1 lj518@bellsouth.net
2 kwalt@mtsu.edu, (615) 898-2660
 
 

 The influence of tidal elevation on snail, Littoraria irrorata, consumption of cordgrass, Spartina alterniflora, leaves from a Georgia salt marsh was determined in a December 1998 experiment.  Differences in tidal elevation, high vs. low marsh, may affect plant physiological stress and the accumulation of phenolic compounds known to inhibit snail foraging.  High marsh plants that are flooded less frequently may experience greater stress, lower phenolic concentrations, and increased consumption by herbivores.  In the field high marsh snails were significantly larger and of greater mass than low marsh snails, possibly resulting from increased consumption of more palatable high marsh leaves.  A 12-d reciprocal cross experiment was conducted in which either low or high marsh green leaves were fed on by no, low, or high marsh snails.  High (5 to 14%) and low (23 to 34%) marsh leaves lost a significant percentage of their ash-free dry mass (AFDM) during the experiment, but low marsh leaves lost significantly greater amounts.  Autogenic AFDM losses in treatments with no snails did not obscure differences in snail foraging.  Low and high marsh snails consumed approximately the same high marsh leaf amounts, but high marsh snails consumed significantly less low marsh leaf tissue.  Snails consumed less in treatments with leaves from a different elevation and the reduced consumption was reflected in a slight (ca. 10%) decline in snail tissue AFDM.  Differences in particulate AFDM collected from washings were opposite to observed feeding patterns; treatments in which snails consumed the least amount of leaf material contained greater amounts of particulate material.  The short-term feeding of L. irrorata on green S. alterniflora leaves is dependent on elevational differences in leaf and snail origins that may be related to leaf phenolic concentration differences and conditioned snail feeding responses.

     The saltmarsh periwinkle, Littoraria irrorata Say, is a common resident and major consumer of smooth cordgrass, Spartina alterniflora Loisel, in the intertidal marshes of the US Eastern and Gulf coasts (Alexander 1979, Newell 1993). Studies suggest that snails prefer to feed on dead rather than live material (Bebout 1988, Kemp et al. 1990) and that the preference may be predicated on a declining concentration of soluble phenolics (Haddad et al. 1992, Wilson et al. 1986). Phenolics are a feeding deterrent for several marsh invertebrates (Rietsma et al. 1988, Valiela et al. 1984).  Senescing S. alterniflora leaves decline in phenolic concentrations as a result of both leaching and fungal colonization (Bergbauer and Newell 1992).  Fungi mediate leaf decomposition (Newell et al. 1986, 1989), however the loss of phenolics appears to influence L. irrorata's preference for dead leaves more than the presence of fungi (Bärlocher and Newell 1993).

     Both L. irrorata and S. alterniflora are distributed widely within intertidal marshes.  Snail distributional patterns appear haphazard (Walters pers. obs.), but in Southeastern marshes the morphological distribution of cordgrass is dependent on tidal elevation.  The tall (>1 m), fast growing form of S. alterniflora dominates low-marsh elevations and the short (<0.5 m), slower growing form dominates high-marsh elevations.  Spartina alterniflora fitness has been linked to tidal elevation and flooding frequency (Smart and Barko 1980, Mckee and Patrick 1988).  Low marsh plants are frequently flooded resulting in reduced soil salinities and lower physiological stress compared to high marsh plants that are subject to infrequent tidal flooding, longer evaporative periods, increased soil salinities, and greater physiological stress.  If the production of phenolics in S. alterniflora is related to the physiological stress experienced by the plant, greater stress equals lower phenolic concentration, then the ability of S. alterniflora to deter herbivory may be dependent on tidal elevation.

    The present study was designed to determine if the tidal elevation of S. alterniflora stems from which leaves were collected influenced L. irrorata consumption.  Snails from different elevations were offered live green S. alterniflora leaves from different elevations and the consumption of leaf material measured.  The choice of live green leaves was designed to ensure minimal fungal presence and maximum phenolic concentrations.

    Littoraria irrorata and S. alterniflora leaves were collected in December 1998 from high and low marsh elevations within the MacKay Marsh located adjacent to the Jose Torres Causeway on St. Simons Island, GA, USA (81° 21' W; 31° 09' N). The MacKay marsh is representative of typical Southeastern US marshes with a monospecific stand of the tall-form (> 1 m) of S. alterniflora distributed from creek-bank to mid-marsh elevations and the short-form (< 0.5 m) of S. alterniflora distributed in higher elevations.  Two sampling sites were selected; one located in the low marsh among 1.5-2.0 m stems ca. 5 m from the MacKay River and one located in the high marsh among 0.3 m stems and 2-3 m from the roadside edge of the marsh. Individual green leaves from multiple, newly emerging stems of S. alterniflora containing <4 green and no yellow-green or brown leaves were excised at the stem ligule.  Leaves from high and low marsh sites were bundled separately, wrapped in foil, placed on ice, and returned to the lab.  Sufficient numbers of snails also were collected from high and low marsh sites, placed in separate 500 ml containers with a small amount of seawater, and returned to the lab.

    Initial field samples of high and low marsh leaves and snails were frozen at -20 °C and processed within 3 mo. of collection. Leaf samples were weighed to determine initial wet mass before freezing.  Frozen leaves were transferred to preweighed scintillation vials, dried at 60 °C for >48 h, and ashed at 450 °C for >4 h to determine the initial leaf ash-free dry mass (AFDM).  A total of 7 multi-leaf samples of 1-2 g wet mass were processed from high and low marsh sites.  Frozen samples of high and low marsh snails (n=50 ea.) were processed to determine shell dry mass and tissue AFDM.  Shells of individual snails were cracked carefully to extract all the soft-body tissue.  Both shell and tissue were placed in preweighed scintillation vials, dried at 60 °C for >48 h, and weighed.  The shell with operculum was removed and the vial reweighed and ashed at 450 °C for >4 h to determine the tissue AFDM.

    In the lab a reciprocal cross experiment was set-up <2 d after collection of leaf and snail material.  Leaves were trimmed, weighed to the nearest 0.1 mg, and ca. 1 g wet mass placed into individual, freestanding 50 ml centrifuge tubes creating high (3 leaves / tube) and low (2 leaves / tube) marsh leaf treatments.  Either a high marsh, low marsh or no snail was placed into each tube containing high or low marsh leaves.  The result was a complete 2 x 3 factorial design with 6 replicates of the following leaf and snail treatments: high-high, high-low, high-no, low-high, low-low, and low-no.  Treatments without snails controlled for the potential autogenic loss of organic carbon by leaves.  Centrifuge tubes with caps loosely seated on top were placed in the lab 0.5 m under fluorescent lights on a 12/12 light/dark cycle.  Caps were removed daily, leaves and snails inspected, and filtered seawater added as necessary to prevent dessication.  Total seawater amounts added did not exceed 5 ml for the entire experiment.

    The reciprocal cross experiment was terminated 12 d after initiation.  Centrifuge tubes were filled to ca. 10 ml with filtered seawater and vigorously shaken to dislodge any loose particulate organic material from leaves and inner surfaces.  Snails were removed and frozen for later processing to determine shell and tissue mass using the procedure outlined for field snails.  Leaves were removed, carefully dried and weighed to determine final wet mass, and then processed as described above to determine final AFDM.  All particulate material in washings were transferred to separate, preweighed scintillation vials and processed to determine AFDM.

    All statistical tests were run using SAS 6.11 for Windows (Joyner 1985).  Reduced major axis regressions (Ricker 1973) using PROC NLP, nonlinear minimization, and Newton-Raphson ridge optimization were calculated to determine the equation describing the linear relationship between wet mass and AFDM for initial leaves.  The generated regression equation was used to predict the initial AFDM of reciprocal cross experiment leaves from initial wet weights.  PROC GLM was used for all ANOVA's to analyze differences in (1) initial field snail size and AFDM between marsh elevations, (2) final snail AFDM between treatments in the reciprocal cross experiment, and (3) the loss of leaf and accumulation of particulate AFDM between experimental treatments.  Leaf AFDM losses were calculated as the difference between regression-estimated initial AFDM and measured final AFDM.  Data either satisfied or were transformed appropriately to satisfy parametric ANOVA assumptions.  A two-factor model was used to test for leaf and snail treatment effects (high or low marsh origins).

    Field snails exhibited elevational differences in size and mass between low and high marsh sites (Fig. 1).  Low marsh snails were significantly longer (F1.87 = 11.71, p < 0.001) but not as wide (F1.87 = 76.58, p < 0.0005) as high marsh snails and had significantly heavier shells (F1, 87 = 11.4, p < 0.002) and greater tissue AFDM (F1, 87 = 5.36, p < 0.03). High and low marsh snails used in the reciprocal cross experiment (Fig. 2) were not significantly different either in shell length or final tissue AFDM (Table 1) even though a 22% difference in AFDM could be detected with 80% power at the 0.05 level of significance (Cohen 1988).  Although no significant difference in snail final AFDM was detected, the mean AFDM of only high marsh snails fed low marsh leaves declined appreciably, 1.8% d-1, during the course of the experiment.

    The wet mass of initial, field-collected leaves was significantly correlated with leaf AFDM (high marsh, Pearson's r = 0.898, p<0.01; low marsh, Pearson's r = 0.946, p<0.002).  Reduced major axis regressions of leaf AFDM by wet mass generated the following predictive equations for high (AFDM = 0.208 * Wet Mass + 0.044) and low marsh leaves (AFDM = 0.314 * Wet Mass - 0.022).

    High and low marsh leaves typically lost AFDM during the 12 d experiment (Fig. 3).  Even leaves in treatments with no snails lost mass (high marsh = 5%, low marsh = 23%), but the autogenic loss did not obscure differences in snail foraging patterns (Fig. 3).  The consumption of leaves by snails resulted in no significant interaction between snail and leaf treatments (F2, 29 = 1.77, p > 0.05), but there were significant snail (F2, 29 = 7.36, p < 0.003) and leaf (F1, 29 = 80.67, p < 0.001) main effects.  Low marsh snails consumed significantly greater amounts and low marsh leaves generally lost greater amounts of plant tissue (Fig. 3).  High marsh snails feeding on low marsh leaves consumed the least; only 5% after correcting for autogenic losses.  Daily leaf consumption rates declined by 82% for high marsh snails feeding on low marsh leaves but only 46% for low marsh snails feeding on high marsh leaves (Table 2).  The AFDM of particulate material collected in treatment washings exhibited no significant interaction (F2, 29 = 0.75, p > 0.05) or leaf treatment effects (F1, 29 = 0.96, p > 0.05), but there was a significant snail effect (F2, 29 = 11.32, p < 0.001).  High marsh snails produced significantly greater amounts of particulate organic material (Fig. 4) and snails that fed the least produced the most particulate material; ca. 1 mg d-1 after correction for amounts collected from no snail treatments.

    Experimental results indicate that elevational differences in the origins of S. alterniflora leaves and associated L. irrorata can affect snail foraging, but connections between green leaf consumption and possible cordgrass chemical defenses are obscure.  Low marsh snails were significantly larger than high marsh snails in the field and high marsh snails consumed ca. 40% less of low compared to high marsh leaves in the reciprocal cross experiment.  If high and low marsh snails represent a single cohort that haphazardly settled within the marsh then size differences may be related to leaf palatability; high marsh leaves are more palatable facilitating snail foraging.  The minimal size difference between high and low marsh snails (16. to 16. mm shell lengths, high vs. low) is consistent with assuming snails represent a single cohort.  The difference in leaf consumption by high marsh snails (high<    Experimental differences in snail tissue mass paralleled differences in leaf consumption.  Mean tissue AFDM of both low and high marsh snails feeding on leaves from the opposite elevation declined.  Low marsh snails feeding on high marsh leaves declined by only 2.8%, but high marsh snails feeding on low marsh leaves declined by 20.7%.  The decline in AFDM of high marsh snails feeding on low marsh leaves is consistent with the reduced consumption of leaf material; snails are not feeding but instead are living off reserves.  Negative growth rates have been previously reported for snails <12 mm shell length feeding on diets that did not include fungi (Bärlocher and Newell 1994).  The AFDM of snails fed leaf and sediment diets not containing fungi declined by 0.5 to 1.5 mg during an ca. 6 wk experiment.  However in our experiment snails in 2 of the 4 treatment levels exhibited positive growth on green leaves that typically should contain minimal fungal biomass.

    The significant difference in leaf-associated particulate AFDM between high and low marsh snails may be related to differences in snail foraging.  Particulate AFDM increased 16-20% compared to only 2-8% in high versus low marsh snail treatments, respectively.  The significantly increased particulate material in high marsh snail treatments suggests a possible reduction in leaf assimilation efficiency relative to low marsh snails.  Leaf digestibility also appears to increase with increased phenolic concentrations; yellow-green leaves are more digestible than standing-dead leaves (Bärlocher and Newell 1994). Low marsh snails may be able to maximize the assimilation of consumed leaf material to counteract reduced feeding rates resulting from increased phenolic deterrents in low marsh leaves.

    Littoraria irrorata consumption rates on green leaves in the reciprocal cross experiment (Table 2) unexpectedly were similar to rates observed on yellow-green or standing-dead leaves in previous studies.  Newell and Bärlocher (1993) reported small (ca. 5 mm shell length) and large snails (ca. 14 mm) feeding on standing-dead leaves in 3 to 6 d experiments consumed between 27 and 67 µg·mg-1·d-1 respectively.  Snails 6 to 12 mm in length consumed 41.7 µg·mg-1·d-1 of standing-dead and 13.3 µg·mg-1·d-1 of yellow-green leaves (Bärlocher and Newell 1994).  The increased consumption of standing-dead
compared to yellow-green leaves was attributed to an increase in fungal biomass and lower phenolic content (Bärlocher and Newell 1994).  Snails prefer and selectively feed on fungi (Bebout 1988, Bärlocher and Newell 1994) and phenolic acids
associated with S. alterniflora are demonstrated feeding deterents (Rietsma et al 1988, Bärlocher and Newell 1994).  Fungal biomass increases (Newell et al. 1986, 1989) and phenolic content declines as leaves age from living green to standing-dead (Wilson et al. 1986, Bergbauer and Newell 1992).  Green leaves will have lower amounts of fungal biomass and increased concentrations of phenolics compared to standing-dead or even yellow-green leaves and snails should exhibit reduced consumption rates on green leaves.  However results from the reciprocal cross experiment suggest snails consume green leaves at rates similar to the highly prized standing-dead leaves (Table 2).  The increased size (>16 mm) of snails used in our experiment alone can not explain the similarities in consumption rates.  Evidence suggests larger snails actually become more selective feeders (Walters in prep.).

    Littoraria irrorata exhibit complex feeding responses on live S. alterniflora leaves from different intertidal marsh elevations. Snails are able to grow on green leaf diets and feed at rates equivalent to aged leaves.  Previously demonstrated snail feeding preferences for standing-dead over live leaves (Bebout 1988, Kemp et al. 1990) and increased snail growth on fungal-infused leaves (Bärlocher and Newell 1994) do not explain observed low and high marsh elevational differences in snail foraging. Hypothesized patterns in phenolic deterrents to foraging also do not explain completely our results.  Snails appear conditioned to feed better on leaves from the elevation of residence.  Conditioning may effect morphological feeding mechanisms (e.g., radula) or physiological digestive processes (e.g., enzyme induction).

    We thank B. Walters for field assistance, the 1999 MTSU Marine Biology class for processing lab samples, and D. Curry, S. Pennings, and anonymous reviewers for their critical evaluation and constructive comments on earlier drafts.  This research was supported in part by Middle Tennessee State University's Department of Biology.

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 Table 1.  Two-way analyses of variance for differences in snail shell length or tissue mass at the termination of the reciprocal cross experiment. (leaf and snail treatments = high or low marsh origin)
 

 Table 2.  Average leaf consumption by snails corrected for autogenic losses and adjusted for snail dry mass (µg leaf / mg snail / d).
 
 

Figure 1.  Initial size, dry mass, and AFDM for L. irrorata collected in the field within high and low MacKay marsh elevations.

Figure 2.  Final shell length and AFDM for experimental L. irrorata from high or low marsh origins fed either high or low marsh S. alterniflora leaves.

Figure 3.  The loss of AFDM from high and low marsh S. alterniflora leaves fed on by no, high, or low marsh L. irrorata.
Initial leaf AFDM amounts were estimated from reduced major axis regressions of AFDM by wet weight.

Figure 4.  The AFDM of particulate material collected from washings of high and low marsh leaf and snail treatments.