Steven Ly Taricha torosa Spring 2010 1 The Threat of Trematodes to the California Newt (Taricha torosa) Steven Ly ABSTRACT In North America, high incidences of malformed amphibians have led to studies into the causes and ecological implications of this phenomenon on already threatened amphibian populations. Current research suggests that the trematode Ribeiroia ondatrae is responsible for inducing malformations in many amphibian hosts. My research investigates whether Ribeiroia induces malformations in the California newt, Taricha torosa, and whether these malformations contribute to newt population declines. This study utilizes field data and preserved biological specimens of California newts collected from 2007 to 2009 at Hog Lake in Hopland, CA. Partitioning of the lake into two sides and experimental elevation of trematode density in one side of the pool allowed for investigation of these hypotheses. Linear regression of the average number of newt malformations to the average parasite load per year yielded an R2 of 0.73, supporting a positive correlation between parasite levels and malformation prevalence. Though a significant difference was found in parasite load between the experiment and control sides of the lake (p < 0.0001), a significant difference was not found in malformation proportions (p = 0.4495). The linear regression slope of average newt density over time was higher for the control side (b = -0.01970, ± 0.01497) than the experiment (b = -0.22853, ± 0.05099) indicating that newt mortality was higher in the experiment side. The difference in mortality cannot be attributed to a difference in malformation proportions, but it could be explained by the difference in parasite load, suggesting that the increased mortality could be due to direct exposure to infection. Overall, it is impossible to say with certainty whether or not Ribeiroia trematodes pose a significant threat to Taricha torosa newts. KEYWORDS Ribeiroia ondatrae, trematode infection, amphibian malformations, predation, incomplete limb regenerationSteven Ly Taricha torosa Spring 2010 2 INTRODUCTION Since as early as the 1970s, the world has experienced a significant decline in its amphibian populations (Stuart et al. 2004). It has been found that worldwide, 32.5% of all amphibian species are threatened (vulnerable, endangered, or critically endangered) and 43.2% of all amphibian species are experiencing population declines (Stuart et al. 2004). Furthermore, the rate of amphibian declines has been increasing in recent years (Alford and Richards 1999). Proposed explanations for these declines include habitat loss, pesticide use, climate change (Alford and Richards 1999), fungal diseases such as chytridiomycosis (Rachowicz et al. 2006), and parasites (Johnson and Lunde 2005). Amphibians species are ecologically important as mid-level vertebrate predators; consuming a large variety of insect species (including pest species such as mosquitoes) and serving as prey to many species of reptiles, fish, birds, and mammals, allowing energy and nutrients to be transferred from small invertebrates to larger predators (Davic and Welsh 2004). Likewise, their amphibious lifestyle acts to transfer energy and matter from aquatic habitats to terrestrial ones (Davic and Welsh 2004). Amphibians are also important to humans as an indicator species, commonly used in metrics for determining ecosystem health and integrity (Davic and Welsh 2004; Marcot and Hayden 2001). In North America, there has been a dramatic increase in reports of malformed amphibians: amphibians with missing or extra digits and/or limbs (Johnson et al. 1999). Studies into the cause of these malformations have demonstrated that infection by the trematode Ribeiroia ondatrae is responsible for these developmental abnormalities in many species across the United States (Johnson et al. 1999; Johnson et al. 2002; Johnson et al. 2006). A commonly suggested hypothesis for why trematodes induce malformations in their amphibian hosts is that it enhances the amphibian’s susceptibility to predation by the definitive hosts of the trematodes (see Fig. 1), thus increasing the transmission rate of trematodes from one host to another (Johnson et al. 2004). Prior studies seem to support this hypothesis, but there still remains much to learn about this complex biotic interaction (Johnson and Lunde 2005). Although it has been shown that Ribeiroia is responsible for inducing malformations in many species of amphibians (Johnson et al. 2004), whether or not such malformations are responsible for increasing predation and thus host-transmission has yet to be firmly established. This hypothesis is not a novel one, as previous studies have shown that parasites are capable of manipulating the physiology andSteven Ly Taricha torosa Spring 2010 3 behavior of their intermediate hosts in order to advance to their definitive hosts (Berdoy et al. 2000; Kunz and Pung 2004). Figure 1. Life cycle of Ribeiroia trematodes. The first stage of life begins in planorbid snails, the first intermediate host, where the trematodes produce and release cercariae to infect their second intermediate hosts, larval amphibians and freshwater fishes. Consumption of the second intermediate hosts by the definitive hosts, birds and mammals, allows the cercariae to mature into metacercariae. Parasite eggs are produced in the definitive hosts and released through their feces to infect snails (Johnson et al. 2004). One of the known host species of Ribeiroia trematodes is Taricha torosa, the California newt. T. torosa newts are one of the host species for which the biological and ecological effects of Ribeiroia trematode infection have not yet been studied. Currently, T. torosa newts are listed as a California Special Concern species (Jennings and Hayes 1994) because of the introduction of non-native predators, such as mosquitofish and crayfish (Gamradt and Katz 1996), and exposure to ultraviolet-B radiation (Anzalone et al. 1998). My research aims to determine if Ribeiroia-induced malformations also pose a risk to populations of T. torosa newts. My hypotheses are that Ribeiroia trematodes induce malformations in Taricha torosa larvae and that such malformations lead to an increase in Taricha mortality levels. I used a combination of three years of monitoring data at a site known to support high infection levels in Pacific chorus frogs (Pseudacris regilla). In particular,
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