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 trinitatis on an exposed sandy beach was examined in relation to moisture content and temperature profiles of the sand. Sand in which animals burrowed always had moisture levels >1.7% of the wet sediment weight throughout the year. Monthly mean moisture values for talitrids ranged from 3.0-5.0%. During winter, talitrids were found high up on the beach far from the reach of the surf and burrowed deeply to avoid freezing. After the thaw in early spring, animals reduced their burrow depth and moved downshore. Large animals began migration earlier than smaller animals. From late spring to summer. the population concentrated near the strandline left by wave action during the preceding high tide, burrowing shallowly. During this period the distribution patterns varied with animal size and reproductive condition. Small animals, especially juveniles, were located shoreward of and burrowed at shallower depths than large animals and ovigerous females, which were located higher on the beach. In late summer, burrowing depth of talitrids increased as temperatures in the surface layers rose. Behavioral responses, such as burrowing and surface migration to cope with the wide fluctuation of environmental factors, mainly sand moisture content and sand temperature, are suggested to be important mechanisms enabling T. trinitatis to survive in the physically severe sandy beach environment.