Armchair rockpooling

Felt a sudden urge to do a bit of rock-pooling at the weekend so called in at Marsden bay near South Shields, to see it there was anything interesting on the shore. This is an exposed coast and it's early in the season so were weren't expecting to find much.

There were plenty of common limpets on top of the rocks and ...

..... lots of chitons aka polyplacophorans aka coat-of-mail shells underneath rocks. But not a lot else. 

It was freezing cold and blowing a gale, which dimmed my enthusiasm when my hands began to turn blue. So I collected a small tuft of red seaweed that I found growing in a rock crevice and carried it home in a vial of seawater (about an egg cup-full) for a closer look under the microscope, in the comfort of an armchair.

Spring comes to the coastal waters just like anywhere else, but it's not immediately obvious until you look really closely. As the spring plankton bloom progresses numerous small animals settle out of the plankton onto the shore, especially amongst the fronds of small red seaweeds that begin to grow now.

To go rockpooling in comfort, all you need is one of these - a binocular microscope with a zoom range between x7 and x45. It costs about as much as an entry level digital SLR camera and can provide hours of amusement.

Almost immediately I found these attached to the seaweed - tiny hydroids, waving their tentacles in the air, until the slightest tremor .....

... sent them into contractions

Then there were these minute mussels, translucent but beginning to acquire their darker pigment at their base, that were anchored to the seaweed after the mussel larvae had settled out of the plankton.

This little animal is an isopod crustacean, hard to spot amongst the seaweed fronds, but best of all there were ...

... loads of these little gammarid shrimps, beautifully pigmented to match the seaweed forest.

Their compound eyes are particularly striking, as are the highly reflected patches embedded in their exoskeleton

This tiny seaweed microcosm was home to about 20 of these gammarids, some of which had highly reflective, sparkly eyes. 

The exoskeleton is this one contained small reflective patches that looked like interior lighting and it had eyes that glowed.

Aside from the mussels, gammarids, hydroids and isopods I also saw a minute sea slug and a flatworm, as well as copepods. 

The photos aren't very good (they were taken by holding a pocket compact camera up against the eyepiece of the microscope) but they do illustrate the wealth of microscopic marine life that's present even on exposed shores, down at the bottom of the inshore marine food chain. And at this time of year it's the most comfortable way to go rock-pooling.

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.