Apparent kleptoparasitism in fish-parasitic gnathiid isopods.

Gnathiid isopods are common external parasites/micropredators that feed on the blood of marine fishes. During the course of processing samples of gnathiid isopods collected from light traps in the central Philippines, we observed a gnathiid attached to and apparently feeding from the abdomen of another gnathiid. Because the abdomens of both gnathiids were enlarged, it was unclear whether one actually fed on the blood meal of the other. Introduction of unfed gnathiids with fed gnathiids revealed that one gnathiid could and did feed on the blood meal of another. This is the first observation of apparent conspecific kleptoparasitism reported for gnathiid isopods.