Beach and Dune - Gower - page 135

Sandybeachesarethehabitatthathasbeenmostsuccessfullyexploitedbythebivalvemolluscs.They
burrow into the sand for protection by means of an extendable foot, maintaining contact with the water
above with siphons that extend from the posterior of the animal. Some bivalves burrow only deep enough
to cover the shell and have short siphons, others burrow very deeply and have long siphons, while the
razor shells descend deep into the sand for protection and can move with ease up and down. There are
three types of deep-burrowing bivalve molluscs, the tellenid bivalves, which have separate and very long
siphons and a large foot, the razor shells, which have short fused siphons and a very large foot, and gaper
bivalves, which have long fused siphons and a very small foot.
Tellenid bivalves found in the Gower sands include thin tellin Tellina tenuis and the similar T.fabula .
Razor shells are widespread and include the razors Ensis ensis and E. arcuatuis and the pod razor shell
E. siliqua . In slightly muddy areas the common razor shell Solen marginatus occurs instead. Unlike the
other razor shells this slowly burrowing animal has a series of constrictions around its siphon, which al-
low it to break off small pieces, perhaps to distract a predatory bird or fish, as it gradually retreats into its
burrow. The gaper bivalves include common otter shell Lutraria lutraria , which is one of the shells most
frequently found on the Gower beaches, but due to its fragility and size by the time it is found it is often
damaged.Otherbivalvemolluscs,suchascommoncockle Cerastoderma edule ,bandedwedgeshell Don-
ax vittatus , striped venus Venus striatula , thick trough shell Spisula solida and rayed trough shell Mactra
corallina , are clearly not adapted for burrowing in sand and only create superficial burrows.
Where stones are mixed with the sand the burrowing anemones, the water anemone Sagartiogeton
undata and daisy anemone Cereus pedunculatus , often occur attached to a fragment of shell or stone bur-
ied slightly below the surface. The common heart urchin or sea potato Echinocardium cordatum is also
abundant, and the near-white fragile limy skeleton, or test, of this species can often be found along the
strandline on Rhossili beach and Whiteford Sands. It is associated with the commensal bivalve Tellimya
ferruginosa , which lives amongst its spines, and the amphipod Urthoe marina , which uses its burrows.
Asmany asfourteen bivalves have been recorded with asingle heart urchin, the adults living freely inthe
burrow while the young are attached to the urchin's spines by byssus threads. Other echinoderms found
on the beaches include the brittlestars Acrocnida brachiata, Ophiura albida and O. texturata .
Sand-burrowing worms include sea worm Glycera convoluta, Notomastis rubicundus, Phyllodoce
maculata , the bristle worm Scoloplos armiger (which can reach lengths of 12 centimetres with 200 or
morebodysegments),thecatworm Nephtyshombergi andoccasionallytheseamouse Aphroditeaculeata ,
which is usually found near the extreme low water mark. Burrowing crustaceans on the Gower beaches
include various species of sandhopper Bathyporeia spp., the isopods Eurydice pulchra and Eurydice af-
finis (relatives of the woodlice that hide in the sand at low water, emerging to feed on plankton as the tide
comes in), the masked crab and the south-claw hermit crab Diogenes pugilator (Fig. 72).
This hermit crab is a small animal that is only found in a limited number of places in Britain and Ire-
land, as it is primarily a warm-water species. As such it may well be worth further study in relation to
climate change. There are already signs that the crab is extending its range, as it was found for the first
timeonthewestcoastofIrelandinthe1990s.Thespeciescaneasilyberecognisedinthefieldandduring
the summer is simple to find. It is distinguished from the common hermit crab Pagurus bernhardus in
that the left claw is the largest one, not the right. The crab was first recorded in Britain at 'Worms Head'
fromspecimenssuppliedbyDillwyntoSpenceBatearound1850anddescribedbyhiminthe Annals and
Magazine of Natural History . It was originally named Pagurus dilwynii after its discoverer by Thomas
Bell, President of the Linnean Society, who had not actually seen it himself. By the time it was included
in Bell's British Stalk-eyed Crustacea (1853) it had also been found in Cornwall. In his topic Bell notes,
'The name applied to this species is one long-known to science, and honoured as the stimulator of natural
history in this locality in the person of L. W. Dillwyn, Esq., Sketty Hall.'