Biodiversity & Conservation

Ocnus planci aggregations on sheltered sublittoral muddy sediment



Image Dominic Counsell - Aggregation of Ocnus planci on broken shell and cobbles. Image width ca 40 cm.
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Distribution map

SS.IMU.MarMu.Ocn recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

  • EC_Habitats

Ecological and functional relationships

Little information on this biotope was found. Shallow records of the biotope are similar to £IMU.PhiVir£ with the addition of epifaunal species including abundant Ocnus, while deeper records share some species with sea pen and burrowing macrofauna communities (see £CMU.SpMeg£). The following information has been inferred from survey records (Erwin et al., 1990; Connor et al., 1997a; Howson et al., 1994; Dipper & Beaver, 1999; Murray et al., 1999; JNCC, 1999), papers on general ecology of Ocnus planci (Ölscher & Fedra, 1977) and reviews of sublittoral mud communities (e.g. Hughes, 1998b) and MarLIN reviews of £IMU.PhiVir£ and £CMU.SpMeg£. Phytoplankton, benthic microalgae and macroalgae present in the shallow extent of the biotope (e.g. Phycodrys rubens, or Saccharina latissima) provide primary productivity within the biotope.

Active epifaunal suspension feeders generate localized currents to collect food such as organic particulates and phytoplankton. They include the sponge Suberites ficus, the soft coral Alcyonium digitatum, tubeworms (e.g. Chaetopterus variopedatus and Pomatoceros triqueter), fanworms (e.g. Sabella pavonina and Myxicola infundibulum), the barnacle Balanus balanus, bivalves (e.g. Pecten maximus, Aequipecten opercularis and Modiolus modiolus) and ascidians such as Ascidiella spp., Ascidia spp. and Ciona intestinalis.

Passive epifaunal suspension feeders collect organic particulates and small zooplankton from the passing water column and include hydroids (e.g. Bougainvillia ramosa), the sea pens Virgularia mirabilis and Pennatula phosphorea, brittlestars (e.g. Ophiothix fragilis and Ophiocomina nigra), and the sea cucumbers Ocnus planci and Ocnus lateus.

Infaunal suspension feeders include burrowing bivalves such as Mya truncata and Abra alba, the commensal Mysella bidentata often found in the burrows of other organisms

and the gastropod Turritella communis. The mud also supports surface deposit feeding terebellid polychaetes (e.g. Eupotymnia nebulosa) and the sea cucumber Leptosynapta sp.

Grazers may include the chiton Leptochiton asellus and the sea hare Aplysia punctata feeding on microalgae and macroalgae, while sea urchins (e.g. Psammechinus miliaris and Echinus esculentus feed on macroalgae, algal fragments and epifaunal crusts (e.g. hydroids).

Infaunal predators include the sea slug Philine aperta feeding on polychaetes, gastropods and bivalves at the sediment surface, and the necklace shell Polinices catera which preys on bivalves.

The burrowing anemone Cerianthus lloydii is a passive carnivore feeding on small invertebrates.

Mobile epifaunal generalist predators include the crabs (e.g. Cancer pagurus and Liocarcinus species) and the starfish Asterias rubens, while the larger starfish Luidia sarsi and Solaster endeca prey on other echinoderms. The sea slug Armina loveni is a specialist predator of Virgularia mirabilis.

Starfish, crabs, hermit crabs (e.g. Pagurus bernhardus) and brittlestars (e.g. Ophiura albida, Ophiocomina nigra and Ophiothrix fragilis) are probably scavengers within the biotope.

Dermersal fish such as the small-spotted catshark Scyliorhinus canicula, and gobies (e.g. Pomatoschistus minutus) are probably generalist predators.

the sea lemon Archidoris pseudoargus feeds on sponges.

The small gastropod Melanella alba parasites holothurians, sucking on their body fluids (Graham, 1988).

Many of the species living in deep mud biotopes are generally cryptic in nature and not usually subject to predation. Evidence of predation on Virgularia mirabilis by fish seems limited to a report by Marshall & Marshall (1882 in Hoare & Wilson, 1977) where the species was found in the stomach of haddock. Many specimens of Virgularia mirabilis lack the uppermost part of the colony which has been attributed to nibbling by fish. Observations by Hoare & Wilson (1977) suggest however, that predation pressure on Virgularia mirabilis is low.

Epifauna probably compete for the limited space for attachment provided by cobbles, pebbles and shell debris, with ascidians, sponges and soft corals probably representing later stages in colonization (succession) (see £MCR.Flu£ for further detail). However, Ocnus species are probably capable of climbing on any available surface, including other epifauna, to raise their feeding tentacles into the prevailing current (see Ölsher & Fedra, 1977; McKenzie, 1991). Bioturbation by deposit feeding or infaunal species is likely to modify the substratum and resuspend sediment, potentially inhibiting suspension feeding organisms, especially small colonies or juveniles.

Seasonal and longer term change

Species such as the sea pen Virgularia mirabilis appear to be long-lived and are unlikely to show any significant seasonal changes in abundance or biomass. Sea pen faunal communities appear to persist over long periods at the same location. Movement of the sea pen Virgularia mirabilis in and out of the sediment may be influenced by tidal conditions (Hoare & Wilson, 1977; Hughes, 1988). The numbers of some of the other species in the biotope may show peak abundances at certain times of the year due to seasonality of breeding and larval recruitment.

Microphytobenthos and algal production may increase in spring, resulting in the formation of mats of ephemeral algae, and be reduced in winter. High summer temperatures may increase the microbial activity resulting in deoxygenation (hypoxia or anoxia), or alternatively result in thermoclines in shallow bays and resultant hypoxia of the near bottom water (Hayward, 1994; Elliot et al., 1998; Hughes, 1998). Flatfish and crabs often migrate to deeper water in the winter months, and therefore, predation pressure may be reduced in this biotope. Mud habitats of sheltered areas are relatively stable habitats, however especially cold winter or hot summers could adversely affect the macrofauna (see sensitivity). In addition, extreme freshwater runoff resulting from heavy rains and storms may result in low salinity conditions in the most shallow parts of the biotope or in haloclines, again potentially causing local hypoxia. No information on long term change was found. But storms events and extreme wave action may resuspend the bottom sediment and move the cobbles, pebbles and shell debris, resulting in loss or burial of epifauna at irregular intervals.

Habitat structure and complexity

This biotope is characterized by a soft to flocculant mud substratum with the presence of hard substrata such a shell debris, living epifauna, rock, cobbles and pebbles. The soft mud supports epifauna and infauna typical of sheltered soft mud habitats (e.g. IMU.PhiVir), while the hard substrata provides habitat for attached epifaunal species and niches and interstices for other epifaunal species (e.g. brittlestars and Ocnus). The habitat can be divided into the following niches:
  • a mobile epifauna of scavengers and opportunistic predators;
  • a sediment surface flora of microalgae such as diatoms and euglenoids, together with aerobic microbes;
  • an aerobic upper layer of sediment (depth depending on local conditions) supporting shallow burrowing species;
  • a reducing layer and a deeper anoxic layer supporting chemoautotrophic bacteria, burrowing polychaetes (e.g. terebellids), burrowing synaptid holothurians (e.g. Leptosynapta sp.) and bivalves (e.g. Abra alba and Mya truncata) that can irrigate their burrows.
  • an epifauna of sea pens, burrowing anemones, scallops or horse mussels sitting in or on the sediment surface;
  • an epifauna of tubeworms, barnacles, sponges, and ascidians attached to hard substrata;
  • a more mobile epifauna of brittlestars on or between hard substrata and Ocnus on any available raised surface.
Burrowing megafauna are generally rare or absent, therefore there will be few burrows available for colonization by other species. Several species, such as the sea pen Virgularia mirabilis, the anemone Cerianthus lloydii, the tubeworm Chaetopterus variopedatus and fan worms Sabella pavonina and Myxicola infundibulum extend above the sediment surface. Apart from a couple of species of nudibranch living on the sea pens and the tubiculous amphipod Photis longicaudata associated with Cerianthus lloydii (Moore & Cameron, 1999) the large species characteristic of the biotope do not provide significant habitat for other fauna. Brittlestars such as Ophiothrix fragilis probably utilize gaps between cobbles and pebbles and inside dead shells of bivalves. Excavation of sediment by infaunal organisms, such as errant polychaetes, bivalves and Philine aperta, ensures that sediment is oxygenated to a greater depth but little information on the infauna was found.


Primary productivity is derived from phytoplankton, benthic microalgae and from macroalgae. However, most of the productivity with the biotope is probably secondary, derived from zooplankton, detritus, dissolved organic material and organic particulates. The biotope is dominated by suspension feeding organisms, especially passive suspension feeders such as brittlestars, sea pens and abundant Ocnus. Ölscher & Fedra (1977) examined passive suspension feeding in the brittlestar Ophiothrix quinquemaculata and the holothurian Ocnus planci (as Cucumaria planci). They noted that passive suspension feeders usually constitute about one third of the community biomass in suspension feeder communities but that they metabolic activity of passive suspension feeders is twice as great due to their small size. They noted the importance of suspension feeding communities to linking the pelagic and benthic ecosystems. Similarly, the importance of bivalve suspension feeding in 'pelago-benthic coupling' has been discussed by Dame (1996) (see also MCR.ModT).

Recruitment processes

Ocnus planci and Ocnus brunneus are dioecious, with separate sexes but are also capable of reproducing asexually by fission. Fertilization is external and spawning occurs in March and April. The eggs are retained after spawning on the tentacles of the female. Development is direct, the larvae adopting the adult body plan without metamorphosis. The larvae are released as a ciliated vitellaria larvae, which is lecithotrophic, completing its development in the plankton (Hyman, 1955; Smiley et al., 1991). No estimate of fecundity was found but other Cucumariidae exhibit clutch sizes between 19 and 340 (Smiley et al., 1991). Planktonic development provides the larvae with potentially long range dispersal capabilities. However, recruitment in echinoderms is known to be sporadic, unpredictable and poorly understood. Ocnus planci and Ocnus brunneus are fissiparous, each individual being able to divide into two or more fragments, over a period of about 14hrs, which then regenerate into complete individuals (Emson & Wilkie, 1980; Smiley et al., 1991). McKenzie (1991) suggested that the large aggregations of Ocnus brunneus may be clones. Fissiparity may provide Ocnus with a mechanism to exploit favourable conditions quickly, although no evidence to this effect was found.

The reproductive biology of British sea pens has not been studied but, in other species, for instance Ptilosarcus guerneyi from Washington State in the USA, the eggs and sperm are released from the polyps and fertilization takes place externally. The free-swimming larvae do not feed and settle within seven days if a suitable substratum is available (Chia & Crawford, 1973). The limited data available from other species would suggest a similar pattern of patchy recruitment, slow growth and long life-span for Virgularia mirabilis.

The associated macroalgae, epifauna and interstitial fauna probably depend on locality and recruit from the surrounding area. Many hydroids, ascidians and probably sponges have short lived planktonic or demersal larvae with relatively poor dispersal capabilities. Exceptions include Alcyonium digitatum and hydroids that produce medusoid life stages and probably exhibit relatively good dispersal potential. Hydroids are opportunistic, rapid growing species, with relatively widespread distributions, which colonize rapidly and are often the first groups on species to occur on settlement panels. Sponges may take longer to recruit to the habitat but are good competitors for space. Recruitment in epifauna communities is discussed in detail in the faunal turf biotopes MCR.Flu, CR.Bug and in Modiolus modiolus beds (MCR.ModT).

Mobile epifaunal species, such as echinoderms (starfish and brittlestars), crustacea, and fish are fairly vagile and capable of colonizing the community by migration from the surrounding areas. In addition, most echinoderms and crustaceans have long-lived planktonic larvae with potentially high dispersal potential, although, recruitment may be sporadic, especially in echinoderms.

Time for community to reach maturity

No information concerning community development was found. Recruitment to available hard substrata by epifauna such as hydroids, and ascidians is probably fairly rapid (see MCR.Flu or CR.Bug), with sponges and soft corals taking longer to develop. 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 long-lived and slow growing with patchy and intermittent recruitment. Other burrowing species representative of this biotope vary in longevity and reproductive strategies. The time taken for the population of Ocnus to grow to the abundances reported in this biotope, by either sexual and/or asexual reproduction, is unknown.

Additional information

None entered

This review can be cited as follows:

Tyler-Walters, H. 2002. Ocnus planci aggregations on sheltered sublittoral muddy sediment. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 21/04/2014]. Available from: <>