Suberites spp. and other sponges with solitary ascidians on very sheltered circalittoral rock
Image David Connor - Suberites spp. and other sponges with solitary ascidians on very sheltered circalittoral rock. Image width ca 20 cm.
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Ecological and functional relationships
The biotopes represented by SCR.SubSoAs are sponge and ascidian dominated. Sponges are noted as being inhabited by a wide diversity of invertebrates. 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).
The hydroid, Nemertesia antennina, also acts as a host for other species. As many as 150 species were found in association with it and examination of single stems revealed clear distributional patterns that correlated with the requirements of each epizoic species, and the range of abiotic conditions from the top to the base of the host (Hughes, 1975).
Sessile organisms are often limited by the space available for attachment and thus competition for such space is intense. Competitive success can result from physical or chemical aggression, 'bull-dozing', smothering and possibly by localized food depletion. 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. Such variation in growth forms may be one form of 'niche partitioning' on homogenous rock surfaces (Sebens, 1985).
The various mobile echinoderms that may be present in the biotope (e.g. Marthasterias glacialis, Henricia oculata) play a role in modifying epilithic populations through predation. Although Henricia oculata can suspension feed by extending its arms in the water to trap suspended particles in mucus, it also feeds on sponges and hydroids by stomach eversion (Fish & Fish, 1996). The anemone, Metridium senile, is heavily preyed upon by the nudibranch Aeolidia papillosa and in most years Aeolidia papillosa is the only important source of mortality to Metridium senile in the absence of rocks or boulders that could move and damage it during storms (Sebens, 1985).
Predatory echinoderms and nudibranchs hunt by chemosensation and touch, tracking their prey along a sensory gradient. Thus it is advantageous for the prey species to posses some means of chemical or tactile camouflage. Wicksten (1983) suggested that encrusting animals such as sponges produce chemical products that mask their metabolic by-products or accumulate secondary metabolites which render themselves distasteful or poisonous. Sessile organisms may also use such biologically active compounds as a form of antifouling to prevent the larvae of other species from settling and growing on their surfaces, and also in interactions with established neighbours (Wood, 1987).
In the SCR.AmenCio.Met biotope, also represented by this review, the interactions among Metridium senile, Alcyonium digitatum and Aplidium spp. can be quite complex, and these three species can each dominate patches of vertical wall indefinitely (Sebens, 1985). Purcell (1977) documented cases of damage inflicted on Alcyonium digitatum by neighbouring Metridium senile, presumably caused by nematocysts in specialized "catch-tentacles" which are used agonistically against other anemones. Large Metridium senile (8 cm tall when contracted) can certainly resist encroachment of Aplidium spp., whilst small Alcyonium digitatum and Metridium senile are frequently overgrown by Aplidium spp. (Sebens, 1986).
Also in the SCR.AmenCio.Met biotope, a symbiotic relationship may exist between the plumose anemone Metridium senile and Modiolus modiolus. Laboratory experiments have shown that the presence of the anemone significantly reduced predation of Modiolus modiolus by starfish (Kaplan, 1984). The anemone was thought to benefit from the feeding activities of its host, which circulated food in its direction.
The small bivalve Modiolarca tumida is sometimes found living as a commensal in the test of the ascidian Acidia mentula (Fish & Fish, 1996).
Seasonal and longer term change
It is likely that some temporal changes in the coverage and shelter provided by the species in the biotope will occur. For instance, there is much seasonal variability in abundance of individual hydroids, e.g. Nemertesia antennina, and experiments have shown that hydroid colony growth is highest over a defined temperature range (Gili & Hughes, 1995).
Habitat structure and complexity
The diversity of species within the biotope can be striking even where the physical heterogeneity of the substratum is not. Whilst the rock surface may be smooth and free of discernibly different micro-habitats, some microhabitats are likely to be provided by cracks and crevices in the rock. However, many of the species characteristic of this community add considerable physical complexity to the biotope. 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 may attract small species such as amphipods, worms and meiofauna.
No photosynthetic species are listed as characterizing species in SCR.SubSoAs, a circalittoral biotope. Consequently, primary production is not regarded as a major component of productivity. The biotope SCR.AmenCio.Met may have a small algal component of coralline crusts. 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 biotopes in general was not found.
- Sponges may proliferate both asexually and sexually. A sponge can regenerate from a broken fragment, produce buds either internally or externally or release clusters of cells known as gemmules which develop into a new sponge. Most sponges are hermaphroditic but cross-fertilization normally occurs. The process may be oviparous, where there is a mass spawning of gametes through the osculum which enter a neighbouring individual in the inhalant current. Fertilized eggs are discharged into the sea where they develop into ciliated larvae, known as planulae. However, in the majority, development is viviparous, whereby the larva develops within the sponge and is then released. Larval life varies from a few hours to a few weeks and metamorphosis follows settlement. Adults may live several years (Fish & Fish, 1996).
- Ascidians are also generally hermaphroditic with self-fertilization, but some rely on cross-fertilization. In solitary forms gametes are released in to the open water for fertilization and larval development to occur, whilst compound forms tend to retain the zygote in the atrium until the 'tadpole' larva is fully developed. The larvae are lecithotrophic and have a free-swimming life varying from a few minutes to a few days. Asexual reproduction by budding is also common and reproduction by fission occurs in some colonial species (Fish & Fish, 1996).
- 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). 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 SCR.SubSoAs and the other biotopes also represented by this review, 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, and Ciona intestinalis
may live up to one and a half years (Fish & Fish, 1996).
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This review can be cited as follows:
Suberites spp. and other sponges with solitary ascidians on very sheltered circalittoral rock.
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=94&code=1997>