Barnacles and Littorina littorea on unstable eulittoral mixed substrata
Ecological and functional relationships
The SLR.BLlit biotope is found in a range of wave exposures. In exposed locations disturbance is high creating small scale succession events so that only fast growing opportunistic algal species such as Ulva are able to grow. However, the abundance of green algae in the biotope is low because of the grazing activity of Littorina littorea which occur in high abundance. In sheltered locations the substrata is more stable and fucoid sporelings may settle but are removed by the grazing activity of limpets and Littorina littorea. Thus, because of the impact of disturbance and/or grazing, algal cover is very low in the whole range of exposure in which the biotope is found.
The pebble and cobble beaches of SLR.BLlit have a poor fauna in comparison to open shore locations on bedrock, presumably as a result of siltation and the instability of the substratum. There is a covering of barnacles on the cobbles and pebbles and on larger stable boulders and rock Patella vulgata is present in high abundance. Littorina littorea, which is tolerant of muddy and silty conditions, can be found in large aggregations and often cluster on the tops of small stones. Although Mytilus edulis is less common on cobbles and pebbles than on larger boulders or bedrock, the species may serve to enhance the stability of the substratum.
Algal cover in the biotope is low and limited mostly to opportunistic green species such as Ulva spp. and Ulva spp.
In extremely sheltered locations, even the smallest stones are relatively stable but remain unoccupied by algal sporelings so that barnacles settle (Lewis, 1964; Raffaelli & Hawkins, 1999).
Littorina littorea is often the dominant grazing gastropod on the lower shore eating soft macrophytes and microalgae. Experiments in Helgoland (Janke, 1990) suggest that Littorina grazing can exclude the green alga Ulva and reduce the settlement and growth of Fucus species. Cover by opportunistic species like Ulva may be kept in check by littorinid grazing.
A dense covering of barnacle species is effective in limiting the efficiency of limpet grazing which adversely affects limpet growth. Bulldozing by grazing limpets may cause high post-settlement mortality of barnacles (Jenkins et al., 2000).
The crab Carcinus maenas is a predator of young Littorina littorea.
The characterizing species of the sediment beneath the pebbles and cobbles are infaunal such as the obligate deposit feeding Arenicola marina.
Seasonal and longer term change
Rocky shore communities are often highly variable in time, due to the combined influences of physical disturbance, competition, grazing, predation and variation in recruitment. Barnacle dominated rocky shores demonstrate dynamic temporal changes, mediated by relatively random events such as recruitment intensity, and the abundance of grazers and predators (Hawkins et al
., 1992; Raffaelli & Hawkins, 1999). Settlement of Semibalanus balanoides
takes place in the spring and Chthamalus
spp. in the summer and autumn. Seasonal fluctuations in the abundance of Ulva
spp. may also be seen.
Habitat structure and complexity
Habitat complexity in this biotope is relatively limited in comparison to some rocky shore biotopes. However, the mixed nature of pebbles and cobbles, boulders, rocks and coarse sediment does create some complexity. Larger cobbles and boulders provide substratum and shelter for a variety of species such as small crabs and gammarid amphipods. Beneath boulders and the largest cobbles and pebbles (if free of sediment) underboulder communities may be present. Smaller pebbles and cobbles will be too small and too unstable (e.g. subject to overturn) for some encrusting species to persist.
In the absence, or low abundance, of macroalgae, production in this biotope is mostly secondary production by suspension and deposit feeders. Primary production will be limited to microalgae growing on rock surfaces. Detrital input will be important for the suspension feeding barnacles and mussels. Rocky shores can make a contribution to the food of many marine species through the production of planktonic larvae and propagules which contribute to pelagic food chains. In general rocky shore communities are highly productive and are an important source of food and nutrients for members of neighbouring terrestrial and marine ecosystems (Hill et al., 1998). However, in the SLR.BLlit biotope, faunal species may not attain the same biomass that may be found on stable rocky substrata on the open coast, so secondary productivity is likely to be lower.
Most species present in the biotope possess a planktonic stage (gamete, spore or larvae) which float in the plankton before settling and metamorphosing into the adult form. This strategy allows species to rapidly colonize new areas that become available such as in the gaps often created by storms. Thus, for organisms such as those present in this biotope, it has long been evident that recruitment from the pelagic phase is important in governing the density of populations on the shore (Little & Kitching, 1996). Both the demographic structure of populations and the composition of assemblages may be profoundly affected by variation in recruitment rates.
- Littorina littorea can breed throughout the year but the length and timing of the breeding period are extremely dependent on climatic conditions. Also, estuaries provide a more nutritious environment than the open coast (Fish, 1972). Sexes are separate, and fertilisation is internal. Littorina littorea sheds egg capsules directly into the sea. Egg release is synchronized with spring tides and occurs on several separate occasions. In estuaries, the population matures earlier in the year and maximum spawning occurs in January (Fish, 1972). Fecundity value is up to 100,000 for a large female (27mm shell height) per year. Female fecundity increases with size. Larval settling time or pelagic phase can be up to six weeks. Males prefer to breed with larger, more fecund females (Erlandsson & Johannesson, 1992). Parasitism by trematodes may cause sterility in Littorina littorea.
- Barnacle settlement and recruitment can be highly variable because it is dependent on a suite of environmental and biological factors, such as wind direction and success depends on settlement being followed by a period of favourable weather. Long term surveys have produced clear evidence of barnacle populations responding to climatic changes. During warm periods Chthamalus spp. predominate, whilst Semibalanus balanoides does better during colder spells (Hawkins et al., 1994). Release of Semibalanus balanoides larvae takes place between February and April with peak settlement between April and June. Release of larvae of Chthamalus montagui takes place later in the year, between May and August.
- Recruitment of Patella vulgata fluctuates from year to year and from place to place. Fertilization is external and the larvae is pelagic for up to two weeks before settling on rock at a shell length of about 0.2mm. Winter breeding occurs only in southern England, in the north of Scotland it breeds in August and in north-east England in September.
- Mytilus edulis recruitment is dependant on larval supply and settlement, together with larval and post-settlement mortality. Recruitment in many Mytilus sp. populations is sporadic, with unpredictable pulses of recruitment (Seed & Suchanek, 1992). Mytilus sp. is highly gregarious and final settlement often occurs around or in-between individual mussels of established populations.
- The infaunal polychaete Arenicola marina has high fecundity and the eggs develop lecithotrophically within the sediment or at the sediment surface. There is no pelagic larval phase and the juveniles disperse by burrowing. Recruitment must occur from local populations or by longer distance dispersal of postlarvae in water currents or during periods of bedload transport.
- Ulva is a rapidly growing opportunistic species which can colonize bare substrata soon after it is created.
Time for community to reach maturity
No specific information was found concerning time taken for the community to reach maturity. However, the characterizing species of the SLR.BLlit biotope are widespread, highly fecund and quick to grow and mature and so the community would be expected to reach maturity within 5 years. For example, Bennell (1981) observed that barnacles that were removed when the surface rock was scraped off in a barge accident at Amlwch, North Wales returned to pre-accident levels within 3 years. However, barnacle recruitment can be very variable because it is dependent on a suite of environmental and biological factors, such as wind direction, so populations may take longer to recruit to suitable areas. Littorina littorea is widespread and often common or abundant. Littorina littorea is an iteroparous breeder with high fecundity that lives for several (at least 4) years. Breeding can occur throughout the year and larvae form the main mode of dispersal. The planktonic larval stage is long (up to 6 weeks) although larvae do tend to remain in waters close to the shore. Most of the other species in the biotope have planktonic larvae and so should colonize suitable areas. Therefore, it seems likely that the biotope would reach maturity within five years. However, in newly created substrata, initial absence of grazing prosobranchs may allow first green, then brown algae to grow and dominate the shore until removed by scour or old age. In such cases the establishment of SLR.Bllit may take longer than five years.
This review can be cited as follows:
Barnacles and Littorina littorea on unstable eulittoral 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=340&code=2004>