could be a hybrid species between the other and their next-closest relative, Actinopyga obesa.
Distribution and habitat
Actinopyga caerulea is found off the coasts of Asia and Africa, in the tropical Indian Ocean and the western Pacific Ocean. Its range extends from Comoros to Indonesia, Papua New Guinea, the Philippines, Taiwan, and other island groups in the western Pacific. It is found on the seabed in deeper tropical water on sand and rubble, as well as coral patches on the edge of coral reefs, at depths between. It is also associated with garden eel plains.Anatomy and description
Actinopyga caerulea is a large sea cucumber, growing to a length of up to and a width of up to across the midsection. The body of this sea cucumber is white, with numerous dark blue tube feet and papillae discontinuously covering its surface. The density of these tube feet and papillae, and therefore an individual's coloration, can vary from sea cucumber to sea cucumber, but density typically increases on the dorsal area.The body itself is stout and loaf-shaped, with a smooth, firm body wall measuring up to thick. The mouth is usually surrounded by 15-18 large shield-shaped feeding tentacles, which are bluish-grey in coloration. The anus is surrounded by five prominent "anal teeth", which are white in coloration. This species lacks Cuvierian tubules. It has one club-shaped Polian vesicle, which is roughly one-seventh of its body length in preserved specimens.