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 marineterrestrial gradient of sand beaches.

Wind-Driven Dynamics of Beach-Cast Wrack in a Tide-Free System

“…However, the major macro-invertebrate beach inhabitant known to bury wrack through digging in the sand, beach fleas (here: Orchestia gammarellus), were rare at our study site (ca 800 m −2 : T. Horstkotte and M. Zimmer, unpubl. ), as compared to densities of >11,500 ind•m −2 (Megalorchestia californiana) or >16,500 ind•m −2 (Traskorchestia traskiana) on Canadian Pacific beaches [12] or even >32,000 ind. m −2 (Orchestia platensis) on northern New England (Atlantic) coasts [17].…”

“…The decomposition of wrack deposits adds to their spatio-temporal dynamics ashore, in turn, being dependent on wrack characteristics, such as chemical composition and fragment size, on micro-climatic conditions and on biotic interactions (c.f. [5] [12]). Thus it is the combination of place of deposition-horizontally along the marine-terrestrial gradient; vertically in the substratum (c.f.…”

Sea wrack delivery and accumulation on islands: factors that mediate marine nutrient permeability

“…Globally, the main avenue thought to facilitate the transfer of seaweed-derived nutrients to the terrestrial environment is the direct consumption of wrack by a large community of semi-terrestrial and terrestrial invertebrates (Lastra et al 2008). Amphipods are considered one of the most abundant and ecologically important residents of beach ecosystems (sand, gravel, or cobble substrates), but are not found on rocky shorelines (Colombini & Chelazzi 2003, Pelletier et al 2011. On islands composed only of rocky shorelines, amphipods would not be able to facilitate the transfer of seaweed-derived nutrients and permeability would be restricted at these sites.…”

“…On islands with sand, gravel, or cobble substrates (capable of hosting amphipod populations), marine nutrient permeability to the islands' interior would be feasible. This vector of nutrient transfer would likely be intensified during the summer months, which is the active season for amphipods (Pelletier et al 2011). Amphipods preferentially feed on aged wrack over freshly deposited seaweed material, although there is no clear amphipod preference for a particular seaweed species (Pennings et al 2000, Mews et al 2006.…”

Flotsam and jetsam: a global review of the role of inputs of marine organic matter in sandy beach ecosystems

“…Mobility is also critical to avoid being washed out to sea with the wrack during high tides, but some species are also reported to have very low mobility (Schooler et al 2017). Not surprisingly, stranded macroalgal deposits are often colonized by mobile macroinvertebrates (particularly amphipods and dipterans) within a few hours of deposition, followed by predatory staphylinid beetles (Pelletier et al 2011, Yanenik, 1980). The subsequent rates of larval development can be enhanced by elevated temperatures within the masses of stranded wrack.…”