31 July 2006
The Amazing life of Nudibranchs or Sea Slugs
by Roger onboard the Rainbow Warrior in the Mediterranean sea.
Nudibranch Flabellina affinis feeding
on a hydroid colony. Some of the
hydroid animals are visible, like
tiny sea anemones. Others have
been eaten. At the top of
the colony is a purple egg string
laid by the nudibranch.
©Greenpeace/Roger Grace
They say "fact is stranger than fiction". Try this one: Jellyfish, sea anemones, corals and hydroids are all related to each other. They have special stinging cells in their tentacles with which they sting and trap small fish or tiny prey animals. In some types the sting is so strong it is dangerous to humans. The stinging cells are usually triggered by touching the tentacles.
Some nudibranchs or seaslugs specialise in eating hydroids. They have developed an amazing technique whereby they can eat the hydroids without triggering the stinging cells and thus don't get harmed by the poisonous stings. But they go even further than that. When they eat the hydroid the stinging cells remain intact and migrate through the stomach wall and into colourful projections on the back of the nudibranch. Here the stinging cells become embedded in the skin of the nudibranch and are ready to sting a fish or other predator which may try to eat the nudibranch! So the nudibranch uses the defence mechanism of the hydroid to protect itself from predators. Nudibranchs are often very brightly coloured and obvious. This serves as a warning to fish that the nudibranch either tastes horrible or can sting. This is the same reason why butterflies are so colourful - they taste horrible and the colours are a warning to birds not to eat them.
This nudibranch has released from the bottom and is in free-fall in open water, perhaps to move to better feeding grounds.
The ringed tentacles on the head are special sensory appendages.
You might be able to see a tiny black eye at the base of these tentacles.
©Greenpeace/Roger Grace
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