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Chemistry and Ecology 123

Downloaded by [Weizmann Institute of Science] at 03:22 26 June 2013 Figure 1. (a) A simplified model of a vermetid reef along a hypothetical transect from shore to open sea. (b) A vermetid
reef from a field study along the Sicilian coast. The numbers refer to the different parts of the reef described in section 2
of the text.

dolomite and flysch [9]. The presence of an abrasion platform is the essential condition for a
true reef. However, Schiaparelli et al. [11] observed that D. petraeum preferentially colonises
granite, rather than limestone, along the coasts of Sardinia. This may be explained by competition
processes acting during the settlement phase, as vermetid larvae may prefer substrata where the
algal cover is reduced.

A second environmental feature influencing the distribution and size of these structures on a
small scale is the hydrodynamism of shallow water layers, because vermetid platforms are rare
along sheltered coasts. Accordingly, large vermetid reefs are present in Sicily along the entire
coast facing north/northwest, whereas only smaller reefs are found along northeast coasts [12].

Finally, the slope of the shore also plays a role in determining the size and shape of biocon-
structions: larger reefs develop where the slope is between 15◦ and 40◦ on the horizon line. By
contrast, vermetid formations growing on flat shores (0–15◦) show reduced thickness and width,
and appear as ledges or thin crusts. Where slopes exceed 40◦ (cliffs), formations rapidly decrease
in size, being completely absent from 50◦ onwards [12].

3. Distribution of vermetid reefs in the Mediterranean Sea

Mediterranean vermetid reefs are found where the temperature of surface coastal waters is no
lower than 14◦C (in winter). The upper latitudinal limit for these formations is around 38◦N,