Little is known generally about warning coloration or aposemetism in marine invertebrates, let alone specifically in west-coast amphipods. The phenomenon is well known in vertebrates and insects, where a toxic, noxious-tasting, or stinging/biting animal sports bright, easily remembered colours to warn off potential predators. The black and yellow/orange markings on bees, wasps, ants, and other insects are thought to have evolved for this purpose. A naive predator eats a warningly coloured potential prey, gets sick or stung, and quickly learns not to eat or touch another of the same, or similar, species. For birds, mammals, and fishes the lesson can be learned and remembered after just a single encounter, but little is known about how aposemetism might work with a marine-invertebrate predator, especially one lacking colour vision or, for that matter, any vision at all. As fishes and birds are likely to be the principal predators of amphipods, both with good colour vision, it should be possible to set up experiments to test an hypothesis of aposematic defense in a certain amphipod species. All that is needed is to find a toxic, brightly demarcated species in sufficient numbers for experimentation.

There seems to be only one documentation of chemical deterrence in a west-coast amphipod, but not associated with warning coloration. This is referenced in the paper cited in Research Study 1 below for an introduced west-coast species Ampithoe longimana. There are a number of brightly coloured west-coast species that invoke the notion of warning coloration, and photos of some of these are shown below. Research Study 1, on an amphipod species in Bilize, is included just for interest sake, as well as for generation of research ideas.

A unique amphipod-host/plant relationship exists in Belize between the amphipod Pseudamphithoides incurvaria and its brown-algal food plant Dictyota bartayresii, a chemically defended species.  The amphipod slices out 2 circular sections from the algal frond, fastens them together, and uses the bivalved product as a portable domicile (see drawing).  The amphipod grips onto its “home” with pereopods 5-7 and can withdraw completely into it.  However, it generally hangs out the front and locomotes by walking or swimming, the latter using both sets of antennae. A study at Carrie Bow Cay, Belize reveals that the amphipod preferentially uses Dictyota bartayresii even when the alga is rare compared with other closely related species of dictyotaceans. Moreover, in laboratory tests, Pseudamphithoides preferentially chooses Dictyota species containing diterpenes that are known to deter feeding by reef fishes.  When presented as food to predatory fishes in the laboratory, amphipods in their algal domiciles are rejected while ones without their domiciles are readily eaten.  Also, when amphipods are forced by necessity to construct their domiciles from the green alga Ulva, which lacks any defensive “chemistry”, they are eaten by reef fishes.  When Ulva is treated with pachydictyol-A in increasing dosages, its use as domiciles by the amphipod increases in proportion to the concentration of metabolite.  The authors sum up by stating that all their experimental data are consistent with an hypothesis of predator deterrence.  Hay et al. 1990 Ecology 71: 733.

NOTE  the alga is rich in the secondary metabolite pachydictyol-A and other terpenes, and is not gemerally eaten by herbivores such as fishes

NOTE  the amphipod appears not to sequester the pachydictyol-A deterrent in their own bodies, relying instead on the protection conferred from their domiciles.  In contrast, the Atlantic-coast species Ampithoe longimana, present on the west coast through introduction, is known by the authors to eat pachydictyol-rich Dictyota dichotoma and does apparently benefit from decreased predation by fishes.  Hay et al. 1987 Ecology 68: 1567.