Biodiversity & Conservation

Mixed fucoids, Chorda filum and green seaweeds on reduced salinity infralittoral rock



Image Sue Scott - Fucoids and Chorda filum submerged. Image width ca 40 cm (foreground).
Image copyright information

Distribution map

IR.SIR.Lag.FChoG recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

  • EC_Habitats

Ecological and functional relationships

The biotope is found in very shallow submerged rocky habitats in lagoons, subject to permanently reduced salinity conditions. These particular habitat conditions lead to a variety of seaweed-dominated communities which include fucoids and green filamentous species. The fucoids, more typical of intertidal habitats, penetrate into the subtidal under the reduced salinity conditions which are not tolerated by kelps.

The very sheltered conditions of the biotope enables the development of algal cover on cobbles and pebbles. In these habitats, infaunal organisms such as Arenicola marina may also be present.

The fucoid species, and the other macroalgae in the biotope, increase the amount of space available for attachment, they provide shelter from wave action and can be an important food source. High abundances of algae may contribute to the oxygen budget of lagoons.

Ectocarpus sp. is often found growing on other seaweeds.

Grazing by littorinid snails such as Littorina littorea may be important in keeping fast growing ephemeral species such as Ulva in check.

The three-spined stickleback Gasterosteus aculeatus is widely distributed throughout north-west Europe and Britain in the intertidal region, particularly in estuaries, and may be present in the SIR.FChoG biotope feeding on a wide range of invertebrates.

Seasonal and longer term change

The biotope occurs in extremely sheltered conditions so temporal changes associated with winter storms are not likely. However, seasonal changes in growth and recruitment are often apparent on rocky shores. Fucus serratus plants, for example, lose fronds in the winter, followed by regrowth from existing plants in late spring and summer, so that summer cover can be about 250% of the winter level (Hawkins & Hartnoll, 1980). Similar growth patterns may also occur in lagoonal habitats. Chorda filum is a summer annual, falling into decay in the autumn and disappearing during winter.

Habitat structure and complexity

Fucoid biotopes provide a variety of habitats and refugia for other species. The beds of Fucus serratus and Fucus vesiculosus and the presence of other algae in the biotope increases 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 desiccation, heat and predation for invertebrates.


Macroalgae rocky shore communities are highly productive and can be an important source of food and nutrients. On rocky shores only about 10% of the primary production is directly cropped by herbivores (Raffaelli & Hawkins, 1996) and this is likely to be similar for lagoonal like habitats. Macroalgae, such as Fucus serratus and other fucoids, 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. Dissolved organic carbon, algal fragments and microbial film organisms are continually removed by the sea and 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. However, in lagoonal like habitats such as the SIR.FChoG biotope, where tidal flows and wave exposure are weak these probably enter the food chain of local ecosystems rather than inshore subtidal or offshore ecosystems.

Recruitment processes

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 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.
  • Recruitment of Fucus serratus from minute pelagic sporelings takes place from late spring until October. There is a reproductive peak in the period August - October and plants can be dispersed long distances (up to 10km). However, weak tidal streams probably results in a smaller supply of pelagic sporelings and most recruitment may come from local populations.
  • Fucus vesiculosus gametes may be produced from mid winter until late summer with a peak of fertility in May and June. Eggs and sperm are released into the seawater and fertilised externally. Zygotes settle to the seabed and begin development wherever they fall. The egg becomes attached to the rock within a few hours of settlement and may adhere firmly enough to resist removal by the next returning tide.
  • Visible Chorda filum sporophytes appear on shores between February and mid-March and develop into secondary sporophytes between April and June. The sporophytes are washed away from October to February, leaving behind zoospores or gametophytes.

Time for community to reach maturity

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. Fucus serratus and Fucus vesiculosus recruit readily to cleared areas, especially in the absence of grazers (Hawkins & Hartnoll, 1985). Chorda filum is an annual species reappearing every year so appearance of the species on cleared areas in suitable conditions will be possible within a year. Green algal species such as Ulva sp. and Ectocarpaceae 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. Therefore, the time for the biotope community to reach maturity is likely to be only a few years although development of a stable community structure may take a little longer due to competitive interactions.

Additional information

This review can be cited as follows:

Hill, J.M. 2001. Mixed fucoids, Chorda filum and green seaweeds on reduced salinity infralittoral rock. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 26/11/2015]. Available from: <>