A new study published in Biology Letters has looked at the cost of hosting ectoparasites to coral reef fish. The paper observed bridled monocle bream (Scolopsis bilineatus), parasitized by an isopod (Anilocra nemipteri), which attaches above the fish’s eye. Model parasites were attached and it was shown that although these did not affect the fish’s metabolic rates (the “live” parasites do), the main effect was reduced streamlining, which in turn reduces swimming performance at high-speed. Previous thought has been that the main cost to the host was the increase in metabolic rate necessary to sustain both fish and parasite. (Image by Valter Jacinto).