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Holothuroidea – Sea Cucumbers
Philip Lambert
859
Phylum Echinodermata
Holothuroidea – Sea Cucumbers
Philip Lambert
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.
Class Holothuroidea
Morphology
Sea 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 Anatomy
The 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. Treelike dendritic tentacles (Fig. 1A) gather small particles
Fig. 1.
Types of
tentacles:
A. dendritic;
B. peltate;
C. pinnate;
D. digitate.
860
A
C
B
D
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 moplike, 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 Anatomy
To dissect a sea cucumber (Fig. 2), determine the dorsal
side (usually slightly darker or with tube feet less welldeveloped) and make a lengthwise incision. Use sharppointed 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.
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
Fig. 2. Anatomy
of a sea cucumber (after
Pawson 1970).
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
Fig. 3. Types of ossicles: A. plates; B.
rods; C. wheel and
miliary granules; D.
tables; E. rosettes.
A
B
Fig. 4.
Types of calcareous rings A. long
posterior tails; B.
anterior projections
only; C. short posterior projections.
A
B
C
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 Preservation
C
E
D
Sea 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
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Bathyplotes moseleyi (Théel, 1886)
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);
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).
100 μm
865
Phylum Echinodermata
Athyonidium chilensis (Semper, 1868)
Class Holothuroidea
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).
B
A
C
100 μm
866
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).
100 μm
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