I must go down to the sea again, to the lonely sea and sky ....

I must go down to the sea again,
to the lonely sea and sky,
I left my shoes and socks there,
I wonder if they're dry

Spike Milligan

Well, I did go down to the sea again today but kept my shoes and socks on, because it was hellish cold - but it was a great way to spend a Saturday.

After a browse around Tynemouth Market, we walked out onto Tynemouth piers ....

.... where we encountered this very handsome pigeon, with beautiful orange eyes and iridescent throat plumage, before we headed along the path to North Shields for fish and chips in the excellent Waterfront fish and chip restaurant ...

... passing these busy purple sandpipers that were feeding along the sea wall while they waited for the tide to turn.

After that we had a quick stroll alone Union Quay where this black-headed gull and ...

..... herring gull posed for portraits

As we walked back to the car park someone threw chips to the herring gulls, so predictably all hell broke loose as they scrambled for them. A  cacophony of herring gulls.

Then it was off along the coat to St.Mary's island, where the sun came out.

Lots of these beautifully patterned grey top shells Gibbula cineraria under the rocks around the rock pools...

 .... and even more of these hermit crabs to tease, by turning them upside down ...

..... and watching their contortions as they were forced to come out of  their shells to right themselves.

One very disgruntled hermit crab.

A spot of rock-pooling

Mid-day low tide so went rock-pooling today at Whitburn rocks, Sunderland. It's not exactly the Great Barrier Reef but you can usually find something interesting.

Who can resist turning over rocks on the seashore? This shanny (?) Lipophrys pholis was under one in the middle shore and wriggled out of the water when disturbed.

Lots of young edible crabs, that adopt this characteristic pose when you pick them up by their crimped-pie-crust carapaces

This one was tiny, smaller than my thumbnail, and judging by its colour had recently moulted.

The rock pools were full of hermit crabs, with their jerky walk that makes them look like automata.

I think this is a very young shore crab, half-hidden in the wet sand under a rock, and .....

....... this is a larger one, very disgruntled and in defensive pose.

This was probably the most interesting find - a grey topshell that had recently spawned - you can see the developing embryos in the jelly, from which they'll hatch and enjoy a brief period as planktonic larvae.

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.