Promoting Intertidal Sciences, Conservation & Education Studies | blog | 15th July | Dr Mike Kent

Last Wednesday I searched for one of my favourite animals, the Celtic sea-slug Onchidella celtica, on the rocks below Tristram car park. It is not outwardly the best looking beast on the planet. It would not win any sea-slug beauty competition, even in Polzeath. But don’t be fooled by its outward appearance; or, as the saying goes: “Don’t judge a book by its cover.”

            Onchidella is one of the most interesting and oddest animals on our surf-swept shores. Its presence, like the punch-line to a laugh-aloud joke, is totally unexpected. On rocks bombarded by Atlantic rollers, you expect to find a tough and tenacious snail, not a slug with little body protection and with powers of adhesion so poor that it can be blown off by a strong puff of breath.

Instead of resisting the elements, Onchidella avoids them by spending most of its life hiding in a crevice. Equipped with a lung, Onchidella can breathe air enabling it to come out to feed when the tide is low. Its diet consists mainly of seaweeds, such as sea lettuce, and small organisms that form a biofilm on rocks and shells.

Its foraging is highly variable, both in distance and duration. However, it has an uncanny ability to find its way home even when travelling relatively long distances along meandering outward and homeward paths that do not overlap. No one knows exactly how it does this, making its behaviour an exciting and challenging topic to study.

Onchidella is a warm-water species found mainly south of Portugal. In the UK, it is found only on some shores in Cornwall and Devon. Its limited geographical distribution combined with its tendency to spend most of its life hidden in rock crevices, means that very few people, even specialists in snails and slugs, have ever seen living Onchidella in Britain. Like silver, gold and fine pearls, rarity makes Onchidella well worth searching for.

Polzeath is one of the places where Onchidella occurs in abundance. On Wednesday conditions seemed ideal, but after 30 minutes of searching I found none. Then Ruth Seadon, on holiday with her husband Philip, approached me curious about what I was doing. After telling Ruth about Onchidella and how difficult it is to find, she was keen to help me search for the elusive creature. I was delighted when we found one, and even more delighted when I saw Ruth smile when she took a close look at this interesting little sea-slug. I’ve seen others respond in a similar way. I’ve even heard Onchidella celtica referred to as cute. This just shows how important it is to look closely at something to fully appreciate it.

Once again searching, but this time for a suitable scriptural text to end this blog, I found the following pearl of wisdom:

The Lord does not look at the things man looks at.

Man looks at the outward appearance,

but the Lord looks at the heart.

1 Samuel 16:7