Crabs at Port Douglas

 

Fiddler crab by its burrow on a muddy seashore in AustraliaOne of many fiddler type crabs (Uca spp.) found on the low-tide mud at Port Douglas in Queensland, Australia. These small colourful crustaceans with their tall stalked eyes emerge from their burrows as the tide goes out to feast on the surface biofilm of the sediments. The one in the video clip is shielding itself, and protecting its territory, with its large right claw while it daintily scoops up mud and food with its tiny left claw and pops it into its mouth. This specimen has a blue patterned carapace about an inch across (2 cm).

Fiddler crab on a muddy seashore

Sea Slater at Fall Bay

Ligia oceanica Linnaeus

Sea Slater or Bilge Bug on the rocksHigh on the dry rocks at Fall Bay in Gower I spotted this fine creature running across the surface. It has many common names: sea slater, bilge bug, littoral wood louse, for example, but it’s scientific name is Ligia oceanica Linnaeus. With a body length of up to 25 mm it is the largest north-west European oniscid isopod. I picked it up to have a closer look at the wonderful texture and markings on its hard segmented covering. The books say that it is a frequent inhabitant of crevices in the upper rocky shore above high water mark but I have only seen it myself a couple of times in my lifetime of mooching on seashores.

Reference

Hayward, P. J., and Ryland, J. S. (2017) Handbook of the marine fauna of north-west Europe, Second Edition, Oxford University Press, ISBN 978-0-19-954945-0, p 350.

Barrel-mouthed Jellyfish at Rhossili

Rhizostoma octopus Linnaeus

Barrel-mouthed Jellyfish washed ashore on Rhossili Bay at the base of cliffs near Kitchen Corner, 27th August 2017. One of several seen on the beach yesterday, each one looking different in size, colour, and arrangement of the organs. This one was pale blue and the domed umbrella surface had become turned inside out. The main shot was taken at ground level and into the sun to show how the light was shining right through the jelly. The inserted thumbnail image shows the same specimen viewed from above.

On the Rocks at Fall Bay

I am familiar with the commonly occurring horizontal stripes of rocky shore zonation where organisms are distributed between the tide levels according to their tolerance of exposure to air – but I  wonder what influences the distribution and arrangement of different species of seashore creatures to result in the irregular patchwork pattern as found on the intertidal rocks at Fall Bay in Gower. The sloping flat surfaces of the limestone strata can be covered with a complete encrusting layer of mussels, limpets, and barnacles, organised by colour, shape, and size to make a patterned carpet.

Seatown Mud Tracks & Trails

The soft smooth almost liquid muds that flow down the cliffs at Seatown after rain, pool and sink into the shingle on the beach. It doesn’t take long to see amazing networks of tracks and trails on the mud surface. These are made by a myriad of small invertebrate seashore creatures like worms, snails, and sandhoppers as they walk across, burrow, and tunnel into it, foraging for food and seeking shelter from exposure. The number of distinct track marks is amazing and I have no idea which mark was made by which animal (that is a whole new project requiring the collection of some mud samples for identification of the occupants of this habitat). Large bird footprints from crows and gulls show that these areas are also good places for them to feed on the creatures in the mud.

Images can be seen in greater detail by clicking on any photograph to view in the gallery, and then clicking “View full size” below the picture.

Seatown Rock with Piddock Holes

A contributory factor in the erosion of the beach rock at Seatown in Dorset is the burrowing activity of the marine bivalve mollusc called the piddock. Low on the shore millions of holes in the soft calcareous mudstones are evidence for the burrows made by Pholas dactylus. The holes are almost circular in shape reaching up to two centimetres in diameter,  and can occur as a scatter or as dense populations wherever the rock remains wet between the tides. They seem to prefer the darker layers rather than the alternating light layers – although they are found in both. The rock on the east Seatown shoreline is composed of alternating almost horizontal layers of pale (carbonate-rich and carbon-poor) mudstone, and darker (carbon–rich and carbonate-poor) mudstone from the Belemnite Marl Member of the Charmouth Mudstone Formation.

Where successive generations of this boring mollusc have colonised the strata, the mudstone has been reduced to an irregular honeycombed mass. Most of the holes seem unoccupied and small pieces of orange-coloured gravel have filled them. In some burrows the empty white shells of the piddock can still be seen. Some of the burrows are undoubtedly still occupied but I did not have an opportunity to locate any for photographs since the area was only exposed for half an hour. The shells of the living animals may not have been visible because they tend to lie deep within the burrow but the living specimens can often be detected by the fact that their siphons extend from the shell to the surface and these periodically squirt out water during low-tide.

Pebbles and beach stones which have neat circular holes in them are frequently wave-washed and beach-tumbled pieces of rock that have broken away from intertidal rock layers that have been riddled with burrows made by rock-boring molluscs such as piddocks, in the way shown in these photographs from Seatown in Dorset, England, along the World Heritage Jurassic Coast.

“Molluscs in Archaeology” Published Today

Aside

The book Molluscs in Archaeology is published today. Here is a link to the publisher’s blog at Oxbow giving details of the delights in store to prospective purchasers and readers of the book.

The book is still available at pre-publication price from Oxbow Books until end July 2017 at:

http://www.oxbowbooks.com/oxbow/molluscs-in-archaeology.html