The Shore Below the New Sea Wall (Part 2)

The rock ledges below the new sea wall at Church Cliffs in Lyme Regis, Dorset, are the upper strata of the Blue Lias limestone. The natural limestone ledges and the smooth artificial substrate of the concrete sea wall,  provide a home for numerous seashore creatures like limpets, winkles, and top shells as well as many commonly attached red, green and brown seaweeds, and encrusting calcareous algae (pink Lithamnion). The brown substance sticking to a lot of limpet shells is also an encrusting seaweed (probably Brown Limpet Paint Ralfsia verrucosa).

Multitudes of small holes penetrating the rocks are the often-occupied burrows of small marine polychaete worms like the Polydora species. Occasional drifts of sandy ripples coating the stone are punctured by largish round holes where bivalved burrowing piddocks living in the rocks below have squirted jets of water from their exhalent siphons and cleared the sand away.

It is interesting to see that the seashore life is equally at home on the old concrete footings from the defunct breakwaters as it is on the limestone.

The Shore Below the New Sea Wall (Part 1)

The shore beneath the new sea wall in Lyme Regis looks very different now the old breakwaters or groynes have been removed. You can still see the linear concrete footings of the old wood and iron structures but most of the pebbles and cobbles that used to accumulate between the walls are now absent (at least for the moment). A bare rock pavement is revealed, comprising a series of steps representing the strata and colonised by seaweeds. Rippled sand sometimes deposits in the valleys between the rock ledges.

More Sand Pellets from Crab Feeding Activities

The topography of the shore near Cape Tribulation varies. At one point it is semi-vegetated with salt-tolerant shrubs. Here the substrate holds water for longer at low tide and the root systems of the plants trap finer sediments and nutrients that are washed down the beach from the nearby forest.

The different composition and consistency of the beach sediments in this place has given rise to a variation on the theme of sand ball patterns made by feeding crabs. The crabs may be the same species or a related species to Scopimera inflata which created the scatters and radiating patterns of drying-out pellets further along the beach (see the previous post). In this location, however, the enriched sand was relatively wet, which meant that the pellets coalesced into discrete linear masses that made very interesting designs on the surface of the beach.

Sand Bubbler Crab Pellets at Cape Tribulation

Small ball-shaped sand pellets produced by the feeding activities of the "Sand Bubbler" Crab

On many sandy beaches along the Queensland coast of eastern Australia there are millions of small sandy balls. Each one is just a few millimetres across. They can form extensive mats or patterns as they cover the shore at low tide. There were many decorating the sand at Cape Tribulation when I visited in 2011. Closer inspection reveals small holes in the sand, usually at the centre of radiating lines of balls. These balls are the result of the feeding activities of small sandy coloured crabs that are themselves rarely seen. The holes are their burrows. They are the “Sand Bubbler” crabs belonging to the genus Scopimera, often S. inflata. The balls are created as the crab feeds on the organic matter attached to the sand grains on the surface around its burrow, and rolls up the cleaned grains into balls before moving on to the next spot.

The patterns vary according to the moisture level of the sand and its organic content, as well as the species concerned, I would imagine. I’ll post some more pictures later showing an interesting variation on the sand ball patterns from the other end of Cape Tribulation beach.

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.