Chemoreceptive foraging in a shallow-water scavenging lysianassid amphipod: role of amino acids in the location of carrion in Scopelocheirus onagawae

Marine scavenging lysianassid amphipods depend on chemoreception for detecting carrion, a scarce resource, efficiently. This study focused on the chemoreceptive foraging of the scavenging amphipod Scopelocheirus onagawae using bioassay. Among the extracted fraction from flounder muscle, neutral and basic amino acids, especially glycine (Gly), alanine (Ala), and arginine (Arg), which are abundant in the tissues of marine animals, showed highly stimulatory effects. Synergism, the enhancement of the stimulatory efficiency by acidic amino acids (e.g. aspartic acid; Asp) that have minimal attractive effects as individual amino acids, was also observed. The threshold concentration of amino acids leading to a locomotory response was 10(-7) mol l(-1), which is relatively low among crustaceans. The locomotory response at the threshold became more sensitive when amphipods were acclimatised to a lower concentration of amino acids (8.0 x 10(-8) M), suggesting that sensory adaptation is involved in their foraging. The chemoreceptive system in S. onagawae, i.e. the detection of common chemicals from animal tissue as feeding stimuli at low threshold concentrations, is advantageous in detecting a wide range of carrion prey items from a broad area. The high sensitivity, sensory adaptation and synergism by uncommon amino acids would be a beneficial adaptation for discriminating a food odour in an environment of chemical 'noise'. Since S. onagawae is a nocturnal suprabenthic forager, the sensitive chemoreceptive ability found in this study would be adaptive for detecting sparse and limited carrion items, the odour plume of which can reach their swimming layer, suggesting that this species is a specialised carrion feeder.