We explore brain responses by Trinidadian killifish, Anablepsoides hartii, to parasitic infection by an intestinal nematode, examining effects on both brain size and plasticity. In the laboratory, stressors such as predators and social aggression commonly alter cell proliferation in the forebrain of fish. In our research, we examine the effect of parasite stress on the brain in natural populations of killifish in parallel streams in the Northern Range of Trinidad. The parasite, Camallanus cotti, invaded some of these streams in 2016 and significantly impacted the populations. Population studies have shown that infected killifish populations have lower survival rates and reproductive output compared with nearby uninfected populations. Infected populations of killifish living in the same environment as guppy populations seem to suffer more severely than infected populations in killifish-only streams. Other studies on mice in natural environments infected with gut nematodes have reported that the infection can alter host behavior, with infected individuals displaying more anxious behavior. If this behavioral effect carries over to killifish, infection may decrease their exploratory behavior and thereby decrease their foraging success. We sought to document brain changes that might underlie such changes in behavior and survival.
We collected specimens of mature killifish (>40 mm) of varying body size and sex from six sites in Trinidad. Each site contained different combinations of predator presence, a competitor presence—guppies— and the presence of a gut parasite, Camallanus cotti. We will label brain sections with immunomarkers for cell proliferation (anti-PCNA), focusing specifically on the telencephalon and midbrain. We will also compare relative brain mass in infected and uninfected streams. To quantify the severity of infection, we will dissect guts of fish to calculate parasite load
This study could reveal brain mechanisms underlying behavioral changes and fitness costs associated with parasitic invasion to natural populations.