|Basic Information||Biotope classification||Ecology||Habitat preferences and distribution||Species composition||Sensitivity||Importance|
Image Dominic Counsell - Aggregation of Ocnus planci on broken shell and cobbles. Image width ca 40 cm.
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SS.IMU.MarMu.Ocn recorded () and expected () distribution in Britain and Ireland (see below)
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.
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.
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.
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.
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.
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 28/11/2015]. Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=325&code=1997>