Shedding new light on the humble 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 bêche 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.