Colonisation of Beach-Cast Macrophyte Wrack Patches by Talitrid Amphipods: A Primer

Abstract

Sandy beaches are characterised by heterogeneously distributed food sources both in time and space. The major energy supply is derived from marine subsidies in the form of beach-cast macrophyte wrack. Wrack patches are short-lived, and their position on the beaches varies with tidal and seasonal cycles as well as weather conditions. Little is known about how sandy beach inhabitants orient themselves towards, and colonise, wrack patches. In a series of field studies on islands off Vancouver Island (British Columbia, Canada), colonisation patterns of wrack patches by beach fleas and sand hoppers (Amphipoda: Talitridae), the most abundant macrofaunal detritivores, were studied. As indicated by colonisation of experimental patches of wrack and wrack surrogates that were either visible or buried in the sand, beach fleas (Traskorchestia traskiana) rely on olfactory cues for locating freshly deposited wrack patches in their patchy and dynamic habitat. Dense colonisation of freshly deposited algal wrack generally occurred within less than 1 h but depended upon the tidal height of wrack patches. Beach flea density in freshly deposited wrack patches increased with increasing tidal height. By contrast, sand hoppers (Megalorchestia californiana) colonised freshly deposited wrack patches in densities that decreased with tidal height. Discussing these interspecific differences, we provide a primer for future detailed studies on transport of matter along the marine–terrestrial gradient of sand beaches.