A ROCK IN MILFORD HAVEN
Jane Lilley reports from a dive on a submerged rock in the sheltered entrance of Milford Haven during 1995.
It was the late August bank holiday weekend. Our diving club had organised
a long weekend's diving in Pembrokeshire, and the weather was not co-operating.
The wind was blowing too strongly from the wrong direction, and the dives
we had planned were impossible until conditions improved. In desperation,
we launched the boat at Dale and went searching for an interesting site
in the shelter of Milford Haven.
On the east side of the Dale peninsula, off Watwick Point between Watwick and Castlebeach Bay, the echosounder showed irregular lumps in an otherwise flat bottom: with luck, areas of rock that might be interesting. Four pairs of divers went in, in slightly different places. Three found nothing very interesting (one pair headed steadily out into the Haven and reported acres and acres of barren sand). My husband and I intended to fin gently inshore and hope to find some interesting life on the rocks at the base of Watwick Point, but there was no need; we dropped onto a large patch of submerged rock, plastered in marine life, and stayed there.
The rock consisted of nearly vertical slabs, with holes of varying sizes underneath fallen slabs and narrow crevices between unbroken ones. At a depth of 11 to 13 metres in rather silty water there was too little light for the kelps, and only a sparse cover of red algae. But animal life, attached and mobile, was varied and abundant.
Many of the exposed rock surfaces were covered in sponges of various
colours. Sponges are a specialist study, and there are not many that I
can identify reliably underwater, but some are distinctive. The fanlike
pale orange upright 'fingers' of Axinella polypoides (which should
apparently now be called Axinella dissimilis) grew on the rocks
in several places, and once we saw the paler A. infundibuliformis,
which I think of as the Cup-Sponge because it forms a conical cup which
is split open down one side. Both are typical south-western species. Much
more widespread in the south and west is the Goosebumps Sponge, Dysidea
fragilis, forming little whitish-grey mounds whose surfaces are covered
all over in tiny projections; I think it is this that I have heard described
as the Mashed Potato Sponge. There was also a single specimen of the Elephant's
Ear (or Elephant's Hide) Sponge, Pachymatisma johnstonia, another
southern and western species which forms grey, smooth-surfaced mounds that
can reach a foot or more across, though this one was only a baby.
Two sponges had been attacked by something. A greenish-grey Halichondria had formed hummocky mounds on two adjacent pieces of rock, joined by a low arch of sponge, 12 mm thick and very uniform, with the oscules (the large holes through which water is pumped out; it is drawn in through very numerous tiny pores all over the sponge surface) arranged along its top. How it had grown into that shape I cannot imagine. The arch was undamaged, but the main mounds had irregular pieces chewed out of the surface. And one of the only two specimens I saw of Cliona celata, the Boring Sponge, had been chewed to destruction: it was detached and drifting in the water, and the entire inside had been eaten away, leaving just the tough surface skin. The Boring Sponge is so called not because it is even less interesting to the casual observer than most sponges, but because it starts life by boring into rocks or shells. All that is visible on the surface are the little yellowish discs of the inhalant papillae and exhalant oscules, while inside it dissolves out a network of tunnels lined with sponge which can result in the rock crumbling away. In this form it is found around much of Britain. But in the south-west it eventually decides to emerge from its shelter, and grows into a massive yellow sponge which can be a foot or more across, quite unmistakable.
Another attached species is the Devonshire Cup Coral, Caryophyllia smithii. This is very common in the area, and was scattered all over the rocks. Large individuals often had one or two of the 'parasitic' barnacle Megatrema (=Boscia) anglicum living on the rim of the calyx. These form pink wart-like bumps which look nothing like barnacles until you see the typical tentacle-fan emerging and sweeping the water for food. Although not as numerous as sometimes - I have seen up to five barnacles on one large cup coral elsewhere - some were very large, as high as the coral itself. There were also smaller cup corals, including the smallest I have met, less than 7 mm across with a circular calyx and only ten or twelve septa, some of them tiny. One individual was defaecating the remains of its last meal, a mass of brownish insubstantial glaire still attached to the mouth.
Unusually, Dead Men's Fingers, Alcyonium digitatum, were uncommon, with only two smallish specimens close together on the rock. There was a single Sea Fan, Eunicella verrucosa, however; rather a battered specimen, one of whose two main branches had been broken off just above the fork. A tuft of red algae grew on the stump. It must have been broken relatively recently, for the remaining branch had made no attempt to fill in the space; it had apparently been the weaker of the two, and now formed a rather small, lopsided segment of a normal fan. Sea Fans will flex a little in a current, when they can be seen vibrating like a tuning-fork, but they apparently break under sudden stress from wave surge or a diver's fin.
The Twin-Fan Worm Bispira volutacornis lived in some of the narrow
cracks in the rocks. This withdraws into its tube with astonishing speed
at any disturbance, leaving only a grey parchment-like tube with the top
covered by a flap, in case you are a hungry crab or fish looking for a
snack. When undisturbed, two fans of tentacles emerge from the tube, so
that each worm looks like two individuals living side by side. In babies,
each tentacle-fan is semicircular, the two fans together forming a single
whorl just overlapping at one side; as they grow larger, each fan becomes
first a complete circle and then a spiral of up to three whorls. They are
very attractive, with the feathery tentacles usually banded in white and
shades of grey, but sometimes orange-brown.
Some individuals were normal Bispira in appearance and behaviour, but others were puzzling: even large specimens had only the single tentacle-whorl, indented where the two half-circles overlapped, which is usually seen only in tiny individuals, and the tentacles seemed limper than usual. When disturbed, these did not withdraw at the usual lightening speed into the tube, but were pulled in rather slowly to a half-retracted position, where they stopped. Even when touched they were reluctant to pull the tentacles in further, and did it very slowly; and the tube remained open at the top, not closed over as usual. There were quite a number of these unusual tubeworms, including one group of at least seven. I have never seen them before, and have no idea whether they are a different species or abnormal specimens of Bispira volutacornis.
There were probably many more crabs living in the holes and cracks than
we saw. Several had moulted recently, leaving their cast exoskeletons drifting
in the currents, including one from a small spider crab, Macropodia
sp We saw several Velvet Swimming Crabs, Necora (=Liocarcinus)
puber, as aggressive as usual, and a large Edible Crab, Cancer pagurus.
The ascidians (sea squirts) on the rocks were not as varied as I have seen in Pembrokeshire, but they included several large solitary individuals of Ascidia mentula and perhaps another similar species, attached by one side so that they lay horizontally on the rock. At least two specimens had been attacked by something which had eaten away the entire upper surface, exposing the internal body cavity and a cross-section of the very thick body walls, irregular and marked with grooves left by the predator's chewing. They may have been victims of two large Edible Urchins, Echinus esculentus, nearby; their teeth are very powerful.
Sea Cucumbers were well represented, with at least four species present. A single large Holothuria forskali, the Cotton-Spinner, was resting in a shallow crevice with a large Tompot Blenny relaxing on it. The others were all crevice-living species. Two of these are very common, and easy to identify once you know what to look for. Both have the body largely or entirely concealed in a crack in the rock, but protrude their tentacles into the water to feed. These are sticky with mucus and collect plankton out of the water; you often see them with the tentacles yellowish with collected food. Each tentacle in turn is curved round and inserted carefully, tip first, into the mouth, then the mouth closes around it and it is withdrawn, wiping off the plankton. The tentacles are usually black-and-grey, but can be partly or entirely white. To tell the two species apart, you have to peer into the crevice at the body. Aslia lefevrei has a short black section below the tentacles (known as the introvert, because it can be turned inwards with the tentacles if these are withdrawn) and then a silty-beige coloured body. Pawsonia saxicola may have a black or a white introvert, but the body below it is pure white unless it has been exposed to a lot of light, when it can become discoloured. Some Pawsonia are entirely white, body, introvert and tentacles.
The last species was unexpected, because it is said to live between boulders in mud, not in silty cracks at the base of rocks; I have seen it in a similar situation elsewhere in Pembrokeshire. It certainly looked like Thyone roscovita, which is illustrated in the Field Guide with similar black-and-grey tentacles to Aslia and Pawsonia, but instead of the axes of the tentacles being angled at each side-branch, they are absolutely straight, and consistently pale-coloured with dark side branches, giving a striking effect. I would like to have the identification confirmed; can any reader help?
Leopard-Spotted Gobies, Thorogobius ephippiatus,
of various sizes were common, always in their usual position: resting on
the bottom on a horizontal surface just outside a hole or crevice into
which they dart at the first sign of danger. There were a few other fish,
mostly wrasse: Goldsinny, occasional Rock Cook,
and female Cuckoo Wrasse, sometimes all three
together in a loose school.
As well as these species, others were present but of less interest to me: patches of barnacles; the predatory gastropod Ocenebra; two nudibranchs Tritonia lineata, unmistakable with glowing-white lines down the sides of a translucent-white body; frequent patches of the zoanthid Isozoanthus and, less commonly, Parazoanthus, the former with tiny brown tentacles, like miniature sea anemones, just emerging from the silt cover on a rock surface and retracting when a torch was shone on them, the latter with almost transparent polyps set close together in groups; a few starfish and sea urchins; and more. My slate was covered in scrawled notes. It was an excellent dive.
We surfaced reluctantly after 75 minutes when we were both getting cold and my husband's air was running low, to be greeted by an ironic cheer from the boat. No-one else, it seemed, had found anything much to interest them. Were we just lucky, or had they not stopped to look at what was there?
Guide to the Echinoderms of the British Isles by Bernard E Picton [Immel
Calyx: hard outer cup of the coral skeleton, within which
the polyp lives.
Epilithic: growing on the outside of rocks.
Septa: radial plates attached to the inside of a coral's calyx
This acorn barnacle grows only on certain species of coral, in Britain
usually the solitary Devonshire Cup Coral Caryophyllia smithii.
It has no common name, and is also known as Pyrgoma or Boscia
Sponges are uncommon on the shore, but many small species and a few large ones live in shallow water attached to rocks, stones or shells.
A simple sponge is a bag of cells surrounding a cavity, supported by microscopic spicules. Water is drawn in through numerous tiny holes all over the surface, and expelled in a steady stream through a large opening (an oscule) at one end. In more complex species the cavity is folded and there may be many oscules. Minute food particles are filtered out as the water passes through, and gas exchange takes place. Sperm is discharged with the water current and may be drawn into another sponge to fertilise the eggs; the larvae have a brief swimming life before they attach and develop into adults.
Some sponges are distinctive in shape or colour, but can vary
in shape sometimes dramatically. This makes them very difficult to identify.
One common species is a thin encrusting sheet on the tops of boulders exposed
to wave action, but becomes much thicker with long slender projections
in sheltered places in the same locality.
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