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The Sea Doc Investigates: Nudibranchs

Dr John Carlin

There is something breathtaking about tropical coral reefs. Of course, if you are a captain of a ship then your view could be slightly skewed as they are also a major cause of shipwrecks. For most, though, the warm, clear water, vibrant colours and the vast array of living organisms that inhabit this spectacular ecosystem are a sight to behold. Coral reefs are said to be the most productive, mature, diverse and complex ecosystems in nature.

'Coral' is a term generally used for several different groups belonging to the phylum (a taxonomic division) called Cnidaria. Cnidarians are entirely aquatic; a few members are found in fresh water but the majority in marine environments. They have a simple body plan, and are most abundant in shallow, warm-temperature or subtropical waters.

Cnidaria comes from the Greek word 'cnidos', which means stinging nettle. All cnidarians are carnivores, capturing prey with the tentacles that ring their mouths. The tentacles, and sometimes their body surface too (such as anemones), bear specialised cells called cnidocytes which occur in no other group of organisms. Within each cnidocyte is a small powerful harpoon, made from a hollow tube with a series of barbed spines, called a nematocyst. Nematocysts, fired by water pressure with enough force to penetrate the shell of a crab, are used both for defence and to capture prey...

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