Habitat requirements

Habitat factor Range of conditions
Salinity Full.

Most brittlestars beds exist in fully marine conditions. However, in the Dutch Oosterschelde Estuary, dense Ophiothrix aggregations have been recorded in areas where normal salinity is only 16.5‰ (Wolff 1968).

Wave exposure Moderately exposed, Sheltered.

Beds are usually sheltered from strong wave action, but examples in moderately exposed situations are known (Ball et al. 1995).

Tidal streams Moderately strong, Weak.

Brittlestar beds can be found in a variety of current regimes. Many sealochs 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 1.5m s-1 during average spring tides (Davoult & Gounin 1995b). Similarly strong tidal streams (1.0 – 1.2 m s-1) were also recorded over beds in the Isle of Man (Brun 1969).

Substratum Bedrock; boulders, cobbles, mixed substrata and sediments.

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 838m-2) on reefs formed by tubes of the polychaete worm Sabellaria spinulosa (George & Warwick 1985). In Strangford Lough, dense Ophiothrix beds overly shells of the horse mussel Modiolus modiolus (Magorrian, Service & Clarke 1995).

Zone Lower Infralittoral, Circalittoral
Depth range 10-30m.

The upper and lower depth boundaries of beds may be very abrupt.

Temperature Within the British Isles the distribution of brittlestars is not 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 3oC, but was eliminated when temperatures fell to 0oC (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.
Water quality 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 sealochs.

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