Effects of detrital food sources on growth of a physid snail

Abstract

Physid snails (Pulmonata: Physidae) are widespread in freshwater habitats throughout the Holartic and are generally considered to feed on periphyton. However, several studies have shown that they also consume living plant material and detritus. In streams where multiple food sources are available, snails may preferentially feed on resources based on palatability or accessibility. It is likely that snail feeding influences food availability and that, when preferable items (e.g. periphyton) are lacking, less desirable food sources can be consumed (e.g. detritus). In Florida, USA, six species of physid snails occur in low-order tributaries of the St Johns River, of which Haitia pomilia (Conrad) has been found to be abundant (M.A. Chadwick, unpublished data). The food sources available in different streams where H. pomilia have been collected vary dramatically due to the presence of a closed or open canopy. Streams with closed canopies are lined with red maple (Acer rubrum ), sweet gum (Liquidambar styraciflua ) and water oak (Quercus nigra ) trees. Streams with an open canopy (due to urbanization and stream-channel modifications) have macrophytes, particularly the invasive Hydrilla verticillata. This plant was introduced from Asia to North America in the 1960s and its role as a potential food source is not well known. The goal of this study was to assess how differences in putative detrital food sources affect H. pomilia growth. Due to their ubiquitous distribution, understanding how snails respond to changes in the availability of different food sources may yield important insights into the processing of organic matter. In June 2004, H. pomilia were collected from small tributaries of the St Johns River in the vicinity of Jacksonville, Florida and transported to the Experimental Mesocosm Facility at the University of Alabama. Snails were kept in temperature-controlled recirculation tanks (800 l, 5 m length, 0.4 m width and depth, 188C, 15 cm/s flow velocity) and provided food sources including live H. verticillata and detritus composed of air-dried red maple, sweet gum and water oak leaves (collected immediately after leaf abscission) and H. verticillata. In October 2004, feeding/growth experiments were initiated. Forty plastic Petri dishes were prepared with one of four treatments composed of detritus from H. verticillata, red maple, sweet gum or water oak (i.e. ten replicates per treatment). In order to homogenize the detritus, air-dried material of each source was conditioned in the mesocosm stream for 2 weeks, re-dried and then ground with a Wiley mill (no. 40 sieve). In each dish, 25 ml of distilled water and 75 mg of fine particulate organic matter (FPOM) were added. Snails were randomly selected from the mesocosm (total shell length 3–6 mm), photographed (RT SE digital camera, Diagnostic Instruments, Inc.), and then randomly assigned to a food source treatment. The treatments were then placed in a randomly blocked arrangement in an 188C incubator, which was used to eliminate light and the potential for periphyton growth. Digital pictures of each snail were then retaken 18 days later. Shell lengths were measured directly from each picture using image analysis software (SPOT, Diagnostic Instruments, Inc.). Snail instantaneous growth rates (g ) were calculated as g 1⁄4 ln(Wfinal/Winitial)/Dt, where W is snail mass (mg) and t is time (d). After 18 days, snails were removed from the dishes and all remaining material was dried and weighed. The remaining material was a mix of dissolved organic carbon, unconsumed FPOM and snail faeces, but this measurement was used as a proxy for food consumption. We developed a predictive model to estimate snail ash-free dry mass from shell length (Table 1). One-way ANOVA was used to assess differences among treatment for both snail growth rates and food consumption. Pair-wise comparisons among treatments were accomplished using 95% confidence intervals. Survival rates for snails among treatments ranged from 70 to 100%. Snail instantaneous growth varied significantly among treatments (F3,32 1⁄4 5.86, P 1⁄4 0.003) with theH. verticillata treatment having much higher growth rates than the other three treatments (Fig. 1). Food consumption also varied significantly among the treatments (F3,32 1⁄4 8.31, P, 0.001) with the least amount of organic matter being found in the H. verticillata treatment (Fig. 2).