Acquisition of particle processing capability in the oyster Crassostrea gigas: ontogeny of the mantle pseudofeces rejection tracts

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Effective particle processing by oysters, in their typically turbid habitats, requires an efficient mechanism of pseudofeces rejection, accomplished by the characteristic oyster mantle rejection tracts. The ontogenetic development of these tracts was studied by means of scanning electron microscopy (SEM) for late larval, postlarval, and juvenile Crassostrea gigas (Thunberg 1793), using hatchery-reared specimens. Pediveliger larvae had a marginal mantle rejection tract, which becomes the inner fold of the 3 mantle folds in postlarval and adult oysters. An ephemeral, secondary marginal ciliary tract was observed in 1.30 mm early postlarvae; this was absent in later postlarval stages. Specimens 2.70 mm in size had a 'tractless' mantle surface, constituting a potentially critical stage with respect to pseudofeces rejection and hence particle processing. The organization of the much more complex adult system, consisting of multiple radial tracts and a marginal collecting tract comprised of 2 contiguous sub-tracts, was not complete until specimens were 10 to 24 mm. The complete re-organization of the mantle rejection system suggests that this is a phylogenetically new character in the Ostreidae. Furthermore, it points to an additional potentially critical event in the acquisition of particle processing capability, especially when considered in the light of previous data on gill development in this species, notably for individuals recruiting in natural habitats.