|Basic Information||Biotope classification||Ecology||Habitat preferences and distribution||Species composition||Sensitivity||Importance|
Image Rohan Holt - Halidrys on flat pebbles and gravel. Image width ca 1 m.
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IR.MIR.SedK.HalXK recorded () and expected () distribution in Britain and Ireland (see below)
Macroalgae provide primary productivity either directly to grazing fish and invertebrates or indirectly, to detritivores and decomposers, in the form of detritus and drift algae or as dissolved organic material and other exudates.
Macroalgal species compete for light, space and, to a lesser extent, nutrients, depending on the growth rates, size and reproductive pattern of each species. For example, large macroalgae such as Halidrys siliquosa and laminarians shade the substratum (depending on density) so that understorey plants tend to be shade tolerant red algae. Understorey algae, by effectively restricting access to the substratum, may also inhibit or restrict recruitment of other species of macroalgae (Hawkins & Harkin, 1985; Hawkins et al., 1992).
Macroalgae compete for space with sessile invertebrates such as sponges, hydroids, ascidians and bryozoans.
Halidrys siliquosa and, when present, laminarians provide substratum for epiphytes, depending on location, including microflora (e.g. bacteria, blue green algae, diatoms and juvenile larger algae), Ulothrix and Ceramium sp., hydroids (e.g. Aglaophenia pluma, Laomeda flexuosa, and Obelia spp.), bryozoans (e.g. Scrupocellaria spp.), and ascidians (e.g. Apilidium spp., Botryllus schlosseri, and Botrylloides leachi) (Moss, 1982; Lewis, 1964, Connor et al., 1997).
Sessile epiphytes, including microflora, may reduce light available for photosynthesis and hence reduce growth and reproduction of the macroalgae, or increase drag and reduce the plants flexibility resulting in increased susceptibility to storm or wave damage (Williams & Seed, 1992).
Amphipods, isopods and other mesoherbivores graze the epiphytic flora and senescent macroalgal tissue, which may benefit the macroalgal host, and may facilitate dispersal of the propagules of some macroalgal species (Brawley, 1992; Williams & Seed, 1992). Mesoherbivores also graze the macroalgae but do not normally adversely affect the canopy (Brawley, 1992).
Gastropods graze epiphytes and macroalgae directly, e.g. Gibbula cineraria, Lacuna vincta and the limpet Tectura spp. Epiphyte grazing by Tectura (as Acmaea) sp. was reported to be important to the survival of an encrusting coralline algae (Hawkins et al., 1992; Williams & Seed, 1992; Birkett et al., 1998b). Where present , laminarians are probably grazed by the blue-rayed limpet Helcion pellucidum.
Sea urchins are important general grazers (grazing drift algae, macroalgae, microalgae, and sessile fauna) in subtidal algal habitats. For example, Echinus esculentus has been shown to control the depth reached by Laminaria hyperborea biotopes in Port Erin (Kain, 1979) (see £EIR.LhypR£) and to significantly affect the biomass of understorey macroalgae (Schiel & Foster, 1986; Hawkins et al., 1992; Vadas & Elner, 1992: Birkett et al., 1998b).
The impact of sea urchin grazing depends on density and hence depth (Hawkins et al., 1992). Although Echinus esculentus and Psammechinus miliaris occur at low density in this biotope (JNCC, 1999), as evidenced by the extent of algal cover, urchin grazing probably increases the diversity of the biotope by clearing small areas for colonization by other species.
Mobile predators include crabs (e.g. Cancer pagurus and Necora puber) feeding on small crustaceans and gastropods, starfish such as Asterias rubens, and fish such as the corkwing wrasse Crenilabrus melops, the butterfish Pholis gunnellus and the dragnet Callionymus lyra feeding on small crustaceans, polychaetes and other invertebrates.
Starfish (Asterias rubens and Henricia oculata), crabs and hermit crabs probably act as scavengers within the biotope.
Epiphytic and benthic suspension feeders include bryozoans, sponges and hydroids together with tube worms (e.g. Pomatoceros triqueter) on boulders or Lanice conchilega or Chaetopterus variopedatus in intervening sediment, the barnacle Balanus crenatus, the long clawed porcelain crab Pisidia longicornis and the starfish Henricia oculata.
The presence of sessile invertebrates (e.g. sponges) or coralline algae, sand or sediment cover and grazing gastropods may inhibit settlement or attachment of propagules and the survival of the germlings. Fucalean algae showed greater recruitment to areas cleared of low lying algae, and coralline algae have been shown to inhibit the settlement of a number of sessile kelp forest species (Schiel & Foster, 1986). Vadas et al. (1992) noted that post-settlement mortality of algal propagules and early germlings was high, primarily due to grazing, canopy and turf effects, water movement and desiccation (in the intertidal) and concluded that algal recruitment was highly variable and sporadic. For example, Sousa et al. (1981) reported that experimental removal of sea urchins significantly increased recruitment in long-lived brown algae. In experimental plots cleared of algae and sea urchins in December, Halidrys dioica colonized the plots, in small numbers, within 3-4 months. Plots cleared in August received few , if any recruits, suggesting that recolonization was dependant on zygote availability and therefore the season. Halidrys dioica did not colonize plots grazed by urchins in their experiments (Sousa et al., 1981).
When bare substratum becomes available for colonization, for instance following storm events, it is expected that algal recruitment and succession would follow a predictable sequence (Hawkins & Harkin, 1985). Initial colonizers on bare rock are often epiphytic species, suggesting that it is competition from canopy forming algae that usually restricts them to their epiphytic habit (Hawkins & Harkin, 1985). Gradually, the original canopy or turf forming species, in this case Furcellaria lumbricalis and Chondrus crispus, then become established. These findings suggest that interactions between macrophytes are often more important than grazing in structuring algal communities (Hawkins & Harkin, 1985).Halidrys siliquosa can float if detached, suggesting another potential route for dispersal. However, although some long range dispersal must occur in macroalgae (resulting in colonization of oil rigs and similar structures), van den Hoek (1987) and Norton (1992) suggested that it is probably ineffective for most species of macroalgae. Wernberg et al. (2001) suggested that the lack of long range dispersal success in Halidrys siliquosa was responsible for its regional distribution in the north east Atlantic. Epiphytic and sessile fauna, such as sponges, hydroids, bryozoans and ascidians, have pelagic but short lived larvae with relatively short effective dispersal ranges, depending on the local hydrography. However, most epiphytic species are widespread and ubiquitous and would probably recruit rapidly from adjacent or nearby populations.
Detailed studies in Norway by Rinde et al. (1992 cited in Birkett et al. 1998b) examined recovery of non-kelp species. The epiphyte community in control areas about 10 years old was richer and more extensive than on replacement plants in harvested areas. Of the epifauna, Halichondria sp. were only found on 10 year old plants and tunicates on plants 6 years post harvesting.Overall, therefore, it is likely that the understorey and large fucoids such as Halidrys siliquosa and laminarians where present may recolonize and recover their biomass within at least 5 years. However, although epiphytic species may recruit rapidly, it may take longer (up to 10 years) for them to recover their original biomass and the biotope to return to its prior species richness.
This review can be cited as follows:
Tyler-Walters, H. 2002. Halidrys siliquosa and mixed kelps on tide-swept infralittoral rock with coarse sediment.. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 25/05/2013]. Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=258&code=1997>