Nature and Importance of the Biotope Complex

General description of the biotope complex

Major constituent species

Importance of the biotope complex


General description of the biotope complex

Brittlestars form a class (Ophiuroidea) within the Phylum Echinodermata. Echinoderms are a major group of exclusively marine invertebrates characterized by five-fold radial symmetry and a body wall containing calcareous skeletal plates. All echinoderms also have a unique water vascular system which communicates with the surrounding sea water and operates, by means of hydrostatic pressure, rows of radially-arranged suckers or ‘tube feet’. Brittlestars have long arms and a relatively-small, well-defined central disc. The tube feet do not have terminal suckers as in other echinoderms, and they are used more for food capture than for locomotion.

The Ophiuroidea is the largest living echinoderm class, comprising about 1800 species, with representatives in all benthic environments from the intertidal zone to the deep ocean, and found in all temperate, tropical and polar seas. There are about 20 species in British coastal and shelf seas (shallow-water species are illustrated in Picton, 1993). In British waters, brittlestars are common members of the benthic fauna of both hard and soft substrata. Some species live epifaunally on or among rocks, others on the surface of sandy or muddy bottoms, and others partially buried below the sediment surface. Brittlestars of some kind can be found in almost any marine benthic biotope in the British Isles, and are often common members of the fauna. In some areas, however, they occur in dense aggregations on the sea bed, with hundreds or even thousands of individuals m-2, and sometimes to the virtual exclusion of any other animals. These brittlestar beds occur on a variety of substrata, from solid bedrock to boulders, gravel, sand or mixed sediments, usually in conditions with fairly strong tidal streams. Beds may be patchy and local in distribution or may cover several km2 of sea bed.

Only a few species of the British brittlestar fauna occur in dense aggregations. Individual beds may be formed by a single species or contain a mixture of several. None of the bed-forming brittlestar species occurs exclusively in dense aggregations. All can be found in smaller numbers in other benthic biotopes. There is therefore no clear-cut population density above which a ‘bed’ can be defined. Nevertheless, there are certain benthic communities around the UK and Ireland so numerically dominated by brittlestars that they have been recognized as distinct biotopes within the MNCR classification (see Chapter II), and it is these biotopes with which this report will be concerned. The bed-forming species are Ophiothrix fragilis, Ophiocomina nigra and, more rarely, Ophiopholis aculeata. Epifaunal, sediment-dwelling brittlestars of the genus Ophiura may also occur in large numbers but do not usually dominate their biotopes to the same extent as the other species. These Ophiura aggregations will be mentioned where relevant. The infaunal brittlestars Amphiura filiformis and A. chiajei live partially buried in muddy sediments and may reach densities of > 1000 m2. However, these infaunal populations are not normally considered as brittlestar ‘beds’ and will not be considered further here.

Subtidal brittlestar beds can be found in several of the habitats defined in Annex I of the EU Habitats Directive. Examples occurring on hard substrata come within the category of ‘Reefs’, while some of those on gravel or mixed sedimentary substrata can be classed within ‘Sandbanks covered by sea water at all times’ (provided these are shallower than 20 m depth). Geographically, examples can be found in ‘Large shallow inlets and bays’.

Major constituent species

Ophiothrix fragilis

This is a large brittlestar with long, spiny arms. The central disc is up to 20 mm in diameter. Colour is very variable, commonly brown or grey, but sometimes patterned with red, yellow or orange. The arms are usually banded with dark and light colours. Ophiothrix fragilis is very common all round the British and Irish coasts from the lower intertidal zone downwards. It occurs in a wide variety of benthic biotopes on hard substrata, among algae or sessile animals, and also on sandy or shelly bottoms. The species is widely distributed in the eastern Atlantic from Norway to South Africa. Ophiothrix fragilis is the species most commonly found in dense aggregations in British waters.

Ophiothrix fragilis is generally considered a single species and is not subdivided by researchers in the UK and Ireland. However, French workers recognize a number of varieties based on morphological differences, termed echinata, pentaphyllum, lusitanica and abildgaardi (Koehler, 1921). It is possible that differences between varieties may explain some of the inconsistencies in findings concerning the life cycle and population dynamics of the species (see Chapter IV).

Ophiocomina nigra

The smooth disc of Ophiocomina nigra is up to 25 mm in diameter. The arms are long, with a less ‘bristly’ appearance than those of Ophiothrix fragilis. The arm spines are prominent, but neatly arranged like the teeth of a comb. Individuals are uniformly coloured with no bold patterns. Colour ranges from jet black, through various shades of brown, to orange. Colour appears to be related to water depth, with darker shades predominating in shallower water (Fontaine, 1962). Ophiocomina nigra is usually found in fairly sheltered sites with some water movement, on bedrock, boulders or gravel, sometimes in mixed populations with Ophiothrix fragilis. It is common around the British Isles apart from the southern North Sea. Total geographic range is from Norway to the Azores and the Mediterranean.

Ophiopholis aculeata

The disc of Ophiopholis aculeata is covered with prominent large plates and is usually about 15 mm in diameter. The arms bear fairly short, robust spines. Colour is brown or reddish with darker bands on the arms. This species is typically an inconspicuous inhabitant of crevices or borings in rock, but forms dense beds in a few localities in Scotland (see Chapter II). Ophiopholis aculeata is a circumpolar cold water brittlestar reaching its southern limit of distribution in the English Channel.  

Ophiura spp.

In British coastal waters, this genus comprises the species O. ophiura (O. texturata is a synonym for this species), O. albida, O. robusta and O. affinis, all of which are active brittlestars living on sand, muddy sand and gravel, or on silt-covered rock surfaces. Ophiura species have narrow, tapering, relatively short arms with inconspicuous spines. The arms are generally held stiffly out from the central disc. Ophiura ophiura is the largest species, with a disk up to 30 mm in diameter, and is a uniform sandy grey-brown colour. Ophiura albida and O. robusta are much smaller (disk to 15 mm diameter) with a pair of conspicuous white spots on the upper disk at the base of each arm. Ophiura affinis is smaller still (disk diameter 8 mm), grey-brown with darker banding on the arms. Ophiura ophiura, O. albida and O. affinis occur off all British coasts and range from Norway to the Mediterranean. Ophiura robusta is a boreal species which reaches only the northern half of the British Isles.

Ophiura species occur at high densities in some situations and are occasionally found as minor components of beds dominated by Ophiothrix or Ophiocomina. Aggregations of Ophiura spp. have received less attention than those of Ophiothrix or Ophiocomina, and much less is known of their ecology and dynamics (Tyler, 1976).

Importance of the biotope complex

Economic importance

Brittlestar beds are currently of no economic importance. No species are harvested from them and they are not thought to be significant feeding or nursery grounds for any commercially-important fish or shellfish.

Scientific importance

The living brittlestar beds have attracted considerable scientific attention as prime examples of an ‘anachronistic’ community, ie. a community type that was common in the distant past but is relatively rare today (Aronson, 1989). In the Paleozoic era (roughly 570 - 245 million years ago), the fossil record shows that sandy or muddy substrata in shallow coastal waters typically supported dense populations of sessile or sedentary animals living above the sediment surface, and feeding on plankton or other suspended matter in the water column. These epifaunal suspension-feeders included many species of brachiopods, stalked crinoids (‘sea lilies’) and other groups that are now either extinct or uncommon in modern marine communities. Brittlestars first appear in the Ordovician Period (roughly 500 - 420 m.y. ago), and the fossil record shows that in some circumstances they formed dense aggregations similar to those that exist today (Aronson & Sues, 1988). A major change in marine benthic communities occurred during the succeeding Mesozoic Era, during which dense populations of epifaunal suspension-feeders largely disappeared from sedimentary habitats in shallow water, which became dominated instead by groups such as bivalves which typically live buried within the substratum. This ‘Mesozoic marine revolution’ (Vermeij, 1977, 1987) appears to be associated with the evolutionary radiation of groups of marine predators able to tackle hard-shelled prey. These predatory groups include the decapod crustaceans (crabs, lobsters), teleost fishes and neogastropod snails (eg. whelks).

Fossilized brittlestar beds begin to decline in frequency during the Jurassic period (roughly 208 - 140 m.y. ago) and are rare in later deposits (Aronson, 1992) The living examples have a restricted geographic distribution today (see Chapter II). The association between the decline of a major community type and changes in predation intensity has attracted much attention from ecologists studying the dynamics of marine ecosystems over evolutionary time-scales. The British brittlestar beds have been studied by workers from as far afield as Japan and the United States, partly to test the hypothesis that modern beds exist where predation pressure is low (Aronson, 1989). Brittlestar beds thus provide a rare opportunity to experimentally test hypotheses concerned with community changes over evolutionary time, as opposed to relying solely on inferences drawn from the fossil record (Aronson, 1992).

Biodiversity and conservation importance

Species existing in dense aggregations consisting of millions of individuals are by definition not rare, and even without these aggregations the three main bed-forming species are widespread and common throughout British and European waters. However, as outlined above, the community type represented by brittlestar beds is fairly uncommon in the modern world. Beds do exist elsewhere (see Chapter II), but are particularly numerous in the British Isles. The British representatives are the best-known examples of their kind and have the longest history of scientific study. They should therefore be considered generically as being of conservation importance.

There is evidence (considered in more detail in Chapter IV ) to suggest that massive aggregations of suspension-feeding brittlestars can have an important effect on water quality in coastal environments and may even help counteract some of the potentially harmful effects of eutrophication (proliferation of planktonic algae) caused by human input of nutrients into the sea. The beds may therefore play a significant role in the ecological functioning of coastal seas. Conversely, the distribution and extent of these conspicuous biological features may be potentially valuable indicators of climatic, oceanographic or human-induced changes in the coastal environment.

The conservation importance of brittlestar beds will be assessed comprehensively in Chapter IX after a full review of their distribution, ecology and sensitivity to change.

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