Corallith beds at the edge of the tropical south atlantic

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Reef sites
Corallith beds at the edge of the tropical South Atlantic
Free-living scleractinian colonies of spheroid
shape, known as circumrotatory colonies or
coralliths, have been reported forthe Atlantic,
Indian, and Pacific Oceans (e.g., Kissling 1973;
Glynn 1974; Roff 2008). Herein, we report the first
corallith site discovered in the subtropical South
Atlantic, a unique formation offree-living colonies
of Madracis decactis (Lyman, 1859) (Scleractinia:
´
Pocilloporidae) at Gale Island off southern Brazil
(Fig. 1). The coralliths spread over 3400 m2 of flat
sand and ruble at6–15 m depth, adjacent to a rocky
´
shore on the westward side of Gale Island
(27°10¢846S; 48°24¢548W). Most colonies are
subspheroid with tissue on their entire surface
(Fig. 1d) and reach up to 15cm in width
(Fig. 1a–c). Within the site, coralliths are patchily
distributed and reach densities of up to 80
colonies m–2 among boulders that may limit their
rotatory movement, resulting inaggregations
designated ‘‘corallith beds’’ herein (Fig. 1a). It was
recently observed (February 2011) that coralliths
are displaced by individuals of the sand dollar
Clypeaster subdepressus (Gray,1825). Thus,
bioturbation may be an important source of
rotatory movement and growth of M. decactis
coralliths, as previously reported for other species
(e.g., Glynn 1974). In addition to corallithsof
M. decactis, the site also contains to a lesser extent
rhodoliths and bryoliths (Schizoporella sp.). At 27°
latitude, this community represents the southernmost reef coral site discovered thusfar in the
Atlantic Ocean and indicates that communities
dominated by free-living corals may be more
common than previously thought.

´
Fig. 1 Madracis decactis coralliths off southern Brazil(Gale Island). a Corallith bed at 10-m depth
(photograph under natural sunlight). b Detail of corallith bed. c Dry corallith. d Detail of living polyps.
Scale bars a 15 cm, b 5 cm, c 1 cm, d 0.5 mm...
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