In contrast, another small invertebrate known as a pteropod is chemically noxious to the fish and is not a prey item. Pteropods are in essence tiny, bright orange snails without shells that move through the water by flapping winglike appendages. Field studies based on dissections of fish have shown that amphipods are common prey for a variety of fish. But pteropods are not eaten, presumably because they are distasteful. Fish that grab a pteropod will shake their heads violently and spit the animal out.
Fish avoid brightly colored, bad-tasting pteropods but readily eat amphipods.
Any tasty animal with no defense whatsoever would be short-lived on the evolutionary scale. After drilling holes in the sea ice, the investigators observed many amphipods carrying pteropods on their backs. Amphipods placed in aquaria were observed to pursue small pteropods then grasp them with their rear appendages and actually hold the smaller animal captive. A scanning electron microscope revealed that an amphipod uses pincers on its rear appendages to grip the pteropod so that it cannot escape. The amphipod places the captured pteropod on its own back in full view, still holding it with the pincers, and goes about its business.