Hydrographic conditions

Substratum type

Temperature

Sedimentation

Most brittlestar beds exist in conditions of full marine salinity. However, in the Dutch Oosterschelde Estuary, dense Ophiothrix aggregations have been recorded in areas where normal salinity is only 16.5 %o (Wolff, 1968), and the species was found to persist down to 10%o. The usual depth range of bed occurrence is roughly 10 - 50 m, but may exceed 70 m in the western English Channel (Holme, 1984). Aggregations on infralittoral rock in Scottish sea lochs can be found as shallow as 5 m. Rare examples are known of Ophiothrix beds extending into the lower intertidal. Beds may have very abrupt upper or lower depth limits. Off the Isle of Man, the Ophiothrix bed studied by Brun (1969) had a sharp upper boundary at 26 m. The lower limit was not determined, but the bed extended to at least 37 m depth.

Beds are usually sheltered from strong wave action, but examples in moderately exposed situations are known (Ball et al., 1995). They can be found in a variety of current regimes. Many sea loch examples experience only weak tidal streams, but on more open coastlines brittlestar beds are generally associated with higher-energy environments. In the Dover Strait, Ophiothrix beds experience current speeds of up to 1.5 m s^-1 during average spring tides (Davoult & Gounin, 1995). Similarly strong tidal streams (1.0 - 1.2 m s^-1) were also recorded over beds in the Isle of Man (Brun, 1969). In both locations, Ophiothrix densities of up to 2000 individuals m^-2 were recorded. Such high densities can probably only be maintained where strong currents can supply enough suspended food. Food requirements probably set a lower limit on the current regime of areas able to support brittlestar beds, but this has not yet been determined. The behavioural responses of brittlestars to changes in current speed will be discussed in Chapter IV.

Brittlestar beds have been recorded on a wide variety of substrata, ranging from bedrock through boulders and cobbles to gravel, sand and mud. Beds on cobbles, gravel and mixed coarse sediments are probably the most common, and these substrata will obviously predominate where strong currents are experienced. In the Bristol Channel, Ophiothrix was recorded at high density (up to 838 m^-2) on reefs formed by tubes of the polychaete worm Sabellaria spinulosa (George & Warwick, 1985). The brittlestars clustered on the reefs and rock outcrops rather than in the gravel-filled hollows between them, probably because the elevated sites offered a more favourable position for suspension-feeding. In Strangford Lough, dense Ophiothrix beds overly shells of the horse mussel Modiolus modiolus (Magorrian et al., 1995).

Brittlestars lie on or cling to the substratum, but are not attached to it. Beds therefore usually occur on upward-facing, level or gently-sloping bottoms, rather than vertical cliff faces or overhangs. In strong currents, the mutual support provided by the interlinked arms of densely-packed brittlestars enhances the stability of the beds and reduces the chance of individuals being swept away (Warner, 1971).

Brittlestar beds exist across virtually the entire geographic extent of the UK and Ireland (Fig. 1). Within the British Isles their distribution is therefore not obviously limited by temperature, although individual species such as Ophiopholis aculeata and Ophiura robusta do show a latitudinal distribution pattern. In the Oosterschelde Estuary, Ophiothrix fragilis was common in areas regularly experiencing winter temperatures down to 3^oC, but was eliminated when temperatures fell to 0^oC (Wolff, 1968). Such extremes are only likely to be found in enclosed situations with very shallow water depths, and will not be experienced by the majority of open-coast brittlestar beds.

High rates of sedimentation are probably unfavourable to brittlestar beds due to the fouling of the animals’ feeding organs (tube feet and arm spines), and in extreme cases suffocation (Aronson, 1992). Beds in current-swept situations will not experience this problem, but it may be a factor in limiting the distribution of beds in semi-enclosed areas such as sea lochs.

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