Slowest of the slow: latitudinal insensitivity of burrowing capacity in the bivalve Laternula
Low temperature limits the rate of biochemical reactions and aerobic scopes of cold water ectotherms. To compensate for this limiting effect, animals living in cold environments often possess physiological or morphological adaptations to maintain vital functions. Cross-latitudinal comparison of aerobic capacities is one method to test which factors constrain activity in thermally distinct environments particularly when congeneric studies are carried out on related species with conservative ecology and habitat. Burrowing is a major aerobic activity of bivalve molluscs that is described here for the first time for the tropical mangrove species Laternula truncata and Laternula boschasina and then compared with their Antarctic congener Laternula elliptica. About 80% of L. truncata (16.3-46.1 mm shell length) and 63% of L. boschasina (11.3-27.7 mm shell length) buried within 24 h at 28 degrees C. The burrowing rate index (BRI = [(3)root wet weight/time to bury]x10(4)) ranged between 1.1 and 20.2 for L. boschasina and 1.1-32.9 for L. truncata. These values are 2-3 orders of magnitude less than other tropical bivalve molluscs and are amongst the lowest recorded for any bivalve. Comparisons with the Antarctic L. elliptica showed little or no differences in BRI (Q(10) of 1.0-1.2 for specimens of the same size). This is contrary to the general pattern over a wide range of bivalves, where BRI increases with a Q(10) of between 2.9 and 6.4 between high latitudes and the equator. L. elliptica has 25-30% longer relative foot length than tropical congeners of the same size, which could be a morphological adaptation compensating for reduced burrowing speeds in a colder environment. Burrowing rates within the genus Laternula could, however, also be maintained by differing habitat, ecological and physiological constraints on burrowing capability.