The Surrounding Sea - Gower - page 113

Offshore the Helwick Bank, a long, shallow subtidal sandbank, is unusual in being very exposed to wave
and tidal action. The only published studies relating to the subtidal area to date are those carried out by
the Field Studies Council (Hiscock, 1979), when 21 sites were dived between Worms Head and Mumbles
Head, and a 1995 survey carried out as part of the Seasearch Project, which involves volunteer divers
recording information about the seabed and associated marine life (Bunker & Hart, 2002). There have
been other limited studies of Carmarthen Bay and a fuller survey of the area was carried out in 1998
as part of a South West Wales Survey. This study will be published as part of the National Museum of
Wales BIOMÔR reports series (Mackie, 2003). It did, however, confirm that areas of sediments in rel-
atively shallow waters within Carmarthen Bay support a range of species including bivalves, amphipods
and worms, many of which spend their time wholly or partly buried in the sediment. Both Carmarthen
Bay and Helwick Bank provide a rich food source for birds and fish.
In the centre of the Bristol Channel south of Gower, extensive areas of the seabed are only covered
with a thin layer of gravel, and to the east bedrock is exposed on the seabed. Subtidal bedrock does not
extend more than a few hundred metres offshore except at East Helwick, where bedrock slopes away into
deep water. The deeper-water rock communities off East Helwick have not been studied in any detail.
The coast to the west of Mumbles Head and around Pwlldu Head is exposed to both wave action and
tidal streams. A shallow forest of northern kelp Laminaria hyperborea leads to low-lying bedrock with a
rich mixed turf of red seaweeds and animals, including sponges. These red seaweeds include flat tentacle
weed Calliblepharis ciliata , iridescent ruffle weed Cryptopleura ramosa , cock's comb Plocamium cartil-
agineum , Irish moss Chondrus crispus and dulse Palmaria palmata . Due to the silty conditions seaweeds
do not grow very deep. The northern kelp for instance extends only down to 1 metre below chart datum,
compared with around 10 metres in west Pembrokeshire. Below this a turf of red seaweeds descends to 3
or 4 metres below chart datum and lower down animals dominate the communities. The water, however,
is rich in food and some species, such as the common blue mussel and the subtidal reef sandworm Sabel-
laria spinulosa , thrive in these conditions.
The common names of sponges and sea squirts are usually fairly descriptive, being often named by
divers rather than biologists, and animals present amongst the red seaweed turf include guarded flask
sponge Scypha ciliata , golf ball sponge Tethya aurantium , white spiky sponge Dysidea fragilis , white
hedgehog sponge Polymastia mamillaris and orange wisp sponge Esperiopsis fucorum . Sea squirts recor-
ded from the area include the star sea squirt Botryllus schlosseri , light bulb sea squirt Clavelina lepadi-
formis (Fig. 61) and orange spot club sea squirt Palladium punctum .
There are a number of gullies, which provide a multiplicity of habitats for marine organisms. The
gully sides are dominated by animals, including sponges, sea firs such as Nemertesia antennina , small
colonies of deadman's fingers and dahlia anemones Urticina felina (Fig. 62). Sea squirts are again nu-
merous and include the star sea squirt, light bulb sea squirt and the no spot sea squirt Morchellium argus .
Thebottomsofthegulliesarecolonisedbyspeciestolerantofthescouringeffectofthetidessuchasjelly
fingers Alcyonium diaphanum and horn wrack Flustra foliacea . The crevices and overhangs also provide
shelter for the edible crab Cancer pagurus , velvet swimming crab Liocarcinus puber , common lobster
and spiny spider crab Maia squinado . A common fish familiar to divers, the tompot blenny Parablennius
gattorugine , also occurs in this area. It is a medium-sized fish that feeds on small invertebrates, including
sea anemones.