Predation on the Endangered Tidewater Goby, Eucyclogobius newberryi, by the Introduced African Clawed Frog, Xenopus laevis, with Notes on the Frog's Parasites

Abstract

We investigated whether the introduced African clawed frog (Xenopus laevis) eats endangered tidewater gobies (Eucyclogobius newberry) where they coexist in the Santa Clara River estuary. The tidewater goby inhabits low flow, shallow, brackish portions of coastal streams, marshes, lagoons, and estuaries in California and is usually abundant where present (Swift et al., 1989). They are small predators (4-5 cm TL) that feed on benthic invertebrates (Swenson 1995; J. Irwin and D. Soltz, unpubl.). Only half of the original tidewater goby populations remain (Swift et al., 1989; US Fish and Wildlife Service, 1994). Habitat loss and degradation (a consequence of land use practices such as conversion of coastal wetlands to marinas, highway, and railroad construction; freshwater diversions; grazing; breaching of coastal lagoons; and flood control practices) appear to have led to several permanent extirpations (Lafferty et al., 1996). Introduced predators may be responsible for some extinctions of tidewater goby populations as well. For example, in the San Francisco Bay area, several extirpations of tidewater goby populations coincided with an invasion of the predatory yellowfin goby (Brittan et al., 1970, McGinnis, 1984) and the aggressive rainwater killifish, Lucania parva (Hubbs and Miller 1965). At least one local extirpation in Southern California is thought to be a result of predation by introduced green sunfish. Here, high waters allowed a connection between the creek and sewage ponds where sunfish were present (D. Holland, unpubl.). The impact of these and other introduced species deserves more study. During a study of tidewater gobies and other fishes, we observed clawed frogs at the mouth of the Santa Clara River in Ventura County, California, where they were not previously documented (although clawed frogs have been abundant 17 km upstream at the Vern Freeman Diversion at Saticoy [E. Ballard, pers. comm.]). The clawed frog was first brought to the US from South Africa for use as a pregnancy assay and later gained popularity with researchers and the pet trade (McCoid and Fritts, 1989), becoming established in Southern California in the mid-1960s (McCoid and Fritts, 1980a) where it breeds year-round (McCoid and Fritts, 1989). In some areas, it reaches high densities (McCoid et al., 1993). The African clawed frog has a mixed reputation as a pest, and it was difficult to predict whether it poses a threat to tidewater gobies. Concern about its impacts on native species derives from reports that it preys voraciously on vertebrates (St. Amant et al., 1973). It is a pest in its native southern Africa where it successfully spreads through disturbed habitats (Picker and De Villiers, 1989) and interferes with aquaculture (Taylor et al., 1992). Anecdotal information from California and South Africa indicate