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Variations in skeletal structure od recent bryozoan Pentapora fascialis from different environments in the Adriatic Sea

Pentapora fascialis (Pallas, 1766) is the largest and most conspicuous calcified bryozoan in the Adriatic Sea. Three different samples of bryozoan P. fascialis were collected at three different localities in the Adriatic: in the Velebit Channel, on Jabuka Shoal and at Korčula Island. Along the coast of the Velebit Channel in the North Adriatic, large colonies of P. fascialis were particularly abundant. There, P. fascialis colonies grew only in the vicinity of submarine freshwater springs on sandy-detritic bottom, between 1 m and 35 m depth. Those colonies are always wide-branched and are the biggest colonies ever found in the Adriatic, up to 200 cm in diameter. On Jabuka Shoal, in the Central Adriatic, only small and narrow-branched colonies (10 to 20 cm in diameter) were observed, within the depth range from 20 m to 50 m. But large colonies of P. fascialis were also observed in the South on Korcula Island where the location was narrow passage between small islands Badija and Lucnjak characterized by constant and very strong currents. There, large and numerous colonies of P. fascialis (50 to 80 cm in diameter) grew in both forms, as wide- and narrow-branched colonies. Morphologic observations of the zooid crossections were carried out with scanning electron microscope (SEM). The same SEM microscope equipped with an energy dispersive spectrometer (EDS) was used for elemental distribution analysis in the samples. On the crossection of the zooid wall according to the substituted cation it is possible to distinguish inner Mg-calcite and outer aragonite structure of the zooid skeleton wall. Calcite shows substitution of calcium for magnesium, while aragonite contains traces of strontium. Variations in the aragonite to calcite ratios between sample from Jabuka Shoal and two other samples emerge from different thickness of the aragonitic part of the zooid wall. Volume of the zooid and calcite part of the zooid wall is constant for all samples and only variation is in the volume of the aragonite. Samples from the Jabuka Shoal built of igneous rocks shows increased content of trace elements. The differences could originate from geological environment, because Jabuka Shoal is built of igneous rocks which are not very common in Adriatic Sea.

Bryozoa; skeletal structure; Adriatic Sea

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