|Basic Information||Biotope classification||Ecology||Habitat preferences and distribution||Species composition||Sensitivity||Importance|
Image Jon Davies - Cobbles with dense Pomatoceros species (ECR.PomByC). Image width ca 50 cm.
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SS.SCS.CCS.PomB recorded () and expected () distribution in Britain and Ireland (see below)
This biotope is characterized by an impoverished fauna, dominated by fast growing epifauna such as the tubeworms, encrusting bryozoans and barnacles. The dominant species probably compete for space on the available hard substrata. While Pomatoceros triqueter may overgrow encrusting bryozoans, encrusting bryozoans tolerate overgrowth and probably subsequently grow over the calcareous tube of Pomatoceros triqueter (Gordon, 1972; Rubin, 1985). Encrusting bryozoans and encrusting corallines also probably compete for space. But this biotope experiences seasonal and sporadic cycles of severe scour that will free space for colonization, so that competition is probably limited. Numerous species have been recorded within this biotope but most are probably opportunistic or are species that are fortunate to find temporary sheltered niches from scour, and the species present probably vary with location. Overall, the community is primarily opportunistic and ephemeral. Primary productivity is provided by encrusting corallines although few species present can probably graze them and few other algae are likely to survive scour in the long term.
The dominant species are suspension feeders on phytoplankton, zooplankton and organic particulates, e.g. the tubeworm Pomatoceros triqueter, barnacles Balanus crenatus and Balanus balanus, encrusting bryozoans (e.g. Parasmittina trispinosa), occasional erect Bryozoa (e.g. Crissiidae, Flustra foliacea and Scrupocellaria species), and occasional hydroids e.g. Sertularia argentea, Nemertesia species and Hydrallmania falcata).
Where present, Urticina felina is a passive predator of zooplankton and small invertebrates.
Mobile predators on epifauna include the starfish Asterias rubens and occasional Echinus esculentus feeding on epifaunal crusts, encrusting corallines, hydroids and bryozoans.
Starfish and hermit crabs (e.g. Pagurus bernhardus) are probably generalist predators and scavengers within the biotope.
The barnacle Balanus crenatus reproduces between February and September, larvae settling in a peak from April to October. Once settled, Balanus crenatus matures within 4 months, so that April settled individuals can produce larvae by July, reaching full size before their first winter (Rainbow, 1984). Balanus crenatus has a life span of only 18 months so that the population requires continuous recruitment. Therefore, dispersal potential is high, depending on the local hydrographic regime. Balanus crenatus also colonized settlement plates or artificial reefs within 1-3 months of deployment in summer, (Brault & Bourget, 1985; Hatcher, 1998), and became abundant on settlement plates shortly afterwards (Standing, 1976; Brault & Bourget, 1985).The brooded, lecithotrophic coronate larvae of many bryozoans (e.g. Flustra foliacea, Parasmittina trispinosa, and Bugula species), have a short pelagic life time of several hours to about 12 hours (Ryland, 1976). Recruitment is dependant on the supply of suitable, stable, hard substrata (Eggleston, 1972b; Ryland, 1976; Dyrynda, 1994). However, even in the presence of available substratum, Ryland (1976) noted that significant recruitment in bryozoans only occurred in the proximity of breeding colonies. Other species, such as Electra and Crisia release long-lived planktonic larvae. Electra pilosa has a planktonic larvae with a protracted life in the plankton and potentially extended dispersal and can colonize a wide variety of substrata. It is probably adapted to rapid growth and reproduction (r-selected), capable of colonizing ephemeral habitats, but may also be long lived in ideal conditions (Hayward & Ryland, 1998). In settlement studies, Electra crustulenta recruited to plates within 5 -6months of deployment (Sandrock et al., 1991). Jensen et al. (1994) reported that encrusting bryozoans colonized an artificial reef within 6-12months. Keough (1983) noted that Parasmittina raigii colonized settlement plates annually. Overall, encrusting bryozoans are probably rapid colonizers of available hard substrata.
Hydroids are often initial colonizing organisms in settlement experiments and fouling communities (Jensen et al., 1994; Gili & Hughes, 1995; Hatcher, 1998). The hydroids (e.g. Hydrallmania falcata and Sertularia argentea) lack a medusa stage, releasing planula larvae. Planula larvae swim or crawl for short periods (e.g. <24hrs) so that dispersal away from the parent colony is probably very limited (Sommer, 1992; Gili & Hughes, 1995). However, Nemertesia antennina releases planulae on mucus threads, that increase potential dispersal to 5 -50m, depending on currents and turbulence (Hughes, 1977). Most species of hydroid in temperate waters grow rapidly and reproduce in spring and summer. Few species of hydroids have specific substrata requirements and many are generalists. Hydroids are also capable of asexual reproduction and many species produce dormant, resting stages, that are very resistant of environmental perturbation (Gili & Hughes, 1995). But Hughes (1977) noted that only a small percentage of the population of Nemertesia antennina in Torbay developed from dormant, regressed hydrorhizae, the majority of the population developing from planulae as three successive generations. Rapid growth, budding and the formation of stolons allows hydroids to colonize space rapidly. Fragmentation may also provide another route for short distance dispersal. Hydroids may potentially disperse over a wide area in the long term as dormant stages, or reproductive adults, rafting on floating debris or hitch-hiking on ships hulls or in ballast water (Cornelius, 1992; Gili & Hughes, 1995).Overall, the dominant species in the biotope, i.e. the tubeworms, encrusting bryozoans and barnacles, are good initial colonizers of hard substrata, capable of rapid growth and reproduction (r-selected) and adapted to ephemeral habitats.
This review can be cited as follows:
Tyler-Walters, H. 2002. Pomatoceros triqueter, Balanus crenatus and bryozoan crusts on mobile circalittoral cobbles and pebbles. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 27/08/2014]. Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=177&code=2004>