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,: "Etymology: The name caerulea, Latin, refers to the unique blue colour of the species." which is also the origin of its vernacular name, the blue sea cucumber.: "COMMON NAMES (English): 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.: "Cherbonnier & Féral (1984) recorded, and later Féral and Cherbonnier (1986) published a photograph of a specimen from New Caledonia which they identified incorrectly as Actinopyga crassa Panning, 1944. Other specimens have been photographed and no voucher material collected, making definitive identification impossible." The species was photographed at a number of Pacific localities throughout the next decade, including Sulawesi, Bali, and Thailand,: "Geographic distribution: Tropical Indo-Pacific; confirmed sightings have been made in Thailand (see Erhardt & Moosleitner 1995, as A. crassa), the Philippines (see Erhardt & Baensch 1998, as A. (?) bannwarthi), Indonesia [Bali (Myers pers. comm.) and Sulawesi (Lane pers. comm.)" 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.: "Erhardt & Baensch (1998) drew upon the expertise of F.W.E Rowe, who put the name Actinopyga (?) bannwarthi to their photographed specimen from the Philippines." 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 recently collected Comoros material, together with a specimen collected earlier in Papua New Guinea by one of us (C.M.), allows a definitive identification of the previously photographed specimens. All are here recognized as a species new to science."
The holotype specimen of this species (RMCA 1803) 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 holotype of the new species is deposited in the collections of the Royal Museum of Central Africa, Tervuren, Belgium (RMCA) [...] Union des Comores (Grande Comore, Ikoni), 22.XI.2003, 37 m depth, coll. Y. Samyn & D. VandenSpiegel, RMCA 1803 (holotype)" The type locality is off the coast of Iconi (Ikoni), Grande Comore, Comoros, at a depth of .: "Union des Comores (Grande Comore, Ikoni), 22.XI.2003, 37 m depth, coll. Y. Samyn & D. VandenSpiegel, RMCA 1803 (holotype)" The four paratype specimens (CNDRS 2004.09, RBINS IG 30376, NMHN EcHo 8081, and NHM 2005.2405) were also collected off the coast of Grande Comore, at depths ranging from .: "Union des Comores (Grande Comore, H.L.M Langouste), 11.X.2004, 28 m depth, coll. Y. Samyn, D. VandenSpiegel & C. Massin, CNDRS 2004.09 (paratype 1); Union des Comores (Grande Comore, Itsandra), 20.XI.2003, 23 m depth, coll. Y. Samyn & D. VandenSpiegel, RBINS IG 30376 (paratype 2); Union des Comores (Grande Comore, Aérodrome), 16.V.2005, 26 m depth, coll. Y. Samyn & D. VandenSpiegel, NMHN EcHo 8081 (paratype 3); Union des Comores (Grande Comore, Itsandra), 16.V.2005, 21 m depth, coll. Yves Samyn & D. VandenSpiegel, NHM 2005.2405 (paratype 4)"
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: "Most Actinopyga species fell into one distinct cluster in the COI analysis (Fig. 1). One notable exception is Actinopyga agassizi, which fell outside this clade. The latter is the only Atlantic species investigated." 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 (today considered to be synonymous with A. echinites).: "TABLE 1. Species and groups as recognized by Panning (1944). [...] Group 2 — 'Act. echinites' A. echinites echinites A. echinites plebeja A. echinites crassa A. serratidens agassizii A. serratidens serratidens A. serratidens bannwarthi [...] Actinopyga caerulea sp.nov belongs to what Panning (1944) has termed the 'echinites group. [...] The more recently described A. flammea also appears to belong to Panning's (1944) 'echinites group', an observation we share with Cherbonnier (1979)."
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%.: "Most other [Actinopyga] species were distinct, but the genetic distance between a specimen of Actinopyga caerulea and one of Actinopyga palauensis was small (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.: "Although most Actinopyga species were well resolved with DNA barcodes, further taxonomic research is required to investigate the species status of Actinopyga caerulea and Actinopyga palauensis (and to some extent Actinopyga obesa) and explore the possibility that one of these might be a hybrid between the other species and third taxon."
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.: "The most notable sea cucumber finds were the recently described (2006) Indo-Pacific Actinopyga caerulea, found on a garden eel plain off west Halmahera (Fig. 4)"
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.: "Very large species; living specimens up to 400 mm long and 140 mm wide mid-body" The body of this sea cucumber is white, with numerous dark blue tube feet and papillae discontinuously covering its surface.: "Colour in life bluish with patches of white devoid of tube feet at anterior and posterior ends and, discontinuously, along sides (Plate 1)." 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,: "Body loaf-shaped with slight ventral flattening (more or less cylindrical with some distal tapering)." with a smooth, firm body wall measuring up to thick.: "Body wall smooth, up to 14 mm thick." The mouth is usually surrounded by 15-18 large shield-shaped feeding tentacles, which are bluish-grey in coloration.: "Mouth ventral, surrounded by 15–18 large, peltate, uniformly bluish-grey tentacles, in turn surrounded by a stout collar of bluish papillae, fused at their base." The anus is surrounded by five prominent "anal teeth" (heavily-calcified tube feet, see picture), which are white in coloration. This species lacks Cuvierian tubules.: "Cuvierian organ absent." It has one club-shaped Polian vesicle, which is roughly one-seventh of its body length in preserved specimens.: "Single, club-shaped Polian vesicle, about one seventh of length of preserved animals."
Body composition In 2010, thirty (30) individual Actinopyga caerulea were purchased from a seafood market in Guangzhou, China as part of a study on the chemical composition and nutritional quality of several commercially important species of sea cucumbers.: "Samples of dried S[tichopus] herrmanni, Thelenota ananas, Thelenota anax, Holothuria fuscogilva, Holothuria fuscopunctata, Actinopyga mauritiana, Actinopyga caerulea and Bohadschia argus products were purchased from a local retail market in Guangzhou, China. There were 30 individuals per sample, with each sample divided into three groups (10 individuals to each group)." The percent composition of A. caerulea was approximately 0.81% (±0.03) moisture, 56.9% (±0.36) protein, 10.1 (±0.25) fat, and 28.4% (±0.32) "ash." Compared to other sea cucumbers, A. caerulea had comparatively higher fat content: "All fat contents of sea cucumber species were less than 2.0%, except for T. anax (9.9%) and A. caerulea (10.1%)." and levels of omega-3 fatty acids,: "T. ananas, A. mauritiana and A. caerulea contained more n-3 PUFA." but less overall nutritional value than Thelenota ananas, Bohadschia argus, and its congener Actinopyga mauritiana.: "The comparison showed that T. ananas, A. mauritiana and B. argus possessed higher nutritional values than other sea cucumber species."
Ecology and behavior Actinopyga caerulea is a detrivore, and forages during the day for organic detritus along sandy plains and coral patches on the outer slopes of coral reefs.: "The species is predominantly a detritus/deposit feeder on coral patches on the outer slope of coral reefs; it forages actively during the day."
Reproduction and life cycle Like many other members of the class Holothuroidea, blue sea cucumbers are gonochoric, and only have a single gonad. During spawning season, eggs and sperm are externally released into the surrounding water by female and male individuals, respectively, and are fertilized when they meet.
Commensal relationships The emperor shrimp (Zenopontonia rex) is known to inhabit the surface of Actinopyga caerulea in a commensal relationship, possibly feeding on ectoparasites or organic detritus on the surface of its skin. There is also at least one recorded instance of Pleurosicya mossambica living on it off the coast of Bitung.
Conservation status As of its latest assessment in 2010, the IUCN Red List considers Actinopyga caerulea to be a data deficient species. It has a wide geographic distribution, but is considered a rare species across its range, which makes the collection of accurate population data difficult.: "This species appears to be widespread in the Indo-Pacific, and is considered rare." It is harvested commercially for food in some parts of its range, and is used in the production of bêche-de-mer in Papua New Guinea;: "This species is harvested for the production of bêche-de-mer in Papua New Guinea (Kinch et al. 2008), and has been purchased from a local retail market in Guangzhou, China (Wen et al. 2010)." however, to what extent commercial activities impact its population is currently unknown.: "The extent that this species is currently fished or will be fished in the future is not known." Its distribution overlaps with at least one marine protected area (MPA).: "Occurs in at least one protected area: Yes"
Notes
All holothurian specimens in the study were incinerated after their moisture, protein, and fat percent compositions were recorded. "Ash" is a catch-all referring to anything not moisture, protein, and fat that was fully incinerated: e.g. ossicles, calcareous rings, gut contents.
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