|Basic Information||Biotope classification||Ecology||Habitat preferences and distribution||Species composition||Sensitivity||Importance|
Image Bernard Picton - Ophiothrix fragilis and/or Ophiocomina nigra beds on slightly tide-swept circalittoral rock or mixed substrata. Image width ca 1 m.
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CR.MCR.EcCr.CarSp.Bri recorded () and expected () distribution in Britain and Ireland (see below)
The main brittlestar bed forming species are Ophiothrix fragilis and Ophiocomina nigra, with occasional examples formed by Ophiopholis aculeata. Beds comprise hundreds or thousands of individuals per m2 formed by a single species only, or a mixture of several and may cover thousands of square metres of the sea bed.
Beds occur on a wide range of substrata from bedrock through to sand and mud. High densities, up to 2000 animals per m2, of brittlestars can probably only be maintained where strong water currents can supply enough suspended food so beds are probably most common on cobbles, gravel and mixed coarse sediments (Hughes, 1998). Food requirements are likely to set a lower limit on the water current regime of areas able to support brittlestar beds.
However, although water currents clearly play a part in determining where brittlestars may congregate Broom (1975) provided strong evidence to suggest that social behaviour is also important. His experiments demonstrated that Ophiothrix spp. can recognise and respond to conspecifics, and that this social response is important for the maintenance of aggregations. Strong evidence of social behaviour is also demonstrated by the abrupt boundaries (Warner, 1971) seen in Ophiothrix beds because it is unlikely that discontinuities in substratum type or water currents could be sharp enough to account for them. Ophiocomina nigra is less tolerant than Ophiothrix fragilis of close contact with conspecifics. Individuals of this species often show a dispersed, non-random spatial distribution, which only breaks down at very high local population densities. Individuals of Ophiocomina nigra will maintain a dispersed distribution from each other even when mixed with much larger numbers of Ophiothrix and Ophiocomina aggregations do not have such distinct boundaries .
Brittlestar beds do not usually have a uniform distribution of animals, instead being made up of patches of different densities of individuals. For example, in one large bed in Torbay, mean density was 309 animals per m2, but denser patches of over 1800 per m2 were present (Warner, 1971). The patchiness of the beds are variable in position over time.
The Ophiothrix fragilis population studied by Warner (1971) showed a bimodal size distribution with large adult animals and tiny juveniles. The juveniles were found clinging to the arms of the adults, where it is suggested they feed on material captured by the adult tube feet. At slightly larger sizes, the juvenile Ophiothrix fragilis appeared to migrate to rock outcrops among Alcyonium digitatum colonies, and into crevices in vertical rock faces. The reasons for this movement, and the return to the dense beds are unknown.
On bedrock, brittlestars tend to have a smothering effect, significantly reducing species diversity and biomass when they are very dense. The brittlestars are mobile and so some areas may appear highly grazed if they previously had brittlestar populations on them. However, on sedimentary substrata, a rich fauna of smaller animals may be found beneath the brittlestar layer. The sediment fauna does not appear to be restricted in numbers or growth by the carpet of brittlestars, and may actually benefit from the increased deposition of organic matter in the beds (Hughes, 1998). Despite the apparent dominance of Ophiothrix fragilis, up to 78 species have been recorded from a brittlestar bed, the most common of which was the bivalve Abra alba (Warner, 1971). Bed-forming brittlestars are not dependent on other species but may crawl onto sessile organisms to gain an elevated position for suspension feeding.
Ophiothrix fragilis may be considered a keystone species in the coastal marine ecosystem of the eastern Channel and a dominant species of gravel communities (Lefebvre & Davoult, 1997). When present in very high densities benthic suspension feeding brittlestars such as Ophiothrix fragilis can have a dominant role in the main nutrient exchanges in estuarine and coastal ecosystems (Dame, 1993 cited in Smaal, 1994; Lefebvre & Davoult, 1997). Brittlestar beds contribute to the cycling of nutrients in two ways, firstly from the output of nitrogenous excretory products and secondly by the removal of large amounts of suspended particulate matter from the water column. Davoult et al. (1991) concluded that the quantities of ammonium excreted by Ophiothrix beds in the Dover Strait made an important contribution to the total flux of ammonium, a nutrient necessary for phytoplankton production, in the water column. In coastal ecosystems brittlestar beds also contribute to nutrient exchange because they can remove large amounts of suspended particulate matter (Davoult & Gounin, 1995) and there is evidence to suggest that massive aggregations can have a favourable effect on water quality in coastal environments and may even help counteract some of the potentially harmful effects of eutrophication (Hughes 1998). The dense carpet of arms raised above the seabed will have a baffle effect on water currents, slowing down flow rates over the bed and producing an increased deposition of food particles (Hughes, 1998). For example, in the Bay of Brest in Brittany, Hily (1991) estimated that Ophiothrix beds with over 400 individuals m-2 can filter the equivalent of 30% of the total water volume of the bay daily. The inflow of nutrient-rich stream water into the bay leads to very high primary productivity, but eutrophication does not occur, apparently because of the removal of particulate matter by the benthic community.
Ophiothrix fragilis has been recorded as representing up to 62 % of the biomass in coarse sediment communities (Migné & Davoult, 1997(b)).
Large mobile animals are often found on brittlestar beds including the starfish Asterias rubens, Luidia ciliaris and Crossaster papposus, the urchins Echinus esculentus and Psammechinus miliaris and a variety of crabs (Hughes, 1998).
Aronson (1989, 1992) has suggested that low predation pressure is a necessary condition for the existence of brittlestar beds. Fish, in particular wrasse, and crabs were the main predators on reefs, whereas starfish (Asterias rubens ) were the main predators in brittle star beds. Mortality was higher on rocky reefs than in the brittlestar beds because of the differences in predation pressure. Starfish were common on both reefs and beds, while fish and crabs were rare outside the reef habitats. Although not an important dietary component, Ophiothrix fragilis may be found in the stomach contents of most common predators (Warner, 1971). Ophiothrix fragilis is not toxic but achieves unpalatability through heavy calcification and possession of glassy spines (Sköld, 1998). There is evidence that fluctuations in Ophiothrix fragilis populations in the western English Channel may be related to changes in the abundance of the large predatory starfish Luidia ciliaris (Holme, 1984; Aronson, 1992). Predation is less likely to be a controlling factor in populations of Ophiocomina nigra because the species secretes a distasteful mucus protecting it from predators.
Brittlestar beds are often separated from rocky reefs by an ophiuroid-free 'halo' zone on the level bottom, several meters wide (Aronson, 1992). The author suggests that the width of the halo probably represents the distance predatory fish and crabs will range from their shelters in the reef to forage on the level bottom. Thus, brittlestar beds cannot persist in the presence of fish and crabs (Aronson, 1992) and so such communities are limited to certain types of habitat. Sea star predation can further limit brittlestar beds as suggested by Holme's (1984) data which shows a decrease in Ophiothrix fragilis beds to be correlated with an increase in abundance of the predatory Luidia ciliaris.
This review can be cited as follows:
Hill, J.M. 2001. Ophiothrix fragilis and/or Ophiocomina nigra beds on slightly tide-swept circalittoral rock or mixed substrata. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 08/03/2014]. Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=278&code=2004>