Effects of short-term exposure to ectoparasites on fish cortisol and hematocrit levels

Abstract

On coral reef systems, ‘client’ fishes combat relentless ectoparasitic infections by engaging in frequent mutualistic interactions with various ‘cleaner’ organisms. Clients are known to adjust their visits to cleaners according to their ectoparasite load. Whether physiological changes due to the ectoparasitic infection, prior to engaging in cleaning interactions, might inform clients of their current need to visit cleaners remains unclear. Here, we tested whether fish blood parameters vary with ectoparasitic infection: we exposed clients (Scolopsis bilineatus) to hematophagous ectoparasite gnathiid isopods (Gnathia aureamaculosa) or a control situation for 30 min, then collected fish blood and counted gnathiids attached to fish. We found that gnathiid-exposed fish had higher blood cortisol hormone levels compared with controls. This suggests gnathiid presence in the experimental situation results in an acute stress hormone response in a fish. In contrast, while hematocrit level, a correlate of blood loss, did not significantly differ between treatments, within gnathiid-exposed fish hematocrit was negatively related to attached gnathiid number. This suggests hematocrit could provide information to a fish about gnathiid load. Thus, short-term exposure to an ectoparasite translates into possible proximate physiological mechanisms underlying client’s decisions involving interactions with cleaners.