Biodiversity & Conservation

Laminaria digitata and piddocks on sublittoral fringe soft rock



Image David George - View across shore showing extensive kelp beds on chalk. Image width ca 4 m.
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Distribution map

IR.MIR.KR.Ldig.Pid recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

  • EC_Habitats
  • UK_BAP

Ecological and functional relationships

Kelp habitats are dynamic ecosystems where competition for space, light and food result in patchy distribution patterns of flora and fauna. Kelp biotopes are diverse species rich habitats and over 1,200 species have been recorded in UK moderately exposed kelp biotopes (MIR.KR) (Birkett et al., 1998b). Kelps are major primary producers; up to 90% of kelp production enters the detrital food web and is probably a major contributor of organic carbon to surrounding communities (Birkett et al., 1998b). Major interactions are thought to be the effects of competition for space, shading, herbivory and predation. In most kelp biotopes there is evidence of strong competition for space, especially for space on a favourable substratum. Competition may between individual plants of the same species, between kelps and substratum-colonizing species of animals and other algae and between colonial animals and encrusting algae. Competition for space between individuals and species is dynamic, resulting in a constantly changing patchwork of species covering any suitable substrata within the biotope.

The blades of Laminaria digitata plants form a canopy layer which may cut off much of the incident irradiance. This restricts the development of species with high light demands so that the understorey of plants becomes dominated by shade tolerant red algae. It also allows species normally restricted to the lower infralittoral in kelp-free areas to compete more effectively in the reduced light levels of the kelp bed and so are found at shallower depths.

Within kelp beds there are relatively few species that are directly grazing either the kelp or the understorey algae as the enzymes required to directly utilise algae as food are not common. Those species able to graze directly on the kelp include the gastropods: Gibbula spp., Littorina spp., Haliotis tuberculata (in the Channel Islands only), Helcion pellucidum, Lacuna spp. and the Rissoidae, together with some amphipods and isopods. Helcion pellucidum grazes epiphytes and the kelp tissue directly, forming pits similar to the home scars of intertidal limpets. The larger, laevis form excavates large cavities in the holdfast of Laminaria spp. which creates tissue damage weakening the adult plant and possibly contributes to its loss due to wave action and storms (Kain, 1979). Infestation with Helcion pellucidum varies between sites and decreases with depth.

Burrowing species such as the piddocks, including the common piddock Pholas dactylus, and the tube worm Polydora ciliata are characteristic of this biotope and contribute to the creation of a relatively high silt environment through burrowing activities. The abundance of filter feeding organisms such as sponges, bryozoans and tunicates within kelp biotopes indicates the importance of planktonic input to the benthic community. Although very little information is available about planktonic communities in kelp beds it can be assumed that there will be larger inputs of larval stages from species with bentho-pelagic life cycles than in the general plankton (Birkett et al., 1998b).

Predation within kelp beds has not been well studied in the UK. Although some species are known to prey on others, such as starfish on mussels, very little is known of the predator-prey relationships for the many species occurring in kelp beds.

Kelp plants are exploited as a habitat; the holdfast, stipe and frond each support a different type of community consisting of possibly thousands of individuals from hundreds of species; holdfasts shelter a particularly rich diversity of animals from a wide range of taxa (see Habitat complexity).

Seasonal and longer term change

Most species in the biotope are perennial and seasonal changes are likely to be in condition of individuals rather than presence or absence.
  • Growth rate of Laminaria digitata is seasonally controlled with a period of rapid growth from February to July and one of slower growth from August to January. Increased wave exposure and storms in winter months are likely to erode Laminaria digitata blades so that they appear tattered in winter months and overall standing biomass is reduced. Periodic storms are also likely to remove older and weaker plants creating patches cleared of kelp and increasing the local turbidity. Cleared patches may encourage growth of sporelings or gametophyte maturation. Growth of understorey algae is also reduced in the winter months.
  • Some species of algae have seasonally heteromorphic life histories spending a part of the year as a cryptic or encrusting growth form and only becoming recognizable in the foliose phase of their life cycles. The occurrence of such algae is often seen as the occurrence of 'ephemeral' algae. Some hydrozoans may be present in the kelp bed in their attached, colonial form only for a part of the year, spending the rest of the year as medusae.
  • With a life span of less than a year and a reproductive period of 3-4 months in the spring or summer numbers of Polydora ciliata are likely to be fairly seasonal with highest abundance of individuals after recruitment in the summer and autumn.
  • Pholas dactylus live to approximately 14 years of age with a maximum shell length of 75 mm (Pinn et al., 2005), although earlier work has recorded maximum shell lengths of 125-150 mm (Jeffries, 1865; Turner, 1954). Spawning usually occurs between May and September with settlement and recruitment of juvenile piddocks occuring between November and February. It is likely that populations of Pholas dactylus will not be subject to significant seasonal changes in abundance.
It should be emphasized that present understanding of the natural fluctuations in the species assemblages, populations, distribution and diversity of species in kelp habitats is very limited.

Habitat structure and complexity

The structure of the biotope is complex with many different microhabitats. They include bedrock, crevices, sediment pockets, the holdfast, stipe and blade of Laminaria digitata plants themselves, undersides of boulders and empty piddock burrows.
  • Holdfasts provide refuge to a wide variety of animals supporting a diverse fauna that represents a sample of the surrounding mobile fauna and crevice dwelling organisms, e.g. polychaetes, small crabs, gastropods, bivalves, and amphipods.
  • Kelp fronds are grazed by molluscs such as the blue-rayed limpet Helcion pellucidum.
  • Older Laminaria digitata stipes provide a substratum for a large number of epiphytic flora and fauna and it has been estimated that rugose stipes provide one and a half times that surface area provided by the bedrock (Jones et al., 2000).
  • Empty burrows of piddocks, such as the common piddock Pholas dactylus, create additional refugia which are recorded as being colonized by vagile species such as Littorina littorea, Gibbula cineraria, Porcellana platycheles and Eulalia viridis (Pinn et al., in press). Sabellidae and Lithothamnia spp. Are examples of sessile species utilising the burrows.
  • The understorey of red algae and crevices in the bedrock provide space for many cryptic fauna.
  • In areas of mud tubes built by Polydora ciliata can agglomerate and form layers of mud up to an average of 20 cm thick, occasionally to 50 cm. These layers can eliminate the original fauna and flora, or at least can be considered as a threat to the ecological balance achieved by some biotopes (Daro & Polk, 1973).


Kelp plants are the major primary producers in the marine coastal habitat. Within the euphotic zone kelps produce nearly 75% of the net carbon fixed and large kelps often produce annually well in excess of a kilogram of carbon per square metre of shore. However, only about 10% of this productivity is directly grazed. Kelps contribute 2-3 times their standing biomass each year as particulate detritus and dissolved organic matter that provides the energy supply for filter feeders and detritivores, such as piddocks and polychaetes like Polydora ciliata, in and around the kelp bed. 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.

Recruitment processes

Most species in this biotope produce planktonic propagules annually and so recruitment is often from distant sources and is frequent.
  • Benthic species, plant and animal, that possess a planktonic stage: gamete, spore or larvae, are likely to be influenced by kelp mediated alteration of fluid and particulate, and consequently larval fluxes. Kelp canopies also affect the physical environment, such as the substratum, experienced by actively settling planktonic larvae. The substrata beneath kelp plants for example, are often dark and sediment laden, conditions likely to affect larval settlement and post settlement survival. Both the demographic structure of populations and the composition of assemblages may be profoundly affected by variation in recruitment rates driven by such factors.
  • Laminaria digitata plants are fertile all year round with maximum production of spores in July - August and November - December. Young sporophytes (germlings) appear all year with maxima in spring and autumn. Chapman (1981) demonstrated that substantial recruitment of Laminaria digitata plants to areas barren of kelp plants was possible up to 600m away from reproductive plants.
  • Pholas dactylus spawns between May and September with settlement and recruitment of juvenile piddocks occuring between November and February (Knight, 1984; Pinn et al., in press).
  • The spawning period for Polydora ciliata varies, from February until June in northern England for example, and from April - September in the Black Sea. Larvae are substrate specific selecting rocks or sediment according to their physical properties settling preferentially on substrates covered with mud
  • 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 living in kelp beds do not have pelagic larvae, but instead have direct development producing their offspring as 'miniature adults'.

Time for community to reach maturity

Kain (1975) examined the recolonization of cleared concrete blocks by kelp plants and other algae and found that Laminaria digitata plants were re-established within 2 years and that red algae returned with a year. Although there is no information available on colonization times or growth rates for the common piddock the other main rock borer, Polydora ciliata is able to rapidly (within months of reproductive period) colonize a suitable area. Recruitment of other species to the kelp bed may take longer. However, maturity is likely to be reached within five years.

Additional information

This review can be cited as follows:

Hill, J.M. 2008. Laminaria digitata and piddocks on sublittoral fringe soft rock. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 20/04/2014]. Available from: <>