Styela gelatinosa and other solitary ascidians on very sheltered deep circalittoral muddy sediment
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 than by interspecific interactions.
Various species including the ascidian species characteristic of this biotope may attach to the clam Pseudamussium septemradiatum. Fourteen species are listed by Allen (1953a) as attached to live individuals of Pseudamussium septemradiatum from the Clyde Sea area including five foramaniferans, one sponge, two polychaetes, one bryozoan, two barnacles, two small bivalves but only one ascidian, Styela clava. Allen (1953a) notes that dead shells had very little attached and suggested that movement of the scallop was important in retaining surfaces clear of silt for settlement of attached fauna.
The biotope supports some burrowing bivalve species and the sea cucumber Paracucumaria hyndmani.
Seasonal and longer term change
One of the key factors affecting benthic habitats is disturbance which in deep sediment habitats such as COS.Sty is minimal and so communities may be relatively stable. However, there may be some seasonal changes in the biotope including recruitment of young, growth rates and abundance of adults. Also, species in the biotope may be attached to terrigenous debris that may degenerate and many of the species in the biotope are short-lived (see 'Recruitment Processes') so that their abundance from year-to-year may vary depending on success of breeding and recruitment.
Habitat structure and complexity
The biotope most likely has very little structural complexity. However, terrigenous debris, especially if that includes twigs and branches, may create some complexity. There is no evidence from the very limited survey data available of burrowing activity that might create complexity in the sediment habitat or burrows that cryptic species can use.
Productivity in subtidal sediments is often quite low. Macroalgae and, most likely, microalgae are absent from COS.Sty and so productivity is secondary, derived from detritus and organic material. Organic material is derived from terregenous debris (vegetation) and water column sources (e.g. plankton, detritus). Organic material is degraded by micro-organisms and the nutrients are recycled.
The species in this biotope predominantly have planktotrophic larvae and are potentially able to recruit from fairly distant sources (>10 km). Critically, no information has been found on recruitment processes of Styela gelatinosa
although (Millar, 1963b) Styela coriacea
produces tadpole larvae that can swim but appear to settle rapidly as do Ascidiella scabra
where settlement time is estimated to be 2-10 days (see Berill, 1950). No information was found for reproduction in Pseudamussium septemradiatum
but scallops generally have a long planktonic stage (for instance 28 days+ for Pecten maximus
: Beaumont & Barnes, 1992). In an enclosed sea loch such as Loch Goil, there is most likely some retention of larvae, local recruitment of species with short-lived larvae such as some ascidians and possibly little recruitment from open coast populations.
Time for community to reach maturity
The community probably has a high turnover rate within individuals of the component species reflecting the likely transitory nature of the biogenic hard substrata available for settlement. For instance, Ascidiella scabra
has a high fecundity and settles readily, probably for an extended period from spring to autumn. Svane (1988) describes it as "an annual ascidian" and demonstrated recruitment onto artificial and scraped natural substrata. The occurrence and longevity of most large solitary ascidians appears similar to that of Ascidiella scabra
. Allen (1953a) demonstrated that Pseudamussium septemradiatum
has a life span of about 3.5 years in populations sampled in the Firth of Clyde. Other species that are recorded as rare or occasional in the biotope (Abra alba
, Ciona intestinalis
, Metridium senile
, Protanthea simplex
, Sabella pavonina
) are all known to settle onto new surfaces within a year or a very few years. Unfortunately, no information has been found for Styela gelatinosa
that can be used to estimate longevity or settlement frequency. Nevertheless, it appears that the community would reach maturity rapidly (possibly within a year or a very few years) after new substrata became available.
Very little information is available on the biology of Styela gelatinosa
has been found and inferences have to be drawn from knowledge of the biology of other ascidians.
This review can be cited as follows:
Styela gelatinosa and other solitary ascidians on very sheltered deep circalittoral muddy sediment.
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=274&code=1997>