For the love of sea slugs…

I love sea slugs a bit more than is probably usual. My other half even made sure I have the t-shirt, which I wear with pride in the Cornish rock pools despite the odd looks it gets me.

Trend setting in my waterlogged wellies and 'I love sea slugs' t-shirt.
Trend setting in my waterlogged wellies and ‘I love sea slugs’ t-shirt. Hannafore beach, Looe.

If you don’t already have your own t-shirt, it might be that you haven’t yet met these amazing little creatures. Unlike land slugs, sea slugs come in a mind-boggling variety of colours and shapes and have cool super-powers.

So, this week I’ve been braving the traditional British summer-holiday weather to find top slugs to convert you to the cause. My lucky t-shirt worked its wonders…

If you follow my blog, you might remember my excitement earlier in the year at finding the rare Calma gobioophaga sea slug, which feeds on the eggs of gobies (fish).

It’s probably fair to say that a woman in an ‘I love sea slugs’ t-shirt has never been so excited as I was when I turned a stone this week in Looe, to find three sea slugs with some clingfish eggs. This was another new species to me – the equally rare Calma glaucoides.

Calma glaucoides and its eggs. See how pretty sea slugs are!
Calma glaucoides and its eggs. See how pretty sea slugs are!

This slug has striking long wavy cerrata (tentacles) down both sides of its back, with blueish glands running through them. The pattern of white circles on its back (part of its reproductive organs) help it to blend in among the white scribble-patterns of its egg strands.

The clingfish eggs on which Calma glaucoides feeds - there weren't many left under this rock.
The clingfish eggs on which Calma glaucoides feeds – there weren’t many left under this rock.
Calma glaucoides sea slugs
Calma glaucoides sea slugs – I have no idea what the collective noun would be.

Most sea slugs are ruthless carnivores, but not the Elysia viridis, which feeds on codium seaweed. For some reason up until now, I’ve tended to find blandly coloured brown specimens which don’t show their magic trick very well.

This week, I’m delighted to bring you an Elysia viridis in its finest regalia of green with vivid blue-green spots.

The photosynthesising sea slug - Elysia viridis, grazing on Codium tomentosum seaweed.
The photosynthesising sea slug – Elysia viridis, grazing on Codium tomentosum seaweed.
Elysia viridis showng the green colours and turquoise spots of the chromatophores it stores
Elysia viridis showng the green colours and turquoise spots of the chromatophores it stores

These colours contain chromatophores from the seaweed that the slug has eaten. In other words, it manages to retain the photosynthesising cells from plants and store them in their bodies where they continue to produce energy for the slug.

I almost popped with excitement when, after finding so many fabulous slugs, yet another one turned up during a rock pool event with Looe Marine Conservation Group. Rummaging on the lower shore for exciting crabs, starfish or squat lobsters to show the kids, I spotted some eggs belonging to a slug, the Sea lemon.

Next to a big clump of Sea lemon eggs (left) is a much smaller spiral of eggs
Next to a big clump of Sea lemon eggs (left) is a much smaller spiral of eggs

This egg spiral, in itself, was fairly commonplace but in its midst and alongside it, I noticed two more tiny spirals of spawn laid by something else that I didn’t recognise. After another minute of staring at the rock I spotted the slug.

A pale-coloured Favorinus branchialis sea slug
A pale-coloured Favorinus branchialis sea slug

This slug, the Favorinus branchialis, has two pretty groups of red-ish cerata along its back and a distinctive bulge in its rhinophores (the tentacles on top of its head). It is variable in colour and it took me another minute to spot the second slug, which was much paler, but with the same body form.

The darker of the two Favorinus branchialis sea slugs, showing the bulge in its rhinophores (the antennae on top of its head)
The darker of the two Favorinus branchialis sea slugs, showing the bulge in its rhinophores (the antennae on top of its head)
Favorinus branchialis swimming upside down because it can
Favorinus branchialis swimming upside down because it can

Cute and pretty though it is, this slug has a rather gruesome habit; it likes feeding on the eggs of other slugs. It was no accident that these slugs had laid their eggs right next to those of the sea lemon.

A slug with some serious weaponry turned up on my next foray into the pools.

Berthella plumula sea slug
Berthella plumula sea slug

The Berthella plumula slug (or Feathered Bertha as I call it), resembles a sherbet lemon, but is far more squidgy. I’d never advise actually squidging one, mostly because it’d be cruel, but also because they can secrete sulphuric acid when attacked.

This sea slug has a feathery gill on its underside - unlike most slugs which have their gills in a tuft on their back or in their cerata.
The Berthella plumula sea slug has a feathery gill on its underside – unlike most slugs which have their gills in a tuft on their back or in their cerata.

Interesting defences are common in sea slugs. Many aeolid sea slugs store the stinging cells from the food they’ve eaten, such as anemones or hydroids. If they’re attacked, the stinging cells fire from the tips of the cerata on their backs into their prey. Sea hares, on the other hand, have a similar defence system to squid and cuttlefish, squirting a cloud of purple ink out their bottoms to ward off predators.

Last week, my lucky t-shirt did me proud, so I’m looking forward to getting it on again to explore the shore on next week’s big spring tides. If you’d like to meet these cool creatures for yourself, there are rock pooling events taking place all round Cornwall through the rest of the summer holidays.

Here's your bonus slug for reading to the end - a Rostanga rubra which eats red sponges and is perfectly camouflaged on them.
Here’s your bonus slug for reading to the end – a Rostanga rubra which eats red sponges and is perfectly camouflaged on them.
A charming view of a Favorinus branchialis's bottom. You're welcome!
A charming view of a Favorinus branchialis’s bottom.
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5 thoughts on “For the love of sea slugs…”

  1. Wow, what a fantastic collection.

    I am taking my daughter on the rocpooling event in Polzeath on Tuesday, thanks to your earlier post. It seems to be extreme rockpooling…I hope we are up to it.

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      1. It was fabulous – really great weather, fabulous staff running the event all with the added bonus of sharing the afternoon with Tony Adams (ex Arsenal and England football captain and personal hero of mine), who was taking his kids on the same trip.

        I was a little rusty, but we found lots of crab species, plenty of blennies and gobies, all sorts of sponges and my favourite blue rayed limpet…dozens and dozens of them. A very happy day for me and my daughter really enjoyed it too.

        Liked by 1 person

  2. Perfect! I’m so pleased to hear it went well. The ‘extreme’ rockpooling events are a fantastic idea and loads of fun. Very cool that you got to meet Tony Adams and lovely to see so many blue-rayed limpets; they’re a favourite of mine and they become particularly abundant this time of year. 🙂

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