Ascophyllum nodosum, sponges and ascidians on tide-swept mid eulittoral rock
Ecological and functional relationships
The moderately strong to very strong tidal streams associated with this biotope support a rich and varied marine life, of which the suspension feeders and autotrophs are the dominant trophic groups. Ascophyllum nodosum, the knotted wrack, forms a canopy in this biotope. The serrated wrack Fucus serratus and bladder wrack Fucus vesiculosus may also form part of this canopy but the long lived Ascophyllum nodosum tends to dominate in terms of abundance and standing biomass. The canopy layer limits light penetration and the understory is dominated by shade tolerant foliose and red seaweeds including Chondrus crispus, Corallina officinalis, Mastocarpus stellatus and encrusting calcareous algae. The filamentous red seaweed Polysiphonia lanosa is possibly the most commonly occurring red algal species in this biotope. It has root like fibres which penetrate the tissue of Ascophyllum nodosum and, less frequently, Fucus sp. (Fish & Fish, 1996). Green algae, especially Ulva sp. and Cladophora rupestris may also be found.
Suspension feeders representing several phyla are commonly associated with this biotope. Representatives of the sponge and ascidian communities, that give the biotope its name, are varied and diverse. Both encrusting sponges, including the breadcrumb sponge Halichondria panicea
and Hymeniacidon perleve
, and solitary forms such as the purse sponge Grantia compressa
are found. Similarly, both colonial and solitary ascidians are found although the baked bean ascidian Dendrodoa grossularia
, for example, is far more abundant and frequently associated with this biotope than the colonial star ascidian Botrylloides leachi
. Large stands of hydroids may be found on the fucoids including Dynamena pumila
and the pink hydroid Clava multicornis
. In general, Ascophyllum nodosum
is remarkably free of epiphytes even when adjacent plants of other species of fucoid algae are heavily infested (Filion-Myclebust & Norton, 1981). This is due to the fact that Ascophyllum nodosum
repeatedly sloughs its entire outer epidermis, and potential epiphytes, including spores and germlings of other algae that had settled on the surface are, therefore, discarded with it (Filion-Myclebust & Norton, 1981).
A variety of winkles, most commonly the common periwinkle Littorina littorea and flat periwinkle Littorina obtusata graze on microorganisms, detritus and algae in this biotope. Whilst the common periwinkle grazes on rock in the biotope, Ascophyllum nodosum is the preferred food for the flat periwinkle (Fish & Fish, 1996). The flat periwinkle lays its eggs on Ascophyllum nodosum, Fucus vesiculosus and Fucus serratus and, occasionally, on the rock surface (Fish & Fish, 1996). The common limpet Patella vulgata can be abundant and grazes on tough plants including Fucus sp. and encrusting red algae. Grazing by Patella vulgata can be an important structuring feature on rocky shores and 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 & Hartnoll, 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 the common mussel Mytilus edulis
, annelids and other crustacea. It is an omnivore and will also consume algal material. The predatory mollusc Nucella lapillus
also feeds primarily on the common mussel, in addition to acorn barnacles (Fish & Fish, 1996) such as Semibalanus balanoides
which can also be abundant in the biotope.
Seasonal and longer term change
can reach an age of 25 years and the community associated with this biotope is usually very stable (Connor et al.
, 2004). There are unlikely to be large visible changes in the biotope throughout the year, especially since the biotope occurs in very sheltered / extremely sheltered habitats where winter storms are unlikely to have as significant an effect on the algal standing biomass as wave exposed shores. However, an increase in ephemeral algae such as Cladophora rupestris
may be observed over the summer months.
Habitat structure and complexity
Owing to the tidally swept habitat with which this biotope is associated, a diverse marine life is supported. The fast currents provide a continual supply of food for both active and passive suspension feeders that dominate the attached fauna. Fine sediment is removed by the current and the settlement of material, that could otherwise be detrimental to the suspension feeders, is prevented. Almost every possible substratum, including the bedrock, boulders, cobbles and overhanging faces, is covered with various flora and fauna. In addition to the luxuriant conditions for suspension feeders, Hiscock (1983) lists some the benefits of strong water movement to include the potential for a greater photosynthetic efficiency, thereby possibly increasing the depth penetration of the algae. Increased water movement has been associated with an increase in photosynthesis in several algal species including Fucus serratus
and Ascophyllum nodosum
(Robins, 1968, cited in Hiscock, 1983).
The algae themselves provide a substratum for epiphytic species including hydroid, sponge and ascidian communities. Leucosolenia sp., for example, are often found on red seaweeds. The hydroid Dynamena pumila may grow on several Fucus species whereas the spirorbid worm Spirorbis spirorbis grows preferentially on Fucus serratus. The fronds of Ascophyllum nodosum, however, are narrow, flexible and slimy, offering a poor support for most encrusting animals. The species is unattractive to most intertidal species with the exception of the pink hydroid Clava multicornis, the bryozoan Bowerbankia imbricata and, on sheltered shores, the tube worm Spirorbis spirorbis. Ascophyllum nodosum also supports the red algae Polysiphonia lanosa, which penetrates its fronds with root-like fibres.
Rock surfaces may, depending on their geology, be broken and include shaded overhangs and damp crevices which, together with the shelter of the algal canopy, allow many animal species to thrive in the damp conditions.
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. Production rates of Ascophyllum nodosum
in Nova Scotia were estimated to be between 0.61 and 2.82 kg/m (Cousens, 1984). Raffaelli & Hawkins (1999) reported an estimate of the productivity of intertidal fucoids as 160 gC/m/year, although this figure was an estimate for moderately wave exposed habitats. The fucoids 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. This 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.
- Ascophyllum nodosum is dioecious and, like all other fucoids, has only a sexual generation. Receptacles are initiated in April, are present on the plant for 12-14 months and ripen in April to June of the following year. Gametes are released from April onwards. In the laboratory, the release of gametes can be triggered by exposing ripe receptacles to air overnight. Fertilization takes place externally and zygotes settle and form a rhizoid within ten days. Recruitment in Ascophyllum nodosum is very poor with few germlings found on the shore. The reason for this poor recruitment is unclear, because the species invests the same high level of energy in reproduction as other fucoids and is extremely fertile every year (Printz, 1959). However, the reproductive period lasts about two months, much shorter than for other fucoids. Printz (1959) suggests that it must be assumed that some special combination of climatic or environmental conditions is needed for effective colonization by Ascophyllum nodosum. The slow growth rate of germlings, which increases the chance of their being covered by diatoms or grazed by gastropods, may also help to explain the scarcity of germlings (Baardseth, 1970).
- 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. Fucus vesiculosus is highly fecund often bearing more than 1000 receptacles on each plant, which may produce in excess of one million eggs. In England, the species has a protracted reproduction period of about six months. Gametes may be produced from mid winter until late summer with a peak of fertility in May and June. Like Fucus serratus, the eggs are fertilized externally to produce a zygote. Zygotes start to develop whenever they settle, even if the substratum is entirely unsuitable. Mortality is extremely high in the early stages of germination up to a time when plants are 3 cm in length and this is due mostly to mollusc predation (Knight & Parke, 1950).
- 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 spores of red algae are non-motile (Norton, 1992) and therefore entirely reliant on the hydrographic regime for dispersal. Hence, it is expected that Chondrus crispus would normally only recruit from local populations and that recovery of remote populations would be much more protracted.
- The breadcrumb sponge Halichondria panicea is likely to have a short, annual season of sexual reproduction (see MarLIN review).
- 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.
- 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).
- The flat periwinkle Littorina obtusata are capable of reproducing through out the year (Graham, 1988). Eggs are laid in a jelly mass, usually on the fronds of Fucus species and hatch three or four weeks later (Goodwin, 1978, cited in Graham, 1988). Recruitment from external sources would therefore rely on the movement of adults into the area.
Time for community to reach maturity
is a long lived, slow growing algae with poor recruitment rates that limit recovery (Holt et al.
, 1997). The lack of recovery of Ascophyllum nodosum
from harvesting is well documented. For example, in their work on fucoid recolonization of cleared areas at Port Erin, Knight and Parke (1950) observed that even eight years after the original clearance there was still no sign of the establishment of an Ascophyllum nodosum
population. In terms of community maturation however, recoverability is likely to take significantly longer. Jenkins et al.
(2004) studied the long term effects of Ascophyllum nodosum
canopy removal on the whole understory community structure on a sheltered rocky shore on the Isle of Man. They reported that, even after twelve years, major effects of the canopy removal were still apparent. At the culmination of the study, the emerging canopy was a mixture of Ascophyllum
and Fucus serratus
with occasional patches of Fucus vesiculosus
. Despite some recovery, mean cover of Ascophyllum nodosum
was still only about 50% of its original level compared with control plots. However, the most alarming changes were evident in the understory community. Previously, this community was characterized by a balance between patches of red algal turf and patches grazed by Patella vulgata
. Removal of the canopy layer broke down the balance between these two 'functional units' to the extent that the community showed no signs of reverting to its pre-disturbance state. In a cascade of events, the red algal turf deteriorated from the lack of canopy protection which in turn increased the available area for limpet grazing, thereby increasing the limpet population.
The time for this biotope to reach maturity is therefore likely to depend on the circumstance under which the community changed in the first place. For example, it would depend on what species had been lost or reduced in abundance. Starting from bare substratum, the time taken for this biotope to reach maturity is likely to be at least fifteen years and under certain circumstances may take significantly longer.
This review can be cited as follows:
Ascophyllum nodosum, sponges and ascidians on tide-swept mid 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=100&code=1997>