Biodiversity & Conservation

The biotope is characterized by fucoid seaweed species. At reduced salinities Fucus ceranoides is a superior competitor and tends to replace Fucus vesiculosus, Fucus spiralis, and Ascophyllum nodosum towards the upper reaches of estuaries and sea lochs. For instance, germlings of Fucus ceranoides developed from zygotes and grew at all salinities from 34 psu to 8.5 psu, whilst those of Fucus vesiculosus did not survive below 24 psu if germlings settled directly at reduced/low salinity. Growth of Fucus ceranoides (germlings and adult plants) was also faster than that of Fucus vesiculosus at all salinities from 17 psu downwards, but the pattern was reversed at full salinity (Khafaji & Norton, 1979). Furthermore, Lein (1984) considered that in addition to salinity stress, grazing by snails and heavy growth of epiphytes in higher salinity conditions confined Fucus ceranoides to estuarine environments.

The presence of a fucoid canopy may inhibit the settlement of barnacles (Semibalanus balanoides and Elminius modestus) by blocking larval recruitment mainly by 'sweeping' the rock of colonizers. However, the canopy offers protection against desiccation which promotes the clumping of adults and the recruitment of young in several species of mobile animals, such as Littorina littorea.

In feeding trials, the preferred food of Littorina littorea was small ephemeral seaweed, such as Ulva (Lubchenco, 1978), and the grazing activity of Littorina littorea may be important in keeping fast growing ephemeral species such as Ulva in check.

Species within the biotope may act as hosts for parasite species. For instance, Littorina littorea hosts trematodes such as Cryptocotyle lingua, Himasthla leptosoma, Renicola roscovita and Ceracaria lebourae, whilst Semibalanus balanoides may be infested by the isopod Hermioniscus balani. Crisp (1960) recorded that 1.7 % of barnacles were infested by the isopod and that infested barnacles had no egg masses and grew nearly 40 % less than un-infested barnacles. King et al., (1993) similarly recorded that 7 % of brooding Semibalanus balanoides were infested by the isopod.

The biotope occurs in extremely sheltered conditions so dramatic temporal changes associated with winter storms are unlikely. However, seasonal changes in growth and recruitment are often apparent on rocky shores, influenced mainly by temperature and day length / degree of insolation. Fucus ceranoides communities may be more exposed to cold air and water temperatures than in locations next to the open sea. Germlings of Fucus ceranoides are found from the end of May to the beginning of August. Receptacles usually drop off by October or November. Some species may show spring seasonal changes, for instance, in northern Britain Littorina littorea migrates down shore as temperatures fall in autumn (to reduce exposure to sub-zero temperatures) and up shore as temperatures rise in spring; migration depends on local winter temperatures.

The beds of fucoids in the biotope increase the structural complexity of the habitat providing a variety of resources that are not available on bare rock. Fronds provide space for attachment of encrusting or sessile epifauna and epiphytic algae and provide shelter from wave action, desiccation and heat for invertebrates.

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). Macroalgae exude considerable amounts of dissolved organic carbon which are absorbed readily by bacteria and may even be taken-up directly by some larger invertebrates. Only about 10% of the primary production is directly cropped by herbivores (Raffaelli & Hawkins, 1996). Dissolved organic carbon, algal fragments and microbial film organisms are continually removed by the sea. This may enter the food chain of local, subtidal ecosystems, or be exported further offshore. Rocky shores make a contribution to the food of many marine species through the production of planktonic larvae and propagules which contribute to pelagic food chains.

• Many rocky shore species, plant and animal, possess a planktonic stage: gamete, spore or larvae which float in the plankton before settling and metamorphosing into adult form. This strategy allows species to rapidly colonize new areas that become available. For these organisms 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). Hence, both the demographic structure of populations and the composition of assemblages may be profoundly affected by variation in recruitment rates.
• The propagules of most seaweeds have little or no control over their destination. When released into the sea, they are distributed by waves and currents. Clearly to disperse beyond the zone that they will be able to inhabit is wasteful of propagules, and some intertidal plants seem able to limit dispersal to a meter or so (Dayton, 1973; Deysher & Norton, 1982). Propagules of Fucus species can colonize up to at least 23 m from the parent plants on a shore devoid of seaweeds (Burrows & Lodge, 1950).
• Littorina littorea can breed throughout the year but the length and timing of the breeding period are extremely dependent on climatic conditions. Estuarine locations, where this biotope may be found, 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 and egg release is synchronized with spring tides, on several separate occasions. In estuaries the population matures earlier in the year and maximum spawning occurs in January. 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. Parasitism by trematodes may cause sterility.
• Barnacle recruitment can be very variable because it is dependent on a suite of environmental and biological factors, such as, wind direction, temperature, latitude, light, feeding, age, size, crowding, seaweed cover and pollution and success depends on settlement being followed by a period of favourable weather. High shore Semibalanus balanoides breed first and low shore specimens last (up to 12 days difference) (Barnes, 1989). Fertilization is prevented by temperatures above 10 °C and continuous light. Differences in breeding times with latitude are probably mediated by temperature and day length, e.g. in Spitzbergen fertilization occurs 2-3 months earlier than in the United Kingdom. Release of Semibalanus balanoides larvae takes place between February and April with peak settlement between April and June. The barnacle Elminius modestus is a cross-fertilizing hermaphrodite which breeds almost continuously throughout the year. Under favourable conditions it has been known for broods to be released every 10 days. Cyprid larvae are found on the shore between May and October. Newly metamorphosed Elminius modestus grow rapidly and can reach maturity in about eight weeks (Fish & Fish, 1996).

Fucoid species are found on all British and Irish coasts so there are few mechanisms isolating populations. With the exception of Ascophyllum nodosum, fucoids are highly fecund, iteroparous, surviving and breeding for protracted periods over 3-4 years. The eggs are broadcast into the water column allowing a potentially large dispersal distance. Green algal species such as Ulva are opportunistic ephemeral species that can recruit rapidly when conditions are suitable and will often be the early colonizers of areas that have been disturbed. Littorina littorea generally reaches maturity between two or three years of age, when shell height is about 12 mm, and the species can live for five or more years (Fish & Fish, 1996). The barnacle, Semibalanus balanoides, grows rapidly in the first season after settlement. Its newly metamorphosed larvae are very squat and only form the adult shape at 3 mm. Semibalanus balanoides may become sexually mature in the first year after settlement although this is often delayed until 2 years of age (Anderson, 1994). Life span of Semibalanus balanoides varies with the position on the shore. Barnacles low on the shore typically die in their third year, whereas those from near the mean level of high water neaps may live for five or six years. Therefore, the community to reach maturity is likely to reach maturity within five years (in terms of the presence of characterizing species and those species being sexually mature), although development of a stable community structure may take a little longer due to competitive interactions.

No text entered.