Biodiversity & Conservation

The characterizing and other species in this biotope occupy space in the habitat but their presence is most likely primarily determined by the occurrence of a suitable substratum rather by interspecific interactions. Virgularia mirabilis and Philine aperta are functionally dissimilar and are not necessarily associated with each other but occur in the same muddy sediment habitats. There is no information regarding possible interactions between any of the other species in the biotope but there seems to little interdependence. Burrowing species which are present create tunnels in the sediment which themselves provide a habitat for other burrowing or inquilinistic species.

Virgularia mirabilis might be adversely affected by high levels of megafaunal bioturbation, perhaps by preventing the survival of newly settled colonies.

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 this species is low. The sea slug Armina loveni is a specialist predator of Virgularia mirabilis.

Nephrops norvegicus is known to be eaten by a variety of bottom-feeding fish, including cod, haddock, skate and dogfish. Symbion pandora, a tiny sessile animal less than 1 mm long, lives commensally on the mouthparts of Nephrops norvegicus.

Brittlestars are common, with Amphiura chiajei predominating on finer muds. Most of these animals are deposit-feeders, ingesting tiny organic particles and feeding on the bacterial layer coating the sediment grains. If present in high abundance the burrowing and feeding activities of Amphiura chiajei can modify the fabric and increase the mean particle size of the upper layers of the substrata by aggregation of fine particles into faecal pellets. Such actions create a more open fabric with a higher water content which affects the rigidity of the seabed (Rowden et al., 1998(b)). Such destabilisation of the seabed can affect rates of particle resuspension.

The hydrodynamic regime, which in turn controls sediment type, is the primary physical environmental factor structuring benthic communities such as IMU.PhiVir. The hydrography also affects the water characteristics in terms of salinity, temperature and dissolved oxygen. It is also widely accepted that food availability (see Rosenberg, 1995) and disturbance, such as that created by storms, (see Hall, 1994) are also important factors determining the distribution of species in benthic habitats.

• Species such as the sea pen Virgularia mirabilis and Amphiura chiajei appear to be long-lived and are unlikely to show any significant seasonal changes in abundance or biomass. Seapen 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).
• 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.

The biotope has very little structural complexity with most species living in or on the sediment. Burrowing megafauna are generally rare or absent, therefore there will be few burrows available for colonization. Several species, such as the sea pen Virgularia mirabilis and the anemone Cerianthus lloydii, extend above the sediment surface. However, 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) these species do not provide significant habitat for other fauna. Excavation of sediment by infaunal organisms, such as errant polychaetes and Philine aperta, ensures that sediment is oxygenated to a greater depth allowing the development of a much richer and/or higher biomass community of species within the sediment.

Productivity in subtidal sediments is often quite low. Macroalgae are absent from IMU.PhiVir and so productivity is mostly secondary, derived from detritus and organic material. Allochthonous organic material is derived from anthropogenic activity (e.g. sewerage) and natural sources (e.g. plankton, detritus). Autochthonous organic material is formed by benthic microalgae (microphytobenthos e.g. diatoms and euglenoids) and heterotrophic micro-organism production. Organic material is degraded by micro-organisms and the nutrients are recycled.

• Philine aperta, Virgularia mirabilis and other major component species in sea pen biotopes appear to have a plankton stage within their life cycle.
• 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). Dispersal of Virgularia mirabilis planulae is therefore likely to be limited to relatively short distances suggesting that populations may not be replenished from distant sources (David Hughes, pers. comm.). The limited data available from other species would suggest a similar pattern of patchy recruitment, slow growth and long life-span for Virgularia mirabilis.

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.

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