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Description
The barnacle Megabalanus sp. is indigenous to coastal Baja California, but migrated north to San Diego during to an El Nino event in the early 1980s (Newman and McConnaughey 1987). This barnacle genus dates back to the Miocene (Doyle et al. 2007), and therefore potentially is useful for paleoenvironmental reconstruction. Megabalanus’ shell structure can potentially indicate exposure and submergence intervals by the production of growth patterns in its calcium-carbonate shell. This barnacle was used to compare tidal cycles with its exposure history in the littoral zone off the coast of San Diego. After four spring tide events, outplant samples were collected from the number 17 piling of the Scripps pier, along with a temperature data logger, and thin sectioned using an Isomet Precision saw. A calcein dye was incorporated to bond with the calcium-carbonate shell before the outplant to mark the initial growth of the experiment. A fluorescence scope showed multiple fluorescence lines (excitation/emission configuration of 495/515nm), not a singular line, indicating discrepancies in dye bonding or shell flourescence. Transmitted light microscopy through the anterior carinal shell section of the specimens revealed darker U-shaped growth lines taken to represent subaerial exposure intervals. For the outplant interval, the relative distribution of growth lines within the shells show a general correspondence with a predicted relative distribution of exposure-related growth lines, though absolute growth rates vary among specimens. These preliminary results, though far from conclusive, call for additional analyses assisted by a greater ability to mark exact outplant dates within the shell. Such studies may provide a framework for geochemical microsampling and paleoenvironmental reconstruction.
