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. Sea pens and burrowing megafauna are functionally and ecologically dissimilar and are not necessarily associated with each other but occur in the same muddy sediment habitats. For example, some sites with abundant burrowing megafauna have no sea pens (and vice versa). It is possible that sea pens might be adversely affected by high levels of megafaunal bioturbation, perhaps by preventing the survival of newly settled colonies. No single species can be considered a keystone species whose activity is essential to the structure of the community. In addition to sea pens and burrowing megafauna, the biotope often supports a rich fauna of smaller less conspicuous species, such as polychaetes, nematodes and bivalves, living within the sediment.

There are however, some interspecific relationships within the biotope. For instance, the shrimp Jaxea nocturna, which often lives in association with the echiuran worm Maxmuelleria lankesteri (Nickell et al., 1995), may benefit from the organic-rich mud pulled into its burrows by the worm. Nickell et al. (1995) found that numerous small bivalves and polychaete worms colonized the walls of Maxmuelleria lankesteri burrows. Mobile polychaetes such as Ophiodromus flexuosus, which normally live out on the sediment surface were also seen to enter burrows. The body of shrimps may offer a substratum for colonization. The ctenostome bryozoan Triticella flava, for example, grows a dense 'furry' covering on the antennae, mouthparts and legs of burrowing crustaceans. It is most commonly found on Calocaris macandreae but has also been found on several of the other crustacean burrowers present in the biotope (Hughes, 1998b). The mouthparts of Nephrops norvegicus harbour a small commensal sessile animal, the newly described Symbion pandora (Conway Morris, 1995). A few organisms have also been recorded in association with British sea pens. Funiculina quadrangularis is often host to the isopod Astacilla longicornis, which clings to the rachis, and the brittlestar Asteronyx loveni, which clings to the sea pen maintaining an elevated position above the seabed. However, Asteronyx is only found in deeper waters, usually below 100 m depth. There are also a few specialist predators of sea pens (see below). Although rare, the tube of the large sea anemone Pachycerianthus multiplicatus, which is only found in this biotope, creates a habitat for attached species (O'Connor et al., 1977).

The species living in deep mud biotopes are generally cryptic in nature. Predation is probably low because many species will be sheltered to some extent from visual surface predators such as fish. 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. Observations by Hoare & Wilson (1977) suggest however, that predation pressure on this species is low. Many specimens of Virgularia mirabilis lack the uppermost part of the colony which has been attributed to nibbling by fish. The sea slug Armina loveni is a specialist predator of Virgularia mirabilis. Nephrops norvegicus is eaten by a variety of bottom-feeding fish, including cod, haddock, skate and dogfish. There are also numerous records of fish predation on thalassinidean mud shrimps such as Calocaris macandreae which has been found in the stomachs of cod and haddock. Maxmuelleria lankesteri has also been recorded in the stomachs of Irish sea cod (Hughes, 1998b). Nephrops norvegicus is carnivorous, feeding on brittle stars, polychaetes, bivalves and other crustaceans such as Calocaris macandreae.

The bioturbatory activities of thalassinidean mud-shrimps such as Callianassa subterranea have important consequences for the structural characteristics of the sediment they inhabit. An important aspect of bioturbatory activity was emphasised by Johnston (1974) who showed that the activity of deposit-feeders results in the production of organic-mineral aggregates which may comprise as much as 70% of the sediment particle total. Such aggregation of particles must greatly increase the porosity of the sediments and so have a considerable influence on the transfer of chemicals by diffusion or other physical processes, as well as critically affecting environmental space for meio- and macrofauna and the bacterial flora. Such influences affect a variety of important ecosystem functions, including nutrient exchange (Nickell et al., 1995), faunal community structure and biogeochemical cycling (e.g. Koike & Mukai, 1983; Waslenchuk et al., 1983; Posey, 1986). Several studies have examined the effects of thalassinidean shrimp bioturbation on sedentary and mobile infaunal species. Tamaki (1988) found that Callianassa japonica had a positive effect on colonization by other mobile taxa, possibly by irrigating and fertilizing the sediment that stimulated the growth of microalgae and bacteria or by loosening up the sediment that eased burrowing and penetration. The abundance of sedentary species such as spionid polychaetes and some bivalves have been observed to be negatively correlated with abundance of Callianassid shrimps (e.g. Posey, 1986). The redistribution of organic matter within the sediment by effective bioturbating species, such as the deep burrowing mud shrimp Callianassa subterranea and the shallower burrowing Nephrops norvegicus, will influence depth distribution and community structure as well. However, the activities of the larger burrowers can either enhance or reduce the overall abundance of sediment macrofauna, depending on the species involved. Megafaunal activity creates a mosaic of disturbance patches which may be important to the maintenance of biodiversity in the sediment community (Hughes, 1998(b)). The presence and activity of Callianassa species has been shown to be linked to significant sediment and radioactive particulate resuspension (Roberts et al., 1981; Colin et al., 1986). Bioturbatory activities of deposit feeding genera such as Nucula and Pectinaria will also actively increase the rate of oxygen diffusion through finer sediments (Pearson & Rosenberg, 1978).

Where several species of burrowing megafauna occur together in the same biotope it is not unusual for burrows to interconnect. Tuck et al. (1994) found that 34% of Nephrops burrows at a site in Loch Sween showed evidence of interactions with other species, including Maxmuelleria lankesteri, Jaxea nocturna and Leseurigobius friesii. These interconnections are probably accidental and not indicative of any close symbiotic relationship between different burrowers. Such interconnections may improve ventilation and nutritional content of the burrows.

Mobile adults, such as Nephrops norvegicus and Callianassa subterranea, often show spacing out phenomena. Such behaviour is usually linked to territorial aggression (Gray, 1974).

The opening of the burrows of Callianassa subterranea provide temporary refuge for fish such as the black goby Gobius niger and Pomatoschistus minutus. Occasional errant polychaetes, particularly polynoid worms, inhabit the burrows (Nickell & Atkinson, 1995).

The burrowing and feeding activities of Amphiura filiformis, if present in high abundance, 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 arms of Amphiura filiformis are an important food source for demersal fish and Nephrops norvegicus providing significant energy transfer to higher trophic levels including to humans. Increased nutrients and eutrophication processes may contribute to increase the accumulation of hydrophobic contaminants in Amphiura filiformis and their transfer to higher trophic levels (Gunnarsson & Skold, 1999).

In their investigation of density dependent migration in Amphiura filiformis Rosenberg et al. (1997) calculated that in areas of high density of the species (3000 individuals per m²), the area of sediment at about 3 to 4cm depth covered by disks of Amphiura filiformis can be estimated as 22%. The capacity of such a density of brittle stars to displace sediment can be calculated at 0.18m² per hour. Thus, movement of Amphiura filiformis should generate a more or less continuous displacement of sediment and be of great significance to the biogeochemical processes in the sediment.

The hydrodynamic regime determines whether a biotope, such as CMU.SpMeg, exists in a particular place by allowing deposition of fine sediment. 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.

• Seapen and burrowing megafaunal communities appear to persist over long periods at the same location. Species such as the sea pen Virgularia mirabilis, the brittle star Amphiura filiformis and the mud shrimp Calocaris macandreae appear to be long-lived and are unlikely to show any significant seasonal changes in abundance or biomass. 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 Liocarcinus depurator, for example, are more frequent in the periods May - September.
• There are daily patterns of activity in some species. Nephrops norvegicus, for example, forages for food at night, returning to their burrows at sunrise. However, in deeper water (> 100m) this activity is reversed suggesting that activity is determined by light intensity. The echiuran Maxmuelleria lankesteri has been observed to feed only at night and so activity may also be related to light intensity. Movement of the sea pen Virgularia mirabilis in and out of the sediment may be influenced by tidal conditions (Hoare & Wilson, 1977).
• Burrowing activity of the mud shrimp Callianassa subterranea in the North Sea appears to be seasonal (Rowden & Jones, 1997). Relatively little activity was observed in the period January - April, before a steady increase through spring and early summer, reaching a maximum in at the end of the summer and a decline in autumn and winter. In January, when bioturbatory activity was low the seabed appeared essentially flat and smooth, whilst in September the bed was littered with numerous mounds and depressions. Tunberg (1986) found that Upogebia deltaura remained inactive in the deepest parts of its burrow during the winter. Maxmuelleria lankesteri is active all year round but seem to show peaks of activity in December and April when the proportion of easily-degradable organic matter at the sediment surface is at its highest (Hughes, 1998b).
• The behaviour of Nephrops norvegicus may be seasonal. In Loch Sween, Nephrops burrows were aggregated in groups during the late summer, which then broke up into a random distribution during the winter (Tuck et al., 1994). Such aggregations may result when burrow complexes formed when juvenile animals settle in pre-existing adult systems, then break up as the juveniles gradually extend their own burrows and lose contact with those of the adults.

• The biotope has little structural complexity above the sediment surface. Burrows and mounds of burrowing megafauna may form a prominent feature of the sediment surface with conspicuous populations of sea pens, typically Virgularia mirabilis and Pennatula phosphorea. However, apart from a couple of species of nudibranch the sea pens do not provide significant habitat for other fauna. Where present, the tube of the rare sea anemone Pachycerianthus multiplicatus, creates a habitat for attached species.
• However, dense populations of burrowers create considerable structural complexity, below the surface, relative to sediments lacking these animals. For example, Callianassa subterranea creates complex burrow systems in sandy mud sediments. The burrows consist of a multi-branched network of tunnels connected to several inhalent shafts, each terminating in a funnel shaped opening to the surface. These burrows allow a much larger surface area of sediment to become oxygenated, and thus enhance the survival of a considerable variety of small species (Pearson & Rosenberg, 1978). Burrows also create habitats for other animals such as clams and polychaetes. Burrows are also created by other crustacean species such as Nephrops norvegicus and Calocaris macandreae although these are not as complex as those of Callianassa. The echiuran worm Maxmuelleria lankesteri produces long-lasting burrows that provide a habitat for a variety of small polychaetes and bivalves but none of these appear to be obligate relationships (Jones et al., 2000). In Scottish sea lochs the black goby Gobius niger will take up residence in burrows belonging to Maxmuelleria lankesteri and other species, frequently extending or changing the shape of the burrow opening. The squat lobster Munida rugosa is frequently found inhabiting burrows on the periphery of megafaunally-burrowed muds, close to coarser sediments. The sediment expelled by Callianassa subterranea forms unconsolidated volcano-like mounds, which significantly modify seabed surface topography (Rowden et al., 1998).
• The bioturbatory activities of callianassids such as Callianassa subterranea has important consequences for the structural characteristics of the sediment. Many infauna are limited to the upper oxygenated layer, however others penetrate deeper in irrigated burrows or possess long siphons capable of transporting oxygenated water into the sediment, which may result in an oxygenated layer around their burrows.

Productivity in subtidal sediments is often quite low. Macroalgae are absent from CMU.SpMeg and so productivity is mostly secondary, derived from detritus and organic material. However, some shallower sites can have an extensive growth of benthic diatoms in the summer. 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.

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). Thus, the limited data available from these other species would suggest a similar pattern of patchy recruitment, slow growth and long life-span. As is typical of decapod crustaceans the female thalassindean burrowers in the biotope carry fertilized eggs on the abdomen before hatching into planktonic larvae. The length of time the larvae spends in the plankton appears to vary between species. The larval stages of Nephrops norvegicus spend about 50 days in the plankton before settlement and it is thought to be about 28 days for Callianassa subterrranea. In Northumberland the life-history of Calocaris macandreae was found to be rather different. Animals produced eggs in January-February which hatched in September-October. Only about 100 eggs were produced in each batch and the large larvae had no free-swimming phase before settling. The larval stage of the echiuran Maxmuelleria lankesteri is completely unknown but the large, yolky eggs suggest that the planktonic stage is brief or absent. However, many of the species in the biotope appear to have planktonic larvae so recruitment to the biotope may often be from distant sources.

There is very little known about community development for this biotope. 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 burrowing decapods that characterise the biotope vary in their reproductive strategies and longevity. In the North Sea the life span of Callianassa subterranea appears to be 2-3 years (Rowden & Jones, 1994) and individuals become sexually mature in their first year. Time to sexual maturity is longer in Nephrops norvegicus, about 2.5 - 3 years, and for the very long-lived Calocaris macandreae individuals off the coast of Northumberland did not become sexually mature until five years of age, and produced only two or three batches of eggs in their lifetime. Although little is known of the life cycle of the echiuran worm Maxmuelleria lankesteri long term observations of populations in situ have provided no evidence of any major fluctuations in population size, and it has been suggested that the species is long-lived with stable populations and low recruitment rates. 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. Since most key species reproduce regularly but take a while to grow, recruitment will be rapid but it will take several years to reach maturity and so it will probably take at least five years for the overall community to reach maturity.