Biodiversity & Conservation

Halidrys siliquosa and mixed kelps on tide-swept infralittoral rock with coarse sediment.



Image Rohan Holt - Halidrys on flat pebbles and gravel. Image width ca 1 m.
Image copyright information

Distribution map

IR.MIR.SedK.HalXK recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

  • EC_Habitats

Ecological and functional relationships

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.

Seasonal and longer term change

Little is known about temporal change in subtidal algal populations (Schiel & Foster, 1986). Most of the dominant algae within the biotope are perennial, present all year round, e.g. Halidrys siliquosa, Delesseria sanguinea, Chondrus crispus, Furcellaria lumbricalis, and Dilsea carnosa. However, they show seasonal variation in reproduction, with Halidrys siliquosa, Furcellaria lumbricalis, Chondrus crispus and Delesseria sanguinea releasing spores in the winter months, potentially enabling them to colonize free space opened up by increased wave action in winter storms and the dying back of annual species (see Kain, 1975). Annual species, e.g. Chorda filum are likely to proliferate in spring, reaching maximum abundance in summer (high insolation and temperature). Winter storms have been reported to damage Furcellaria lumbricalis plants (Austin, 1960b) and presumably could potentially damage or remove other members of the community, potentially opening space for colonization.

Habitat structure and complexity

  • Halidrys siliquosa, together with laminarians present, form an upper canopy shading the understorey algae and substratum.
  • Halidrys siliquosa, and to a lesser extent Saccharina latissima when present support a diverse assemblage of epiphytes (see above). If present, Laminaria hyperborea may also support a diverse array of epiphytes on its stipe (see species review).
  • The understorey of smaller macroalgae is dominated by a variety of sand-scour tolerant red algae, which probably varies with location. However, Phyllophora sp., Chondrus crispus, Polyides rotundus, Delesseria sanguinea, Dilsea carnosa and Furcellaria lumbricalis typically occur. The understorey includes brown seaweeds, e.g. Dictyota dichotoma, Chorda filum and Desmarestia aculeata.
  • The surface of the substratum may support sessile invertebrates that are effective space occupiers, e.g. sponges, and barnacles (e.g. Balanus crenatus) and some anemones e.g. the dahlia anemone Urticina felina.
  • The surface of boulders or cobbles support a sparse fauna of encrusting sponges (e.g. Esperiopsis fucorum), tubeworms (e.g. Pomatoceros triqueter) barnacles, crabs and ascidians (Botryllus schlosseri, Clavelina lepadiformis, and Ascidiella spp.). The underboulder surface may support encrusting sponges, the porcelain crabs and brittlestars.
  • The substratum typically includes mobile, coarse sediment (e.g. pebbles, gravel and sand), which may support burrowing polychaetes such as Lanice conchilega or Chaetopterus variopedatus.
  • The interstices between understorey macroalgae may act as shelter or refuge for larvae and juveniles of the organisms found in the community (Birkett et al., 1998). Laboratory evidence (Johns & Mann, 1987) suggested that Irish moss (Chondrus crispus) and habitat complexity attract juvenile lobster, presumably as a refuge from predation. However, Vadas & Elner (1992) suggested that field evidence for large invertebrates or fish using macroalgal habitats as refuges or nurseries was conjectural.


Studies of subtidal seaweed communities in Nova Scotia suggested that seaweed annual production exceeded the consumption rates of herbivores about 10-fold. It was suggested that most of the productivity was exported in the form of suspended particulate matter (Miller et al., 1971; cited in Vadas & Elner, 1992). A large proportion of the primary productivity of seaweeds in subtidal algal stands is, therefore, probably exported in the form of drift algae (onshore or onto the strand line), particulates, exudates of dissolved organic matter, and contributes to the productivity of surrounding communities. However, no information concerning productivity within this biotope was found.

Recruitment processes

The propagules of most macroalgae tend to settle near the parent plant (Schiel & Foster, 1986; Norton, 1992; Holt et al., 1997). For example, the propagules of fucales are large and sink readily and red algal spores and gametes and immotile. Norton (1992) noted that algal spore dispersal is probably determined by currents and turbulent deposition (zygotes or spores being thrown against the substratum). For example, spores of Ulva sp. have been reported to travel 35km, Phycodrys rubens 5km and Sargassum muticum up to 1km, although most Sargassum muticum spores settle within 2m. The reach of the furthest propagule and useful dispersal range are not the same thing and recruitment usually occurs on a local scale, typically within 10m of the parent plant (Norton, 1992).

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.

Time for community to reach maturity

Kain (1975) noted that on a single block cleared every two months, most biomass belonged to Rhodophyceae in winter, Phaeophyceae in spring and Chlorophyceae in late summer. On blocks cleared and monitored for five years, the red algae colonized quickly and the community (including Laminaria hyperborea) had reached a condition similar to the pre-clearance community within 2 years and nine months (Kain, 1975). Furcellaria lumbricalis species grows very slowly compared to other red algae (Bird et al., 1979) and takes a long time to reach maturity. For example, Austin (1960b) reported that in Wales, Furcellaria lumbricalis typically takes 5 years to attain fertility. This would mean that, following perturbation, recovery to a mature reproductive community would take at least 5 years. Similarly, Halidrys siliquosa does not reproduce until the end of its second year, and the population would therefore, take at least 2 years to begin recovery if removed. However, it grows rapidly, a maximum summer growth rate of 2cm/month being reported by Moss & Lacey (1963), so that damaged but surviving individuals would probably regain prior condition is within a year, depending on season. Recovery of Chondrus crispus was monitored after a rocky shore was totally denuded by ice scour in Nova Scotia, Canada, its original biomass returning within 5 years. (Minchinton et al., 1997). Several fucoids have been shown to recolonize cleared areas readily, especially in the absence of grazers (Holt et al., 1995, 1997). For example, Fucus dominated areas may take 1-3 years to recolonize in British waters (Holt et al., 1995).

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.

Additional information

None entered.

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 17/04/2014]. Available from: <>