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The identification of bivalve larvae and early postlarvae in plankton and benthic samples has long been a challenge, hampering both basic and applied research efforts in marine, estuarine, and freshwater environments. The usefulness of published optical micrographs of the early life-history stages of bivalves is limited because of the great morphological similarity of the imaged articulated shells, particularly at the early (straight-hinge) developmental stages. While a number of techniques have been refined in recent years and show promise for use in routine identifications of larval and post-larval bivalves (e.g., single-step nested multiplex polymerase chain reaction; in situ hybridization protocols through color coding with taxon-specific, dye-labeled DNA probes; coupled fluorescence in situ hybridization and cell sorting; and image analysis techniques using species-specific shell birefringence patterns under polarized light), no adequate comprehensive reference source exists that accurately depicts the morphology and morphometry of the shells of larval and post-larval stages of target bivalve species in a consistent format to assist in identification of such stages. To this end, scanning electron micrograph (SEM) sequences are presented of the disarticulated shell valves of laboratory-reared larval and post-larval stages of 56 species of bivalve molluscs from a wide spectrum of marine, estuarine, and freshwater habitats. Emphasis is placed on the usefulness of the morphology and morphometrics of consistently-oriented, disarticulated shell valves and associated hinge structures in discriminating the early life-history stages of these various bivalve species. Although the scanning electron micrograph sequences presented accurately depict the gross morphologies/ morphometrics and hinge structures of the disarticulated shell valves of the larvae and/or postlarvae of the 56 species of bivalves, it is important to emphasize that a scanning electron microscope is not necessary to observe even fine hinge structures associated with the early ontogenetic stages of these species. Such structures are readily visible using a wide range of optical compound microscopes equipped with high-intensity reflected light sources, although the disarticulated shell valves must be viewed in several planes of focus to discern the often subtle details seen clearly in the scanning electron micrographs. These morphological characters provide researchers with invaluable aids for the routine identification of the early life-history stages of these species isolated from plankton and benthic samples.
From the 1980s through 1995, scientists at numerous marine, coastal, estuarine, and freshwater laboratories spawned bivalves to provide larvae for use in identifying species based on larval hinge structures and gross shell morphometry. These larvae were preserved in 95% ethanol and stored in sample vials, many of which dried out over the years. Advantage was taken of 50 of 56 species from this collection (and two additional species that were not in the collection for a total of 52 species) to explore the use of optical techniques (polarized light and a full-wave compensation plate) to highlight birefringence patterns of larval shells to discriminate individual species. Representative images of various developmental stages of 77% (40/52) of the larval bivalve species in the collection were successfully imaged. Similarities across birefringence patterns were observed at the taxonomic ordinal and familial level. Molecular polymerase chain reaction techniques were used in an effort to sequence many of the dried-out specimens and they successfully identified 19% (10/52) of the larval bivalve species with matches in GenBank. Here it has been demonstrated that optical techniques are efficient for imaging dried-out larval bivalve shells for classification purposes and we present successful sequences of 10 species of bivalve larvae from the preserved collection.