||Range of conditions
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).
||Moderately exposed, Sheltered.
Beds are usually sheltered
from strong wave action, but examples in moderately exposed situations are known (Ball et
||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).
||Bedrock; boulders, cobbles, mixed substrata and sediments.
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).
||Lower Infralittoral, Circalittoral
The upper and lower depth boundaries of beds may
be very abrupt.
||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.
||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.