Lunulitiform Bryozoa from 136 bottom samples collected on the shelf from the Amazon river mouth to British Guyana were studied. Seven species were identified of which 2 (Cupuladria surinamensis and Cupuladria lagaaiji) are described as new. Lunulitiform Bryozoa were absent from the inner shelf zone (0 to 20 m depth). Rapid sedimentation of soft fine muds, almost no suitable grains for the settlement of the larvae, and high sediment load of the water appear to prohibit distribution in this area. Lunulitiform Bryozoa are abundant alive and dead in the middle shelf zone (20 to 60 m) off Guyana where they constitute 10 to 30% of the skeletal remains in the coarse fraction (>2 mm) of the samples. Sedimentation here is low to absent, sediment load of the water is much less and the clayey to sandy calcareous sediment is stiffer and coarser than in the inner shelf zone. Bottom currents are moderate, and phytoplankton primary production is highest in this zone. The combination of these factors together with the tropical temperature of the bottom water (23° to 28°C) makes this habitat favourable for lunulitiform Bryozoa. They surpass other zoarial forms of Bryozoa in this zone. In the outer part of the shelf (60 to 90 m) lunulitiform Bryozoa were found alive at much lower densities, while they were absent from the shelf edge (90 to 100 m). Sediment here may be too coarse and food may be limited. Other zoarial forms are relatively better represented, they will be competitors for food. Sedimentation is absent in this zone, as a consequence (sub)fossil specimens of lunulitiform Bryozoa were frequently encountered, dating from times when the sealevel was lower. Cupuladria pyriformis (Busk) and C. lagaaiji sp. nov. were uncommon, only dead specimens being found in the outer shelf zone. The other 5 species were more common, and were all found alive, sometimes the 5 species together in one sample. As dead specimens they were frequently found together. Living specimens of Mamillopora cupula (Smitt) were evenly distributed over the middle and outer shelf zone. Cupuladria surinamensis sp. nov. was confined alive to the middle shelf, Cupuladria monotrema (Busk) prefers deeper water, Cupuladria biporosa Canu & Bassier and Discoporelia umbellata depressa (Conrad) occupy an intermediate position.
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