One of the first things a dive master pointed out to me on
my very first saltwater dive was a dark segmented length of something
resembling what I shoveled out of my horse barn. He seemed to think I should be
interested and might want to touch it. I signaled in return, ‘thanks but, no
thanks’ since I had plenty of that in a manure spreader back home. I wondered
why he thought the droppings of some large marine animal would attract me.
When we boarded
the boat, he explained that it was a sea cucumber. OK… a plant, I thought. I
soon learned it wasn’t a plant but an animal. Years later, when diving with my
students, I too search for sea cucumbers. Remembering my initial reaction, I
chuckle when even the most macho student looks abhorred at the prospect of
touching a Donkey Dung sea cucumber.
There are an
estimated 1,250 species of sea cucumbers in oceans around the world. They are
invertebrates, meaning they have no backbone or spinal column, and they are
classified as Echinoderms, “spiny skinned”, as are sea stars and sea urchins.
Ranging in size from 1 inch to over 3 feet in length, sea cucumbers are also
known as the earth worms of the sea.
From the ugly
Donkey Dung to the beautiful Beaded Sea Cucumber, they provide an invaluable
service in keeping the sand clean and uncompressed so that other bottom
dwelling creatures may survive. In areas where sea cucumbers have been
over-hunted for the Asian gourmet trade, researchers have discovered that once
sea cucumbers disappear, so do many other bottom dwelling species, as well as
various water column species that prey upon the bottom dwellers. Without
cucumbers to aerate the sand, it becomes too hard for bottom dwelling creatures
that must have loose sand in which to build their homes.
All sea cucumbers
feed on algae, detritus and plankton. Some are filter feeders, using tentacles
to capture food floating in the water. The three most common Caribbean species,
the Donkey Dung, the Three-Rowed, and the Furry sea cucumber feed by slowly
inching their way along sand bottoms as they suck in nutrient-rich sand,
leaving tubular casts of processed sand behind. In doing so, they’ve earned
their nickname, earthworms of the sea. Just as earthworms clean, aerate, and
enrich dirt, the sea cucumbers perform the same beneficial process underwater.
While sea
cucumbers move very slowly on suction-cupped podia, they have one of the
fastest digestive systems, giving them the ability to take in food and expel
cleaned sand casts within an hour. Some scientists believe that one sea
cucumber may clean and process 500-1000 tons of sand per year. When observing
sea cucumbers, it is often difficult to determine the head from the tail unless
one looks for the sand casts. And, offensive as it may sound to humans; the sea
cucumber itself provides shelter to the tiny Pearl Fish that takes up residence
by entering the cucumber through its anus and lives inside the body cavity.
When threatened,
sea cucumbers can eviscerate themselves by expelling their internal organs
along with sticky thread-like cuvierian tubes; giving their predators something
to feed upon as the cucumber slowly creeps away and rapidly regenerates its
vital organs, including the respiratory trees. Without gills and living their
entire lives on the bottom, sea cucumbers have developed ‘trees’ of filtering
membranes within their body cavities. In order to breathe they take water in
through their anus where it passes over the respiratory trees and the oxygen is
extracted.
So, the next time you look down in the water
and see an ugly elongated lump, don’t swim away. Stay awhile and marvel at one
of the most beneficial animals in the sea.
Tweet