''Pelagothuria'' is a genus of

sea cucumbers Sea cucumbers are echinoderms from the class Holothuroidea (). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothuri ...

in the family

Pelagothuriidae Pelagothuriidae is a family of deep-sea swimming sea cucumbers. They are somewhat unusual in appearance, in comparison with other sea cucumbers, having numerous appendages, including conical papillae and leaf-like tentacles. Most of them are bent ...

. It is

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispec ...

, being represented by the single species ''Pelagothuria natatrix''.

Characteristics

This sea cucumber is somewhat unusual in appearance in comparison with other sea cucumbers (and even within its family), as it looks more like a

jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrella- ...

with its large umbrella-like swimming structure supported by a ring of around 12 highly modified oral tentacles, its small tapered body and its swimming position with the mouth on top. The body is translucent with a pale purple pigmentation. The mouth is surrounded by around 15 short feeding tentacles like any sea cucumber, and the veil can be contracted like jellyfishes do (it is interrupted at the central ventral radius). The animal seems to reach around 16 cm in total diameter. This species constitutes the only true pelagic holothurian (and even echinoderm) known to date. However, its swimming seems mostly passive, more like slightly controlled drifting. Image:Pelagothuria natatrix (cropped).png, ''Pelagothuria natatrix'' observed in 2011 off the Galápagos islands. Image:Pelagothuria natatrix Samoa.jpg, 2017 observation off the

Samoa Samoa, officially the Independent State of Samoa; sm, Sāmoa, and until 1997 known as Western Samoa, is a Polynesian island country consisting of two main islands (Savai'i and Upolu); two smaller, inhabited islands (Manono Island, Manono an ...

islands. Image:Pelagothuria natatrix Samoa 1.jpg, Closer view from same 2017 NOAA Okeanos Explorer observation

Habitat and repartition

This sea cucumber is extremely rare, but its geographic range seems very wide: it has been collected in the Atlantic, Pacific and Indian oceans, between 200 and 4,433 m of depth.

Discovery

This species was described by Hubert Jacob Ludwig in 1893 based on trawled specimens collected in 1891 by the USS ''Albatross'' between the Gulf of Panama and the Galapagos Islands (605–3,350 m deep). It was not until 1989 that the first ''in situ'' footage of the species was obtained thanks to a scientific expedition in the Galapagos (542 m deep off San Cristóbal Island), followed by a scientific review of deep-sea swimming sea cucumbers from John Miller and David Pawson in 1990.Miller, J. E.; Pawson, David L.
Swimming Sea Cucumbers (Echinodermata: Holothuroidea): A Survey, with Analysis of Swimming Behavior in Four Bathyal Species
''Smithsonian contributions to the marine sciences'', no 35, 1990. In 2011, the American scientific expedition

NOAA Okeanos Explorer The National Oceanic and Atmospheric Administration (abbreviated as NOAA ) is an United States scientific and regulatory agency within the United States Department of Commerce that forecasts weather, monitors oceanic and atmospheric conditio ...

photographed what scientists first believed to be an unknown jellyfish, but the picture was formally identified in 2014 by

Smithsonian Institution The Smithsonian Institution ( ), or simply the Smithsonian, is a group of museums and education and research centers, the largest such complex in the world, created by the U.S. government "for the increase and diffusion of knowledge". Founded ...

experts Christopher Mah and David Pawson as ''Pelagothuria natatrix''. A second observation was made in March 2017 by the same mission off the

Samoa Islands The Samoan Islands ( sm, Motu o Sāmoa) are an archipelago covering in the central South Pacific, forming part of Polynesia and of the wider region of Oceania. Administratively, the archipelago comprises all of the Independent State of Samoa an ...

(443 m deep near Howland Island), identified by NOAA expert Steve Auscavitch, and this time included a high-resolution video of the animal swimming in the water column. Since then, more attention has been brought to this unusual species, and it has been observed more than 100 times by NOAA.

References

{{Taxonbar, from=Q2570953 Pelagothuriidae