The influence of temperature and spatial distribution on the susceptibility of southern leopard frog eggs to disease

Abstract

Since host defenses to infectious disease are often costly, one would expect hosts to use their defenses only when the threat of infection is high. Southern leopard frogs (Rana sphenocephala) at Ellenton Bay in South Carolina (USA) have an extended breeding season and their eggs are exposed to a wide range of temperatures depending on the time of year when they are laid. Adults aggregate their egg masses in cold temperatures, but separate them in warm temperatures. The spatial aggregation of egg masses may insulate eggs from cold temperatures, but may also affect the transmission of pathogens between the eggs. I examined the effects of temperature, pathogens, and spatial distribution on the survival of R. sphenocephala in the egg stage. Field observations found that temperature had little effect on the number of infected eggs within egg masses, but that egg masses in colder water were more likely to be aggregated together. In a controlled laboratory experiment, the presence of aquatic oomycetes led to higher mortality in cold temperatures than they did at warmer temperatures. Infectious disease may be a selective force that favors R. sphenocephala adults that aggregate their offspring when the threat of disease is high. Host aggregation can reduce the risk of infection when pathogens have a slow generation time that hinders them from responding to an abundance of hosts.

Water molds of the genera Saprolegnia and Leptolegnia are pathogenic to the North American frogs Rana catesbeiana and Pseudacris crucifer, respectively

“…3). This may have not been enough time for hyphae from infected eggs to reach neighboring eggs (Ruthig 2006(Ruthig , 2008. Therefore, the infected eggs in the experiment may have been primarily infected by the water mold zoospores from the initial inoculation and host density would not have affected their probability of being infected.…”

“…Parents may lay their eggs in locations where they are less likely to become infected (Ruthig 2006(Ruthig , 2008. The eggs also contain their own defense mechanisms that protect them (Sagvik et al 2008a,b), including the jelly surrounding the eggs (Gomez-Mestre et al 2006) and phenotypic plasticity in hatching time of eggs exposed to water molds (Touchon et al 2006).…”

“…The embryo within the egg had stopped developing and was consumed by white water mold hyphae. Once these hyphae were isolated (Ruthig 2008), this culture was considered an isolate of a single strain and was the parent line in all of the experiments using R. catesbeiana eggs as hosts.…”

“…Water mold infections have been associated with egg mortality on species of amphibians in many places around the world, including: Crinia signifera in Aus-tralia (Williamson & Bull 1994), Rana arvalis, R. temporaria and Bufo bufo in Europe (Robinson et al 2003, Sagvik et al 2008a, R. sphenocephala in southeastern North America (Ruthig 2008), B. americanus and R. sylvatica in northeastern North America (GomezMestre et al 2006) and R. cascadae and B. boreas in northwestern North America (Blaustein et al 1994, Kiesecker & Blaustein 1995. To date, the eggs of Hyla regilla (Kiesecker & Blaustein 1995), R. clamitans (Karraker & Ruthig 2009) and Ambystoma maculatum (Gomez-Mestre et al 2006, Karraker & Ruthig 2009) have been found to be resistant to infection when exposed to water molds in controlled experiments.…”

“…For example, salamander adults are more susceptible to Saprolegnia parasitica in water polluted by motor oil (Lefcort et al 1997), and frog eggs are more susceptible to infection under exposure to ambient levels of UV-B radiation in shallow water (Kiesecker & Blaustein 1995, Kiesecker et al 2001b. Temperature also affects the susceptibility of amphibian eggs to infection (Ruthig 2006, 2008, Sagvik et al 2008b. Water molds are also important pathogens of overcrowded and injured fish in fisheries (El-Sharouny & Badran 1995) that may potentially carry pathogenic strains of water molds to naïve amphibian populations (Kiesecker et al 2001b).…”