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Sea Sabres
Aquatic Biodiversity & Biosecurity
Shedding new light on the humble sea cucumber
|
The detritus-feeding sea cucumber.
An example of a “cup” ossicle.
The strawberry sea cucumber. |
A major study of New Zealand’s sea cucumbers that is nearing
completion, and recent taxonomic work, are providing a fuller picture of the
diversity of this remarkable group of creatures.
Literally in a class of their own (Class Holothuroidea or
holothurians), sea cucumbers are the less charismatic worm-like cousins of
sea-stars, sea-urchins, and sea-lilies. One writer has described them as
creatures that “ooze along the bottom with the speed of an hour-hand, or slurp
their way through organic mud”. They are generally slow, sluggish, and not very
attractive, which is why they have historically been largely ignored. However,
interest in them is increasing because they are of great ecological importance
and make up 90% of the biomass in parts of the deep sea. They are also an
important food source in many parts of the world, known as trepang or
b꣨e de mer, and have renowned aphrodisiac properties.
At a time when interest in New Zealand’s sea cucumbers is
increasing because of their role in our marine ecosystems (they can sometimes
dominate bycatch) and as a food item (using the local species Stichopus
mollis), it helps to know that we now have 100 or more species, up 43%
since the last major study in 1970, and we are better equipped to identify
them.
Sea cucumbers, like their vegetable namesake, are cylindrical.
Their intestines, a gonad, and a stone canal of uncertain function are enclosed
by the body wall. Being echinoderms, they have tube-feet and the typical
five-radial symmetry, but the latter is usually only evident internally or
around the mouth. Limited fossil evidence shows that sea cucumbers were once
essentially U-shaped, with a skeleton of overlapping plates. This body form
later became cylindrical and the skeleton was reduced to microscopic ossicles.
Today these ossicles are one of the main identification features of sea
cucumbers, and the various forms of these calcified structures can be stunning.
There are five orders of holothurians; however, only one, the Aspidochirotida,
is commonly known. This is partly because of their conspicuous nature, but also
because this order is a popular food species and is sold in some parts of the
world as a natural health supplement because of its alleged aphrodisiac properties.
In New Zealand there is only one well-known species – the aspidochirotid Stichopus
mollis. This species lives in the rocky intertidal and sandy or muddy
subtidal zone and is frequently seen by people while they are diving and
snorkelling. This species is harvested on a small scale and has aquaculture
potential, so parts of its life cycle and ecology have been studied; however,
we do not know enough about its reproductive biology to say whether a
substantial fishery could be maintained.
Although only one species of sea cucumber is conspicuous, 100 or
more species occupy various habitats in New Zealand, ranging from intertidal
zones through to shallow subtidal zones and right down to the abyssal depths.
They have various feeding methods, including churning through sand and
engulfing everything in their path, or protracting their tentacles into the
water column and catching small detritus as it floats by.
Some of these species, such as Squamocnus brevidentis,
can form dense populations. We recently studied this species in collaboration
with Mark O’Loughlin of Museum Victoria in a revision of the New Zealand
species of the taxonomically difficult family Cucumariidae. This species lives
in southern New Zealand on rocky walls at densities of up to about 1000 individuals
per square metre. It defies the uncharismatic, sluggish properties typical of
sea cucumbers, and is a beautiful bright red animal with white dots and
stunning branching red tentacles. The masses of this “strawberry sea cucumber”
form a mat on a rock wall in Preservation Inlet, Fiordland, now commonly known
as “Strawberry Fields”. Another holothurian, Paracaudina chilensis, a
sand-burrowing species, is also regularly found in New Zealand dredge samples.
This species burrows its oral end in the sand and leaves its posterior end at
the surface for respiration (the ‘ostrich holothurian’ perhaps?).
The reproductive strategies of holothurians are also diverse and
interesting. Most species disperse their gametes into the sea, where
development follows completely independently of the adult. However, we
discovered rare parental care in Squamocnus brevidentis, an external
brooder that incubates 70–100 young under the ventral surface of the female for
between 1 and 3 months. Tiny replicas of the adults emerge and settle nearby,
creating the huge densities mentioned above. David Pawson of the Smithsonian
Institution found that another brooder, Psolidocnus sacculus, a
deep-sea holothurian found off the Otago coast, displays a type of coelomic
brooding where the brood pouches have no permanent contact with the exterior.
Apart from these studies, NIWA’s involvement with holothurians to
date has been small. Yet, there is much research potential, especially given
the possibility for aquaculture of Stichopus mollis, the dominance of
holothurians in the deep sea, and the interesting reproductive mechanisms.
Traditionally, holothurians have often been omitted from benthic
surveys, put into jars labelled “worms” or washed down the drain. However,
various NIWA surveys, including the Ross Sea biodiversity project and recent
ports and harbours surveys, have produced small collections that are indicative
of larger numbers in these environments.
The foremost authority on New Zealand holothurians is New
Zealander David Pawson of the Smithsonian Institution, Washington, D.C., who
will soon be submitting a monograph for publication in the NIWA
Biodiversity Memoirs series.
Recipe
for Braised Sea Cucumber with Mushrooms
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Posted August 21, 2003