Causes and consequences of aggregation

Feeding

Structure of brittlestar beds

Reproduction and recruitment

Brittlestars are not the only echinoderm group known to occur in dense populations: aggregations of crinoids, sea urchins and sea cucumbers have also been recorded (Warner, 1979). The relatively simple nervous and sensory systems of echinoderms have been thought incapable of producing true social behaviour, ie. the intentional association of individuals with others of their own species. Reese (1966) suggested that aggregations arose from the individual responses of echinoderms to features of the physical environment, so for example, brittlestars might congregate in areas where strong currents bring an abundant supply of food. Broom (1975) investigated this question on an Ophiothrix bed in Torbay, Devon, by removing individual brittlestars from the bed, placing them on bare substratum and observing their responses. Isolated animals began ‘walking’ across the prevailing current, pausing and changing direction at intervals until other brittlestars were encountered. The animals rejoined the beds and resumed feeding. Movement across the bottom was only terminated by contact with conspecifics, and not by contact with stones, weed, hydroids or Alcyonium colonies. These experiments demonstrate that Ophiothrix can recognize and respond to conspecifics, and that this social response is important for the maintenance of aggregations. The abrupt boundaries frequently shown by Ophiothrix beds (Brun, 1969; Warner, 1971) are also strong evidence of social behaviour, as it unlikely that discontinuities in substratum type or water flow could be sharp enough to account for them.

Broom’s (1975) experiments also demonstrated the advantages of social aggregation for individual Ophiothrix. In the prevailing current speeds at the site, isolated brittlestars could not maintain their position on the substratum and were swept away. The minimum group size seen to persist for one day was 35 individuals. Broom estimated that at least 100 would be necessary for long-term group survival. The enhanced stability of aggregations in fast currents was also observed by Warner (1971). At current speeds above 0.25 m s^-1, aggregated Ophiothrix ceased feeding, flattened themselves against the substratum and linked arms, this mutual support allowing the bed to maintain its position on the sea floor. At slower current speeds, the support provided by neighbouring brittlestars will allow each animal to extend more feeding arms than would otherwise be possible.

Bed formation also has reproductive advantages for the animals involved. Brittlestars reproduce by external fertilization (see below), and the success of this will be enhanced by the close proximity of large numbers of individuals.

The presence of such dense concentrations of suspension-feeding brittlestars might be thought to cause serious competition for food, but in fact the density of animals may actually increase the supply of suspended food to them. The dense forest of arms raised into the current will have a baffle effect, slowing down the rate of water flow over the bed and producing an increased deposition of food particles. Dense aggregations would be expected to provide optimum conditions for the spread of parasites or disease, but nothing is known of this aspect of brittlestar ecology.

Ophiothrix fragilis feeds on plankton and suspended detritus intercepted by the tube feet of arms extended into the water column (Warner & Woodley, 1975). While feeding, the animal’s disc is held just off the bottom, supported by several flexed arms, the remaining arms being raised into the passing current (Warner, 1971). More arms can be used in feeding in dense aggregations where support is provided by neighbouring animals. Material trapped on the tube feet is collected into a bolus and passed down the arm to the animal’s mouth. Warner & Woodley found that the diet of Ophiothrix in Torbay consisted mostly of silt and detritus particles, but Brun (1969) found that diatoms were the main food on Isle of Man beds.

In the Dover Strait, Davoult & Gounin (1995a) found that current speeds below 0.2 m s^-1 were optimal for suspension-feeding, and that feeding activity ceased if velocity exceeded 0.3 m s^-1. These values agree with those found by Warner (1971). In the Dover Strait, currents exceed 1.5 m s^-1 during average spring tides, and the time available for feeding therefore varies according to the tidal cycle. Suspension-feeding by Ophiothrix is more or less continuous during neap tides, but the flux of particles is small because the slow current speed inhibits resuspension of material from the sea bottom. During spring tides, current speeds increase rapidly, bringing about a large resuspension of particles. Feeding bouts at these times are very brief, but more profitable to the brittlestars owing to the increased concentration of suspended matter. Growth rate of Ophiothrix in the Dover Strait is maximal in April/May, coincident with the spring phytoplankton bloom (Davoult & Gounin, 1995).

Ophiocomina nigra is a highly versatile feeder, able to exploit almost any available food source (Fontaine, 1965). Like Ophiothrix, it can capture small suspended particles, but in this case using a net of mucus strands secreted between the arm spines. Slightly larger particles are captured by the tube feet. Large particles or invertebrate prey can be actively grasped by the arms. Ophiocomina will also deposit-feed on benthic detritus or algal films, and will scavenge from carcasses on the sea floor. Individuals living intertidally will also use the arms to graze the detrital film at the air-water interface.

Ophiopholis aculeata has been recorded suspension-feeding on phytoplankton (Roushdy & Hansen, 1960). Brittlestars of the genus Ophiura are omnivorous, feeding on organic detritus, microalgae and small sediment-dwelling organisms. In Denmark, Feder (1981) found the large Ophiura ophiura to be quite predatory in its feeding habits, eating a wide variety of small bivalves, polychaetes and crustaceans. Tyler (1977) recorded a similar diet for specimens from the Bristol Channel, whereas the smaller O. albida was found to rely more on microalgae and detritus.

Although they may have sharply-defined boundaries, brittlestar beds do not usually comprise a uniform carpet of animals of unvarying density. Off the southern Isle of Man, the brittlestar bed studied by Brun (1969) had a mean density of 1347 animals m^-2, with local concentrations up to 2196 m^-2. Approximately 90% of these were Ophiothrix fragilis, the remainder being Ophiocomina nigra and Ophiopholis aculeata. In Torbay, the bed studied by Warner (1971) and Broom (1975) measured at least 1 km long by 200 m wide. Boundaries were sharp, but brittlestar distribution within the bed was very patchy. Mean density was 309 individuals m^-2, but denser patches of up to 1864 m^-2 were present. Within the bed, only about 23% of the sea floor was actually covered by brittlestars. The denser patches were up to 30 m across, irregular in shape, and variable in position over time. The bed was largely composed of Ophiothrix fragilis, but small numbers of Ophiocomina nigra also occurred in discrete aggregations of their own. A third brittlestar species, Amphipholis squamata, was also found among small Ophiothrix on rock outcrops. The Ophiothrix population in the bed showed a bimodal size distribution (Warner, 1971), being composed of large adult animals (disc diameter 8 - 12 mm) and tiny juveniles (disc diameter < 2 mm). Animals of intermediate size were absent from the gravel plain supporting the dense bed, but occurred on rock outcrops among Alcyonium colonies, and in crevices on nearby vertical rock faces. In the brittlestar bed, the tiny juveniles were found clinging to the arms of the adults, where they were probably feeding semi-parasitically on material captured by the adult tube feet. At slightly larger sizes, the juvenile Ophiothrix apparently migrated to rock outcrops and clumps of sessile epifauna. The factors leading to this movement, and the eventual reverse migration back to the level plain environment have not been determined.

In Kinsale Harbour, southern Ireland, Ball (1991) found that Ophiothrix of intermediate size could be found within the genital bursae (slits on the underside of the disc) of larger adult individuals. Between 9 and 22% of adult Ophiothrix were found to be carrying smaller animals in this way. This phenomenon of ‘pseudo-brooding’ was also recorded by Smith (1938). Its significance is unknown.

In the Gulf of St Malo, Ophiothrix beds were composed entirely of two-year old animals (Allain, 1974). Animals in the centre of the beds had a larger disc diameter than those at the periphery. This difference in growth rate may reflect the suitability of conditions for suspension-feeding in different parts of the bed. In the centre, individuals can raise more arms to capture food because of the greater support provided by neighbours.

In Strangford Lough, brittlestar beds are composed overwhelmingly of Ophiothrix fragilis, mixed with smaller numbers of Ophiocomina nigra (Erwin, 1977). At the fringes of the community, the Ophiothrix population often ends abruptly, but Ophiocomina continues for some distance at about the same density as in the Ophiothrix bed. This phenomenon arises from a difference in the social behaviour of the two species. The highly aggregative tendencies of Ophiothrix have already been described. Ophiocomina appears to be less fond of close contact with others of its species and does not occur in such dense aggregations as Ophiothrix. Ophiocomina often adopts a regularly-spaced distribution, in which individuals with arms spread horizontally do not quite touch each other (Warner, 1979). In the western English Channel, Ophiocomina aggregations up to 300 m across were observed using towed underwater televison on extensive offshore plains of rippled gravel and on the smooth tops of massive rock outcrops (Wilson et al., 1977). Within these patches, individual Ophiocomina showed a dispersed, non-random distribution on the sea bed. The regular spacing-out of individuals was confirmed by laboratory experiments, in which animals never came to rest in close proximity to one another, and sometimes showed antagonistic arm movements when encountering conspecifics. The regular spacing of individual Ophiocomina broke down only at very high population densities (200 m^-2). Observations suggested that when mixed with Ophiothrix, Ophiocomina individuals maintain the same type of spacing among themselves as when they occur alone.

Ophiothrix fragilis has an extended breeding season running roughly from April to October (Smith, 1940; Ball et al., 1995). Eggs and sperm are shed into the water column and fertilization is external. Larvae are planktonic. In the Dover Strait, the main period of larval settlement is in September/October, but some settlement also occurs in February, April and June (Davoult et al., 1990). Maximum population densities (approximately 2000 individuals m^-2) are found during the main recruitment period in September (Davoult, 1990). A similar seasonal pattern was found by Brun (1969) in the Isle of Man, where newly-settled juveniles were found in August and September. Peak juvenile numbers occurred in November in a Bristol Channel population (George & Warwick, 1985). In Kinsale Harbour, Ireland, post-settlement juveniles could be found throughout the year, with maximum numbers (up to 1000 juveniles m^-2) in October (Ball et al., 1995). Mortality was high, leading to low levels of recruitment into the adult population. All studies agree that recruits initially settle on the arms of adults.

There is some disagreement concerning the life span of Ophiothrix fragilis. Davoult et al. (1990) suggested a life span of 9 - 20 months. Taylor (1958, quoted in Gorzula, 1977) recorded that Ophiothrix reached a disc diameter of about 14 mm in two years, and that most individuals died after spawning in their second summer. However, other researchers have considered the animals to be much longer-lived. Gorzula (1977) quotes evidence that Swedish Ophiothrix can live for up to eight years. A life span of over nine years has been suggested from counts of growth bands in the skeletal arm plates of Ophiothrix (Gage, 1990). It is possible that growth rates may vary widely in different areas, or that the different varieties of Ophiothrix fragilis recognized by French workers may have contrasting population dynamics. These inconsistencies show that even in a species as well-known as this, many basic questions still remain to be resolved.

The breeding system of Ophiocomina nigra appears to be somewhat more complex than that of Ophiothrix fragilis (Gorzula, 1979). Male and female individuals grow to about the same size, but in the spring, large female brittlestars (disc diameter 10 - 20 mm) are often found in close association with small males (mostly 5 - 10 mm diameter). The small males cling to the upper or lower disc surface of the females. In the Clyde, spawning took place over a limited period in late June. Spawning was not confined to the paired males and females, but it is possible that these acted as a ‘trigger’ for synchronized spawning of the whole population (Gorzula, 1979). Ophiocomina nigra appears to be a slow-growing, long-lived species spawning annually after reaching an age of 3 - 4 years, and living for up to 14 years (Gorzula, 1977). In contrast to the pattern in Ophiothrix, juvenile Ophiocomina appear not to settle among adults. The Clyde populations studied by Gorzula (1977) were each dominated by a single size-class of animals, suggesting that each Ophiocomina bed is formed by a single settlement of juveniles which thereafter receives little or no recruitment.

There is no detailed information available on the life cycle or population dynamics of Ophiopholis aculeata. Tyler (1977) found that populations of Ophiura albida in the Bristol Channel had a well-marked annual reproductive cycle, with spawning taking place in May and early June. Spent adults and planktonic larvae were found up to early October. This short annual reproductive period led to the occurrence of distinct size cohorts in the adult population. In contrast, the larger Ophiura ophiura had a more protracted breeding season, and adult size classes were less distinct. Gage (1990) suggested a life span of 5 - 6 years for O. ophiura from the west of Scotland. Studies of growth bands in the arm plates suggested that the animals enter a phase of rapid growth in early spring, continuing until late autumn/early winter. Skeletal growth ceases during the winter (Wilding & Gage, 1995). Further details of growth rates in Ophiura ophiura and O. albida are given by Dahm (1993).

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