Fucus distichus and Fucus spiralis f. nana on extremely exposed upper shore rock
Image Sue Scott - View down shore showing upper shore bedrock with Fucus distichus and green algae. Image width ca XX cm.
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Ecological and functional relationships
In general exposed conditions favour the growth of barnacles, limpets and mussels rather than fucoid algae. However, the ELR.Fdis biotope includes seaweeds that are able to tolerate the extreme conditions of wave exposed rocky shores, primarily the physical stresses caused by wave action. The strong holdfast and short tufted structure of Fucus distichus and Fucus spiralis f. nana allow these fucoids to survive on extremely exposed shores in the north and north-west. Other seaweeds able to tolerate the wave-wash are the red encrusting algae Hildenbrandia rubra and seasonally occurring Porphyra spp.
In Britain and Ireland, Fucus distichus has only been recorded attached to bedrock in the mid to upper eulittoral zone on exposed rocky shores in northern Scotland and Ireland. It is thought to be prevented from growing further south due to its poor tolerance of desiccation and inability to compete with plants growing further down the shore. However, on the east coast of North America, Fucus distichus is only found in rock pools and is incapable of growing on emergent rock surfaces in the mid to upper eulittoral. The isolated and dispersed occurrence of Fucus distichus together with a greater abundance on more northerly shores of the North Atlantic suggest that it may be a relic form surviving only in habitats which are unsuitable for the main fucoids found at these latitudes (Lewis, 1964). A critical factor in the distribution of Fucus distichus is probably day length. Short day lengths stimulate the onset of receptacle formation (Bird & McLachlan, 1976).
Grazing on rocky shores can exert significant controlling influences on the algal vegetation, particularly by patellid limpets and littorinid snails which are usually the most prominent grazers. There may also be effects caused by 'mesograzers' - amphipods such as Hyale prevostii and isopods, which are much smaller but can occur in high densities.
The surf-swept conditions under which both Fucus distichus and Fucus spiralis f. nana occur are not always conducive to the formation of well-defined zones. Scattered plants or thick ankle-deep carpets can often lie somewhat randomly placed (Lewis, 1964).
The presence of a fucoid canopy inhibits the settlement of barnacles 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. The number of limpets increases with maturing fucoid clumps.
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. However, the communities on wave exposed shores tend to be less variable than on moderately exposed shores and are therefore more stable. The wave exposed conditions in this biotope seems to favour the development of a relatively stable covering of wave tolerant fucoids plus a patchy covering of barnacles and limpets. However, seasonal changes are apparent on rocky shores with seasonal variation in growth and recruitment. For example, Fucus distichus plants lose fronds in the autumn after reproducing and are then removed from the rock by wave action during their third winter.
Habitat structure and complexity
The ELR.Fdis biotope provides a variety of habitats and refugia for other species. Macroalgae increases the structural complexity of the habitat providing a variety of resources that are not available on bare rock. Algal fronds provide space for attachment of encrusting or sessile epifauna and epiphytic algae and give shelter from wave action, desiccation and heat for invertebrates. Empty barnacle shells can shelter small littorinids such as Littorina neglecta
and Littorina saxatilis
. If present mussels can increase habitat complexity and species diversity because the gaps between interconnected mussels form numerous interstices for a variety of organisms. The barnacles may be covered by Porphyra
sp. on the upper shore although few other species can attach to them.
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 taken up 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 such as in the gaps often created by storms. 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). Both the demographic structure of populations and the composition of assemblages may be profoundly affected by variation in recruitment rates.
- Receptacles of Fucus distichus are initiated in December, they become ripe in April and gametes are released from April to August. The species produces gametes of both sexes within each conceptacle. When released, ova can survive and disperse for several days. Antherozoids can only live for several hours. Self-fertilization is thought to be high in the species and once a zygote is formed it can only be dispersed over limited distances (Rice et al., 1985).
- Fucus spiralis is also hermaphroditic. Receptacles are initiated during late January to February, gametes discharged during July and August, and the receptacles shed by November, although exact timing of reproduction depends on location and the form of the plant.
- Among sessile organisms, patterns fixed at settlement, though potentially altered by post settlement mortality, obviously cannot be influenced by dispersal of juveniles or adults. Some of the species that may be found living in the biotope, such as amphipods, do not have pelagic larvae, but instead have direct development of larvae producing their offspring as 'miniature adults'.
Time for community to reach maturity
The time for the biotope to reach maturity should be relatively rapid because recruitment of key species is good. For example, Fucus distichus
and Fucus spiralis
have been observed to readily recruit to cleared areas (Ang, 1991) and have fast growth rates, so recovery rates are expected to be high. Fucus distichus
has a lifespan of about 3 years. Colonization by other species found in the biotope, such as Littorina neglecta
and Melarhaphe neritoides
, is also likely to be quite rapid. Therefore, it seems likely that the biotope should reach maturity within a few years.
This review can be cited as follows:
Fucus distichus and Fucus spiralis f. nana on extremely exposed upper shore 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=234&code=1997>