Halichondria bowerbanki, Eudendrium arbusculum and Eucratea loricata on reduced salinity tide-swept circalittoral mixed substrata
Ecological and functional relationships
ECR.HbowEud is sponge, barnacle and ascidian dominated with Bryozoa (Eucratea loricata, Alcyonidium diaphanum) and Hydrozoa (species of Eudendrium) in particular forming an erect canopy. Sponges can provide hard substrata for attachment, refugia and shelter, an enhanced food supply in feeding currents and a potential food source themselves (Klitgaard, 1995; Koukouras et al., 1996).
Sponges are noted as being inhabited by a wide diversity of invertebrates. For instance, Biernbaum (1981) describes the amphipod community associated with Halichondria bowerbanki.
Hydroids are also likely to act as a host for other species, for instance caprellid amphipods.
Sessile organisms in the biotope are likely to be in intense competition for space and overgrowth (for instance, of barnacles by sponges and ascidians) is likely to occur. Whilst some sessile organisms form flat sheets over the surface, others retain a small point of attachment and grow upwards and form a canopy above the substratum, particularly the bryozoans Eucratea loricata and Alcyonidium diaphanum and hydrozoans of the genus Eudendrium. Such variation in growth forms may be one form of 'niche partitioning' on homogenous rock surfaces (Sebens, 1985).
There appear to be very few active predators in this biotope and they occur infrequently. Exceptions are the starfish Asterias rubens and the crab Carcinus maenas. Any reduction in predation pressure is likely to favour some species such as solitary ascidians (see Schmidt & Warner, 1984).
Seasonal and longer term change
No information has been found on seasonal or temporal change in this biotope.
Habitat structure and complexity
The range of growth forms of species that dominate or occur frequently in this biotope provides some habitat complexity. Species such as sponges and hydroids can provide substrata for attachment, refugia and shelter for a number of animals (Klitgaard, 1995; Koukouras et al., 1996) The biotope occurs in very sheltered conditions and any upward facing surfaces are likely to accumulate silt which, despite strong tidal streams, may attract small species such as amphipods, worms and meiofauna.
No photosynthetic species are listed as characterizing species in ECR.HbowEud, a circalittoral biotope. Consequently, primary production is not regarded as a major component of productivity. Nevertheless, the biotopes represented by this review are often species rich and may contain quite high animal densities and biomass. Specific information about the productivity of characterizing species or about the biotope in general was not found.
- The majority of the species in the biotope are sessile and rely on planktonic larval stages for recruitment. In the case of barnacles and hydroids and possibly sponges, larval survival in the plankton is likely to be long. In the case of ascidians, it is much shorter. Sponges may also proliferate asexually.
- Two sets of hypotheses explaining patterns of larval settlement have become established. The first proposes that active habitat selection determines the distribution of newly settled larvae. The second suggests that distribution and abundance are determined by passive deposition of competent larvae (i.e. purely hydrodynamic processes) (Havenhand & Svane, 1991) (see Meadows & Campbell,1972; Scheltema, 1974; Butman, 1987). Although these two hypotheses have been regarded by some authors to be conflicting, they are not necessarily mutually exclusive (Butman, 1987). For example, the presence of conspecific adults can be an important factor in determining habitat selection. Long-term data from populations of the ascidian Ascidia mentula occurring on subtidal vertical rock indicated that recruitment of Ascidia mentula larvae was positively correlated with adult population density, and then by subsequent active larval choice at smaller scales. Factors influencing larval settlement were light, substratum inclination and texture (Havenhand & Svane, 1989). The presence of hydroids may also be important in recruitment of ascidians. Schmidt (1983) describes how the hydroid Tubularia larynx (which has a similar structure to Eudendrium arbusculum) attracted a 'bloom' of the ascidians Ciona intestinalis and Ascidiella aspersa on settlement panels. However, the swimming power of an ascidian tadpole larva is relatively low (Chia, Buckland-Nicks & Young, 1984), and on a larger scale hydrodynamic variables will most probably determine distribution (Olson, 1985; Young, 1986).
Time for community to reach maturity
No information concerning the development of this specific community was found. However, many of the species present in ECR.HbowEud are present in the biotopes described by Sebens (1985) which were considered to be dynamic and fast growing. Many sponges recruit annually, growth can be quite rapid, with a life span of one to several years. Other species present can be relatively long-lived. For example, the ascidian Ascidia mentula
has been reported to live seven years in some populations, whilst Ascidiella aspersa
may live between one to one and a half years around the British Isles compared with two to three years in Norwegian waters (Fish & Fish, 1996).
No additional information.
This review can be cited as follows:
Halichondria bowerbanki, Eudendrium arbusculum and Eucratea loricata on reduced salinity tide-swept circalittoral mixed substrata.
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=45&code=2004>