|Basic Information||Biotope classification||Ecology||Habitat preferences and distribution||Species composition||Sensitivity||Importance|
Image Keith Hiscock - Underboulder community dominated by sponges from a rock pool habitat. Wembury, South Devon. Image width ca 1 m.
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LR.MLR.BF.Fser.Bo recorded () and expected () distribution in Britain and Ireland (see below)
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).
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.
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 29/05/2015]. Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=371&code=2004>