''Pelagothuria'' is a genus of

sea cucumbers Sea cucumbers are echinoderms from the class Holothuroidea ( ). They are benthic marine animals found on the sea floor worldwide, and the number of known holothuroid species worldwide is about 1,786, with the greatest number being in the Asi ...

in the family

Pelagothuriidae Pelagothuriidae is a family of deep-sea swimming sea cucumber Sea cucumbers are echinoderms from the class (biology), class Holothuroidea ( ). They are benthic marine animals found on the sea floor worldwide, and the number of known holothur ...

. 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 "unisp ...

, 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, also known as sea jellies or simply jellies, are the #Life cycle, medusa-phase of certain gelatinous members of the subphylum Medusozoa, which is a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animal ...

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 and known until 1997 as Western Samoa, is an island country in Polynesia, part of Oceania, in the South Pacific Ocean. It consists of two main islands (Savai'i and Upolu), two smaller, inhabited ...

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 Hubert is a Germanic masculine given name, from ''hug'' "mind" and '' beraht'' "bright". It also occurs as a surname. Saint Hubert of Liège (or Hubertus) (c. 656 – 30 May 727) is the patron saint of hunters, mathematicians, opticians, and m ...

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 San Cristóbal Island (), also known as Chatham Island, is the easternmost island in the Galápagos archipelago, as well as one of the oldest geologically. It is administratively part of San Cristóbal Canton, Ecuador. Names ''San Crist� ...

), 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 NOAAS ''Okeanos Explorer'' 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, Education center, education and Research institute, research centers, created by the Federal government of the United States, U.S. government "for the increase a ...

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 () 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 and most of Americ ...

(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