Infectious diseases are a significant threat to worldwide biodiversity. Amphibian declines, a significant part of current biodiversity losses, are in many cases associated with infectious disease. Water molds are one group of pathogens affecting amphibians on a worldwide basis. Although water molds have been studied extensively for their effects on host embryos, little information is available about how they affect post-embryonic amphibians. We tested the effects of one species of water mold, Saprolegnia ferax, in a comparative study of larvae of 4 amphibian species: Pseudacris regilla (Pacific treefrog), Rana cascadae (Cascades frog), Ambystoma macrodactylum (long-toed salamander), and R. aurora (red-legged frog). S. ferax can kill amphibians at the embryonic and juvenile life history stages, depending on the amphibian species. In the present study, a 1 wk exposure to S. ferax killed P. regilla larvae and a 2 wk exposure killed R. aurora larvae. Larvae of the other host species were unaffected after 1 wk of exposure to S. ferax. Our results suggest that S. ferax can kill amphibian larvae and further suggest that evaluation of how pathogens affect amphibians at the population level requires investigation at various life stages.KEY WORDS: Water mold · Saprolegnia · Amphibian · Ambystoma macrodactylum · Pseudacris regilla · Rana aurora · Rana cascadae Resale or republication not permitted without written consent of the publisherDis Aquat Org 83: [187][188][189][190][191][192][193] 2009 multiple species of both Achlya and Saprolegnia caused mortality and sublethal effects in embryos of Rana sylvatica (wood frog) and Bufo americanus (American toad) . The effects of water molds on amphibian embryos appear to be influenced by several environmental factors, including temperature, pH, ultraviolet-B radiation, precipitation, plant cover, and oviposition behavior (Kiesecker & Blaustein 1995, Green 1999, Kiesecker et al. 2001a, Ruthig 2006, additional references in Romansic et al. 2006.In contrast to the extensive research on the effects of water molds on amphibian embryos, there is relatively little information available about how water molds affect post-embryonic amphibians. There is experimental evidence that Saprolegnia influences competitive interactions between larvae of Rana cascadae (Cascades frog) and Pseudacris regilla (Pacific treefrog), apparently by mediating densities of the competing species through differential effects on embryos (Kiesecker & Blaustein 1999). Water mold infections have been reported in amphibian larvae in nature (Bragg & Bragg 1958, Bragg 1962, Berger et al. 2001, and occur occasionally in larvae of North American species . Berger et al. (2001) reported massive mortality of Bufo marinus (cane toad) larvae associated with Aphanomyces sp. infection, and Saprolegnia has been found on dead amphibian larvae after mass mortality events (Bragg & Bragg 1958, Bragg 1962. Romansic et al. (2006) demonstrated that Saprolegnia can kill larvae of R. aurora (northern red-legged frog), and Rom...

We tested for a synergism between nitrate and Saprolegnia, a pathogenic water mold, using larvae of 3 amphibian species: Ambystoma gracile (northwestern salamander), Hyla regilla (Pacific treefrog) and Rana aurora (red-legged frog). Each species was tested separately, using a 3 × 2 fully factorial experiment with 3 nitrate treatments (none, low and high) and 2 Saprolegnia treatments (Saprolegnia and control). Survival of H. regilla was not affected significantly by either experimental factor. In contrast, survival of R. aurora was affected by a less-than-additive interaction between Saprolegnia and nitrate. Survival of R. aurora was significantly lower in the Saprolegnia compared to the control treatment when nitrate was not added, but there was no significant difference in survival between Saprolegnia and control treatments in the low and high nitrate treatments, consistent with increased nitrate preventing Saprolegnia from causing mortality of R. aurora. Survival of A. gracile followed a similar pattern, but the difference between Saprolegnia and control treatments when nitrate was not added was not significant, nor was the nitrate × Saprolegnia interaction. Our study suggests that Saprolegnia can cause mortality in amphibian larvae, that there are interspecific differences in susceptibility and that the effects of Saprolegnia on amphibians are context-dependent. KEY WORDS: Pathogen · Saprolegnia · Amphibian Resale or republication not permitted without written consent of the publisherDis Aquat Org 68: [235][236][237][238][239][240][241][242][243] 2006 Saprolegnia, a water mold, is one important pathogen found in many amphibian populations (e.g. Strijbosch 1979, Banks & Beebee 1988, Blaustein et al. 1994. Furthermore, Saprolegnia-associated mortality appears to increase in the presence of abiotic stressors (Strijbosch 1979, Banks & Beebee 1988, Kiesecker & Blaustein 1995, Kiesecker et al. 2001a.Saprolegnia (family Saprolegniaceae) is both saprobic and parasitic, obtaining nutrition from decaying organic matter or living hosts (Seymour 1970). Saprolegnia infects a wide variety of organisms, including insects, turtles, fishes, and amphibians (MacGregor 1921, Seymour 1970. In amphibians, embryos and larvae can become infected (Bragg & Bragg 1958, Walls & Jaeger 1987, Blaustein et al. 1994. Saprolegnia-infected embryos of fishes and amphibians become covered with visible white hyphal filaments and usually do not hatch (Blaustein et al. 1994). Infection can spread via contact from growing hyphae (in the case of immobile hosts such as amphibian egg masses) or through colonization by free-swimming zoospores (Wood & Willoughby 1986). Transmission can occur between species, for example, between fishes and amphibians (Kiesecker et al. 2001b). Fishes and amphibians may also be infected by Saprolegnia via contact with infected soil (Kiesecker et al. 2001b). Host species show strong interspecific variation in their susceptibility to infection (Richards & Pickering 1978, Wood & Willoughby 1986, Kiesecker & Blau...