Temporal dynamics of epidermal responses of guppies Poecilia reticulata to a sublethal range of waterborne zinc concentrations

Microparasite dispersal in metapopulations: a boon or bane to the host population?

Although connectivity can promote host species persistence in a metapopulation, dispersal may also enable disease transmission, an effect further complicated by the impact that parasite distribution may have on host-parasite population dynamics. We investigated the effects of connectivity and initial parasite distribution (clustered or dispersed) on microparasite-host dynamics in experimental metapopulations, using guppies and Gyrodactylus turnbulli We created metapopulations of guppies divided into four subpopulations and introduced either a low level of parasites to all subpopulations (dispersed) or a high level of parasites to one subpopulation (clustered). Controlled migration among subpopulations occurred every 10 days. In additional trials, we introduced low or high levels of parasites to isolated populations. Parasites persisted longer in metapopulations than in isolated populations. Mortality was lowest in isolated populations with low-level introductions. The interaction of connectivity and initial parasite distribution influenced parasite abundance. With low-level introductions, connectivity helped the parasite persist longer but had little effect on the hosts. With high levels, connectivity also benefited the hosts, lowering parasite burdens. These findings have implications for disease management and species conservation.

Waterborne zinc alters temporal dynamics of guppy Poecilia reticulata epidermal response to Gyrodactylus turnbulli (Monogenea)

The present study assessed the histological changes in the epidermis of Poecilia reticulata induced by the combined effects of an ectoparasite Gyrodactylus turnbulli and differing concentrations of waterborne zinc (Zn). Infected guppies were exposed to 0, 15, 30, 60, or 120 µg Zn l-1 and monitored over 3 wk during the exponential increase in parasite numbers on the fish. The fish epidermis responded within 3 d to G. turnbulli infection with a rapid increase in epidermal thickness and a modest increase in number, but not size or composition, of mucous cells. In contrast, in the presence of combined waterborne Zn and infection, mucous cell numbers declined rapidly. As the parasite numbers increased, the epidermis remained thicker than normal, and the number and size of mucous cells decreased. The addition of Zn led to a dramatic thickening of the epidermis during the exponential growth of the parasite population. Mucous cell numbers remained depressed. Temporal changes in mucous cell size were Zn concentration dependent. At 60 µg Zn l-1, cells returned to normal size as infection progressed, whereas they remained extremely small at 120 µg Zn l-1. Changes in mucin composition previously reported in response to Zn alone were subdued in the presence of the parasite except at 60 µg Zn l-1, where all cells contained only acidic mucins. Together these results demonstrate that, on exposure to both Zn and G. turnbulli infection, the epidermal response is initially a protective response to both stressors, and then mainly driven by the increased parasite burden.