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Lissarca notorcadensis, a small pallial-brooding mollusc with a circum-Antarctic distribution, is frequently found attached by stout byssal threads to the spines of sea urchins. Here, complete ontogenetic series can be found. Using standard, structural definitions, the prodissoconch I of L. notorcadensis is small compared to the large prodissoconch II, a feature atypical of brooded young and possibly reflective of a planktotrophic past. Fine commarginal striae are found on prodissoconch II of this subtidal clam, which also possesses more pronounced commarginal striae on the dissoconch prior to brood release. The provinculum is retained and remains functional well into adulthood. A small central ligament pit is present through the postlarval stages. This develops into a large, triangular pit in adulthood that shows obvious growth lines. Calcareous, presumably aragonitic fibres develop in the ligament of specimens still retained in the adult brood. Five to six adult hinge teeth are formed on either side of the ligament. Alignment of shell valves by these well-developed teeth is augmented by retention of larval denticles. The functional and structural demise of the provinculum occurs between 4 and 7 mm shell height and results from denticle erosion and subumbonal shell overgrowth. Many canals permeate larval and adult valves, but external pores in the adult shell differ from those of the larval shell and could indicate mode of formation. Prodissoconch form, provinculum retention, and shell canals are considered primitive traits.
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Lyonsia hyalina Conrad possesses “radial mantle glands” of a complex structure located deep within the periostracal groove. They occur along the mantle edge in alignment with slightly raised striations of the periostracum, are deeply staining and are composed of three cell types. Secretory and supportive cells, which are flask-shaped, alternate throughout the gland, while a third cell type is ovoid and borders the gland. A sulfated mucopolysaccharide is secreted over the periostracum by the glands and functions in adhesion of sand grains to the shell.
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Spiral and near-spiral growth patterns of the peripheral crossed-lamellar shell microstructure of the bivalve Corbicula cf. fluminea have been discovered. This is the first report of spiral microstructures outside of nacreous and "foliated calcite" deposits. The spiral growth surface in this corbiculid is underlain by a conical crossed-lamellar microstructure. This form of lath deposition is usually associated with high concentrations of organic material located beneath the undertucked periostracum along the valve's periphery. These structures can offer the bivalve additional resistance against predators that enter the valves through the shell's edge. Similar spiral structures have not been found in all "white" forms of North American Corbicula, in the "purple" form of North American Corbicula, nor in the corbiculids Polymesoda caroliniana and P. (Geloina) erosa.
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Aqueous-phased xenobiotic contaminant exposure can biochemically modify newly generated periostracum of the Asian freshwater bivalve,Corbicula fluminea. Laser-induced desorption of partially polymerized periostracum produces spectra distinguishable from mass spectral images generated from uncontaminated periostracum. Organic xenobiotic contamination putatively impedes full polymerization of the periostracin protein. The detection of the effects of pollution on periostracum by the laser microprobe mass analyzer constitutes a novel bioprobe for the definitive but qualified detection of xenobiotic contamination.
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