What on earth is this a picture of? Try to guess and then read on.
It is the siphon or outlet water-tube of a giant clam that
is part of the feeding and respiration system. Seawater flows in one siphon,
through the gills and a trap for catching plankton, and then out the other
siphon.
The giant clam is a mollusc and the
largest living bivalve. Giant clams can weigh more than 200 kilograms and measure
as much as 1.2 metres from one end to the other. If they are not taken by
fishermen or killed by predators, they can live for about 100 years or more. In
nature, very young giant clams are eaten by loggerhead turtles whose beaks are
strong enough to smash the thick shells.
Hippopus the horse foot clam.
Although giant
clams filter plankton from the water for food, tropical seas do not provide
enough nutrient for these big animals. To help with the food supply giant clams
keep symbiotic algae in their tissue that convert sunlight to sugars; the algae
get a safe home in exchange for the extra food they produce.
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In the early days
of diving, giant clams were thought to be dangerous, trapping men under the
sea. Although they are big, they are not dangerous. The larger clams are
reluctant to close their shells and probably would never trap a diver.
The IUCN lists
the giant clams as vulnerable. The numbers of wild clams have been greatly
reduced by over-fishing for food and the aquarium trade. Many giant clams are
now grown on farms through a practice known as aquaculture. Giant clams are
protected in most parts of the Pacific but many are still taken. The meat,
called Himejako in Japan,
is prized and very expensive.
Diving down to see a giant clam is one of the 'experiences of a lifetime'. Just imagine how
lucky we are to have these gentle giants living in the ocean today. We should all do what we can to protect them?
(Photos from Wikimedia used here for
educational purposes).
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