The Hair Curler Sea Slug (Spurilla neapolitana) Photo by Carlos Fernandez-Cid at https://www.flickr.com/photos/78557484@N02/page5

Dear Readers, I am a long-term fan of sea slugs (known as nudibranchs by those of us with a scientific bent) – they are spectacularly coloured, and they eat sea anemones – as if being eaten wasn’t bad enough, the sea slugs actually harvest the stinging cells from their prey to grow their own ‘armoury’. The Hair Curler Sea Slug (Spurilla neapolitana) is in the news because it has been spotted in the waters off of Cornwall, whereas it usually lives in the Caribbean and Mediterranean. The fact that it can survive here probably relates to the warming of the oceans, sadly, but let’s at least acknowledge that this is a very fine mollusc indeed, though probably not quite as fine as some of its relatives:

Berghia coerulescens (Photo by By Géry PARENT – Own work, cropped, Public Domain, https://commons.wikimedia.org/w/index.php?curid=8647592)

Chromodoris lochi from Dungon Wall divesite, Puerto Galera, the Philippines. There is some colour variation among this species. Photo by By Alexander R. Jenner – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8699877

Blue Dragon (Glaucus atlanticus) Photo  By Sylke Rohrlach from Sydney – Blue dragon-glaucus atlanticus, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=39934058

These very attractive animals are not very closely related to land slugs – for a start they are exclusively carnivorous (and while a few land slugs also eat other animals, mostly they just munch through leaves, mostly dead ones). Sea slugs eat sponges, corals, sea anemones, barnacles and even other sea slugs. However the Blue Dragon (pictured above) is a specialist predator of Portuguese Man O’War jellyfish, nibbling off the stinging tentacles and absorbing the nematocysts (the stingy bits) into their own skin, making them quite the proposition.

Unlikely as it seems, in 1884 a scientist observed that the sea slugs that he was keeping in a tank emitted a sound that is like:

‘the clink of a steel wire on the side of the jar, one stroke only been given at a time, and repeated at intervals of a minute or two; when placed in a large basin of water, the sound is much obscured and is like that of a watch, one stroke being repeated, as before, at intervals.’

The scientist, one Professor Grant, speculated that the sounds could indicate communication between the sea slugs: he maintained that the sounds could be heard at a distance of twelve feet from the tank. Goodness! I cannot find any evidence that anyone else has ever heard the song of the sea slug, but that doesn’t mean that Professor Grant was wrong.

Hermissenda crassicornis Photo By Brocken Inaglory – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2202919

I would speculate that the bright colours of many sea slugs advertise to other creatures that although they are soft-bodied and slow-moving, they are not to be messed with. They have a few other tricks up their sleeves too: some of them can absorb the photosynthetic cells from algae and use them so that they can photosynthesise themselves, while others will sever chunks of their bodies if these become infested with parasites. Some sea slugs can grow an entire new body from just their head if decapitated, so the loss of a mere tentacle is as nothing. What amazing animals sea slugs are! It’s almost worth taking my diving qualification just to see some.

Goniobranchus kuniei (Photo by By Heavydpj – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=98478518)