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
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).
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.
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.
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,
Importance of the biotope complex
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.
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.