I Must Go Down to the Sea Again....

Today's Guardian Country Diary is an account of a little experiment in providing a new home for a hermit crab, which I've conducted many times before in aquarium tanks but not so often in a rock pool on the seashore.

Every now and then I get an irresistible urge to do a bit of rock-pooling on the seashore. Does anyone every grow out of the childhood compulsion to turn over a few rocks and see what lurks underneath? On this occasion we visited Whitburn Rocks, just north of Sunderland, on a lowish tide where the sea had retreated as far as the kelp beds - but I was so engrossed that I never got all the way down to the tide line, where the most interesting stuff lives, before the tide turned. 

This isn't the most biodiverse location but on this occasion the most striking feature was the sheer number of hermit crabs Pagurus bernhardus in the mid-shore rock pools. Even the smaller, shallower ones were home to dozens, sidling up to one another with their jerky, mechanical walk.

I supplied this one with this new, larger periwinkle shell to move into, which it immediately defended with this aggressive stance and menacing claw. Hermit crabs' growth is limited by the size of available empty gastropod shells and at Whitburn the vast majority of shells are periwinkles of similar size. Larger homes are hard to find. The ultimate prize - the hermit crab equivalent of winning the lottery and buying a mansion - is a whelk shell but they are rare on this part of the coast. 

Some of the most interesting research ever done on hermit crabs involved the use of replica glass shells, that the crabs would occupy and which allowed biologists to watch the intimate details of their lives inside the shell. Ian Lancaster, at Penwith Sixth Form College at Penzance in Cornwall used this technique and about 20 years ago he also published an excellent paper in Field Studies on hermit crab biology, which you can download free if you click here. You can watch a video of a hermit crab in a glass shell by clicking here.

Incidentally, if there's nothing else available hermit crabs will occupy homes made from Lego bricks - click here for video.

There were also plenty of chitons - primitive molluscs with eight articulated shell plates that give them the alternative name of coat-of-mail shells - like this one under the rocks, which I think is Leptochiton asellus. Their shell plates contain light-sensitive nerve endings called aesthetes, which are the earliest precursors of eyes and although that can't form images they can tell the chiton whether it's exposed on top of a rock or safe underneath it - invaluable information if there are hungry gulls about. I think it was Richard Dawkins who commented that, in relation to the evolution of eyes, 'half an eye is better than none'. The evolution of aesthetes in chiton shells was the first step in the evolution of mollusc eyes that culminated in the eyes of complex camera-type in squid, octopus and cuttlefish that are in some respects superior to those in humans. 

There are two other organisms in this picture: the keel worm Pomatoceros lamarcki, that has secreted its triangular-in-cross-section calcareous shell on one of the chiton's shell plates (click here for info for more seashore worms that live in tubes); and an acorn barnacle which looks like it might be Semibalanus balanoides.

The edges of these pools on the middle shore are fringed with forested of the red seaweed Corallina officinalis, whose gritty fronds are encrusted with calcium deposits, and it's in these swaying forests that much of the most interesting animal diversity lives - but much of it is microscopic. You can see some examples by clicking here, here and here. 

The sides of the pool were also home to beadlet anemones Actinia equina, ready to catch a paralyse prey with their stinging tentacles. If you poke their tentacles with your finger, thanks to the tiny, barbed nematocysts (also known as cnidocytes - for details click here) whose barbs catch in the surface layer of human sking but can't penetrate it, unlike the jellyfish nematocysts in this video that can.

I think these are the eggs of dog whelks and you can see from the ragged tops that most of the juveniles have chewed their way out, but the two in front seem to be intact.

There aren't many soft sediments for polychaete worms to burrow into on this part of the shore but there are several species that are flattened and live under rocks. I haven't identified this one yet but it looks like a scaleworm .

Aside from winkles and dog whelks, the most numerous gastropod molluscs are these grey top shells Gibbula cinerea, with their distinctive striped pattern. When that's abraded the underlying iridescent mother-of-pearl layer is exposed. 

..... and finally, a souvenir of the seashore to take home - the carapace of an edible crab Cancer pagurus.

Flower Animal

Sea anemones belong to a class of animals known to science as the Anthozoa – a word that literally means ‘flower animal’, and these beadlet anemones (Actinia equina), photographed on the beach at Warkworth in Northumberland, are typically flower-like. There’s some debate about whether the green individuals like the one in the photo below are just a colour form or a separate species. On the upper shore the purple form is by far the commonest.

The ring of tentacles encircling the mouth are equipped with barbed stinging cells called nematocysts, triggered when prey brushes against them, that inject paralysing poison into their victim, which is then manoeuvred through the mouth and into the stomach by the tentacles. The nematocysts of British sea anemones are too small to penetrate human flesh but you can feel the barbs. Poke you finger into a beadlet anemone and it feels sticky – the 'stickiness' is due to the microscopic barbs hooking onto your skin. Incidentally, these pictures were taken with the little Pentax W20 pocket camera, that I use for most of the pictures on this blog, fully submerged in the rockpool; so far the manufacturer’s claim that it’s waterproof has proved to be sound. It’s ideal for taking pictures of animals in rockpools, avoiding the problems of reflections that are inevitable when photographing through the surface.

One of the finest books on sea anemone’s was Philip Henry Gosse’s Actinologia Britannica; A History of the British Sea-Anemones and Corals, published in 1860 and illustrated with beautiful hand-coloured plates showing all the then-known British species. A copy of this would now cost you somewhere between £250 and £600, depending on condition, but you can read the book for nothing on the web (or even download a pdf of the whole book) at http://www.archive.org/details/actinologiabrita00goss
For some more up-to-date information on beadlet anemone, take a look at http://www.marlin.ac.uk/speciesinformation.php?speciesID=2359