Distribution and behavior of scavenging amphipods from the central North Pacific

Abstract

Deep-sea scavenging amphipod distributions, abundance, and behavior were studied with five-vehicle baited traps. The species composition and numerical abundances were comparable in the areas sampled. All size classes of the four predominant species ( Paralicella caperesca, P. tenuipes, Orchomene gerulicorbis, and Eurythenes gryllus) were trapped including sexually mature individuals, but no females were ovigerous. The central North Pacific E. gryllus is morphologically distinct from Atlantic E. gryllus and may represent a different species. Pelagic and demersal amphipod guilds exist, as determined by body size and vertical and horizontal distributional patterns. The demersal guild is composed of P. caperesca, P. tenuipes, and O. gerulicorbis, all of which are <2 cm in total body length and occur within 1 m of the sediment. E. gryllus, the only representative of the pelagic guild, ranges between 1.7 and 14 cm in total body length and has its greatest abundance several to tens of meters above the sediment. Members of the demersal guild probably can detect and exploit both large and small food falls or autochthonous organic particles because of their proximity to the sediment. They reside within the area of the benthic boundary layer where currents are slowest, and the diffusion of odor is slower than higher in the water column, thus reducing the area from which they can be attracted to a food item. The vertical distribution of E. gryllus correlates well with several physical features of the benthic boundary layer. Their vertical abundance increases an order of magnitude near the top of the Ekman layer, correlating with the increase in current velocities. It continues to increase up to 20 m, corresponding with the probable vertical extent of more than two-thirds of an odor trace. Their abundance decreases significantly above 20 m. The above correlations imply that the vertical distribution of E. gryllus is an adaptation that provides a wide chemosensory overview of the sediment, and that they may be adapted to use the swifter background currents above the Ekman layer in their search for food. Finally, their vertical distribution implies that they feed primarily on relatively large food particles. Individuals found above 50 m are likely to be unaware of food falls and probably feed by predation. Species in the demersal guild are very patchy and probably occur in groups even when not feeding; film data indicate that E. gryllus occurs singly when not at bait. Near an odeor source all species tend to choose bait with other amphipods on it, neglecting similar bait nearby. When given a choice of traps, individual E. gryllus prefer to enter traps containing individuals of similar age and sex. At least at close range, locating a food item is probably based more on information received from other amphipods rather than food odor. It is unknown how this communication might occur.