Fucus serratus, sponges and ascidians on tide-swept lower eulittoral rock
Ecological and functional relationships
Due to the moderately strong to very strong currents associated with this biotope, suspension feeders are the dominant trophic group, indicating the importance of a planktonic input to the benthic community. Suspension feeders frequently associated with this biotope include the sponges Halichondria panicea and Hymeniacidon perleve, ascidians such as Ascidiella scabra and Dendrodoa grossularia, hydroids including Dynamena pumila, bryozoans, spirorbid and serpulid worms, and barnacles.
Herbivores include the common periwinkle Littorina littorea, the grey top shell Gibbula cineraria and common limpet Patella vulgata. The common periwinkle grazes on microorganisms and fine green algae including Ulva sp., apparently rejecting the brown seaweed Ascophyllum nodosum (Fish & Fish, 1996). The common limpet can graze on tough plants including Fucus sp. and encrusting red algae whereas the grey top shell is unable to consume the tough cell walls and feeds mainly on detritus and microalgae (Fish & Fish, 1996). Grazing by Patella vulgata can be an important structuring feature on rocky shores and it is often considered to be a keystone species on north-east Atlantic rocky shores. Reductions in limpet density have been observed to have a significant impact on rocky shore community composition, particularly of fucoid algae and barnacles (Hawkins & Hartnol, 1985; Raffaelli & Hawkins, 1999).
The common shore crab Carcinus maenas is the largest mobile predator frequently associated with this biotope and is likely to move between the boulders and pebbles feeding primarily on small molluscs, especially Littorina sp. and Mytilus edulis, annelids and other crustacea. It is a true omnivore and will also consume algal material. The predatory mollusc Nucella lapillus, the dog whelk, is also frequently associated with this biotope and feeds primarily on the common mussel Mytilus edulis and acorn barnacles (Fish & Fish, 1996) such as Semibalanus balanoides which may also be found.
Autotrophs in the biotope are varied and include representatives from the brown, green and red algal groups such as Fucus serratus, Cladophora rupestris and Mastocarpus stellatus respectively. The algae themselves, especially the Fucus serratus canopy, may provide substratum for epiphytes including hydroids, sponges and ascidians. The distribution of epifauna into different areas on the Fucus serratus is such that competition for space is likely to be reduced. On heavily encrusted Fucus serratus fronds tunicates and sponges are largely basally located, most bryozoans, hydroids and spirorbids occur further out on the central parts of the plants whilst Electra is predominantly found distally (Seed, 1985). In addition, clumps of algae are likely to provide refuge for smaller crabs and periwinkles which may otherwise be washed away by the strong currents.
Due to the eulittoral position of this biotope, the associated fauna are likely to experience some predation from larger predators, namely birds, when exposed at low tide and shallow water fish at high tides.
Seasonal and longer term change
The plants in this biotope are likely to experience some seasonal change in abundance, the general pattern being a lower percentage cover over the winter months. Periodic storms may remove older and weaker plants and reduce the overall biomass of the plants. If the forces were strong enough, the cobbles and boulders may also be moved around, to the detriment of the epilithic fauna. For example, if colonies of sponges and ascidians landed face down on the bedrock, parts of the colony may be crushed and lost. However, this biotope is limited to habitats that are sheltered to extremely sheltered from wave exposure and therefore, increases in wave exposure during winter and the occurrence of winter storms are unlikely to affect it to the same extent that more exposed habitats would be affected. In some habitats, the surface cover of Fucus serratus
may reach 95% in the summer months. Ephemeral green algae especially, increase in abundance over the summer months.
Habitat structure and complexity
The substratum within this biotope is varied and offers a wide variety of potential habitats including bedrock, and the cracks and crevices therein, boulders and cobbles. In addition, the various seaweeds including Fucus serratus
and foliose red seaweeds such as Mastocarpus stellatus
offer a substratum for colonization by epiflora including bryozoans, sponges, ascidians and spirorbid worms. 91 taxa of associated fauna were found on 65 specimens of Fucus serratus
in Strangford Lough, Northern Ireland (Boaden et al.
, 1975). Clumps of seaweed also offer refuge for Carcinus maenas
and the grazers Gibbula cineraria
and Littorina littorea
. The empty shells of the molluscs also provide some heterogeneity to the substratum.
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). 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.
Raffaelli & Hawkins (1999) reported an estimate of the productivity of intertidal fucoids as 160 gC/m²/year, although this figure was an estimate for moderately wave exposed habitats. The Fucus serratus
canopy and other macroalgae associated with this biotope can exude dissolved organic carbon, which is taken-up readily by bacteria and may even be taken-up directly by some larger invertebrates. Dissolved organic carbon, algal fragments and microbial film organisms are continually removed by the sea, which may enter the food chain of local subtidal ecosystems, or be exported further offshore. Many of the species associated with this biotope make a contribution to the food of many marine species through the production of planktonic larvae and propagules, which contribute to pelagic food chains. The productivity in this biotope is likely to be greater than SLR.FserX.T (Fucus serratus
with sponges, ascidians and red seaweeds on tide-swept lower eulittoral mixed substrata) that is similar is terms of exposure, water flow and species composition but with a mixed substrata as opposed to bedrock.
For the majority of important characterizing species and other important species within this biotope, reproduction and recruitment is an annual process. For some of the species, various stages in the reproductive process, including gametogenesis, the timing of spawning and/or recruitment, are variable depending on, for example, environmental factors and geographic location. Recruitment in the major groups present is summarized below.
- Reproduction in Fucus serratus commences in late spring and continues until November, with a peak in August and October. Eggs and sperm are produced separately and fertilized externally to form a planktonic zygote. Recruitment is therefore possible from sources outside the biotope.
- Chondrus crispus has an extended reproductive period (e.g. Pybus, 1977; Fernandez & Menendez, 1991; Scrosati et al, 1994) and produces large numbers of spores (Fernandez & Menendez, 1991). The sexual life cycle of Mastocarpus stellatus involves the upright gametophyte plants developing carpospores that settle to produce a tetrasporophyte crust phase. An apomictic cycle has also been noted whereby upright fronds produce carpospores (without fertilization) which give rise to further apomictic plants (Dudgeon et al., 1999). This species (studied as Gigartina stellata) had a peak in mature carposporangia in winter in Galway Bay, Ireland (Pybus, 1977). The spores of red algae are non-motile (Norton, 1992) and therefore entirely reliant on the hydrographic regime for dispersal. Hence, it is expected that both Chondrus crispus and Mastocarpus stellatus would normally only recruit from local populations and that recovery of remote populations would be much more protracted.
- There is some debate as to the nature of reproduction in the breadcrumb sponge Halichondria panicea but it is likely that it has a short, annual season of reproduction (see MarLIN review).
- Ascidiella scabra has a high fecundity and settles readily, probably for an extended period from spring to autumn. Eggs and larvae are free-living for only a few hours and so recolonization would have to be from existing individuals no more than a few km away. It is also likely that Ascidiella scabra larvae are attracted by existing populations and settle near to adults (Svane et al., 1987).
- Hayward & Ryland (1995a) and Dons (1927) stated that growth in Pomatoceros triqueter is rapid and that sexual maturity is reached in approximately 4 months. Hayward & Ryland (1995a) and Segrove (1941) suggested that breeding probably takes place throughout the year although a breeding peak in spring and summer has been noted and records from Port Erin by Moore (1937) indicated that breeding only took place in April in this location. Castric-Fey (1983) stated that only very rare settlement was observed during winter and maximum settlement occurred in April, June, August and Sept-Oct. 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, 1995a). The settlement of the tubeworm Spirorbis spirorbis (studied as Spirorbis borealis) on Fucus serratus was reported to occur over the summer months in the north east of England (Daly, 1978, cited in Seed et al., 1981).
- Patella vulgata become sexually mature as males aged about nine months. Reproduction is an annual process with peaks within a defined spawning season (October - January) depending on location. Planktonic trophic larvae are produced although the larvae are only planktonic for a few days.
- Dispersal of the hydroid Dynamena pumila is restricted to the planula stage which usually settles and starts to metamorphose within 60 hours of release (Orlov, 1996). Orlov (1996) that long-distance dispersal was further restricted by the dense bushes of neighbouring algae which serve to trap the larvae in the area. Seed et al. (1981) reported that the reproductive zooids of Dynamena pumila were in abundance between May and August in Strangford Lough, Northern Ireland.
- The larvae of Alcyonidium gelatinosum have only a brief planktonic life and brooding of the embryos has been reported from several localities during spring or autumn (Fish & Fish, 1996).
Time for community to reach maturity
No information was found concerning the development of this biotope. However, the important characterizing species all reach sexual maturity within a few years and have annual reproductive episodes suggesting that the time taken for the community to develop is likely to be less than five years. However, if adverse environmental conditions prevail, time taken to reach maturity could take significantly longer.
This review can be cited as follows:
Fucus serratus, sponges and ascidians on tide-swept lower eulittoral rock.
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=42&code=1997>