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Rocky shores and nearshore rocky reefs form extensive
habitats along several coastlines, being an important
marine biotope with considerable socio-ecological significance
(Raffaelli and Hawkins 1996). They arise as a result
of marine erosion of the over-burden and bedrock due to a
combination of sea level rise and wave action in areas
where there is low sediment supply (Ruwa 1996). These
natural habitats may be extended by a plethora of manmade
structures such as jetties, breakwaters, docks,
groynes, dykes and seawalls, which essentially function as
artificial rocky reefs (Crowe and others, 2000, Moschella
and others, 2005). The nature and properties of rocky
shores and reefs are primarily related to the mode of their
formation (Yorath and Nasmith 2001), with most being
comprised of limestone, basalt and granite. Their physical
properties such hardness and porosity vary, with limestone
and basaltic rocks being more porous than granite. This, in
turn, determines the patterns of settlement and establishment
of marine epibenthic organisms on them (UNEP/
Nairobi Convention Secretariat 2009). In many tropical
regions where such reefs are mostly formed from raised
fossilised coral, the rock may be pitted, cracked and/or
creviced. This creates heterogeneous environments with
numerous rock pools, overhangs, gullies and caves. However,
in the arctic and cold temperate regions, such reefs
are often formed from glacial action and, hence, are mostly
dominated by boulders with deep interstitial spaces. In
spite of some uniformity within similar latitudinal settings,
rocky reefs are highly variable and, depending on local
geology, they can range from steep overhanging cliffs to
wide, gently-shelving platforms, and from smooth, uniform
slopes to highly dissected, irregular masses or even extensive
boulder beaches (Steffani 2000). Rocky reefs are,
therefore, rarely smooth slabs of rock, but are instead complex
environments which abound with cracks, crevices,
gullies and pools, creating special habitats with their own
sets of advantages and limitations (Raffaelli and Hawkins
1996). |
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