Biodiversity & Conservation

Pomatoceros triqueter, Balanus crenatus and bryozoan crusts on mobile circalittoral cobbles and pebbles



Image Jon Davies - Cobbles with dense Pomatoceros species (ECR.PomByC). Image width ca 50 cm.
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Distribution map

SS.SCS.CCS.PomB recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

  • EC_Habitats
  • UK_BAP

Ecological and functional relationships

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.

Seasonal and longer term change

This biotope probably experiences seasonal variation in scour, which is most severe in winter storms. Holme & Wilson (1985) suggested that the fauna of his Balanus-Pomatoceros assemblage in the central English Channel was restricted to rapid growing colonizers able to settle rapidly and utilize space in short periods of stability in the summer months. The biotope will probably exhibit spring and summer peaks in hydroids, erect bryozoa and fast growing ascidians. Species richness is probably highest in the spring and summer. Inter-annual variation in storms and wave action is likely to remove the majority of epifauna in some years but allow more species to become established in others. However, the biotope is dominated by opportunistic species and effectively annual and ephemeral. Off Chesil Bank, the epifaunal communities dominated by Pomatoceros triqueter, Balanus crenatus and Electra pilosa, decreased in cover in October, was scoured away in winter storms, and was recolonized in May to June (Warner, 1985). Warner (1985) reported that the community did not contain any persistent individuals, being dominated by rapidly colonizing organisms but, while larval recruitment was patchy and varied between the years studied, recruitment was sufficiently predictable to result in a dynamic stability and a similar community was present in 1979, 1980 and 1983.

Habitat structure and complexity

The surface of cobbles and pebbles support tubes of Pomatoceros species, encrusting coralline algae, encrusting bryozoans and barnacles. Boulders and more stable hard substrata may support more delicate species such as the hydroids, erect bryozoans (e.g. Bugula spp. and Flustra foliacea) and fast growing ascidians (e.g. Ascidiella species and Dendrodoa grossularia). Patches of gravel and sand overlying bedrock may support the large dahlia anemone Urticina felina. The sand and gravel infauna probably supports meiofauna and some polychaetes but no information was found. Mobile species such as squat lobster (e.g. Galathea spp.) may use spaces between boulders as temporary refuges. Brittlestars (e.g. Ophiocomina nigra and Ophiothrix fragilis) may utilize spaces between cobbles and pebbles. The biotope may be surrounded by more species rich biotopes. For example, the biotope may grade into MCR.Flu.SerHyd with increasing substratum stability (Connor et al., 1997a). Holmes & Wilson (1985) noted that raised bedrock, above the main area affected by scour, in the English Channel was characterized by Flustra foliacea communities (see MCR.Flu for more information).


This biotope is dominated by secondary producers. Food in the form of phytoplankton, zooplankton and organic particulates from the water column together with detritus and abraded macroalgal particulates from shallow water ecosystems are supplied by water currents and converted into faunal biomass. Their secondary production supplies higher trophic levels such as mobile predators (e.g. starfish, sea urchins, and fish) and scavengers (e.g. starfish and crabs) and the wider ecosystem in the form of detritus (e.g. dead bodies and faeces). In addition, reproductive products (sperm, eggs, and larvae) also contribute to the zooplankton (Hartnoll, 1998). No estimates of productivity were found in the literature but the biotope is impoverished so that productivity is likely to be low.

Recruitment processes

Pomatoceros triqueter probably breeds throughout the year with a peak in spring and summer, although breeding was reported to only occur in April at Port Erin (Moore, 1937; Segrove, 1941; Hayward & Ryland, 1995). Larvae are pelagic for about 2-3 weeks in the summer. However, in the winter this amount of time increases to about 2 months (Hayward & Ryland, 1995). Settlement was reported to be rare in winter but maximum settlement occurred in April, June, August and Sept-Oct (Castric-Fey, 1983). Once settled juveniles grow at about 1.5mm/month, and become sexually mature with about 4 months (see MarLIN review). Pomatoceros triqueter may live for up to 4 years, although 1.5-2.5 years is probably more usual and most die after reproduction (Castric-Fey, 1983; Hayward & Ryland, 1995), so that life span probably depends on location and environmental conditions. Dispersal potential is high, depending on local hydrographic condition, and tubeworms, such as spirorbids and Pomatoceros triqueter are commonly the initial recruits to new substrata (Sebens, 1985, 1986; Hatcher, 1998). For example, Pomatoceros triqueter colonized artificial reefs soon after deployment in summer (Jensen et al., 1994), settlement plates within 2-3.5 months and dominated spring recruitment (Hatcher, 1998). However, in the mobile stone communities of Chesil Bank, Warner (1985) suggested that Pomatoceros triqueter did not reach sexual maturity in the population he studied.

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.

Time for community to reach maturity

This biotope has a impoverished community consisting of rapid colonizing, rapid growing and reproducing species (see above). After winter storms or other severe disturbance, the dominant species would probably recolonize the habitat within a few months, and the community probably develops annually by recruitment from surviving individuals or colonies and recruitment from adjacent or upstream habitats. The biotope would probably be recognizable within less than 6 months. Hydroids and erect bryozoans may take longer to establish, probably from surviving fragments or hydrorhizae but would either regrow or re-colonize within 6-12 months in most cases. Holme & Wilson (1985) suggested that the fauna of his Balanus-Pomatoceros assemblage was restricted to rapid growing colonizers able to settle rapidly and utilize space in short periods of stability in the summer months, and develop within less than a year.

Additional information

None entered

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 17/04/2014]. Available from: <>