Burrowing anemones in sublittoral muddy gravel
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. No single species can be considered a keystone species whose activity is essential to the structure of the community. In addition to the anemones the biotope may support a rich fauna of smaller less conspicuous species, such as polychaetes, nematodes and bivalves that live within the sediment.
There are however, some interspecific relationships within the biotope. Associations between amphipods and anemones are well known. The burrowing anemone Peachia hastata Gosse has been observed to be associated with the lysianassid amphipod Acidostoma neglectum Dahl (Ansell, 1969) and more recently Moore & Cameron (1999) identified a tubiculous amphipod Photis longicaudata associated with Cerianthus lloydii.
Anemones have few predators, the most notable being nudibranchs (sea slugs), some of which feed only on a single species. Anemones are found amongst the stomach contents of fishes but whether they constitute a regular item of diet is uncertain (Manuel, 1988). Cerianthus lloydii secretes a soft, felt-like, mucous tube up to 400mm in length in which it lives. The species is able to move freely within the tube and can contract rapidly when it comes into contact with other organisms. The other species that may live in the biotope, such as infaunal organisms, are often cryptic in nature and not usually subject to predation.
The density of anemones is probably determined by sediment type, current conditions and food availability.
The hydrodynamic regime, which in turn controls sediment type, is the primary physical environmental factor structuring benthic communities such as IMX.An. 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
Burrowing anemone communities probably persist over long periods at the same location. There are no reports of significant seasonal or temporal changes in the biotope.
- Continuous observations over several 24 hour periods by divers and underwater TV indicated the absence of any diurnal rhythmic activity in Cerianthus lloydii. Observations over a tidal cycle gave no evidence of a tidal feeding rhythm (Eleftheriou & Basford, 1983). An absence of an activity rhythm has also been described in edwardsiid anemones (Ellehauge, 1978).
- Anemones are generally very slow growing and long lived species so the biotope is not likely to undergo any significant seasonal changes. For example, Stephenson (1935) observed some specimens of Cereus pedunculatus survived in the laboratory for 70 years and suggested that it seems probable that in the wild, anemones could live for hundreds of years under suitable conditions.
- Some individuals may be washed out if there are severe winter gales in shallow water but temporal changes are expected to be minimal.
Habitat structure and complexity
The biotope has very little structural complexity with most species living partially buried in the sediment. There are no prominent features on the sediment surface such as burrows or mounds. With the exception of a few associations between anemones and other invertebrates (see ecology) burrowing anemones do not provide significant habitat for other fauna.
Productivity in subtidal sediments is often quite low. Macroalgae are absent from IMX.An and so productivity is mostly secondary, derived from planktonic organisms.
- Burrowing anemones are generally opportunistic omnivorous suspension feeders living on a range of zooplankton including copepods, cladocerans, ostracods and bivalves. Cereus pedunculatus feeds mainly on crustaceans. Larger anemones such as Cerianthus lloydii are able to take larger prey and can be considered to be predatory, although the species has also been observed to capture and phagocytose fine detrital particles and bacteria (Tiffon, 1974 cited in Chintiroglou & Koukouras, 1992).
- The role of sea anemones in the marine benthic food web may be important and they should be considered as both primary and secondary consumers.
- Eleftheriou & Basford (1983) suggest that the higher densities of meiofauna in the immediate vicinity of Cerianthus lloydii was because an enrichment of the sediment with digestible wastes and dissolved organic components stimulated a bacterial growth near the tube aperture where nematodes, gastrotrichs, tardigrades and copepods were attracted and multiplied.
- Cereus pedunculatus contains populations of unicellular zooxanthellae (dinoflagellates) which can make an important contribution to the cnidarian metabolism.
- The literature (e.g. Stephenson, 1935) suggests that most sea anemones occupy preferred locations in particular ecological situations. However, there is little information available about the means by which these animals come to settle in such locations. However, there is some evidence to suggest that behaviour patterns do exist which makes it possible for certain sea anemones to select particular habitats (Ross, 1967). However, other species may encounter a random assortment of locations but that survival depends on a few of the young coming into contact with the location in which they are normally found.
- Sexual and asexual reproduction occurs in anemones, however, such is the complexity of reproduction in sea-anemones that in some species the method differs from one population to another (Fish & Fish, 1996). There is no asexual reproduction in cerianthid anemones.
- Cerianthus lloydii has pelagic larvae, the arachnactis, which has been recorded in the plankton from January to August having a planktonic life of about 3 months (Fish & Fish, 1996).
- For the daisy anemone Cereus pedunculatus young are brooded in the parent and Stephenson (1935) reports it to be prolific even after 70 years of growth in the laboratory.
Time for community to reach maturity
There is very little known about community development for this biotope. Almost nothing is known about the life cycle and population dynamics of British burrowing anemones. However, many are slow growing and very long lived and may have patchy and intermittent recruitment. For example, in many localities burrowing anemones lost with the disappearance of eelgrass beds in the 1930's have not returned despite the recovery of Zostera in some regions (Manuel, 1988). Therefore, it seems likely that it could take many years for such a community to develop.
This review can be cited as follows:
Burrowing anemones in sublittoral muddy gravel.
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=8&code=2004>