Holothuroidea

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Experts


Synallactidae morphotypes

Synallactes

Benthothuria

Paelopatides morphotype

Pseudostichopus morphotype

Molpadiodemas morphotype

Mesothuria morphotype

Paroriza morphotype


Deimatidae morphotypes

Deimatidae gen. sp.

Deima morphotype

Oneirophanta morphotypes

Orphnurgus


Laetmogonidae morphotypes


Psychropotidae

Psychropotes morphotypes

Benthodytes morphotypes


Elpidiidae morphotypes

Peniagone morphotypes

Amperima morphotype

Double velum morphotype


Pelagothuriidae

Enypniastes morphotype

Introduction

Holothuroidea Selenka, 1867 (sensu Smirnov, 2012) is one of the five Recent Classes in the phylum Echinodermata. Holothurians, also known as sea-cucumbers, are among most common (abundant) members of the abyssal megafauna. Abyssal depths sometimes are called “the kingdom of holothurians”.

Classification

The following six Orders are recognised by most authors in Holothuroidea: Apodida Brandt, 1835; Dendrochirotida Grube, 1840; Aspidochirotida Grube, 1840; Elasipodida Théel, 1882; Molpadida Haeckel, 1896 and Dactylochirotida Pawson & Fell, 1965. In the deep-sea most common are the elasipodid holothurians and representatives of the family Synallactidae (Aspidochirotida). In the CCFZ area all six orders are represented. Approximately 1500 species of sea-cucumbers are estimated worldwide. More than 700 species occur in the bathyal-abyssal zones. Approximately 80 species are expected to occur in the North-East Pacific.

Basic anatomy

Sea cucumbers are typically 10 to 30 cm in length, although the smallest forms are just few mm long and the largest can reach several meters in length. The body shape ranges from almost spherical to worm-like. Many deep-sea forms are soft, gelatinous with great variety of shapes. The anterior end of the animal containing the mouth corresponds to the oral pole of other echinoderms (which in most cases is the underside), while the posterior end containing the anus corresponds to the aboral pole. Thus, compared with other echinoderms, sea cucumbers can be said to be lying on their side.

The body of a holothurian is roughly cylindrical. It is radially symmetrical along its longitudinal axis and has a bilateral symmetry transversely with a dorsal and a ventral surface. There are five ambulacra separated by five ambulacral grooves, the interambulacra. The two dorsal ambulacra make up the bivium while the three ventral ones are known as the trivium. In many deep-sea forms there are additional clearly distinguished features of bilateral symmetry: flattened body, papillae in two rows along the bivium, dorsal appendages built of fused pairs of papillae etc.

At the anterior end, the mouth is surrounded by a ring of tentacles, in some groups they are retractable into the mouth. Tentacles may be dendritic, digitate (with finger-like projections), pinnate (feather-like), or peltate (flattened and shield-like). Posterior to them there is an internal calcareous ring. This ring serves as an attachment point for muscles operating the oral tentacles and for the anterior ends of five bands of muscles running internally longitudinally along the ambulacra.

Some sea cucumbers possess organs not found in other invertebrates. In some Aspidochirotida, the respiratory trees display Cuvierian tubules. In most species, these are apparently defensive structures. They can be expelled through the anus, whereupon they dramatically expand in length and become sticky, entangling or deterring potential predators, such as crabs and gastropods. Many forms, with the exception of members of Elasipodida and Apodida, possess respiratory trees used in gas exchange. These are paired, heavily branched tubes attached to the intestine near the anus. This type of breathing is known as "cloacal breathing". Inside the body wall is the coelom divided by three longitudinal mesenteries which surround and support the internal organs

Holothurians with the exception of members in Elasipodida have a madrepore that opens into the coelom (body cavity). In contrast, elasipodids and nearly all other echinoderms have a madrepore that opens externally.

A key feature of holothurians is the reduction of the skeleton to microscopic ossicles. They come in a great variety of shapes: from complex 3D forms – tables, towers, anchors, to perforated plates (with variations), cross-shaped elements, simple or tripartite rods, C-shaped brackets etc. Ossicles vary in size from tens to hundreds of µm, some exceed in length 1mm.

Ecology

Sea-cucumbers are found in nearly every marine environment, they are especially diverse on tropical shallow-water coral reefs. On the vertical scale, they range from the intertidal, where they may be exposed briefly at low tide, to the floor of the deepest oceanic trenches. Many deep-sea species can swim and there are even forms that live their entire lives as plankton, floating with the ocean currents. Most of deep-sea species occur on soft sediments, however there are also specialists of rocky substrates. Shallow-water holothurians feed from the sediment (deposit-feeders) or from the water column (suspension-feeders). Deep-sea species are mainly deposit-feeders with different ability to selectivity of material.

Sea-cucumbers of the CCFZ area

The number of species/morphospecies recognized in the CCFZ area in publications is around 30 (Pawson & Foell, 1983; Tilot, 1992; 2006). Most common species in the CCFZ (ranked by abundance) are Mesothuria murrayi (40 ind. ha-1), Benthodytes incerta (26), Enypniastes eximia and Synallactes profundi (both 25).

Limitations of images

Reliable identification of holothurian species in most cases is based on calcareous ossicles . As an exception, single characteristic species (such as Enypniastes eximia) can be recognized based on their external morphology (moreover in this species ossicles are lacking). Some characters easily seen on images are shared even between higher taxa of holothurians. Thus, most common in the deep-sea synallactid and elasipodid holothurians both can be pinkish in colour, flattened and bare visible dorsal papillae (in rows or not). Several genera of elpidiid holothurians (Elasipodida) share such characteristic features as anterior dorsal appendage (velum) and large easily distinguished tubefeet. Adding to difficulties of identification is the poor knowledge of the whole taxon Synallactidae (waiting for taxonomic revision) and overall poor knowledge on the North-East Pacific deep-sea holothurian fauna.

References

  • Pawson D. and Foell E. 1983. Atlas of photographs of megafauna from the study area. Report №MS-200-146 of Deep-sea Ventures Inc., Gloucester Point, Virginia6 pp. 1-120.
  • Smirnov A.V. 2012. System of the Class Holothuroidea. Paleontological Journal, 46 (8): 793–832.
  • Tilot V. 1992. La structure des assemblages mégabenthiques d’une province de nodules polymétalliques de l’océan Pacifique tropical Est, Doctorat (nouveau régime) en Sciences et Techniques, Université deBretagne occidentale, Brest, France, pp.1-380.
  • Tilot V. 2006. Biodiversité et distribution de la mégafaune. Ecosystème de nodules polymétalliques de l’océan Pacifique Est equatorial. Paris, UNESCO/IOC. (IOC Technical Series, 69, Vol.1).