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Holothuroidea – Sea CucumbersPhilip Lambert

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General Introduction

The first confirmed fossil evidence of sea cucumbers

dates from the late Silurian period, about 400 million

years ago. About 1,400 species of sea cucumbers are

described worldwide.

MorphologySea cucumbers, technically known as the Holothuroidea,

have a flexible body wall containing circular and

longitudinal muscles and a skeleton made up of isolated

calcite particles, called ossicles. The ossicles may

toughen the skin and represent a vestige of the normal

echinoderm skeleton. Typically, a sea cucumber is an

elongate cylinder lying on its side with a mouth at one

end and an anus at the other. Five rows of tube feet run

the length of the body. Around the mouth there are one

or two circles of feeding tentacles, which are actually

modified tube feet.

External AnatomyThe tube feet, also known as podia, usually consist of

a cylindrical shaft with a sucker at the tip. Of the five

longitudinal rows of tube feet along the body, typically

the three ventral rows are more robust than the two on

the dorsal side. But, the tube feet can be quite variable.

In some species, such as Chiridota pisanii, the tube feet

are absent, or like Bathyplotes moseleyi the tube feet

on the dorsal side are modified into pointed bumps, or

papillae.

The feeding tentacles, being part of the water vascular

system, can be extended by hydraulic pressure. Tree-

like dendritic tentacles (Fig. 1A) gather small particles

suspended in the water. Particles adhere to a coating of

mucus on the tentacle, and then the sea cucumber places

it into its mouth and removes the food. Heterocucumis

godeffroyi is a common suspension feeder.

Sea cucumbers that ingest sediment as they roam across

the ocean floor, (e.g. Bathyplotes moseleyi) have mop-

like, peltate tentacles (Fig. 1B). The animal presses the

tentacles onto the substratum and particles adhere to the

mucus. It then retracts the tentacle and inserts it into

its mouth to remove the particles. The organic matter is

digested from the sediment as it passes through the gut.

Sea cucumbers that ingest sediment as they burrow

below the surface, have pinnate (Fig. 1C) (e.g. Chiridota)

or digitate (Fig. 1D) tentacles. These short, finger-like

tentacles push sediment into the mouth.

Internal AnatomyTo dissect a sea cucumber (Fig. 2), determine the dorsal

side (usually slightly darker or with tube feet less well-

developed) and make a lengthwise incision. Use sharp-

pointed scissors and make the cut to the left of the

midline in order to leave the dorsal mesentery intact.

The main internal organs are a coiled digestive tract

that may be two or three times the length of the body, a

pair of respiratory trees attached to the posterior cloaca,

and a gonad made up of a cluster of genital tubules.

Five bands of longitudinal muscles attach to the inside

of the body wall, which consist of circular muscles,

connective tissue and skin. The action of these circular

and longitudinal muscle layers produces a worm-like

or peristaltic movement like squeezing a water-filled

balloon and causing the end to extend. Waves of

contraction move the animal along the ocean floor or

through the mud.

The ossicles of the skeleton include: the microscopic

pieces of calcite embedded in the outer layers of the skin,

the scales on the back of some groups, and the calcareous

ring around the anterior gut. All are extremely important

for verifying the identity of a species. The microscopic

ossicles are complex and varied in shape. Taxonomists

use their shapes as one way of telling the species apart.

Holothuroidea – Sea CucumbersPhilip Lambert

Fig. 1. Types of tentacles: A. dendritic; B. peltate; C. pinnate; D. digitate.

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A typical ossicle is a flat plate perforated by holes,

but ossicles occur in a large variety of shapes (Fig. 3).

A description or illustration of them is an important

part of a scientific account. Closely related species that

have recently evolved from a common ancestor have

very similar ossicles. Ossicle shapes also vary within

a single animal. Those in the dorsal skin can be larger

on average than those in the ventral

skin. Tube foot and tentacle ossicles can

also be quite different. Furthermore,

as the animal matures, the ossicles

change from a complex juvenile form

to a simpler adult form. In the family

Psolidae greatly enlarged ossicles form

the exposed overlapping scales on the

dorsal side of the animal (they also

have microscopic ossicles in the ventral

sole). The calcareous ring is comprised

of a series of plates, usually ten, joined

side by side like a collar around the

oesophagus. The ring varies in shape in

different species; some plates have long

tails (Fig. 4A), others have short tails (Fig.

4C) or only anterior projections (Fig.

4B). The shape of the ring is important

in the classification of sea cucumbers.

For example, all those that have long

posterior tails on the ring are placed in

the same family. Each plate may be a

solid piece, or in some species, a mosaic

of smaller segments. Being one of the

few hardstructures in a sea cucumber, the calcareous

ring is often the only part that fossilizes, thus providing a

way of relating extinct and living forms.

Collection and PreservationSea cucumbers can be collected by hand while SCUBA

diving or at low tide. Most tend to be cryptic and live

under rocks or in crevices. When the tide is out they

retract their feeding tentacles and are difficult to see.

When the tide returns they extend their feeding tentacles

revealing their hiding places. Biologists collect deep sea

cucumbers with a dredge or trawl towed behind a boat.

It should be possible to identify most of the species in

this chapter from external characters; however, to verify

some of the more obscure ones it may be necessary

Fig. 4. Types of calcare-ous rings A. long posterior tails; B. anterior projections only; C. short pos-terior projections.

Fig. 2. Anatomy

of a sea cu-

cumber (after

Pawson 1970).

Fig. 3. Types of os-sicles: A. plates; B. rods; C. wheel and miliary granules; D. tables; E. rosettes.

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Bathyplotes moseleyi (Théel, 1886)

colour yellow to white with brown spots; ossicles are

4-armed tables with a spire of 4 rods and 2 or more

crossbars.

Habitat: Rocks in shallow water and mud in deeper

water. Depth: 24–690 m. Abundance: Infrequent

in shallow water. Distribution: NW Pacific (Japan);

Antarctica; Subantarctic Islands (South Sandwich); SE

Pacific (Peru; CPZ). Chile: 48°S–50°S. Biology: Little is

known about the biology of this species, but like other

Synallactids it is probably a deposit feeder. Comments:

B. natans may be a junior synonym of B. moseleyi.

Ossicle figure from Théel (1886).

Main references: Théel (1886); Perrier (1902); Pawson

(1965); Solis-Marin & Laguarda-Figueras (2004).

Common name: Spiky sausage sea cucumber;

Salchicha del mar con puntas.

Description: Size ~10 cm long. Colour in life reddish

purple to transparent whitish. Body cylindrical, slightly

flattened on ventral surface with ca. 20 tentacles

surrounding the mouth. Ventral podia in 3 series, middle

series forms a thin double row, 2 outer series form a

simple zigzag row. Dorsal podia modified into 2 narrow

double rows of small conical papillae and some low

whitish warts. Ossicles: Dorsal skin contains star or cross

shaped ossicles with 4–8 arms radiating from centre

with enlarged flattened ends; usually with a central spire

composed of 4 rods that join at top and form a spire of

4 teeth. No other crossbars. Possibility for confusion:

Bathyplotes natans lives in deeper water (200–1.600 m);

100 μm

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Athyonidium chilensis (Semper, 1868)

Common name: Burrowing shaggy sea cucumber;

Pepino excavante velludo.

Description: Size ~25 cm long. Colour in life greyish,

tentacles brownish, reddish, greenish or black. Colour

in alcohol dorsally dark brown to blackish, ventrally

lighter with dark tentacles. Body with thick, soft skin and

20 tentacles (5 pairs of large outer and 5 pairs of small

inner tentacles). Podia numerous and densely spread

over body. Ossicles: No ossicles in body wall except

for small rods (A) and large complex end plates (B) in

podia. Tentacles contain only rosettes (C). Possibility

for confusion: A. chilensis is distinct with a larger body

densely covered with podia and dark greenish or reddish

black tentacles that cannot be confused with any other

common Chilean species.

Habitat: Near kelp holdfasts (Macrocystis) and

in a variety of habitats from sand to rock. Depth:

Intertidal–7 m. Abundance: Common. Distribution:

SE Pacific (Peru; PP–NPZ). Chile: 18°S–42°S. Biology:

Usually buried in sand with tentacles extended on the

surface. Intestinal contents included pieces of brown

algae (e.g. Macrocystis), green algae, small crustaceans

and hydroids. Begins shedding gametes in the spring and

continues for 4–6 months.

Main references: Deichmann (1941; 1947); Pawson

(1964; 1965).

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Psolus squamatus segregatus Perrier, 1905

Common name: Red squamous rock sea cucumber;

Pepino rojo escamoso de roca.

Description: Size large (up to 80 mm). Colour in life light

brown or light red with light red tentacles with darker

red spots. Body with 10 tentacles. Ventral podia in a

double row around periphery with a few at each end

of central row; occasionally with a staggered row down

centre. Dorsal scales varying from smooth to granular.

Numerous oral and anal valves. Between mouth and

anus are 10 to 15 scales, difficult to see edges of scales.

Ossicles: Sole of smaller specimens (25 mm) contains

a few thin, smooth perforated plates with large holes

and partially formed holes around edge (150–200 µm

diameter); some simple X-shaped plates appear to

be precursors of the more complete plates. Medium

specimens (40 mm) contain more complex and robust

plates, with scattered knobs on surface and around

edge (shown). Large specimens (70 mm) contain similar

plates but larger and more complex, some knobs on

plates merge together into bars on surface of plate (not

shown). Possibility for confusion: Psolus patagonicus

is usually much smaller (20 mm) and flatter; but to be

sure one would need to analyse the ossicles in the sole.

P. antarcticus is usually clean and whitish pink with 5

distinctive triangular valves covering the mouth opening.

P. paradubiosus has 5 triangular oral valves and 8–10

clearly visible scales between mouth and anus.

Habitat: Usually attached to stones or rock. Body often

covered with sediment. Depth: 7–207 m (P. squamatus:

7–1.087 m). Abundance: Common, locally dominant.

Distribution: Psolus squamatus: Widespread; Atlantic;

Pacific (57°N–56°S). Southern form, P. squamatus

segregatus: SW Atlantic (Argentina; Falkland Islands);

SE Pacific (PP–SPZ). Chile: 18°S–56°S (P. squamatus

segregatus, 41°S–56°S). Biology: Some specimens

overgrown with epizoic organisms such as sponges

(e.g. Clathrina sp.) or ascidians (e.g. Didemnum studeri).

During ROV transects in the fjord Comau dense

aggregations of P. squamatus segregatus between

polychaete tubes were observed between 70 and 100 m

depth.

Main references: Ekman (1925); Deichmann (1941;

1947); Pawson (1969).