If you read this blog regularly, you’ve probably noticed a pattern: I bimble about the Cornish rock pools looking for an exciting creature, fail completely, then find something unexpected. Well, hopefully you like the format because this week is no exception. I go on a quest to find fish eggs and discover this rare sea slug.

Fish eggs are amazing. If you catch them just as they’re nearing hatching you can see each baby fish staring out, its tail curled tight around its head like a scarf.

So, when Junior announces he wants to go for a big walk, I suggest Port Nadler. This slightly exposed rocky bay is ideal for Cornish clingfish. Their distinctive yellow eggs usually carpet the underside of the rocks and their developing babies are especially beautiful.

Only the tide today isn’t low enough to access the clingfishes’ favourite gully.

I look in the pools and lose count of how many rocks I lift. There don’t seem to be any fish eggs. As the tide drops a little further, I come across some Berthella plumula sea slugs and a sea hare.

There’s an anemone I don’t recognise. Translucent white all over with a base so wide it looks like a bowl. I later realise it must be the white form of Sagartia elegans (var. nivea).

Junior finishes his digging in the sand at the top of the beach and wanders over to join me. We lift a rock together and finally here are some eggs. They’re not the yellow clingfish eggs I was looking for, they’re smaller goby eggs, forming black-specked carpet of grey. Under the camera, the specks become a sea of eyes looking up at me.

I remember rockpool expert David Fenwick, who runs the fabulous Aphotomarine site, telling me a year or two back that there was a species of sea slug that specialises in eating these eggs. I peer into the greyness and see nothing, apart from a thin yellowish patch in the centre which looks like a piece of sponge or sea squirt.

I look some more. The eggs around the edge of the yellow patch look longer than the others.

I stare, stare some more then focus my camera on the patch and do yet more staring. Even then I’m not sure, but it could be…

It’s only when I see a tentacle move that I begin to see the slug properly. It’s over a centimetre long, but most of its body is covered in pointed cerrata the colour and shape of goby eggs, right down to the black dots that ressemble eyes.

This is the weirdly named Calma gobioophaga egg-eating sea slug. It’s the first I’ve seen and only the second record of this species in Cornwall.

The yellowish patch I thought was a sea squirt is the slug’s back. It’s covered in pale circles, which my books tell me afterwards are the mature gonads. Who’d have thought?

The books also tells me that the slug absorbs the fish eggs so well into its gut that it has no need of an anus. That’s right; it doesn’t poo. I’m not sure how that works, but it’s the sort of fact that gets Junior’s attention.

It’s not the easiest thing to photograph: a grey blob on a mat of grey eggs on a grey day in silty water. As I start to get my eye in to the outline of the slug, it glides towards me, feeling the eggs with its tentacles and swinging its long rhinophores forwards.

Tucked immediately behind each rhinophore is a distinct black eye, one of the characteristic features of this species.

The books suggest this species only eats the eggs of the black goby (Gobius niger), but slugs are not great readers and the other records from Cornwall and Brittany are, like this one, probably on rock goby (Gobius paganellus) eggs. In the Mediterranean this species has also been recorded on giant goby (Gobius cobitis) eggs. There’s another, closely related, species of sea slug, Calma glaucoides, that feeds on a wider range of eggs, including clingfish eggs and has been recorded in Cornwall too. Hopefully I’ll find that one soon!

The slug’s life cycle intigues me. As fish eggs are available for such a short period of the year, I’m not sure what happens to these slugs during the remaining months.

It comes on to rain heavily and as we’re already soaked from exploring the rock pools, we call it a day. I haven’t found a single clingfish egg, but, as is the way with rockpooling, I’ve discovered something even better.