I learned this week that sea cucumbers slink along the sea floor with a hidden skeleton. Known to most of us as those sloppy, sausage-like things that sometimes wash-up on our beaches, sea cucumbers are pretty much a tube of muscle with a mouth at one end and an anus at the other. Underwater, they bury in sand or camouflage themselves against rocky reefs.
A colourful sea cucumber (or holothuroid).
Image: Julian Finn
Source: Museum Victoria
Rather than running through the middle of the body, the skeleton effectively surrounds the body to reinforce the muscular body tube. It is made up of tiny structures called ossicles, which can be fifty times smaller than a millimetre. They are like miniature fish scales, but more intricate in design and not usually visible. Some of the structures make some animals sticky to touch.
Here’s an example of the ossicles of an Antarctic species:
Ossicles from Sigmodota contorta, a species misidentified under about ten different names. Wheel and hook forms on the left from the body wall, and branched rods on the right from the tentacles.
Source: O’Loughlin and VandenSpiegel (2010) Memoirs of Museum Victoria 67: 61–95.
These weird and spectacular structures vary in form. Not only do they prevent the body from turning into a mush of intestine and muscle, but they are also a microscopic key to identify many species – so don’t be too disappointed if you can’t identify a sea cucumber when diving or looking in a rock pool!
Oh and if you're interested...
Sea cucumbers belong to a group of animals called holothuroids, part of the wider group of echinoderms – more commonly known for its sea stars and sea urchins. MV Honorary Associate Mark O’Loughlin is a world expert in identifying sea cucumbers. He has shown me a few tricks of the trade on his way to describing over 20 new species in recent years from Victoria and its neighbouring oceans. He is currently sorting out whether the common local species, Paracaudina australis, is actually multiple undescribed species. His work was recently published in the Memoirs of Museum Victoria.
O’Loughlin, P. Mark and Didier VandenSpiegel. A revision of Antarctic and some Indo-Pacific apodid sea cucumbers (Echinodermata: Holothuroidea: Apodida) Memoirs of Museum Victoria 67: 61-95 (2010)
Question of the Week: Aboriginal-Indonesian trade in sea cucumber
Reef Education Network: Sea cucumbers