This is not a title I’d every imagined myself writing. Slugs in the garden are brown and squidgy, and we respond with a shudder and an “Errr, yuck!”.
But sea slugs are a different group of beasties altogether. As you can see from the pictures on this page, they’re not the boring brown critters we find eating the lettuce. And one species has a surprise that I’ll save for the end of this post.
The sea slug above, Flabellina iodinea or Spanish shawl, is brilliantly coloured partly as camouflage (yes, camouflage – the bright orange blends in with the colour of food that they sit on while eating) and partly to warn off predators – those pretty red appendages, used to extract oxygen from sea water, are also used to store stinging cells from the anemones they eat.
Then there’s Glaucus atlanticus. This is another stunning, although small, sea slug, and it too stores stinging cells from its food, which includes the Portugese man o’ war. It, too, looks quite distinctive, again to warn potential predators that it’s likely to be more trouble than it’s worth.
I do find myself wondering about a couple of things. Firstly, how they manage to ingest the stinging cells without triggering them, and are then able to store and use these cells. And secondly, did the ability to do this evolve concurrently with the distinctive appearance, or did one precede the other? If slugs that store stinging cells looked just like any others, there would be no evolutionary advantage to them, because predators wouldn’t be able to tell the difference, and if they had the appearance but not the stinging ability, predators wouldn’t be dissuaded from eating them.
You can find a great deal more information about these fascinating slugs (and others) from this post on Ecologica. Look for the Elysiella pusilla, a slug that can take chloroplast cells from the algae that it feeds on and incorporate them into its own body, which not only turns it green but also allows it to photosynthesise. In this case it is easy being green.
But the big surprise is a species called Chromodoris reticulata. This species, like many nudibranchs, is simultaneously hermaphroditic: that means that the creatures mate in pairs using both male and female parts at the same time.
Chromodoris reticulata goes further than other nudibranchs, however, by detaching its penis after mating. Now it’s not the only species that does this: orb-weaving spiders, periwinkles, and some land slugs also lose their male part after mating. The big surprise with this one is that it can grow another.
Japanese scientists from the Osaka City University and Tokyo’s Nihon University discovered that this species has a coiled spiral inside the body that rapidly uncoils after mating and grows into a new penis. Think of it like a propelling pencil or a Stanley knife (I believe American readers call these ‘boxcutters’): a few hours to add some finishing touches and it’s ready to go again.
I don’t know about you, but I can’t help being reminded of this song (warning: NSFW).