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Sea slug defences

Sea slugs go without a hard protective shell into which they can retreat. They therefore need other forms of defence.

 
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This sea hare (Aplysia punctata) had been grabbed by an edible crab (Cancer pagurus) but the crab was handling it tentatively at first.

 
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When the crab then tried to tear its potential prey apart, the sea hare released copious clouds of purple slime. The startled crab stopped attacking but kept hold of the sea hare and walked off, perhaps waiting for the slime to dissipate before trying again.

 
 
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In another example of sea slug defence, this large black goby (Gobius niger) attacked an elegant sea slug (Okenia elegans) and tried to bite at it.

 
 
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The repellant chemicals in the sea slug's skin seemed to function well, as the goby rejected it very quickly.

 
 
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The grey sea slug (Aeolidia papillosa) has an even more impressive defence system. It preys on sea anemones (such as the snakelocks Anemonia viridis here) and passes their stinging cells through its own digestive system until they reach the tips of the projections on its back. From here, the slug can discharge the stinging cells to ward off fish attack.

 
 
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This beautiful nudibranch sea slug (Coryphella browni) operates a similar system but its prey is the oaten pipe hydroid (Tubularia indivisa). This slug had crawled up the stem of a hydroid and was munching on its tentacles.