Psychropotidae is a family of deep-sea swimming
sea cucumbers. The geographic range of some psychropotids is very extensive at abyssal depths, whereas other species are found within more restricted ranges.
[P. A. Tyler & D. S. M. Billett (1988) The Reproductive Ecology of Elasipodid Holothurians from the N. E. Atlantic, Biological Oceanography, 5:4, 273-296, DOI: 10.1080/01965581.1987.10749518]
Description
"Psychropotidae are elongate in shape, with the anterior part of the dorsal surface depressed. Many psychropotids have a large posterior appendage. They usually have 10-20 tentacles and ventrolateral radii, each with a single row of many small
tube feet
Tube feet (technically podia) are small active tubular projections on the oral face of an echinoderm, whether the arms of a starfish, or the undersides of sea urchins, sand dollars and sea cucumbers; they are more discreet though present on br ...
." Like all holothurians, psychropotids have calcareous deposits, vestigial elements of the usual
Echinoderm
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...
skeleton. "In psychropotids, These are essentially similar to other
Holothuroidea
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 holothurian ...
, differing in a very low degree of calcification", instead consisting mainly of connective tissue.
The calcareous ring and spicules of Holothuroidea are useful in determining phylogeny, as they vary greatly and are believed to have evolved independently as species adapted to various
biotopes. In psychropotidae there are usually four armed spicules with arms projecting inward and a single large central process projecting outwards. The calcareous ring is incomplete and has 5 independent parts.
[Llano, G. A., Schmitt, W. L. (1967). Biology of the Antarctic Seas III. United States: American Geophysical Union of the National Academy of Sciences-National Research Council.] "The longitudinal axis of sclerites in papillae is transverse to the longitudinal axis of the papilla".
[ "Concave sclerites of Laetmogonidae are probably homologous to concave cruciform sclerites of Elpidiidae and Psychropotidae".][ ''Scler ites can be absent in some specimens of the species Benthodytes typica and Benthodytes sanguinolenta.][''
"In Elasipodida all three mesenteries suspending the intestine in the anterior and middle part of the body are attached to the body wall dorsally and only in the posterior part of the body, the mesentery holding the second descending part of the intestine enters the right ventral interradius and is attached to the body wall near the mediadorsal muscular band".][
]
Behavior
All psychropotidae can swim short distances, yet certain species, such as Psychropotes longicauda spend much of their juvenile phase in the pelagic zone.[Echinodermata. (2020). United States: CRC Press.] "Mortensen, 1927, speculated that 'The long tail probably is a swimming apparatus"; his suggestion is apparently true for juveniles of this species, but not for adults".[Miller, J. E., Pawson, D. L. "Swimming Sea Cucumbers (Echinodermata: Holothuroidea): A Survey, with Analysis of Swimming Behavior in Four Bathyal Species", Smithsonian Institution Press, Washington D.C.] Several authors describe the ability of a number of Psychropotes
''Psychropotes'' is a genus of sea cucumbers in the family Psychropotidae. The members of this genus possess the ability to swim, although this is only facultative.
Description
As adults, species of ''Psychropotes'' are roughly cylindrical in sh ...
species to use a large, unpaired dorsal appendage as a 'sail' to move with the prevailing current, contorting their body so as to steer their movement. The great variation of this appendage throughout the Holothuroid family suggests that this adaptation is significant in allowing species to access food sources in the food-scarce environments in which they live.[Echinoderm Research 1995. (1995). Netherlands: Taylor & Francis.]
"Ostergren (1938) noted that most species in the Family Psychropotidae and some synallactids such as genus Paelopatides are darker in color ventrally than dorsally. He suggested that the differing colors may conceal swimming animals from predators above and below.
Taxonomy
Using traditional taxonomic methods, Psychropotidae have been placed in the same taxon
In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular n ...
as the family Deimatidae. Recent molecular phylogenetic data, however, has suggested that the taxon is polyphyletic, with Deimatidae separated from all other Elasipoda families.[Miller, A. K., Kerr, A. M., Paulay, G., Reich, M., Wilson, N. G., Carvajal, J. I., & Rouse, G. W. (2017). Molecular phylogeny of extant Holothuroidea (Echinodermata). Molecular phylogenetics and evolution, 111, 110–131. https://doi.org/10.1016/j.ympev.2017.02.014]
Reproduction
"In the psychropotid species the ovary is a large thick-walled nodular structure containing oocytes with an interspecific maximum size of 1.2 mm to 3 mm".[Hansen, B., 1975. Systematics and biology of the deep-sea holothurians. Part 1. Elasipoda. Galathea Report 13:1-262.] noted a maximum egg size of 4.4 mm for Psychropotes longicauda, the largest egg known in holothurians.
References
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