Virgularia mirabilis and Ophiura spp. on circalittoral sandy or shelly mud
Image Bernard Picton - Virgularia mirabilis, Cerianthus lloydii with Ophiocomina nigra on shelly muddy gravel. Image width ca XX cm.
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Ecological and functional relationships
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 brittlestars 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 these species. In addition to Virgularia mirabilis and brittlestars the biotope supports a fauna of smaller less conspicuous species, such as polychaetes and bivalves, living within the sediment.
Virgularia mirabilis might be adversely affected by high levels of megafaunal bioturbation, perhaps by preventing the survival of newly settled colonies. Seapens and various species of burrowing megafauna certainly coexist but no investigation of the interaction between them has been found. Burrowing species create tunnels in the sediment which themselves provide a habitat for other burrowing or inquilinistic species.
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. If present in high abundance, the arms of Amphiura filiformis are an important food source for demersal fish providing significant energy transfer to higher trophic levels. Brittlestars of the genus Ophiura are known to be a common prey for flatfish such as plaice (Downie, 1990 cited in Hughes, 1998b). There are also epibenthic predators/scavengers, such as Liocarcinus depurator and Pagurus prideaux, in the biotope. An increase in the numbers of predators can have an influence on the abundance and diversity of species in benthic habitats (Ambrose, 1993; Wilson, 1991). For example, enclosure experiments in a sea loch in Ireland have shown that high densities of swimming crabs such as Liocarcinus depurator, that feed on benthic polychaetes, molluscs, ophiuroids and small crustaceans, led to a significant decline in infaunal organisms (Thrush, 1986).
The majority of the species are suspension feeders so competition for food may occur.
When present in high abundance the burrowing and feeding activities of Amphiura filiformis 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). 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 CMS.VirOph. 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.
Seasonal and longer term change
- Species such as the sea pen Virgularia mirabilis and Amphiura filiformis 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. Immature individuals of Liorcarcinus depurator, for example, are more frequent in the periods May - September.
Habitat structure and complexity
The biotope has very little structural complexity with most species living in or on the sediment. 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, 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 CMS.VirOph 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. The high surface area of fine particles provides surface for microflora. If present in high abundance the arms of Amphiura filiformis
can be an important food source for demersal fish and Nephrops norvegicus
providing significant energy transfer to higher trophic levels.
- Virgularia mirabilis and the other major component species in this biotopes appear to have a plankton stage within their life cycle, so colonization is likely to occur from distant sources.
- 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.
- 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 observed up to early October. In contrast the larger Ophiura ophiura had a more protracted breeding season.
- Studies of Amphiura filiformis suggest autumn recruitment (Buchanan, 1964) and spring and autumn (Glémarec, 1979). Using a 265µm mesh size Muus (1981) identified a peak settlement period in the autumn with a maximum of 6800 recruits per m2. Muus (1981) shows the mortality of these settlers to be extremely high with less than 5% contributing to the adult population in any given year. In Galway Bay populations, small individuals make up ca. 5% of the population in any given month, which also suggests the actual level of input into the adult population is extremely low (O'Connor et al., 1983). The species is thought to have a long pelagic life so recruitment can come from distant sources.
- The scallop Pecten maximus appears to have a long breeding period with peaks in spring and autumn (Fish & Fish, 1996). The veliger larvae are planktonic for about three to four weeks and settle on a wide range of algae, bryozoans and hydroids.
Time for community to reach maturity
No evidence on community development was found. Almost nothing is known about the life cycle and population dynamics of British sea pens, but data from other species suggest that they are likely to be long-lived and slow growing with patchy and intermittent recruitment. The other key species, Amphiura filiformis
and Pecten maximus
are also long lived and take a relatively long time to reach reproductive maturity. It takes approximately 5-6 years for Amphiura filiformis
to grow to maturity so population structure will probably not reach maturity for at least this length of time. In addition, Muus (1981) shows the mortality of new settling Amphiura filiformis
to be extremely high with less than 5% contributing to the adult population in any given year. Pecten maximus
reaches sexual maturity within the first two to three years and has a life span of 10-20 years. The suggested life span for Ophiura ophiura
in the west of Scotland was 5-6 years (Gage, 1990). Many of the other species in the biotope, such as polychaetes and bivalves, are likely to reproduce annually, be shorter lived and reach maturity much more rapidly. However, because the key species in the biotope, Virgularia mirabilis
and Amphiura filiformis
are long lived and take several years to reach maturity the time for the overall community to reach maturity is also likely to be several years, possibly in the region of 5-10 years.
This review can be cited as follows:
Hill, J.M. & Wilson, E.
Virgularia mirabilis and Ophiura spp. on circalittoral sandy or shelly mud.
Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line].
Plymouth: Marine Biological Association of the United Kingdom.
Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=66&code=1997>