Biodiversity & Conservation

Foraminiferans and Thyasira sp. in deep circalittoral soft mud


Foraminiferans and <i>Thyasira</i> sp. in deep circalittoral soft mud
Distribution map

COS.COS.ForThy recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

  • UK_BAP

Ecological and functional relationships

Community structure
The presence of the characterizing and other species in this biotope is primarily determined by the occurrence of a suitable substratum rather than by interspecific interactions. However, the component species modify the habitat and, in that way, affect each other. The following points may be relevant to this biotope. Deposit feeders sort and process sediment particles and may result in destabilization 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 (Petersen, 1977). Polychaetes also significantly influence nutrient fluxes of nitrogen and phosphorus at the sediment-water interface, owing to their burrowing activity promoting oxygenation of the substrata. The burrowing and feeding activities of the macrofauna are likely to 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 sediment fabric with a higher water content which affects the rigidity of the seabed (Rowden et al., 1998). Such alteration of the substratum surface can affect rates of particle resuspension.

Bioturbation is particularly important in controlling chemical, physical and biological processes in marine sediments, especially when the influences of physical disturbances such as wave action or strong currents are minimized (Widdicombe & Austen, 1999).

Another factor determining the distribution of assemblages is the annual variation of temperature in bottom layers, influenced by the amount of stratification in the water column. COS.ForThy occurs in water depth greater than 100 m in the North Sea and Celtic Sea, i.e. deeper than the seasonally stratified water. Differences in stratification north and south of the Dogger Bank might explain why cold water species do not go further south than the Dogger Bank (Kunitzer et al., 1992).

In Loch Nevis there is greater vertical mixing and primary production, therefore a higher rate of deposition of organic material would be present and able to support greater populations of benthic animals (McIntyre, 1961).

Predator-prey relationships
Most of the species living in deep mud biotopes are generally cryptic so are protected to some extent from visual surface predators. However, some species of foraminifera, such as Astrorhiza sp. usually live on the substratum surface. The arm tips of Amphiura chiajei, which is often present in this biotope, are also an important food source for demersal species.

Foraminifera are able to move along the sediment surface. Feeding takes place when the animal is stationary, by developing a network of numerous thin extensions of cytoplasm called ‘reticulopodia’ or ‘pseudopodia’ (Buchanan & Hedley, 1960; Wetmore,1995). Buchanan & Hedley (1960) noted that the pseudopodia of Astrorhiza lamicola ramify over the sediment surface and through the interstitial spaces to a depth of 2-3mm, extending to a distance of ~7cm from the animal.

Depending on size and available food, foraminifera, prey on dissolved organic molecules; bacteria, diatoms and other single-celled phytoplankton; small crustacea and recently metamorphosed Echinocardium flavesens (Buchanan & Hedley, 1960; Wetmore, 1995; Rivkin & DeLaca, 1990).

Buzas (1978) suggested that foraminiferans probably also represent an important food source for benthic macrofauna. Predation was thought mainly to be by demersal fish species (McIntyre, 1961).

Dando & Southward (1986), Southward (1986), and Spiro et al. (1986) found that different species of Thyasira species show a range of nutritional dependence on bacteria in their gills; from none (heterotrophs) to complete dependence (chemoautotrophs).

Seasonal and longer term change

Large areas of the southern North Sea are not stratified during most of the year and the summer temperature of bottom waters is high (>10°C) (Tomczak & Goedecke, 1964), while in the stratified areas north of the Dogger Bank summer temperatures are <7°C. In winter the southern North Sea is colder (4°C) than the rest of the North Sea (5-7°C). Phytoplankton productivity increases during the summer, which may lead to more available food for macrofauna. However, in the North Sea large stocks of copepods develop, which consume the summer production of phytoplankton (Fransz & Gieskes, 1984). The faecal pellets do not reach the deep water, being recycled higher in the water column (Krause, 1981) so limiting this source of food to benthos in the summer months. This could explain the low biomass of infauna in the northern North Sea (Kunitzer et al., 1992).

Habitat structure and complexity

The biotope has very little surface structural complexity as most species are infaunal, however, the bioturbating megafauna can create considerable structural complexity below the surface, relative to sediments that lack such animals.
  • The sediment surface may appear pitted by small burrows of infaunal species, with arm tips of Amphiura chiajei stretching out over the surface but these are not likely to provide a significant habitat for other fauna. Infaunal and epifaunal species colonize the area and foraminifera tests may also be present in large numbers on the surface of the sediment.
  • Most species living within the sediment are restricted to the area above the anoxic layer, the depth of which will vary depending upon sediment particle size and organic content. Some structural complexity is provided by the burrows of macrofauna. Burrows and the bioturbatory activity that creates them allows a much larger volume of sediment to become oxygenated, enhancing the survival and diversity of a considerable variety of smaller infaunal species (Pearson & Rosenberg, 1978).


Macroalgae are absent from COS.ForThy and consequently productivity is mostly secondarily derived from detritus and organic material. Allochthonous organic material is therefore derived from plankton including dead plankton sinking to the seabed and other animal productivity. Autochthonous organic material is also 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 recycled.

Recruitment processes

No information is known about the reproduction and recruitment of foraminifera within this biotope.

Larval development of Thyasira equalis is lecithotrophic and the pelagic stage is very short or quite suppressed. This agrees with the reproduction of other Thyasira sp., and in some cases (Thyasira gouldi) no pelagic stage occurs at all (Thorson, 1946). This means that larval dispersal is limited. No information relating to fecundity of Thyasira species within the biotope was found, however information is available for another Thyasira sp., and it is possible that fecundity is similar in species within the COS.ForThy biotope. Spawning of Thyasira gouldi occurs throughout the year, with up to 750 eggs produced each time. No information is available on the mechanism of spawning or the number of spawnings per year.

Other species that usually occur in the biotope, such as polychaetes and brittlestars usually have planktonic development, an annual reproductive cycle and are fecund.

Time for community to reach maturity

Little is known about the mode of reproduction and recoverability of foraminifera. All other characteristic species within the biotope are fecund and species such as polychaetes and brittlestars are likely to recover fairly quickly. However, the larval development of Thyasira equalis is lecithotrophic and the pelagic stage is very short or quite suppressed. This agrees with the reproduction of other Thyasira sp., and in some cases (Thyasira gouldi) no pelagic stage occurs at all (Thorson, 1946). This means that larval dispersal is limited.
  • Between 1979 and 1980, deoxygenation of bottom waters resulted in the depletion of Thyasira equalis and Thyasira sarsi from 550/m² to almost zero. However, by 1987 200/m² were present (Dando & Spiro, 1993).
  • After a decline in the abundance of Thyasira flexuosa in Penobscot Bay, Maine, after trawler disturbance, populations were reported to recover within 3.5 months (Sparks-McConkey & Watling, 2001).
Explanations for the high recovery of these populations could be due to high post-settlement survival, or new populations of adults washed in by bedload transport to colonize the area.

Additional information

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This review can be cited as follows:

Riley, K. 2002. Foraminiferans and Thyasira sp. in deep circalittoral soft mud. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 01/12/2015]. Available from: <>