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The concept that the formation of CaCO3 coccoliths functions as a photosynthetic adaptation for the use of bicarbonate is evaluated in the coccolithophorids Coccolithus huxleyi and Cricosphaera carterae by two new methods. In the first, carbon fixation is measured at 10‐s intervals in the first 2 min after addition of 14CO2 and H14CO3− to buffered cultures; this method exploits the relatively long half‐time for the hydration or dehydration of dissolved CO2. In the second, shifts in pH and alkalinity resulting from carbon fixation by cells growing in liquid culture are assessed to indicate fluxes of CO2 and HCO3− into cells and these values compared to measurements of 14C incorporation in photosynthesis and carbonate deposition. The data are interpreted in terms of one of several net inorganic reactions of deposition considered. In this reaction, CO2 is the substrate of photosynthesis and HCO3− is the form of carbon supplied to the calcification site. CO2 resulting from carbonate deposition supplements the CO2 from the medium that diffuses into cells as a source of carbon for photosynthesis.

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The exoskeletons of pancrustaceans, as typified by decapod crustaceans and insects, demonstrate a high degree of similarity with respect to histology, ultrastructure, function, and composition. The cuticular envelope in insects and the outer epicuticle in crustaceans both serve as the primary barrier to permeability of the exoskeleton, preventing loss of water and ions to the external medium. Prior to and following ecdysis, there is a sequence of expression and synthesis of different proteins by the cuticular epithelium for incorporation into the pre-exuvial and post-exuvial procuticle of insects and the exocuticle and endocuticle of crustaceans. Both exhibit regional differences in cuticular composition, e.g., the articular (intersegmental) membranes of insects and the arthrodial (joint) membranes of crustaceans. The primary difference between these cuticles is the ability to mineralize. Crustaceans' cuticles express a unique suite of proteins that provide for the nucleation and deposition of calcium carbonate. Orthologs of genes discussed in the present review were mined from a recently completed cuticular transcriptome of the crayfish, Cherax quadricarinatus, providing new insights into the nature of these proteins.

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In order to investigate the control of the onset of postecdysial mineralization of the cuticle of the blue crab, Callinectes sapidus Rathbun, EDTA-soluble proteins and glycoproteins were analyzed at short intervals after ecdysis by SDS-PAGE. Gels were either stained with silver, blotted to polyvinylidene difluoride membranes and stained with periodic acidlschiff reagent (PAS), or blotted to nitrocellulose membranes and probed with a variety of lectins [particularly the mannose-binding lectins concanavalin A (Con A) and GaZanthus niualis agglutinin (GNAjI.Pieces of cuticle were also tested for their ability to nucleate calcite in vitro. A major change occurred in the biochemistry of the cuticle between 1 and 3 hr after ecdysis. This change, which we term the post-ecdysial cuticle alteration (PECA), involved 1) loss of a major 66 kDa glycoprotein band, 2) decrease in Con A binding by a 79 kDa protein band, 3) complete loss of PASreactivity of both 66 and 79 kDa proteins, 4) gain in Con A affinity of an existing band a t 73 kDa, and 5) an increase in the intrinsic ability of cell-free cuticle pieces to mineralize, as measured by the in vitro calcite crystal nucleation assay o 1995 WiIey-Liss, Inc.

Individual gill filaments of the freshwater crayfish Procambarus clarkii were determined to be either predominantly respiratory or transporting. Silver staining revealed that the filaments within the central bed of the gills formed silver deposits whereas filaments at the margins and the entire sixth pleurobranch formed no deposits. Designation of the silver staining gills as predominantly transporting and unstained filaments as predominantly respiratory was substantiated by ultrastructural analyses and measurements of ATPase and transepithelial potentials. Presumptive transporting filaments had an epithelium subjacent to the cuticle that was relatively thick and dominated by abundant mitochondria. Lacunae were delineated by pillar structures and served as collateral pathways for the movement of blood from the afferent to efferent blood channels, which were separated by a thin septum. Presumptive respiratory filaments had an extremely thin epithelium with few organelles, but a relatively thick septum. Present in both types of filaments were nerves and podocytes. The values for Na, K-ATPase were significantly higher in the transporting filaments than in those designated as respiratory. The measurement of transepithelial potentials showed both filaments to be cation selective with the respiratory filaments slightly more positive and the transporting filaments slightly more negative than the diffusion potential for Na.