Biodiversity & Conservation

Underboulder communities



Image Keith Hiscock - Underboulder community dominated by sponges from a rock pool habitat. Wembury, South Devon. Image width ca 1 m.
Image copyright information

  • #
  • #
  • #
  • #
Distribution map

LR.MLR.BF.Fser.Fser.Bo recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

  • EC_Habitats
  • UK_BAP

Ecological and functional relationships

The fauna are dominated by a variety of active and passive suspension feeders such as encrusting sponges (e.g. Halichondria panicea), solitary ascidians (e.g. Dendrodoa grossularia), barnacles (e.g. Balanus crenatus), spirorbid worms (e.g. Spirorbis spirorbis), hydroids (e.g. Dynamena pumila), bryozoans (e.g. the sea mat Electra pilosa and the encrusting bryozoan Umbonula littoralis) and colonial ascidians (e.g. Botryllus schlosseri). There is likely to be competition for space between many of the encrusting species. Gordon (1972) noted that competition between bryozoans and ascidians always favoured ascidians. He also noted that Halichondria sponges, even after they had died, prevented the spreading and survival of bryozoans trying to overgrow them.

Herbivores include the common periwinkle Littorina littorea, the grey top shell Gibbula cineraria, the green sea urchin Psammechinus milaris and, less frequently, the common limpet Patella vulgata. Herbivorous grazers most likely use the underboulder habitat for shelter but emerge from under the boulders to feed.

There are few species that prey on other members of the community but, for instance, dog whelks Nucella lapillus may feed on barnacles and the European cowrie Trivia monacha feeds on the star ascidian Botryllus schlosseri. The common shore crab Carcinas maenas is probably the largest mobile predator associated with MLR.Fser.Fser.Bo. It will move between the boulders and pebbles feeding primarily on small molluscs, especially Littorina spp., annelids and other crustacea. It will also consume algal material. Non-mobile carnivores include the beadlet anemone Actinia equina which feeds passively.

Some resident mobile species are detritivores such as the hairy porcelain crab Porcellana platycheles and some brittle stars.

Several species that occur under boulders gain shelter from insolation and predators when the tide is out but are not an integral part of the community; for instance, blennies, crabs and shrimps e.g. the common prawn Palaemon serratus.

Species diversity and disturbance
Boulder communities are subject to frequent wave-induced disturbance, especially during the winter months as storm and wave energy increases. Due to the varying size of boulders likely to be found in MLR.Fser.Fser.Bo, some boulders will be moved around and turned-over more frequently than others.

Larger boulders remain undisturbed for longer periods of time and, consequently, the community on them is likely to be dominated by a few late successional species. In MLR.Fser.Fser.Bo, large stable boulders may be dominated by a few prolific species such as Dendrodoa and Halichondria (Foster-Smith, pers. comm.). In contrast, small boulders are tossed around regularly and are unlikely to reach a 'climax' community as disturbance is too frequent. The frequency of disturbance determines the interval of time over which recolonization can occur (Sousa, 1985) and small boulders sample the available pool of spores and larvae more often (Sousa, 1979a) and they are likely to be characterized by hardy species capable of rapidly colonizing bare space e.g. barnacles, spirorbid worms and bryozoans. Sousa (1979a) noted that, in an algal dominated boulder field in California, boulders subjected to intermediate disturbance frequencies were usually less dominated than those which are frequently disturbed, and always less dominated than boulders which were seldom disturbed. Furthermore, intermediate boulders remained undisturbed for long enough that several species had become dominant but not so long that species had been competitively displaced, resulting in dominance. In other words, intermediate size boulders are likely to be more diverse in terms of species diversity. For this reason, the species composition under boulders within the MLR.Fser.Fser.Bo classification can vary considerably which can be problematic when assessing sensitivity (see Species Composition).

Seasonal and longer term change

Some species of bryozoans and hydroids demonstrate seasonal cycles of growth in spring/summer and regression (die back) in late autumn/winter, over wintering as dormant stages or juvenile stages (see Ryland, 1976; Gili & Hughes, 1995; Hayward & Ryland, 1998). Many of the bryozoans and hydroid species are opportunists adapted to rapid growth and reproduction (r-selected), taking advantage of the spring/summer phytoplankton bloom and more favourable (less stormy) conditions (Dyrynda & Ryland, 1982; Gili & Hughes, 1995). Henry (2002) reported a drastic decline in Dynamena pumila over the winter months in the Bay of Fundy. Foster-Smith (1989) recorded that many encrusting ascidians increased in abundance by late summer under boulders on the Northumbrian coast.

On the boulder shores with which MLR.Fser.Fser.Bo is associated, the increased storm and wave energy over the winter months are likely to significantly influence both the flora and faunal components of MLR.Fser.Fser.Bo. Many boulders and cobbles will be thrown around creating bare patches in encrusting species, ripping seaweed off the boulders and overturning boulders to the detriment of species previously on top of the boulders which may suffer from anoxia and crushing etc. These species are likely to perish if left under the boulder which will mean that the proportion of 'bare' rock will increase. Over the winter months therefore they may be an increase in opportunistic species such as Pomatoceros triqueter. However, the winter months also giver the late successional species a chance to colonize the rocks as other e.g. algae such as Ulva die back. Sousa (1979b) found that Ulva sp. inhibited the colonization of mid-successional species such as Fucus serratus and that these mid-successional species, in turn, inhibited the recruitment of late-successional species such as Gigartina canaliculata. Therefore, these winter months are important for the development and diversity of the biotope as a whole.

Habitat structure and complexity

  • The epilithic community usually occurs as a single layer although competition between encrusting species may result in overgrowth and smothering.
  • Habitat complexity increases where soft rocks are bored by bivalve molluscs creating holes for other species to nestle.
  • Variation occurs especially in relation to the degree of influence of underlying sediments. Physical complexity is increased where boulders lie on top of other loosely-packed boulder creating interstices whilst siltation under the boulders means that deeper silty layers may support detritus-feeding polychaetes (Foster-Smith, pers. comm.). Faunal diversity on the boulder surface will be decreased where the boulders are embedded or partly in sediment. In contrast, there may be flowing water under some boulders (for instance, overflows from pools or lagoonal habitats draining at low water) which creates rich communities.


Insufficient information

Recruitment processes

The majority of important and characteristic species associated with MLR.Fser.Fser.Bo have planktonic larvae which recruit frequently. Recruitment in the important species is summarized below.
  • The breadcrumb sponge Halichondria panicea is likely to have a short, annual season of sexual reproduction. Most sponges are hermaphroditic but cross-fertilization normally occurs. The process may be oviparous, where there is a mass spawning of gametes through the osculum which enter a neighbouring individual in the inhalant current. Fertilized eggs are discharged into the sea where they develop into a planula larva. However, in the majority, development is viviparous, whereby the larva develops within the sponge and is then released. Larvae have a short planktonic life of a few hours to a few weeks, so that dispersal is probably limited.
  • In Botryllus schlosseri, up to eight eggs are produced per zooid. After fertilization and development to a tadpole stage, the tadpole is released and is free swimming for up to 36 hours (Berril, 1950; Berril, 1975). This short planktonic stage therefore limits recruitment to nearby colonies.
  • Ingle (1997) indicated that the eggs of Pisidia longicornis were present from March to August in southern England and from February to September in the Mediterranean. The planktonic larvae and highly mobile nature of this crab mean that this species does not necessarily rely on recruitment from ,local sources. Underboulder areas may be important refuges for young crabs, especially Cancer pagurus.
  • The dispersal phase of Umbonula littoralis is probably brief and larvae probably do not travel far therefore recruitment is dependant on local sources. Embryos were recorded as present in the Plymouth area in June and August (Marine Biological Association, 1957), from October and November on the north-east coast of England (Hastings, 1944) and from September to February in Manx waters (Eggleston, 1969).
  • Balanus crenatus releases planktotrophic nauplii larvae between February and September, with peaks in April and late summer when phytoplankton levels are highest. They pass through six nauplii stages before eventually developing into a cyprid larva which is are specialized for settlement. Peak settlement occurs in April and declines until October. The larvae may not settle for a month after release and therefore, dispersal potential is relatively high as is recruitment from distant sources. Metamorphosis usually takes place within 24 hours of settlement.
  • Although asexual reproduction occurs in many ascidians, reproduction in the baked bean ascidian Dendrodoa grossularia is entirely sexual (Millar, 1954). Millar studied reproduction in Dendrodoa grossularia in two locations (the River Crouch in Essex and the Isle of Cumbrae in the Firth of Clyde) and found that reproduction was bi-polar in nature with one peak in spring and another in late autumn, the spring episode being more intense. The average number of eggs produced per individual (over 7 mm in length) was only ca 25-100 eggs in the Clyde and River Crouch respectively. Furthermore, the eggs are brooded internally until the larval stage is reached thereby compressing the free swimming stage.

Time for community to reach maturity

Settlement panels, which attract similar communities to underboulder habitats, may be fully colonized within about 18 months of being placed into the environment (extrapolated from Sutherland & Karlson, 1977; Todd, 1994). Development of 'mature' communities under boulders is likely to occur within two years and there will be dynamic stability, i.e. composition of the community will remain much the same although individual organisms and colonies will die and be replaced by the same species.

Additional information

No text entered

This review can be cited as follows:

Hiscock, K. 2005. Underboulder communities. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 28/11/2015]. Available from: <>