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.

Beachcombing 3

Rough weather in the Humber estuary a couple of weeks ago scoured a lot of interesting marine life from the seabed and cast it up on the shore. Last week, during a walk along the strand line Cleethorpes beach we found ....

.... numerous cuttlefish' bones' from common cuttlefish. These have been washed ashore in numbers all along the North East coast lately. I've found them on Warkworth beach in Northumberland recently and they've also been reported from the Yorkshire coast at Runswick bay.

There were plenty of the familiar common cockles, with the two valves of this one still joined by their ligament....

....... and some fine oyster shells.

I think this spindle-shaped whelk is the red whelk Neptunea antiqua rather than the common whelk Buccinum undatum.

An edible crab carapace

A cluster of slipper limpets Crepidula fornicata - you can read more about their bizarre sex life by clicking here

Common whelks Buccinum undatum - the lower image shows the operculum attached the the muscular foot, that seals the shell entrance when the foot is retracted.

Common starfish Asterias rubens, in an advanced state of decay

This strange object, again in an advanced state of decay, is the underside of a sun star, probably Crossaster papposus .....

... and these are the eggs of a common whelk, together with the bryozoan Flustra foliacea, also known as hornwrack (click here to see examples of living bryozoans)

But perhaps the most interesting stranding was this necklace shell, Polinices catenus, with its distinctive hollow spire ( known as an umbilicus) visible on the underside. The snail that lives in the necklace shell has a broad foot and uses it to plough through soft substrates, exposing buried bivalve molluscs whose shells it drills with its abrasive tongue. Its bivalve victims have a tell-tale, neat round hole in one of their valves..........

... like this banded wedge shell Donax vittatus



For more beachcombing, click here, here and here.

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.

Beachcombing

There are few more pleasant ways to idle away an afternoon than to indulge in a spot of beachcombing, discovering what the falling tide has left behind, and when we arrived at Warkworth beach last Saturday lunchtime the tide had just turned, leaving a trail of marine artefacts, like the crimped pie-crust carapace of this edible crab .....

... and the saw-toothed carapace of a shore crab.

Recent rough seas had scoured the sand and cast up thousands of these sand mason worms, with their tubes constructed from grains of sand ....

... but this dog whelk, with its tough, thick shell may have been rolling around in the surf for weeks.

This is a mollusc that doesn't belong on the shore at all - it's a grove or brown-lipped snail Cepaea nemoralis, which is an air-breathing species that lives amongst the marram grass on the sand dunes behind the beach. Maybe a bird caught it and dropped it on the beach, or it might have been dislodged when a spring tide eroded the edge of the dune. This species comes in a variety of background colours and banding patterns, but many on the dunes here are either pure yellow or sometimes pink.

 

This is the delicate mauve interior of the rayed trough shell, which is decorated with markings ....

..... like the rays of the setting sun on its outer surface.

Banded wedge shells come in a range of colours, including brown and purple, but the pure yellow ones are particularly attractive.  

The star find was this tower or auger shell, Turritella communis. It's common enough around much of Britain, especially on the west coast, but this is the first time that I've found it here in thirty years of walking along this beach.

And finally, a modern archaeological curiosity - a clay pipe. I wonder who lost this, and how long ago - maybe a fisherman who dropped it overboard? Whoever it belonged to, it must have remained buried in the sand until a convergence of rough weather and high tide scoured it out and left it on the tide line.

For more beachcombing on Warkworth beach, visit

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