The Surrounding Sea - Gower - page 115

rock is occupied by sand-tolerant seaweeds and mossy clumps of red rose grass weed occur together with
larger species including little forked worm weed Furcellaria lumbricalis , slender red filament weed Gra-
cilaria gracilis and red bottlebrush weed Halurus equisetifolius . Animals include common lobster and
common shore crab Carcinus maenas . Conspicuous species in the shallow sandy areas include sand ma-
son worms Lanice conchilega , razor shells and hermit crabs. Patches of mussels are widespread, as is the
white horseshoe worm Phoronis hippocrepia and various sea squirts.
The Strombus and other wrecks in Oxwich Bay provide substrates raised above the generally low-ly-
ing and sediment-covered rock on the seabed and therefore support species that are not found elsewhere
alongthecoast,orwhichareuncommoninotherlocations.Theseincludethespinyantlersponge Raspail-
ia hispida , white hedgehog sponge and plumose anemone Metridium senile . The Strombus , in particular,
is the only known Gower locality for Devonshire cup coral Caryophyllia smithii and is also frequented
by large conger eels Conger conger and by ocean triggerfish Balistes balistes . The ocean triggerfish was
once a rare summer visitor to the area, but now breeds in the Bristol Channel, probably as a result of the
warmer temperatures. The upper surfaces of this wreck, which lies in shallow water to the south of Ox-
wich Bay, are covered in foliose seaweed, including equally divided net weed Dictyota dichotoma . Con-
spicuous animals on the wreck include spike barnacle Balanus crenatus , rosy feather-star, candy-striped
flatworm Prostheceraeus vittatus and bird's head coralline Bugula turbinata .
To the west of Oxwich Bay, between Port-Eynon and Worms Head, the coast is again more exposed.
Much of the bedrock on this section has a dense covering of common blue mussels, which provide food
for the common starfish. Large mussels tend to be found only in depths shallower than 3 metres; below
this depth the beds are full of small individuals. Although mussels far outnumber any other species in
terms of numbers and biomass the area is very species-rich, with many other organisms present in low
numbers.Therichnessofthefaunaassociatedwithmusselbedsinintertidalareasiswelldocumentedand
the same seems to be true on the subtidal rock. Mussel beds are in fact among the most productive hab-
itats on earth, often rivalling the productivity of tropical rainforests and kelp beds, due to their complex
three-dimensional structure. They can often reach a considerable thickness and both the living and dead
mussel shells increase the surface available for colonisation.
Mobile organisms move freely through the complex matrix of shells and interconnecting byssus
threads,bywhichmusselsattachthemselvestohardsurfaces.Thesespeciesincludecommonlobsters,bib
Trisopterus luscus , ballan wrasse Labrus bergylta , pollack Pollachius pollachius , an anglerfish Lophias
piscatorius , the uncommon ghost sea slug Okenia adspersa and the orange and black spotted sea slug
Thecacera pennigera . There are large numbers of the latter at Mumbles and it is thought to be an intro-
duced species that has developed a self-sustaining population.
Atthetime ofthe1978surveylargepopulations ofcommonstarfish werepresent onthemussel beds,
and at some sites there was a distinct line of densely packed animals working its way across the bed,
leaving dead mussel shells behind. Where the mussels have been removed, by starfish or other agencies,
blue-mouthed red sea squirts Polycarpa scuba occur along with a range of other species including Tao-
nia atomaria, Obelia dichotoma and Sertularia argentea . Anemones include the cave-dwelling anemone
Sagartia troglodytes . The anemone uses its tentacles to prey upon small shrimps and crabs. In turn it is
preyed upon by the grey sea slug Aeolidia papillosa and the tompot blenny. Colour is not a good method
of identifying sea anemones, and this is especially the case with Sagartia , which is more variable in col-
our than any other British sea anemone. There are two forms, a large form ( decorata ), which is typically
up to 50 millimetres across the base and 100 millimetres tall, and a small variety ( ornata ) that is frequent
amongst rocks or mussels; this form is usually green or brown and rarely exceeds 15 millimetres across
its base.