SEASONAL HORIZONTAL AND VERTICAL-DISTRIBUTION PATTERNS OF THE SUPRALITTORAL AMPHIPOD TRINORCHESTIA-TRINITATIS IN RELATION TO ENVIRONMENTAL VARIABLES

The seasonal distribution of the talitrid amphipod Trinorchestia trini tatis on an exposed sandy beach was examined in relation to moisture c ontent and temperature profiles of the sand. Sand in which animals bur rowed always had moisture levels >1.7% of the wet sediment weight thro ughout the year. Monthly mean moisture values for talitrids ranged fro m 3.0-5.0%. During winter, talitrids were found high up on the beach f ar from the reach of the surf and burrowed deeply to avoid freezing. A fter the thaw in early spring, animals reduced their burrow depth and moved downshore. Large animals began migration earlier than smaller an imals. From late spring to summer. the population concentrated near th e strandline left by wave action during the preceding high tide, burro wing shallowly. During this period the distribution patterns varied wi th animal size and reproductive condition. Small animals, especially j uveniles, were located shoreward of and burrowed at shallower depths t han large animals and ovigerous females, which were located higher on the beach. In late summer, burrowing depth of talitrids increased as t emperatures in the surface layers rose. Behavioral responses, such as burrowing and surface migration to cope with the wide fluctuation of e nvironmental factors, mainly sand moisture content and sand temperatur e, are suggested to be important mechanisms enabling T. trinitatis to survive in the physically severe sandy beach environment.