South American channeled applesnail (Pomacea canaliculata) has been introduced at locations throughout the Indo-Pacific Region where it has become a major threat to rice and taro crops. First reproducing population of channeled-type applesnails was first discovered in Texas in 1989. This invader is of special concern for the Texas coastal ecosystems and Texas rice belt that supports a billion dollar industry annually. The goals of this research are to determine its current distribution and reproductive potential, to define its physiological limits and to predict the potential spread of channeled applesnails in Texas. Thus far the data from this project has been used to produce three M.Sc. theses (by David Hollas and Leah Cartwright, 2006, and Kevin Nichol, 2008) and are currently in preparation for several publications. The first paper will be published in 2008 in Journal of Wetland Restoration.
Objectives and Brief Results:
Determine current distribution in southeastern Texas. The distribution surveys are conducted annually; in 2005 and 2006 the presence of Pomacea insularum was confirmed in seven southeast Texas counties (Austin, Brazoria, Chambers, Fort Bend, Harris, Galveston, and Waller). Of 394 total sites surveyed in 13 counties, snails were found in 53. Snails were found in 18 waterways including canals, bayous and streams (Brazos River, Horspen, Sims, Bessie's, Armand, Buffalo, Mustang, Chocolate, Disckinson, New, Whites Bayou, American Canal), and 2 reservoirs (Barker and Addicks). In 2005 snails were first found in four new waterbodies (Sims, Bessie's, Horsepen Bayous, and Bear Creek), all outside of the region of previous distribution.
Determine current densities at selected locations. Population densities in Texas have never been defined before the beginning of our study although these estimates could provide an indication of the potential extent of possible environmental impact and serve a bench marks to determinations of future increases or declines in numbers. 20 infested sites in 13 waterways in 5 counties (Brazoria, Galveston, Harris, Waller, and Ford Bend) were sampled in August 2005. Average densities were from <0.1 to 4 snails/m2. The highest densities found were up to 24 snails /m2; biomass up to >800 g/m2 (Mustang Bayou, Fort Bend). In October 2005 much higher densities (in average 826 snails/m2; 6,689 g/m2) were found in a rice irrigation canal in Brazoria County.
Study monthly population dynamics of snail populations in several permanent lotic and lentic waterbodies. Changes in densities, size structure, growth rate, and reproduction were monitored at permanent sites (3 ponds in Chambers Co Golf Course, and a ditch near Alvin) from November 2004 to January 2005. Snail densities in ponds were up to 44 m-2, biomass: up to >1,700 g m-2 (wet total live mass). Average densities in ponds were from 0.2 ± 0.03 to 2.4 ± 0.3 snails m-2. In the ditch, densities were lower (average 0.7 ± 0.2 m-2, max 8 m-2, biomass 400 g m-2).
Determine reproduction potential (number of eggs per egg mass; number of egg masses/female/unit of time; reproductive temperature range in Texas). The number of eggs per clutch and annual clutches per female can help predict population growth as it relates to possible future environmental impacts. Reproduction in Texas starts in March and finishes at the end of October – beginning of November. Number of egg masses was proportional to the size of the population and increased through the season from 8 to 426 in August. The average number of eggs per egg clutch was app. 1,000; max: app. 2,500 eggs.
Define selected physiological limits to some abiotic parameters. Understanding tolerance limits to temperature, pH, salinity, calcium levels and emersion are critical to understanding which areas may be most at risk from applesnail invasions. These studies are currently ongoing in the University of Texas in Arlington (Dr. Robert McMahon and his students).
Determine potential food items in southeastern Texas. Feeding laboratory trials were conducted in summer 2005 to determine the willingness of snails to consume plant taxa present in and near the Galveston Bay drainage area. Over 15 native and exotic plant species were tested up to date. Non-choice laboratory feeding experiments revealed that palatability of aquatic plants to the snails varies depend on plant species.
Objectives for 2006-2009:
Determine the rate of applesnail spread in southeastern Texas. Surveys of P. insularum distribution conducted in 2005 and 2006 provided us with a unique opportunity to determine the rate of P. insularum spread in Texas. David Hollas and Leach Cartwright are going to continue the survey in summer 2008.
GIS analysis of the current distribution of applesnails. Data on the current distribution and densities of applesnails combined with abiotic data will allow us to determine the speed, vectors and factors affecting snail spread and distribution in Texas.
