Subtidal vermetids reefs were recorded in the nuclear power plants discharge area, fouling tens of boulders (1 to 6 m in diameter) in the sea bottom between one and six meters depth. This vermetid reefs are formed by clusters of living individuals cemented around a peripheric area of the rocks from the bottom to near the top, which is normally inhabited by unidentified algae (Fig. 1a, b). In some rocks, because of their high development, vermetids also inhabited the top.
Such features resemble the “microatolls” described by Safriel (1974, 1975) for Bermuda and Israel. However, there are differences concerning the species composition, origin, depth, water temperature and others. The vermetid reefs recorded in the present paper are monospecific, formed by clusters of P. varians incrusted on subtidal rocks, in an area with unusual high temperature. According to Safriel (1974), the Shikmona reefs from Israel are situated on the intertidal and made of limestone encrusted by vermetids. The limestone relief is leveled off and surrounded by an elevated rim, covered by a crust of Dendropoma petraeum, while the basin is covered by a crust of Vermetus triquetrus. In the Bermuda “microatolls”, Dendropoma irregulare provides the bulk of their crust, growing all over the reefs surface. Moreover, the reefs are made of biogenic material, at least to the depth of 5 m into the reefs (Safriel, 1974). Considering the sea-surface temperatures, there is similarity between Israel and Bermuda, since both are located on the same latitude. In the coldest month the mean temperature in Bermuda, being 19 °C, is higher than in Israel, while summer temperatures in both places are the same (Safriel, 1974).
The highest density of the subtidal P. varians were around 100,000 ind.m−2 and similar values were found on the intertidal in the same area (Breves-Ramos, 2012). On the other hand, values up to 300,000 ind.m−2 were found on intertidal rocky shores in the same bay around 8 km of the impacted area (Breves-Ramos, 2012). The effluent of the nuclear power plant seems to impact their density, since it may cause the decrease in benthic cover (Teixeira et al., 2009) and lack of macro-organisms (Mayer-Pinto et al., 2012).
Mainly high chlorine input and high temperatures seem to be responsible for the drastic low diversity in this area creating a desert landscape in the sea bottom. This condition, added to lack of competitors, facilitate the intertidal dominant species P. varians to attach free spaces on subtidal rocks in this area. Furthermore, in general vermetids are known for their high tolerance to adverse environmental conditions (Schiaparelli & Cattaneo-Vietti 1999; Strathmann & Strathmann 2006), with a great ability to attach to different artificial substrates (Schiaparelli et al., 2003). Besides vermetids few organisms are found; the sea urchin Lytechinus variegatus the sponge Hymeniacidon heliophila and the ascidian Phallusia nigra. The geniculate calcareous algae Jania sp., the brown algae Sargassum spp. and the soft coral Palythoa caribaeorum can also be found (Teixeira et al., 2012).
Vermetids feed on particles, such as microscopic plants and animals and detritus. For predominantly mucous feeders, the mucous trap hangs undisturbed while gathering food material (Al Barash & Zenziper 1985). Hydrodynamics might be affecting these populations, since according to Schiaparelli & Cattaneo-Vietti (1999), vermetids are warm-water sessile gastropods capable of building upright tubes (feeding tubes) to take advantage of the water flow, as exploratory tubes. In a power plant channel at Planta Centro in Puerto Cabello, Venezuela, dense mass aggregation of P. varians lives in the intertidal, with a permanent flow of water (Weinberger et al. 2010).
These subtidal P. varians reefs were recorded spreading over about 1,000 m2 into the Piraquara de Fora cove (Fig. 2), restricted to the nuclear power plant discharge area, under continuous influence of strong currents and high temperature. There is no other subtidal P. varians reefs in anywhere else in Ilha Grande Bay.
