LACK OF METABOLIC TEMPERATURE COMPENSATION IN THE INTERTIDAL GASTROPODS, LITTORINA-SAXATILIS (OLIVI) AND LITTORINA-OBTUSATA (L)

Two intertidal snails, Littorina saxatilis (Olivi, 1792) (upper eulitt oral fringe/maritime zone) and Littorina obtusata (Linnaeus, 1758) (lo wer eulittoral) were collected from a boulder shore on Nobska Point, C ape Cod, Massachusetts, in July and acclimated for 15-20 days at 4 deg rees or 21 degrees C. Oxygen consumption rate (VO2) was determined for 11-15 subsamples of individuals at 4 degrees, 11 degrees and 21 degre es C with silver/platinum oxygen electrodes. Multiple factor analysis of variance (MFANOVA) of log(10) transformed values of whole animal VO 2 with log(10) dry tissue weight (DTW) as a covariant revealed that in creased test temperature induced a significant increase in VO2 in both species (P<0.00001). In contrast, MFANOVA revealed that temperature a cclimation did not affect VO2 in either L. saxatilis (P = 0.35) or L. obtusata (P = 0.095). Thus, neither species displayed a capacity for t he typical metabolic temperature compensation marked by an increase in VO2 at any one test temperature in individuals acclimated to a lower temperature that is characteristic of most ectothermic animals. Lack o f capacity for metabolic temperature acclimation has also been reporte d in other littorinid snail species, and may be characteristic of the group as a whole. Lack of capacity for respiratory temperature acclima tion in these two species and other littorinids may reflect the extens ive semi-diurnal temperature variation that they are exposed to in the ir eulittoral and eulittoral fringe/maritime zone habitats. In these h abitats, any metabolic benefits derived from longer-term temperature c ompensation of metabolic rates are negated by extreme daily temperatur e fluctuations. Instead, littorinid species appear to have evolved mec hanisms for immediate metabolic regulation which, in L. saxatilis and L. obtusata and other littorinids, appear to centre on a unique abilit y for near instantaneous suppression of metabolic rate and entrance in to short-term metabolic diapause at temperatures above 20-35 degrees C , making typical seasonal respiratory compensation mechanisms characte ristic of most ectotherms of little adaptive value to littorinid speci es.