information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

Ocnus planci aggregations on sheltered sublittoral muddy sediment



UK and Ireland classification


Dense aggregations of Ocnus planci (or Ocnus brunneus?) on various substrata, typically muddy but sometimes with stones or shells, in sheltered conditions such as sea lochs. Associated species vary but are typical of very sheltered muddy habitats. Melanella alba, which parasitises holothurians, was found in large numbers at one site. (Information taken from the Marine Biotope Classification for Britain and Ireland, Version 97.06: Connor et al., 1997a, b).

Depth range

0-5 m, 5-10 m, 10-20 m, 20-30 m

Additional information

The description of this biotope is based on only four records (Erwin et al., 1990; Connor et al., 1997a). The taxonomic status of Ocnus planci and Ocnus brunneus is under review and the species may have been confused. However, the two species are ecologically similar and have been discussed together for the sake of this review.

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Habitat review


Ecological and functional relationships


Seasonal and longer term change


Habitat structure and complexity




Recruitment processes


Time for community to reach maturity


Additional information


Preferences & Distribution

Habitat preferences

Depth Range 0-5 m, 5-10 m, 10-20 m, 20-30 m
Water clarity preferencesNo information found
Limiting Nutrients Not relevant
Salinity preferences Full (30-40 psu), Variable (18-40 psu)
Physiographic preferences Enclosed coast / Embayment, Sea loch / Sea lough
Biological zone preferences Circalittoral, Infralittoral
Substratum/habitat preferences Cobbles, Mud and sandy mud, Pebbles, Small boulders
Tidal strength preferences Very Weak (negligible), Weak < 1 knot (<0.5 m/sec.)
Wave exposure preferences Extremely sheltered, Moderately exposed, Sheltered, Very sheltered
Other preferences Hard substrata

Additional Information

The biotope is characterized by the presence of hard substrata (cobbles, pebbles, gravel, and shell debris) in muddy or sandy habitats.

Species composition

Species found especially in this biotope

Rare or scarce species associated with this biotope


Additional information

The MNCR recorded 131 species within this biotope, although not all species occurred in all records of the biotope.

Sensitivity review


This community is characterized by the abundance of Ocnus planci (or Ocnus brunneus). Loss of the Ocnus population would result in loss of the biotope as described and, therefore Ocnus has been considered to be important characterizing.

The other species in the community are common and characteristic of the wave sheltered muddy habitats in which the biotope is found. Therefore, the dominant associated species vary with location and have little significant association with the Ocnus population itself. Reference has been made to Nemertesia ramosa to represent hydroids, Virgularia mirabilis to represent sea pens, Pomatoceros triqueter to represent tubeworms, Ciona intestinalis to represent ascidians, and Alcyonium digitatum to represent anthozoans, and Echinus esculentus, Ophiothrix fragilis, Amphiura filiformis and Neopentadactyla mixta to represent echinoderms.

Species indicative of sensitivity

Community ImportanceSpecies nameCommon Name
Important characterizingOcnus planciA sea cucumber

Physical Pressures

 IntoleranceRecoverabilitySensitivitySpecies RichnessEvidence/Confidence
High Moderate Moderate Major decline Low

Removal of the substratum would result in the loss of the community as a whole, and hence the biotope. Therefore an intolerance of high has been recorded. Recoverability is probably high once favourable conditions return.

Intermediate High Low Decline Low

Deposition of a 5 cm layer of sediment is unlikely to disturb burrowing infaunal species. Kukert & Smith (1992) noted that deposition of 5-6 cm of sediment resulted in a 32% reduction in macrofaunal abundance in less than 1 month but that deposit feeders had returned to background levels within 3 months, while scavengers and carnivores took longer. Suspension feeding epifauna are likely to be most intolerant. Species that project above the sediment surface, such as sea pens and Sabella pavonina are probably tall enough to avoid the effects, while burrowing species such as Myxicola infundibulum, Chaetopterus variopedatus, Cerianthus lloydii and Amphiura filiformis could probably burrow up through the sediment. Ocnus planci may be large enough to avoid the effects of smothering but Ocnus brunneus is a small species, however, holothurians are generally burrowing species and it is probably able to regain the sediment surface rapidly. Raise areas of sediment may even provide this species with a new feeding perch.

Other epifauna e.g. hydroids, tubeworms, ascidians are fixed to the substratum, while the brittlestar Ophiothrix fragilis and smaller sea urchins are unable to burrow effectively and would probably be smothered, potentially succumbing due to clogged respiratory apparatus and temporary anoxia.

Overall, Ocnus and most of the species typical of mud communities would probably survive smothering by 5cm of sediment for a month. However, some epifaunal species, and juveniles of several species may be adversely affected, resulting in a short term decrease in species richness. Therefore an intolerance of intermediate has been recorded. Recovery of the affect species would probably be rapid (see recoverability below).

Low Very high Very Low No change Low

The majority of the species present in the biotope are characteristic of sheltered muddy habitats, and hence probably tolerant of relatively high suspended sediment loads and siltation. For example, Ocnus planci (as Cucumaria planci) maintained a high filtration efficiency even at food concentrations (suspension densities) three times average natural levels. Burrowing species are unlikely to be affected and suspension feeding species are likely to experience, and hence tolerate, periodic re-suspension of suspended sediment due to wave or current surges, storms or digging by predators. Therefore, they are likely to tolerate an increase in suspended sediment for a month, although feeding efficiency of some species, especially epifaunal suspension feeders, may be impaired.

Low Moderate Moderate Major decline Low

A decrease in suspended sediment may result in decrease in food availability for suspension feeding species, and may allow more silt intolerant epifaunal species, e.g. hydroids and bryozoans to colonize the available hard substrata. Overall, a decrease in suspended sediment for a month (see benchmark) may impair food availability in the short term but have little adverse effect. Therefore an intolerance of low has been recorded.

Not relevant Not relevant Not relevant Not relevant Not relevant

This biotope is found in the infralittoral zone and so is not likely to be exposed to the air or desiccation. If any of the characterizing species were exposed to the air, e.g. by being washed ashore, they are likely to be highly intolerant. However, desiccation has been recorded as not relevant.

Tolerant Not relevant Not relevant No change Low

This biotope was recorded from ca 5-30m depth (Connor et al., 1997a). An increase in emergence may effectively reduce the average depth of the biotope, allowing macroalgae to colonize. In Loch Goil, Ocnus planci was absent from the upper part of the habitat surveyed where the macroalgae Phycodrys rubens and the coralline algae Lithothamnium glaciale were present (JNCC, 1999). But, Ocnus planci has been observed using macroalgae as a perch when feeding (see Picton, 1993).

While an increase in algal growth may not adversely affect Ocnus planci, epifauna may experience increased competition with for space, especially on upper surfaces and become smothered by algae. The biotope would probably remain although a few intolerant epifaunal species may be replaced by macroalgae. Therefore, not sensitive has been recorded at the benchmark level.

Not relevant Not relevant Not relevant Not relevant Not relevant

The biotope is found in the infralittoral zone so a decrease in emergence is not relevant.

High Moderate Moderate Major decline Low

The biotope is only found in areas of weak or very weak tidal streams and so is likely to be intolerant of increases in water flow. Some tidal flow is necessary for the supply of food in the form of organic particles by resuspension and advective transport, gaseous exchange and the removal of wastes, influencing the growth rate of suspension-feeding benthos (Dauwe, 1998). However, some suspension feeders in the biotope will be unable to feed if the water flow rate increases by two categories in the water flow scale (see benchmarks) form e.g. weak to strong flow. The sea pen Virgularia mirabilis for example, will retract into the sediment at water currents speeds greater than 0.5m/s (i.e. 1 knot) (Hiscock, 1983). If water speeds remain at this level or above, sea-pens will be unable to extend above the sediment, will be unable to feed and will probably die. In the same study, the brittlestar Ophiothrix fragilis was swept off the surface in flume experiments in currents above 0.3m/s (Hiscock, 1983). The sea slug Philine aperta, which lives on the surface of the sediment would probably be washed away by strong water movement. Ocnus planci has been recorded from moderately strong to very weak tidal streams. The feeding efficiency of suspension feeding species may be impaired in strong flow, while allowing other species to colonize.

An increase in water flow will probably result in a marked change in the community due to modification of the substratum, removing finer particulates such as muds and favouring the deposition of coarser deposits. The resultant sediment scour may adversely affect epifaunal species. Increased water flow may also cause small stones, pebbles and cobbles to roll along the substratum resulting in further abrasion for epifauna. Therefore, a long term increase in water flow rates would probably result in the loss of many of the species and marked changes in the substratum on which the community depends, and hence the biotope. Therefore, an intolerance high has been recorded. Once the substratum returns to prior condition a recognizable biotope will probably recover within 5 years although the full diversity of species may take longer to develop.

Low Very high Very Low Not relevant Low

The biotope exists in habitats such as sea lochs, where tidal streams are already very weak so a decrease in flow rate would result in almost non-moving water. Water movement is essential for suspension feeders such as hydroids, bryozoans, sponges, amphipods and ascidians to supply adequate food, remove metabolic waste products, prevent accumulation of sediment and disperse larvae or medusae. In the enclosed or semi-enclosed water bodies occupied by the biotope, negligible water flow may result in stratification and some deoxygenation (see below). But since de-oxygenation is a rare phenomenon in sea lochs, it seems likely that sufficient water flow will persist. Therefore, an intolerance of low has been recorded.

Low Very high Very Low No change Low

Hughes (1998) suggested that in shallow sea lochs, sedimentary biotopes would probably experience a seasonal temperature range between about 5-15 °C. Greater temperature extremes may be experienced in unusually warm summers or cold winters, although the effects on sedimentary communities is unknown (Hughes, 1998). Burrowing species would probably be able to avoid extreme temperatures within the sediment, so that epifauna were likely to be more vulnerable.

Ocnus planci and Ocnus brunneus reach their northern limit in northern Europe and the British Isles, Ocnus brunneus is reported from France and Denmark while Ocnus planci is reported from the Mediterranean and Senegal. Therefore, they are unlikely to be adversely affected by a long term temperature rise. Similarly, most other dominant species within the biotope are found to the south of the British Isles and unlikely to be adversely affects by long term temperature rise. The growth and fecundity of Amphiura filiformis may increase with increased average temperatures while Ophiothrix fragilis was reported to recruit in high numbers after mild winters (see MarLIN reviews).

Overall, most species in the biotope are unlikely to be adversely affected by long term temperature rise and are buffered from extremes of temperature by their depth. Therefore, an intolerance of low has been recorded to represent effects on growth rates and metabolism. In shallow, isolated waters (e.g. sea lochs) increased summer temperatures may result in stratification of the water column (a thermocline) and increased oxygen demand resulting in deoxygenation of the bottom waters (Hughes, 1998)(see below).

Low Very high Moderate Minor decline Low

Ocnus planci and Ocnus brunneus reach their northern limit in British waters, however no information on their temperature tolerance was found. Most of the other common species within the biotope are recorded north of British and Irish waters and are unlikely to be affected by long term decreases in temperature. However, shallow waters population may be more vulnerable to low temperatures in extreme winters (Hughes, 1998). For example, a population of Amphiura filiformis at 27m depth off the Danish coast was killed by the winter of 1962-63 (Muus, 1981) and a population at 35-50m depth in the inner German Bight was killed in the winter of 1969-1970 and a new population did not re-establish until 1974 (Gerdes, 1976). Ursin (cited in Gerdes, 1978) suggested that Amphiura filiformis did not occur in areas with winter temperatures below 4°C although in Helgoland waters it can tolerate temperatures as low as 3.5°C. Low temperatures are a limiting factor for breeding in this species, which takes place during the warmest months in the British waters. Similarly, Ophiothrix fragilis occurs in shallow, enclosed waters that regularly drop to 3 °C but is absent from areas where temperatures drop to 0 °C (see review). In addition, temperature influences growth and reproduction in many species of hydroids and ascidians (see species reviews).

Overall, most species will probably not be adversely affected by a long term decrease in temperatures, although short term acute decreases in temperatures associated with extreme winters may result in loss of a few species. Therefore an intolerance of low has been recorded.

Tolerant Not relevant Not relevant No change Low

An increase in turbidity will decrease the extent of macroalgae in shallower instances of the biotope but otherwise not adversely affect the community. Therefore, not sensitive has been recorded.

Low Very high Moderate Minor decline Low

A decrease in turbidity may enable macroalgae to colonize to greater depths, increasing competition with epifaunal species and perhaps smothering smaller species, e.g. hydroids. However, Ocnus can use macroalgae as a perch during feeding and may not be adversely affected (see Picton, 1993). Therefore, an intolerance of low has been recorded, to represent increased competition for hard substratum within the biotope.

High Moderate Moderate Major decline Low

Ocnus planci was recorded from moderately wave exposed to extremely wave sheltered habitats (JNCC, 1999). But IMU.Ocn was only recorded from wave sheltered to ultra sheltered conditions. An increase in wave exposure for e.g. sheltered to exposed is likely to have a marked effect on the community. The resultant increase in oscillatory water movement at the sediment surface is likely to resuspend fine particulates, and deposit coarser sediment types, e.g. sands and gravels resulting in loss of many of the mud dwelling species. Water movement is also likely to roll, or move stones, cobbles , pebbles and shell fragments, resulting in increased sediment scour and abrasion, and probably loss of many of the epifaunal species. Ocnus species may survive on coarser substrata but are rare in wave exposed environments (JNCC, 1999). Overall, the community is likely to change significantly and the aggregations of Ocnus lost. Therefore, an intolerance of high has been recorded. Recoverability may be high (see additional information below).

Not relevant Not relevant Not relevant Not relevant Low

This biotope occur in wave sheltered to ultra wave sheltered environments. A further decrease in wave exposure is unlikely.

Tolerant Not relevant Not relevant Not relevant High

Marine invertebrates are thought to respond to noise or vibration similar to the hydrographic flow of currents and eddies, and are unlikely to be adversely effected by anything but close proximity to powerful sound sources such as seismic survey arrays or explosions (Vella et al., 2001). Therefore, not sensitive has been recorded at the benchmark level.

Tolerant None Not relevant Not relevant High

Many of the species within the biotope probably respond to light levels, detecting shade and shadow to avoid predators, and day length in their behavioural or reproduction. However, their visual acuity is probably very limited and they are unlikely to be intolerance of visual disturbance at the benchmark level.

Intermediate High Low Decline Low

Erect epifaunal species are particularly vulnerable to physical disturbance. Hydroids are likely to be detached or damaged by bottom trawling or dredging (Holt et al., 1995). Veale et al. (2000) reported that the abundance, biomass and production of epifaunal assemblages decreased with increasing fishing effort. Hydroid and bryozoan matrices were reported to be greatly reduced in fished areas (Jennings & Kaiser, 1998 and references therein). Mobile gears also result in modification of the substratum, including removal of shell debris, cobbles and rocks, and the movement of boulders (Bullimore, 1985; Jennings & Kaiser, 1998) on which many of the species in this community depend. The removal of rocks or boulders to which species are attached results in substratum loss (see above). Magorrian & Service (1998) reported that queen scallop trawling flattened horse mussel beds and removed emergent epifauna in Strangford Lough. They suggested that the emergent epifauna such as Alcyonium digitatum, a frequent component of this biotope, were more sensitive than the horse mussels themselves and reflected early signs of damage. Species with fragile tests such as Echinus esculentus and the brittlestar Ophiocomina nigra and edible crabs Cancer pagurus were reported to suffer badly from the impact of a passing scallop dredge (Bradshaw et al., 2000). But brittlestars such as Ophiothrix fragilis probably have good powers of regeneration and infaunal species such as Amphiura filiformis may avoid the effects of a passing beam trawl. Scavengers such as Asterias rubens and Buccinum undatum were reported to be fairly robust to encounters with trawls (Kaiser & Spencer, 1995) may benefit in the short term, feeding on species damaged or killed by passing dredges. However, Veale et al. (2000) did not detect any net benefit at the population level.


Sea pens project above the surface of the sediment floor and so are likely to be sensitive to physical damage. Species obtained by dredges were invariably damaged (Hoare & Wilson, 1977) and during SCUBA diving observations occasional colonies were seen to be broken, presumably as a result of accidental contact with large animals or possibly due to anchoring. Eno et al. (1996) found that even if damaged, the sea pen, Funiculina quadrangularis, appeared to remain functional and this could also be true of Virgularia mirabilis. In experiments, sea pens were seen to bend away in response to the pressure wave travelling ahead of a dropping pot/creel (Eno et al., 1996) This means the top of the sea pen is less likely to be struck causing fracture of the colony. On removal of the pots sea pens consistently righted themselves. In addition, during a manipulative experiment to examine the effect of extensive and repeated experimental trawl disturbance over an 18 month period Tuck et al. (1998) reported no effects on the abundance and distribution of Virgularia mirabilis. The authors suggested that this may be due to the species' ability to rapidly withdraw into the sediment, thereby avoiding damage.

Ocnus species are the most important characterizing species within the biotope, and may be damaged by physical disturbance due to an anchor or passing fishing gear. But they probably have good powers of regeneration (given their fissiparous habit) and would survive if the damage were not too severe. However, they may be displaced in the process (see below). Overall, physical disturbance by an anchor or mobile fishing gear is likely to remove a proportion of all groups within the community and attract scavengers to the community in the short term. Therefore, an intolerance of intermediate has been recorded. Recoverability is likely to be high due to repair and regrowth of hydroids and echinoderms and recruitment within the community from surviving individuals (see additional information below).

Low Very high Very Low Minor decline Low

Ocnus species are unlikely to be adversely affected as long as they are not damaged during displacement, although their regenerative powers are probably high. Ocnus occurs on a wide variety of substrata and will probably survive as long as the habitat includes surfaces raised above the substratum, moderate water movement and an adequate supply of organic particulates and plankton in suspension. Ocnus will probably migrate and aggregate on raised surfaces. Most of the burrowing species, including Virgularia mirabilis, brittlestars and Cerinathus lloydii will probably be able to re-burrow effectively.

Displacement of rocks, stones and pebbles is likely to physically damage epifaunal species by abrasion, e.g. brittlestars and hydroids. Most permanently fixed, sessile species, sponges, ascidians (e.g. Ascidella spp.) and hydroids cannot reattach to the substratum if removed, and may be damaged or destroyed in the process. Hydroids and sponges may be able to grow from fragments, aiding recovery. Mobile species, such as amphipods, gastropods, small crustaceans, crabs and fish are likely to survive or avoid displacement.

Overall, the dominant species are likely to survive displacement, while a proportion of the epifauna may be lost. But the biotope may still be recognizable so an intolerance of low has been recorded. Recovery is likely to be very high (see additional information below).

Chemical Pressures

Intermediate High Low Decline Very low

Little information on the toxicity of synthetic chemicals to holothurians was found. Newton & McKenzie (1995) suggested that echinoderms tend to be very intolerant of various types of marine pollution but gave no detailed information. Cole et al. (1999) reported that echinoderm larvae displayed adverse effects when exposed to 0.15mg/l of the pesticide Dichlorobenzene (DCB). Smith (1968) demonstrated that 0.5 -1ppm of the detergent BP1002 resulted in developmental abnormalities in echinopluteus larvae of Echinus esculentus. Smith (1968) also noted that large numbers of dead Echinus esculentus were found between 5.5 and 14.5 m in the vicinity of Sennen, presumably due to a combination of wave exposure and heavy spraying of dispersants in that area. TBT was shown to inhibit arm regeneration in the brittlestar Ophioderma brevispina, at 10ng/l and produce significant inhibition at 100 ng/l. It was suggested that TBT acts via the nervous system, although direct action on the tissues at the point of breakage could not be excluded (Bryan & Gibbs, 1991) Therefore, holothurians and their larvae may also be intolerant of synthetic chemicals.

The species richness of hydroid communities decreases with increasing pollution (Boero, 1984; Gili & Hughes, 1995). Alcyonium digitatum at a depth of 16m in the locality of Sennen Cove (Pedu-men-du, Cornwall) died resulting from the offshore spread and toxic effect of detergents e.g. BP 1002 sprayed along the shoreline to disperse oil from the Torrey Canyon tanker spill (Smith, 1986). Possible sub-lethal effects of exposure to synthetic chemicals, may result in a change in morphology, growth rate or disruption of reproductive cycle.

Tri-butyl tin (TBT) has a marked effect on numerous marine organisms (Bryan & Gibbs, 1991). Bryan & Gibbs (1991) reported that virtually no hydroids were present on hard bottom communities in TBT contaminated sites and suggested that some hydroids were intolerant of TBT levels between 100 and 500 ng/l. Copepod and mysid crustaceans were particularly intolerant of TBT while crabs were more resistant (Bryan & Gibbs, 1991), although recent evidence suggests some sublethal endocrine disruption in crabs. The effect of TBT on Nucella lapillus and other neogastropods is well known (see review), and similar effects on reproduction may occur in other gastropod molluscs, including nudibranchs. Rees et al. (2001) reported that the abundance of epifauna had increased in the Crouch estuary in the five years since TBT was banned from use on small vessels. Rees et al. (2001) suggested that TBT inhibited settlement in ascidian larvae. This report suggests that epifaunal species (including, bryozoan, hydroids and ascidians) may be at least inhibited by the presence of TBT.

Therefore, hydroids, crustaceans, gastropods, and ascidians are probably intolerant of TBT contamination. Echinoderms, including Ocnus may be intolerant of synthetic chemicals. Overall, therefore, members of this biotope may be intolerant of synthetic chemicals to varying degrees and adverse effects on larvae may reduce recruitment in the long term resulting in the loss of as proportion of the population. Therefore, an intolerance of intermediate has been recorded, albeit at low confidence. A recoverability of high has been recorded (see additional information below).

Heavy metal contamination
Intermediate High Low Minor decline Very low

Bryan (1984) reported that early work had shown that echinoderm larvae were sensitive to heavy metals, e.g. the intolerance of larvae of Paracentrotus lividus to copper (Cu) had been used to develop a water quality assessment. Kinne (1984) reported developmental disturbances in Echinus esculentus exposed to waters containing 25 µg / l of copper (Cu) and heavy metals caused reproductive anomalies in the starfish Asterias rubens (Besten, et al., 1989, 1991). Sea-urchins, especially the eggs and larvae, are used for toxicity testing and environmental monitoring (reviewed by Dinnel et al. 1988). Crompton (1997) reported that mortalities occurred in echinoderms after 4-14day exposure to above 10-100 µg/l Cu, 1-10 mg/l Zn and 10-100 mg/l Cr but that mortalities occurred in echinoderm larvae above10-100 µg/ l Ni.

Various heavy metals have been show to have sublethal effects on growth in the few hydroids studied experimentally (Stebbing, 1981; Bryan, 1984; Ringelband, 2001). Gastropod molluscs have been reported to relatively tolerant of heavy metals while a wide range of sublethal and lethal effects have been observed in larval and adult crustaceans (Bryan, 1984). Bryan (1984) suggested that polychaetes are fairly resistant to heavy metals, based on the species studied. Short term toxicity in polychaetes was highest to Hg, Cu and Ag, declined with Al, Cr, Zn and Pb whereas Cd, Ni, Co and Se the least toxic.

Overall, while some invertebrate groups are tolerant, many of the other groups, including echinoderms are probably intolerant, especially their larvae. Therefore an intolerance of intermediate has been recorded, albeit with very low confidence.

Hydrocarbon contamination
High Moderate Moderate Major decline Low

Sheltered embayments and lagoons, where this biotope is found, are particularly vulnerable to oil pollution, which may settle onto the sediment and persist for years (Cole et al., 1999). Subsequent digestion or degradation of the oil by microbes may result in nutrient enrichment and eutrophication (see nutrients below). Although, protected from direct smothering by oil by its depth, the biotope is relatively shallow and would be exposed to the water soluble fraction of oil, water soluble PAHs, and oil adsorbed onto particulates.

Suchanek (1993) reviewed the effects of oil spills on marine invertebrates and concluded that, in general, on soft sediment habitats, infaunal polychaetes, bivalves and amphipods were particularly affected. Crude oil and refined oils were show to have little effect on fertilization in sea urchin eggs but in the presence of dispersants fertilization was poor and embryonic development was impaired (Johnston, 1984). Sea urchin eggs showed developmental abnormalities when exposed to 10-30mg/l of hydrocarbons and crude oil : Corexit dispersant mixtures have been shown to cause functional loss of tube feet and spines in sea urchins (Suchanek, 1993). Olsgard & Gray (1995) found the brittlestar Amphiura filiformis to be very sensitive to oil pollution. During monitoring of sediments in the Ekofisk oilfield Addy et al. (1978) suggest that reduced abundance of Amphiura filiformis within 2-3km of the site was related to discharges of oil from the platforms and to physical disturbance of the sediment. Although acute toxicity test showed that drill cuttings containing oil based muds had a very low toxicity (LC50 52,800 ppm total hydrocarbons in test sediment) Newton & McKenzie (1998) suggest these are a poor predictor of chronic response. Chronic sub-lethal effects were detected around the Beryl oil platform in the North Sea where the levels of oil in the sediment were very low (3ppm) and Amphiura filiformis was excluded from areas nearer the platform with higher sediment oil content. Similarly, in Ophiothrix fragilis, exposure to 30,000 ppm oil reduces its load of symbiotic bacteria by 50 % and brittle stars begin to die (Newton & McKenzie, 1995). Crude oil from the Torrey Canyon and the detergent used to disperse it caused mass mortalities of echinoderms; Asterias rubens, Echinocardium cordatum, Psammechinus miliaris, Echinus esculentus, Marthasterias glacialis and Acrocnida brachiata (Smith, 1968).

Suchanek (1993) reported that the anemones Anthopleura spp. and Actinia spp. survived in waters exposed to spills and chronic inputs of oils. Similarly, one month after the Torrey Canyon oil spill the dahlia anemone, Urticina felina, was found to be one of the most resistant animals on the shore, being commonly found alive in pools between the tide-marks which appeared to be devoid of all other animals (Smith, 1968). If the physiology within different animals groups can be assumed to be similar, then amphipods, echinoderms and soft corals may be intolerant of hydrocarbon contamination, while hydroids may demonstrate sublethal effects and anemones and some species of sponge are relatively tolerant. Therefore, an intolerance of high has been recorded, albeit with low confidence.

Radionuclide contamination
No information Not relevant No information Insufficient
Not relevant


Changes in nutrient levels
Intermediate High Low Decline Low

Hughes (1998b) suggested that sea loch sediments, where this biotope occurs, were naturally rich in organic matter. Moderate enrichment, is likely to increase food availability and hence, the abundance of deposit feeding organisms. Increases in nutrients and low oxygen conditions result in anaerobic conditions within the sediment. Nutrient enrichment may also result in algal blooms. For example:

  • nutrient enrichment of sediment results in exclusion numerous macrofauna and dominance by a few, tolerant species such as Capitella capitata;
  • Cerianthus lloydii was found near the centre of sewage sludge dumping groups at ca 10% organic carbon but more abundant at intermediate nutrient enrichment (Hughes, 1998b);
  • Virgularia mirabilis and Pennatula phosporea were found to be abundant in the vicinity of distillery effluent in sediment enriched to <5% organic carbon. But Virgularia mirabilis was absent from areas of Holyhead harbour heavily affected by sewage (Hoare & Wilson, 1977; Hughes, 1998b), and
  • echinoderms were shown to be intolerant of the effects of algal blooms, resulting in mortalities of the sea urchins Echinus esculentus and Paracentrotus lividus, and the holothurian Labidoplax digitata amongst other echinoderms, probably due to hypoxia caused by death of the algal bloom algae (Boalch, 1979; Forster, 1979; Griffiths et al., 1979; Lawrence, 1996).

Therefore, while moderate enrichment may be beneficial, sea pens in particular are probably intolerant of the effects of eutrophication, while echinoderms are indirectly intolerant due to sudden hypoxia resulting form the death of algal blooms (see below). Therefore, an intolerance of intermediate has been recorded at the benchmark level. Recoverability is probably high (see additional information below).

Not relevant None Not relevant Not relevant Not relevant

This biotope is recorded from full to variable salinity and a further increase in salinity is unlikely.

High Moderate Intermediate Major decline Low

Echinoderms are generally regarded as stenohaline and most species are exclusively marine (Binyon, 1966; Pawson, 1966; Stickle & Diehl, 1987; Lawrence, 1996). Lawrence (1996) cites several examples of mass moralities in echinoderms due to sudden increases in river discharge or localized heavy rains. However, some euryhaline species have been reported and local adaptation may occur in some species (see Binyon, 1966 and Stickle & Diehl, 1987 for reviews). In addition, Jones et al. (2000) suggested that Virgularia mirabilis appears to be somewhat tolerant of occasional lowering of salinity. Most other species are likely to be adapted to full saline conditions. A decrease in salinity from e.g. full to reduced for a year (see benchmark) is likely to result in marked changes in community composition, including loss of the Ocnus aggregations. Therefore an intolerance of high has been recorded with a recoverability of high (see additional information below).

High Moderate Moderate Major decline Moderate

Lawrence (1996) reported mass mortality of echinoderms in the Gulf of Trieste due to hypoxia caused by a strong thermocline combined with high pelagic productivity and eutrophication. The brittlestar Ophiura quinquemaculata were killed with a few days, holothurians including Ocnus planci (as Cucumaria planci), starfish Asteropecten sp. and the remaining brittlestars were killed within a week. In experiments Amphiura filiformis only left its protected position in the sediment when oxygen levels fell below 0.85mg/l (Rosenberg et al., 1991). Mass mortality of Amphiura filiformis has been observed during severely low oxygen events (<0.7 mg/l) (Nilsson, 1999). However, at oxygen concentrations between 0.85mg/l and 1.0mg/l Rosenberg et al. (1991) observed the species was able to survive for several weeks. Echinoderms were shown to be intolerant of the effects of algal blooms, resulting in mortalities of the sea urchins Echinus esculentus and Paracentrotus lividus, and the holothurian Labidoplax digitata amongst other echinoderms, probably due to hypoxia caused by death of the algal bloom algae (Boalch, 1979; Forster, 1979; Griffiths et al., 1979; Lawrence, 1996). Diaz & Rosenberg (1995, Figure 5)) suggested that shrimp and crustaceans were lost as oxygen levels dropped below ca 0.75ml/l and that the macroinfauna was reduced below ca 0.4ml/l. The sea pen Virgularia mirabilis was also reported to be absent from anoxia sediments, heavily affected by sewage in Holyhead harbour (Hoare & Wilson, 1977).

Overall, based on the evidence cited above, echinoderms, including the Ocnus planci, appear to be intolerant of hypoxic conditions. Tolerance of hypoxia varies between invertebrate species groups but an overall intolerance of high has been recorded. Recoverability is probably high.

Biological Pressures

Low Very high Very Low No change Moderate

The gastropod snail Melanella alba parasites holothurians, probably by sucking their fluids (Graham, 1988) and occurs at high abundance in this biotope (Connor et al., 1997a). Any parasite is likely to reduce the viability of the host species, so an intolerance of low has been recorded.

No information Not relevant No information Insufficient
Not relevant

No information found.

Not relevant Not relevant Not relevant Not relevant Not relevant

It is extremely unlikely that any of the species indicative of sensitivity would be targeted for extraction and we have no evidence for the indirect effects of extraction of other species on this biotope.

Not relevant Not relevant Not relevant Not relevant Not relevant

Additional information

Very little is known about recruitment in Ocnus. In holothurians with indirect development, the pelagic period from spawning to settlement usually ranges from two weeks to two months. But Ocnus has direct development, releasing a pelagic vitellaria larvae, so that the pelagic period is probably shorter, possibly a few weeks to a few days. Therefore, local recruitment may be good but long distance recruitment will depend on the local currents. Recruitment in echinoderm species is poorly understood and often sporadic. In the isolated waters in which this biotope occurs, long range dispersal is likely to be low. However, Ocnus is a mobile species, using raised surfaces to aid feeding, and would probably recruit to and aggregate in areas of raised hard substrata from the surrounding area. Subsequent development of large populations is probably aided by fissiparous asexual reproduction, although the rate of reproduction is unknown. Ocnus is probably fairly long-lived. A specimen of Ocnus planci (as Cucumaria planci) was maintained in an aquarium for three years and four months (Hyman, 1955). Recruitment is likely to be slow and recovery of a large population may be prolonged by sexual reproduction alone, however its ability to reproduce asexually may allow the population to grow relatively quickly. Therefore, where a proportion of the population remains recoverability may be high but take longer recovery is dependant on recruitment alone.

Very little is known about the population dynamics and longevity of Virgularia mirabilis in Britain. However, information from other species suggest that this species is likely to be slow growing with patchy and intermittent recruitment and so recovery from loss of this species is likely to longer than five years. Philine aperta is thought to live for 3-4 years and spawns egg masses, which release pelagic larvae, for several months between the spring and summer so recovery is likely to fairly rapid. Individuals can also migrate in from outside areas.

Many epifaunal species, e.g. hydroids, colonial ascidians, some sponges and Metridium dianthus are capable of asexual reproduction and colonize space rapidly. For example, in studies of subtidal epifaunal communities in New England, Sebens (1985, 1986) reported that cleared areas were colonized by erect hydroids, bryozoans, crustose red algae and tube worms within one to four months in spring, summer and autumn. Tunicates such as Dendrodoa carnea and Aplidium spp. appeared within a year, Aplidium sp., and Halichondria panicea achieved pre-clearance cover within >two years, while only a few individuals of Metridium dianthus and Alcyonium sp. colonized within four years.

Brittlestars vary in their recruitment ability. Ophiothrix fragilis produces numerous, long-lived planktonic larvae capable of long distance dispersal, and reach reproductive maturity with six to ten months, and are probable capable of recovery within less than five years. Amphiura filiformis also produces numerous planktonic larvae with high dispersal potential but exhibit high juvenile mortality, so that net recruitment is probably low. Adults also take five to six years to reach maturity, so that recovery may be prolonged. However, Amphiura filiformis was reported to recover to a population of 100/m² within two years. Brittlestars are mobile, potentially able to migrate from the surrounding area but the above species are not thought to be actively mobile. Therefore, the recovery of brittlestars will depend on the sporadic good larval recruitment, local hydrography, and will vary between species, some recovering with a few years while other may take longer.

Overall, Ocnus may be able to recruit and generate a large population size within five years in favourable conditions. Most epifaunal species will probably recover within five years, with some slower growing species, or slow to mature species taking longer.


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This review can be cited as:

Tyler-Walters, H., 2016. [Ocnus planci] aggregations on sheltered sublittoral muddy sediment. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 18-06-2018]. Available from:

Last Updated: 31/05/2016