Amphiura filiformis and Echinocardium cordatum in circalittoral clean or slightly muddy sand
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. Amphiura filiformis and Echinocardium cordatum are functionally dissimilar and are not always / 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 Amphiura filiformis and Echinocardium cordatum the biotope supports a fauna of burrowing species such as Callianassa subterranea and smaller less conspicuous species, such as polychaetes, nematodes and bivalves, living within the sediment.
There are however, some interspecific relationships within the biotope. The bivalve Tellimya (=Montacuta) ferruginosa is a commensal of Echinocardium cordatum, and as many as 14 or more of this bivalve have been recorded with a single echinoderm. Adult specimens live freely in the burrow of Echinocardium cordatum, while the young are attached to the spines of the echinoderm by byssus threads (Fish & Fish, 1996). The amphipod crustacean Urothöe marina (Bate) is another common commensal (Hayward & Ryland, 1995).
Most of the species living in deep mud biotopes are generally cryptic in nature and not usually subject to predation. However, the arms of Amphiura filiformis are an important food source for demersal fish and Nephrops norvegicus providing significant energy transfer to higher trophic levels. Increased nutrients leading to eutrophication processes (increased primary production) may contribute to increase the accumulation of hydrophobic contaminants in Amphiura filiformis and their transfer to higher trophic levels (Gunnarsson & Skold, 1999). 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.
In their investigation of density dependent migration in Amphiura filiformis Rosenberg et al. (1997) calculated 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 0.18 m2 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 burrowing and feeding activities of Amphiura filiformis 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 destabilization of the seabed can affect rates of particle resuspension. At a permanent monitoring station in Galway Bay, the brittle star Amphiura filiformis consistently ranks third among the numerically dominant species. On this basis and due to its effect on the sediment (Ocklemann & Muus, 1978), it is tentatively given 'keystone' status within the community in question (O'Conner et al., 1983).
The openings of the burrows of Callianassa subterranea provide temporary refuge for fish such as the black goby Gobius niger and the sand goby Pomatoschistus minutus. Occasional errant polychaetes, particularly polynoid worms, inhabit the burrows (Nickell & Atkinson, 1995).
The bioturbatory activities of thalassinidean mud-shrimps such as Callianassa subterranea have important consequences for the structural characteristics of the sediment they inhabit.
The hydrodynamic regime, which in turn controls sediment type, is the primary physical environmental factor structuring benthic communities such as CMS.AfilEcor. 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 Amphiura filiformis and Echinocardium cordatum are 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.
- 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 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.
- One of the key factors affecting benthic habitats is disturbance, which in shallow subtidal habitats will increase in winter due to weather conditions. Storms may cause dramatic changes in distribution of macro-infauna by washing out dominant species, opening the sediment to recolonization by adults and/or available spat/larvae (Eagle, 1975; Rees et al., 1977; Hall, 1994) and by reducing success of recruitment by newly settled spat or larvae (see Hall, 1994 for review). For example, during winter gales along the North Wales coast (Rees et al., 1976) northerly gales threw piles of Echinocardium cordatum on to the strand line and the author suggests these events are not uncommon. Lawrence (1989) also reports that Echinocardium cordatum and other organisms such as bivalves and brittlestars can be washed out of the sediment by water currents generated by gales.
Habitat structure and complexity
- The biotope has very little structural complexity with most species living in or on the sediment. The sediment expelled by Callianassa subterranea forms unconsolidated volcano-like mounds, which can significantly modify the seabed surface topography (Rowden et al., 1998).
The sea pen, Virgularia mirabilis, and the anemone Cerianthus lloydii extend above the sediment surface although these do not occur in high numbers and apart from a couple of species of nudibranch living on the sea pens these species do not provide significant habitat for other fauna.
- Some structural complexity is provided by animal burrows although these are generally simple. The burrows of Echinocardium cordatum, for example, provide a habitat for other species such as the small bivalve Tellimya (=Montacuta) ferruginosa. Most species living within the sediment are limited to the area above the anoxic layer, the depth of which will vary depending on sediment particle size and organic content. The mud shrimp Callianassa subterranea creates complex burrow systems consisting of a multi-branched network of tunnels connected to several inhalant shafts, each terminating in a funnel shaped opening to the surface. The presence of burrows of species such as Echinocardium cordatum and Callianassa subterranea allows 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.
- Deposit feeders manipulate, sort and process sediment particles which may result in destabilization and bioturbation of the sediment which inhibits survival of suspension feeders. This can result in a change in the vertical distribution of particles in the sediment that may facilitate vertical stratification of some species with particle size preferences. Vertical stratification of species according to sediment particle size has been observed in some soft-sediment habitats (Peterson, 1977).
Productivity in subtidal sediments is often quite low. Macroalgae are absent from CMS.AfilEcor 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. However, the arms of Amphiura filiformis
are an important food source for demersal fish and Nephrops norvegicus
providing significant energy transfer to higher trophic levels.
Studies of Amphiura filiformis suggest autumn recruitment (Buchanan, 1964) and spring and autumn (Glmarec, 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.
- In Echinocardium cordatum the sexes are separate and fertilization is external, with the development of a pelagic larva (Fish & Fish, 1996). The fact that Echinocardium cordatum is to be found associated with several different bottom communities would indicate that the larvae are not highly selective and discriminatory and it is probable that the degree of discrimination in 'larval choice' becomes diminished with the age of the larvae (Buchanan, 1966). Metamorphosis of larvae takes place within 39 days after fertilization (Kashenko, 1994). On the north-east coast of England a littoral population bred for the first time when three years old. In the warmer waters of the west of Scotland breeding has been recorded at the end of the second year (Fish & Fish, 1996). Buchanan (1967) observed that offshore populations were very slow growing and did not appear to reach sexual maturity so recruitment may be sporadic in places. However, since Buchanan (1967) also found that intertidal populations bred every year, recruitment should take place on an annual basis.
- Many of the other species in the biotope, including Callianassa subterranea and Virgularia mirabilis appear to have planktonic larvae so much recruitment to the biotope may be from distant sources.
Time for community to reach maturity
No evidence on community development was found. However, the two key species Amphiura filiformis and Echinocardium cordatum are 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. In Galway Bay (O'Connor et al., 1983) 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. Echinocardium cordatum breed for the first time when two to three years old. Recruitment of Echinocardium cordatum is often sporadic with reports of recruiting in only 3 years over a 10 year period (Buchanan, 1966) although this relates to subtidal populations. Intertidal individuals reproduce more frequently. Many of the other species in the biotope, such as polychaetes and bivalves, are likely to reproduce annually. However, because the key species in the biotope, Amphiura filiformis and Echinocardium cordatum, 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.
This review can be cited as follows:
Amphiura filiformis and Echinocardium cordatum in circalittoral clean or slightly muddy sand.
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=368&code=1997>