Habitat influences snail community structure and trematode infection levels in a spring-fed river, Texas, USA

Abstract

The spring-fed Comal River in Texas, USA, has been impounded and channelized resulting mainly in a lentic environment with four headwater spring runs. We sampled two spring runs (lotic sites) and two lentic sites seasonally from April 2001 through April 2002 to assess (1) co-occurrence of native and exotic snail species (as determined by interspecific association), (2) the importance of habitat conditions in structuring relationships among these species, and (3) the distribution of snails infected with exotic trematode parasites. Three exotic and four endemic species of aquatic snails were collected, but only Elimia comalensis (Prosobranchia: Pleuroceridae, native), Melanoides tuberculatus, and Tarebia granifera (Prosobranchia: Thiaridae, exotic) were in sufficient densities for further analyses.Tarebia granifera was positively associated with both M. tuberculatus (v 2 = 18.5, P \ 0.001) and E. comalensis (v 2 = 7.3, P \ 0.01), although the cooccurrence between the two exotics was much stronger. Melanoides tuberculatus and E. comalensis exhibited a strong, negative association (v 2 = 10.9, P \ 0.001). The weaker co-occurrence between E. comalensis with the thiarids appeared to be driven by differences in habitat use by the thiarids and native E. comalensis. In lentic habitats, densities of M. tuberculatus and T. granifera were similar but differed significantly from E. comalensis whose densities were 200 times less than the exotic snails. In lotic spring runs, densities of T. granifera and E. comalensis were similar, but differed significantly from M. tuberculatus whose densities were 10 times fewer. Lower densities of M. tuberculatus and T. granifera in habitat conditions common to the spring runs may explain why exotic snail interactions were less with the native E. comalensis than with each other. The native snail, E. comalensis, was not infected with any trematodes, while 6.1% of M. tuberculatus and 4.8% of T. granifera were infected with exotic trematodes. Distributions of infected snails were aggregated; such that most infected snails were found in lentic habitats with silt substrates and moderate to high levels of detritus. Continued declines in spring-flows due to aquifer withdrawals and droughts will increase lentic habitats that may lead to increased densities of T. granifera and M. tuberculatus and their concomitant parasites.

Introduction, distribution, spread, and impacts of exotic freshwater gastropods in Texas

“…However, initially high densities of Marisa cornuarietis in Comal Springs were followed by a population crash in 1990 (Horne et al, 1992). In 2001-2002 the population density of Marisa cornuarietis in Comal Springs and Landa Lake was \1 m -2 (Tolley-Jordan & Owen, 2007). During our survey, the density of Marisa cornuarietis in Comal Springs and Landa Lake was also very low (only 2 individuals found).…”

“…Neck (1984) also suggested that Marisa cornuarietis impacts E. comalensis in the San Marcos River. Melanoides tuberculatus and T. granifera also co-occur with E. comalensis in the Comal River, but apparently differ in their habitat use, with E. comalensis preferring higher flows in lotic habitats (Tolley-Jordan & Owen, 2007). We found that in some localities, very low densities of M. tuberculatus were associated with high (over 4,000 m -2 ) densities of E. comalensis (e.g., Barton Springs in Austin, Travis Co).…”

“…We also looked for other published research through web-based searches including searches of invasive species databases, primary literature in scientific journals, books, reports, and the authors' personal holdings. Published records included Murray (1964Murray ( , 1971Murray ( , 1975, Clench & Fuller (1965), Murray and Wopschell (1965), Murray & Haines (1969), Abbot (1973), Dundee (1974), Kotrla (1975), Fullington (1978), Lindholm & Huffman (1979), Jokinen (1982), Neck (1984Neck ( , 1985Neck ( , 1986, Horne et al (1992), Neck & Schultz (1992), McDermott (2000), Howells (2001a, b, c), Howells & Smith (2002), , Howells et al (2006), andTolley-Jordan &Owen (2007).…”

“…Because quantitative studies of native snails in aquifer-fed waterbodies of Texas were never conducted before the introduction of exotic snails, it is impossible to evaluate the invasion effect on local species. Tolley-Jordan & Owen (2007) suggest that declines in spring flows due to water withdrawal and drought may favor exotic mollusks, which are more likely than natives to prefer lentic habitats and silt substrates.…”

“…However, in Texas both M. tuberculatus and T. granifera are intermediate hosts for the exotic trematode Philophthalmus gralli, which infects waterfowl (Murray & Haines, 1969;Nollen & Murray, 1978), and Centrocestus formosanus, which infects fish and piscivorous birds, including federally protected species (Mitchell et al, 2000. While P. gralli is not considered a serious threat to animal health (Tolley-Jordan & Owen, 2007), C. formosanus, the gill trematode, is known to cause serious economic losses to the tropical fish industry and is harmful to wild fish (reviewed in .…”