General distribution

Occurrence within ‘demonstration’ candidate SACs

Occurrence within other candidate or possible SACs

Assessment of the biotope complex within SACs

Figure - The general distribution of brittlestar beds around the British Isles.

The typical Ophiothrix/Ophiocomina beds (MCR.Oph) are found widely around the northern, western and southern British Isles (Aronson, 1989). Examples are known from Shetland (Howson, 1988), from many localities on the western Scottish coasts (Howson et al., 1994) and in the Clyde Sea (Gorzula, 1976). Extensive beds occur around the Isle of Man (Chadwick, 1886; Brun, 1969), off the Llyn Peninsula (Hiscock, 1984), Pembrokeshire (Aronson, 1989), and in the Bristol Channel (George & Warwick, 1985). Inshore brittlestar beds have been studied in south-west England (Warner, 1971), while those further offshore in the western English Channel have yielded data on long-term fluctuations in spatial extent (Holme, 1984). Further east in the Channel, Ophiothrix beds are a dominant community type in the Dover Strait (Davoult, 1990; Migné & Davoult, 1997).

On the east coast of the UK, Ophiothrix/Ophiocomina beds are known from Berwickshire (Pagett, 1984; Foster-Smith et al., 1996), but appear to be absent from the stretch of coast between this area and the Dover Strait. In Ireland, beds have been recorded from several localities on the southern, eastern and western coasts (Keegan et al., 1985; Ball et al., 1995).

The occurrence or non-occurrence of subtidal brittlestar beds in the 12 ‘demonstration’ SACs included within the UK Marine SACs Project is summarized in the table below.

 SACs probably lacking the biotope complex

The subtidal biotopes of the Solway Firth have been surveyed quite recently, but no brittlestar aggregations have been reported (Covey, 1992; Cutts & Hemingway, 1996). None are known from Cardigan Bay (Mackie et al., 1995). Brittlestar beds are well-known from south-west England but have not been recorded from within the confines of Plymouth Sound (Hiscock & Moore, 1986; Devon Wildlife Trust, 1993) or the Fleet Lagoon (Dyrynda, 1984).

The locations of ‘demonstration’ amd other candidate SACs known to support brittlestar beds are shown in the linked figure. The sites themselves are discussed individually below.

Brittlestar beds are known from several localities around Shetland (Howson, 1988). In the area of the Papa Stour candidate SAC, Howson recorded an Ophiocomina bed from coralline algal-encrusted rock ridges at 25m depth in the Sound of Papa, between the island and the Shetland mainland (Figure).

The complex system of Loch nam Madadh in North Uist contains a variety of hard and soft substratum biotopes (Entec, 1996). No extensive beds of brittlestars (biotopes MCR.Oph or MCR.Oph.Oacu) have been found, but local aggregations of Ophiothrix and Ophiocomina occur on rock outcrops, boulders and gravel patches, mainly in the outer parts of the loch (Figure). The absence of extensive aggregations suggests that Loch nam Madadh is probably not an important site for the biotope complex.

The benthic biotopes of the Sound of Arisaig candidate SAC have been mapped recently by acoustic methods, towed video and diving (Davies & Hall-Spencer, 1996). Areas of grazed bedrock and boulders with Ophiothrix and Ophiocomina were found scattered in the entrances to Loch Ailort and the south channel of Loch Moidart. Howson (1990) found the same community on deep (up to 39 m) reefs in the Sound of Arisaig proper and further north in the entrance to Loch nan Uamh, although most of her localities fall outside the northern boundary of the proposed SAC. Both brittlestar species were also found abundantly around bedrock adjacent to a muddy sand plain at 8m depth near the head of Loch Ailort (Howson. 1990).

Brittlestar aggegations thus appear to be quite common within and just outside the Sound of Arisaig SAC (Figure), but are spatially restricted to bedrock outcrops and boulder patches.

This large, virtually land-locked marine inlet on the coast of Northern Ireland contains a diverse range of benthic biotopes. Very dense aggregations of brittlestars, mainly Ophiothrix, but with some Ophiocomina, cover an extensive area of the lower lough (Erwin, 1977). A recent acoustic mapping of benthic biotopes in Strangford Lough (Magorrian et al., 1995) estimated that the Ophiothrix beds occupied 0.4 km² of the 22.9 km² surveyed (Figure).

The Morecambe Bay candidate SAC supports at least one dense Ophiothrix bed, at Roa Island at the south end of Walney Channel (C. Lumb, personal communication). The bed extends into the intertidal, covering an area of about 0.1 ha. Its subtidal extent has not yet been determined. The bed appears to be increasing in size.

Details of the occurrence of brittlestar beds in this area are scarce. Hiscock (1984) recorded a dense Ophiothrix bed covering an ‘extensive area’ at Porth Colmon off the north coast of the peninsula (Figure). Ophiothrix aggregations have also been found during surveys associated with exploratory oil drilling at a site 9.5 km north of the headland (Braich y Pwll) at the tip of the peninsula (Mills, 1997). The substratum in this area was composed of tide-swept cobbles and pebbles embedded in a sediment matrix.

No Ophiothrix or Ophiocomina beds are known from this area. However, a sediment community characterized by Ophiura albida, and with O. ophiura also present, has been recognized in the Wash (Covey, 1991). Foster-Smith et al. (1997) recorded Ophiura spp. on muddy sand, associated with the urchin Psammechinus miliaris and the starfish Asterias rubens. Ophiura ophiura and O. albida had overlapping but generally well-defined distributions. Both species were found on a range of substrata, but O. ophiura was confined to the south-east Wash and often associated with silty substrata containing a variable shell component. Densities of up to 1500 O. albida m^-2 have been recorded (Dipper et al., 1989).

Foster-Smith et al., (1996) used acoustic methods (see Chapter VII) to map the benthic biotopes of the Berwickshire coast. Extensive Ophiothrix (with smaller numbers of Ophiocomina) were found on bedrock and boulders from 10 - 20 m depth, occasionally down to 30 m. Beds occurred on urchin-grazed rocks with the tubeworm Pomatoceros triqueter and sparse soft corals, Alcyonium digitatum, (this species was sometimes absent), and on shallow cobbles with encrusting coralline algae. The beds occurred in a patchy but extensive belt running parallel to the coastline (Figure).

In an earlier survey of the St Abbs area, Pagett (1984) reported beds of three brittlestar species, Ophiothrix fragilis, Ophiocomina nigra and Ophiura albida. The brittlestars occurred in a variety of species combinations:

• Ophiothrix beds occurred on bedrock, boulders or gravel, and usually had some Ophiocomina, and sometimes also Ophiura albida, mixed in.
• Ophiura albida beds occurred on gravel or sand, and frequently contained some Ophiura robusta.
• Ophiocomina beds occurred on bedrock, gravel or coarse sand. This was the only brittlestar found in monospecific aggregations, but in most beds one or more of the other species were also present.

Several other candidate or possible SACs are known to support brittlestar beds. Ophiocomina - dominated beds are known from around the island of Mousa off the south-east coast of Shetland (Howson, 1988). The upper reaches of Loch Alsh have a cobble and pebble substratum in depths of 11 - 33 m, experiencing moderately strong tidal streams (Connor, 1989). This supports the rare variant biotope MCR.Oph.Oacu, characterized by high densities of the usually cryptic brittlestar Ophiopholis aculeata (Figure). Ophiothrix and Ophiocomina also occur mixed with Ophiopholis in these beds. In south-west Britain, Ophiothrix/Ophiocomina beds have been recorded from the Pembrokeshire Islands candidate SAC (Bunker & Hiscock, 1987).

Of the various candidate or possible SACs, those with the most extensive brittlestar beds appear to be Strangford Lough, Berwickshire/North Northumberland and Loch Alsh. Loch Alsh is particularly noteworthy as the sole example of the Ophiopholis - dominated biotope within an SAC. Large beds may also be present around the Llyn Peninsula, but their distribution in this area has been less thoroughly mapped.

Brittlestar aggregations in Loch nam Madadh and the Sound of Arisaig appear to be patchy and not very extensive. Good examples of the biotope complex are found in Shetland, but the best recorded there is not located within a proposed SAC. Howson (1988) found a rich bed at 30 m depth off the east coast of the island of Foula. A substratum of rounded, coralline-encrusted cobbles supported very high densities of all three main bed-forming species (Ophiothrix fragilis, Ophiocomina nigra and Ophiopholis aculeata), while large numbers of Ophiura albida, O. robusta and possibly O. affinis were found beneath the stones. This is an unusually high species diversity for a single brittlestar bed.

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