Snail Melampus Bidentatus Research Articles

The Common Snail Melampus bidentatus Occurs Throughout the Salt Marsh in Its Northern Range

The Common Snail Melampus bidentatus Occurs Throughout the Salt Marsh in Its Northern Range

Molecular Species Delimitation and Morphometry in the Melampus bidentatus (Panpulmonata, Ellobiidae) Cryptic Species Complex

The coffee bean snail Melampus bidentatus occurs in coastal salt marshes along the North American Atlantic and Gulf coasts, and the Caribbean. It was recently found that this large geographical span is actually occupied by a complex of three apparently cryptic species (preliminary called “North”, “South” and “Gulf”) with partially overlapping distributions. Until now it was not clear whether there are any morphological differences between the three species or which of the available names can be applied to which of the cryptic species. We used the already known distribution patterns of the cryptic species, as well as new barcode sequences to assign available names to the three cryptic species. We then compared morphological characters from 264 specimens using two approaches: an analyses based on 11 landmarks and another based on semi-landmarks of the entire shell outline. We were able to assign a nominal name to each of the three cryptic species: Melampus bidentatus for “North”, Melampus jaumei for “South” and Melampus gundlachi for “Gulf”. The morphometric analyses did not yield any diagnostic differentiating features; these cryptic species are hence diagnosible solely by genetic analysis, but may further phenotypically differ in some unseen internal features or in their physiology.

Population ecology of the snail Melampus bidentatus in changing salt marsh landscapes

We studied the population ecology of the snail Melampus bidentatus in relation to patch composition and landscape structure across several salt marsh systems in Connecticut, USA. These marshes have changed significantly over the past 40–50 years including loss of total area, increased areas of short Spartina alterniflora, and decreased areas and fragmentation of Spartina patens. These changes are consistent with tidal inundation patterns that indicate frequent flooding of high marsh areas. Melampus bidentatus densities were highly variable, both among different salt marsh systems and locations within specific marshes, but were generally similar among short Sp. alterniflora and Sp. patens patches within locations. Densities were lowest where the marsh was regularly inundated at high tide and only remnant Sp. patens patches remained. Almost no snails were found in bare patches. Areas that had large Sp. patens patches adjacent to short Sp. alterniflora supported the highest M. bidentatus densities. Population size-structure varied significantly among patch types, with higher proportions of large individuals in short Sp. alterniflora and hummocked Sp. patens patches than in large and remnant Sp. patens patches. This was likely due to size-selective predation and/or higher snail growth rates due to better food resource conditions in short Sp. alterniflora patches. Egg mass densities and the number of eggs per egg mass were highest in short Sp. alterniflora. Our results indicate that M. bidentatus is resilient to the level and patterns of salt marsh change evident at our study sites. Indeed, snail densities were significantly higher than reported in other field studies, suggesting that increased patch areas of short Sp. alterniflora and associated environmental conditions at our study sites may provide more favorable habitats than previously when marshes were dominated by extensive Sp. patens meadows. However, there may be threshold conditions that could overwhelm the ability of M. bidentatus to maintain itself within salt marsh systems where changes in hydrology, sedimentation and other factors lead to increased numbers of bare patches and ponds and loss of short Sp. alterniflora and Sp. patens. Studies of the responses of resident and transient fauna to salt marsh change are critically needed in order to better understand the implications for salt marsh ecosystem dynamics and services.

Diverse Dietary Responses by Saltmarsh Consumers to Chronic Nutrient Enrichment

We examined the effect of whole-ecosystem nutrient enrichment on herbivory in saltmarsh creek-wall habitats in the Plum Island Estuary (Massachusetts, USA). Located between the macrophyte-dominated high marsh and adjoining mudflats, creek walls are steep vertical habitats vegetated with productive filamentous algae and associated epiphytes. Annual nitrate and phosphate loading rates were increased approximately ×10–15 in creeks mimicking short-term (2-month) and chronic (6-year) eutrophication. We assessed the diets of epifaunal invertebrates (three gastropods and one amphipod species) that potentially graze on benthic algae using natural isotope abundance data and per capita grazing rate measurements derived from 13C prelabeled algae. Substantial dietary contributions from benthic algae were observed in all consumers even though previous research has indicated most rely on Spartina detritus as the principal food resource. The amphipod Orchestia grillus and the snail Melampus bidentatus grazed benthic algae in excess of 500 μg algal C g C−1 h−1, whereas the snail Nassarius obsoletus and hydrobiid snails grazed at lower rates. Few dietary changes were detected with short-term enrichment. Algal grazing rates of N. obsoletus and M. bidentatus increased with chronic enrichment probably as a functional response to increased algal productivity. O. grillus grazed at a high rate and parasitic infection did not affect its consumption of benthic algae. The abundance and frequency of occurrence of O. grillus on creek-wall habitats increased with chronic nutrient enrichment suggesting amphipods contribute to top–down control on benthic algae and slow algal growth as nutrient enrichment occurs.

Ecological partitioning among parapatric cryptic species

Geographic range differences among species may result from differences in their physiological tolerances. In the intertidal zone, marine and terrestrial environments intersect to create a unique habitat, across which physiological tolerance strongly influences range. Traits to cope with environmental extremes are particularly important here because many species live near their physiological limits and environmental gradients can be steep. The snail Melampus bidentatus occurs in coastal salt marshes in the western Atlantic and the Gulf of Mexico. We used sequence data from one mitochondrial (COI) and two nuclear markers (histone H3 and a mitochondrial carrier protein, MCP) to identify three cryptic species within this broad-ranging nominal species, two of which have partially overlapping geographic ranges. High genetic diversity, low population structure, and high levels of migration within these two overlapping species suggest that historical range limitations do not entirely explain their different ranges. To identify microhabitat differences between these two species, we modelled their distributions using data from both marine and terrestrial environments. Although temperature was the largest factor setting range limits, other environmental components explained features of the ranges that temperature alone could not. In particular, the interaction of precipitation and salinity likely sets physiological limits that lead to range differences between these two cryptic species. This suggests that the response to climatic change in these snails will be mediated by changes to multiple environmental factors, and not just to temperature alone.

Salt Marsh Restoration in Connecticut: 20 Years of Science and Management

AbstractIn 1980 the State of Connecticut began a tidal marsh restoration program targeting systems degraded by tidal restrictions and impoundments. Such marshes become dominated by common reed grass (Phragmites australis) and cattail (Typha angustifolia and T. latifolia), with little ecological connection to Long Island Sound. The management and scientific hypothesis was that returning tidal action, reconnecting marshes to Long Island Sound, would set these systems on a recovery trajectory. Specific restoration targets (i.e., pre‐disturbance conditions or particular reference marshes) were considered unrealistic. However, it was expected that with time restored tides would return ecological functions and attributes characteristic of fully functioning tidal salt marshes. Here we report results of this program at nine separate sites within six marsh systems along 110 km of Long Island Sound shoreline, with restoration times of 5 to 21 years. Biotic parameters assessed include vegetation, macroinvertebrates, and use by fish and birds. Abiotic factors studied were soil salinity, elevation and tidal flooding, and soil water table depth. Sites fell into two categories of vegetation recovery: slow, ca. 0.5%, or fast, more than 5% of total area per year. Although total cover and frequency of salt marsh angiosperms was positively related to soil salinity, and reed grass stand parameters negatively so, fast versus slow recovery rates could not be attributed to salinity. Instead, rates appear to reflect differences in tidal flooding. Rapid recovery was characterized by lower elevations, greater hydroperiods, and higher soil water tables. Recovery of other biotic attributes and functions does not necessarily parallel those for vegetation. At the longest studied system (rapid vegetation recovery) the high marsh snail Melampus bidentatus took two decades to reach densities comparable with a nearby reference marsh, whereas the amphipod Orchestia grillus was well established on a slow‐recovery marsh, reed grass dominated after 9 years. Typical fish species assemblages were found in restoration site creeks and ditches within 5 years. Gut contents of fish in ditches and on the high marsh suggest that use of restored marsh as foraging areas may require up to 15 years to reach equivalence with reference sites. Bird species that specialize in salt marshes require appropriate vegetation; on the oldest restoration site, breeding populations comparable with reference marshland had become established after 15 years. Use of restoration sites by birds considered marsh generalists was initially high and was still nearly twice that of reference areas even after 20 years. Herons, egrets, and migratory shorebirds used restoration areas extensively. These results support our prediction that returning tides will set degraded marshes on trajectories that can bring essentially full restoration of ecological functions. This can occur within two decades, although reduced tidal action can delay restoration of some functions. With this success, Connecticut's Department of Environmental Protection established a dedicated Wetland Restoration Unit. As of 1999 tides have been restored at 57 separate sites along the Connecticut coast.

Macroinvertebrate and fish populations in a restored impounded salt marsh 21 years after the reestablishment of tidal flooding.

During the last two decades, the State of Connecticut has restored tidal flow to many impounded salt marshes. One of the first of these and the one most extensively studied is Impoundment One in the Barn Island Wildlife Management Area in Stonington, Connecticut. In 1990, twelve years after the re-establishment of tidal flooding, the density of the marsh snail Melampus bidentatus, the numerically dominant macroinvertebrate of the high marsh, in Impoundment One was about half that in reference marshes below the breached impoundment dike. By 1999 the densities of Melampus above and below the dike were not significantly different, but the shell-free biomass was greater above the dike as a result of the somewhat larger number and size of the snails there. Twenty-one years after the renewal of tidal flooding, three marsh macroinvertebrates (the amphipods Orchestia grillus and Uhlorchestia spartinophila and the mussel Geukensia demissa) were significantly less abundant in the previously impounded marsh than in the reference marshes, whereas another amphipod (Gammarus palustris) was more abundant above the breached dike where conditions appeared to be somewhat wetter. In 1991 the fish assemblage in a mosquito-control ditch in Impoundment One was similar to that in a ditch below the breached dike; however, the common mummichog Fundulus heteroclitus appeared to be less abundant in the restoring marsh. By 1999 the number of mummichogs caught in ditches was significantly greater in Impoundment One than in the reference marsh, but the numbers of mummichogs trapped along the tidal creek were comparable above and below the dike. The results obtained in this study and those of other restoring marshes at Barn Island indicate the full recovery of certain animal populations following the reintroduction of tidal flow to impounded marshes may require up to two or more decades. Furthermore, not only do different species recover at different rates on a single marsh, but the time required for the recovery of a particular species may vary widely from marsh to marsh, often independently of other species.

Osmoregulation and FMRFamide-related peptides in the salt marsh snail Melampus bidentatus (Say) (Mollusca: Pulmonata).

The pulmonate snail Melampus bidentatus occupies the high intertidal zone of salt marshes in a nearly terrestrial environment. The hemolymph osmolarity of the snails collected in the field paralleled that of the adjacent water and was affected by the tides and precipitation. The snails initially gained or lost weight when submerged in hypo- or hyperosmotic media, respectively, but returned to their original weight after 24 h. The content of their immunoreactive (IR)-FMRFamide-Related Peptides (FaRPs) was measured in various tissues by radioimmunoassay, and IR-FaRPs were found in every tissue analyzed. The subesophageal part of the central nervous system (CNS) contained more IR-FaRPs than the supraesophageal part, and the kidney and the tissues of the reproductive tract contained more than other peripheral tissues. The levels of IR-FaRPs in the CNS, kidney, and hemolymph were higher in snails that were immersed in higher concentrations of seawater. Many IR neurons are present in all ganglia of the CNS except the pleural ganglia, and IR neurites are extensively distributed within the CNS and its connective tissue sheath. The visceral nerve from the visceral ganglion is immunoreactive and could be seen to innervate the kidney, which contains IR-varicosities. An osmoregulatory role for the FaRPs is suggested.

Ice formation and freezing damage in the foot muscle of the intertidal snail Melampus bidentatus

Freeze injury to the foot muscle of the intertidal pulmonate gastropod, Melampus bidentatus was examined by light microscopy using a technique which allowed fixation of the tissues at subzero temperatures. Exposure of snails to −5 or −10 °C (above the lower lethal temperature) resulted in no apparent damage to the foot muscle. Exposure of the snails to −13 °C (the lower lethal temperature), however, resulted in sloughing of the epithelium and possible loss of membrane material. Ice was present in the extracellular spaces of foot muscle which was fixed at −10 and −13 °C but not in tissues exposed to −5 °C. The percentage of the area occupied by the cells in muscle tissues decreased significantly when ice was present and increased to control levels upon warming in all cases except for the tissue exposed to −13 °C.

LOSS AND RECOVERY OF LOCOMOTOR BEHAVIOR AFTER CNS LESIONS IN THE SNAILMELAMPUS BIDENTATUS

To analyze neural pathways underlying the control of locomotion in the snail Melampus bidentatus, we lesioned connectives and commissures involved in locomotor control. The results suggest the presence of an independent oscillator in each pedal ganglion which is connected primarily via the pedal commissure. The bilaterally paired cerebropedal and cerebropleural connectives carry redundant information concerned with initiation of locomotion. In the 2-4 weeks after lesions were made, the snails usually regained locomotor coordination.

Alcian blue-alcian yellow mapping of neurosecretory cells in the central nervous system of the salt marsh pulmonate snail Melampus bidentatus

1. 1. The neurosecretory system of the primitive ellobiid Melampus bidentatus (Pulmonata: Basommatophora) was investigated using Alcian blue-Alcian yellow histochemistry. 2. 2. Putative neurosecretory cells within the central ganglia were distinguished by the criteria of cell size, position, and staining reaction. 3. 3. The cerebral ganglia, with attached lateral lobes, contained the greatest diversity of neurosecretory cell types (at least seven), inclucling single cells and cell clusters ranging from two to 40 cells. 4. 4. Four neurosecretory cell types were identified in the parietal and visceral ganglia, two in the pedal ganglia, and one each in the buccal and pleural ganglia. 5. 5. Neuroseeretory system homology among pulmonate gastropods is suggested by the close similarity of the Melampus AB/AY cell map to those reported in the literature for two limnic basommatophorans and for four terrestrial stylommatophorans.

Behavioral recovery associated with central nervous system regeneration in the snail Melampus

The pulmonate snail Melampus bidentatus regenerates central nervous tracts following commissurotomy, connective transection, and cerebral ganglion ablation. Our goal was to determine whether or not neural regrowth within the central nervous system restored behaviors disrupted by lesions. One behavior that is disrupted by commissurotomy is retraction of facial structures that are contralateral to a stimulated facial region, a response that normally accompanies the ipsilateral retraction. Tentacle withdrawal on the side contralateral to stimulation reappeared on a timescale that was correlated with growth of a commissural link (8-19 days post-lesion). Electrophysiological recordings from a labial nerve pathway that has a contralateral component similar to the contralateral tentacle response showed that development or strengthening of an alternative pathway could also mediate contralateral responses. Thus, a major conclusion of this study was that both tract regeneration and changes in existing CNS pathways can underlie recovery. The percentage (approx. 75%) of snails that regenerate the cerebral commissure and show behavioral recovery is established early in the period following commissure transection. Behavioral recovery and anatomical evidence of regeneration were also correlated in the other two operations: single cerebral ganglion removal and unilateral cerebropleural and cerebropedal connective transection. We conclude that Melampus is able to regenerate neuronal connectivity that can restore normal behavior.

Tidal migrations of the littoral salt marsh snail Melampus bidentatus Say

Quantitative field observations and experiments on populations of the snail Melampus bidentatus Say revealed tidal-related shifts in their distribution within the intertidal zone of a New England salt marsh. Adult M. bidentatus were found at lower marsh levels during periods of neap tidal cycles and moved to higher ground on days of spring tides. Juveniles did not participate in these periodic migrations and remained in the middle portion of the overall M. bidentatus range, often concealed under the stems and exposed roots of marsh grasses. Some adult snails, but not juveniles, also climbed grass stems during high tides, but did so irregularly and in variable numbers. Since only one-third of the climbers went high enough to avoid flood waters, this vertical migration is unlikely to serve essential respiratory or temperature control functions. Experiments on the locomotory behavior of snails in a laboratory chamber in which humidity and slope of crawling surface were controlled complemented field observations. Wet (high relative humidity, rH) conditions induced up-slope movement at greater speeds and lower turning rates than observed in dry (low rH) conditions. Partially dehydrated (10% loss of body water) snails showed strong geotactic responses (down-slope) but moved up-slope when placed in wet conditions. Juvenile snails exhibited no preferential movement in any chamber conditions but did reduce their crawling speeds and increase their turning rates in response to dry conditions. These combined field and laboratory studies emphasize that M. bidentatus adults respond strongly to specific environmental conditions with locomotory behavior that may help reduce the risk of potentially dehydrating exposure. As is discussed, M. bidentatus possesses a number of physiological capabilities for dealing with water stress. The behavior described herein adds a behavioral adaptation of use to this air-breathing snail in persisting as one of the few resident animals in the highly variable conditions of the salt-marsh intertidal zone.

Nitrogen, phenolic acids, and other feeding cues for salt marsh detritivores.

A number of feeding cues determine the palatability of detritus to detritivorous invertebrates. In salt marsh detritus the feeding cues include flavor provided by phenolics such as ferulic acid, pH, salinity, and nitrogen content. We examined the feeding responses to each cue by using experimental manipulations where we changed concentrations of these chemical cues. The palatability of detritus of the salt marsh grass Spartina alterniflora to the detritivorous snail Melampus bidentatus is reduced by increases in ferulic acid in the detritus. The acidity of the acid is partly responsible for inhibition of feeding, but other flavors of the ferulic acid are the major factor. Changes in salinity makes detritus more or less palatable to different species of detritivores. Available nitrogen confers greater palatability to detritus.In the field the feeding cues are all present simultaneously, and detritivores feed based on a hierarchy of cues. For M. bidentatus the presence of sufficient available nitrogen overwhelms the response to ferulic acid, the second most important cue. Salinity and pH, although used as discriminatory cues by themselves, are located lower in the hierarchy than nitrogen and ferulic acid.As detritus ages in the marsh, nitrogen content changes. Similarly, detritus from eutrophied environments shows enhanced nitrogen content. The differences in palatability of new and old detritus, and from eutrophic and non-eutrophic environments, suggests that detritivores respond not to total nitrogen but rather to the availabel nitrogen fractions. The increase in lignin found in old and eutrophic detritus may increase the binding of available nitrogen, and hence reduce the palatability of detritus.

Generation of new cerebral ganglion neurons in the snail Melampus: an ultrastructural study.

Reports in the literature have established that reconnection of central neural tracts occurs following commissurotomy and cerebral ganglion excision in the primitive pulmonate snail Melampus bidentatus and have suggested the possibility that long-term regeneration might result in the appearance of new neurons in the ganglion bud. We have used electron microscopy to examine the ganglion buds that form by reconnection of cerebral nerves, commissure, and connectives following cerebral ganglion excision in adult Melampus. The buds were examined from 2.5 to 12 months postoperatively. By 2.5 months, ganglion buds consist of a mixture of axon tracts that travel through the bud region and some dendritic processes; a few synaptic contacts can be identified at this stage, scattered throughout the bud. By 5--6 months, some of the most advanced ganglia have undifferentiated cells that are distinct from glia. By 7 months, differentiated neurons with clear, small dense-core or neurosecretory vesicles are present, although these cells are not all concentrated in a rind on the ganglion surface. Another cell type, the pigment-sheath cell, is present by this stage. By 11--12 months, the most advanced regenerating ganglia have neurons which form a cell rind on the ganglion surface. The gross appearance of a regenerated ganglion at this stage is similar to that of the intact contralateral cerebral ganglion, although the regenerated ganglion is smaller. One 12-month ganglion was found to possess fairly normal intraganglionic morphology, with lobes and cell types that were recognizable. Hence, nerve cell regeneration can occur in the absence of body part regeneration in adult members of one species of pulmonate snail.

Implanted cerebral ganglia produce supernumerary eyes and tentacles in host snails

AbstractIn the pulmonate snail Melampus bidentatus, cerebral ganglia from adult donors implanted into the hemocoel of adult host snails form connections with the host nervous system and periphery. In addition, approximately 50% of the implanted ganglia develop an eye on the surface of the ganglion, and approximately 25% of the host snails develop a supernumerary tentacle above the site where the implanted ganglion lies. The morphology of these sensory structures was examined by light and electron microscopy. Comparison of implant eyes with adult control eyes and regenerated eyes indicates that the same elements are present (cornea, lens, microvillar layer, pigment layer and cell layer), although implant eyes at four months and regenerated eyes at two months do not have pigment and microvillar layers as thick as the controls. The receptor cell structure is similar in all cases. The surface of the supernumerary tentacles is not as convoluted as that of control tentacles, but sensory cells and a neural link to the implant ganglion are present. An eye is often produced in the skin at the base of the supernumerary tentacle.

Latitudinal variation in freezing tolerance of Melampus bidentatus (Say) (Gastropoda:Pulmonata)

The limiting factor hypothesis was examined in relation to the latitudinal distribution of the marsh snail Melampus bidentatus (Say). Populations from Massachusetts and Delaware were more resistant to freezing than were the animals from South Carolina. Patterns of cold acclimation also differed among the populations. Mass mortality in local populations of Melampus from South Carolina was observed during the severe winter of 1976–1977 but not during the winter of 1977–1978. Transfer experiments showed that snails from South Carolina could not resist the winter in Massachusetts and had reduced survival compared to native animals in Delaware. Delaware animals survived in Massachusetts but were compromised in their reproductive potential. It is suggested that M. bidentatus is represented by at least two physiological races and the southern populations are limited in their range expansion at least in part by low temperature.

Water relations and physiological ecology of the salt marsh snail, Melampus bidentatus say

Field and laboratory studies were conducted on the water relations of the high intertidal snail Melampus bidentatus Say (Pulmonata: Ellobiidae) in the Little Sippewisset Marsh, Cape Cod (Mass., U.S.A.). Densities of Melampus in the upper 12% of the intertidal zone averaged about 1300 snails/m 2. A size gradient of snails was found, with large snails (5–12 mm shell length) occupying both upper and lower extremes of tidal height. Adult snails (>5 mm) undergo semilunar migrations, such that during neap tidal cycles the center of peak density shifts 2 m lower in the marsh from that of spring tidal cycles. Juvenile snails appear to remain in the dense vegetation of mid-range. During dry conditions in the field, snails contained 15% less body water compared with normally hydrated snails. Laboratory studies on desiccation (controlled temperature and humidity, still air) showed water loss to be inversely proportional to shell length and relative humidity (RH). Immature snails regulate water loss poorly; few survive for more than 12 h in 0–75% RH (20°C). In contrast, adults survive for 48 h or more in ⩽ 50% RH and remained alive in 97% RH for at least 14 days, despite the loss of up to 40% body water. Adult snails placed in ≈0% RH for 27–36 h were remarkably tolerant to desiccation, with an LD 50 near 80% body water loss. Melampus rehydrates rapidly when provided with free water; from a level of 50% body water loss, snails reached 90% normal water content within 0.75 h. Adult Melampus can survive submergence in sea water for at least 10 days and in fresh water for 3 days and can tolerate −12°C for several days. These studies demonstrate some of the physiological and behavioral adaptations to the rhythmicity of the tides and to the extremes in other physical factors found in the high intertidal zone. The significance of these adaptations in Melampus, and the fitting of its primitive, marine reproductive strategy to an amphibious lifestyle, are discussed in relation to the transitional status of this snail in the movement of molluscs from marine origins to land and freshwater habitats.

Effects of the organophosphorous insecticides temephos (Abate�) and chlorpyrifos (Dursban�) on populations of the salt-marsh snail Melampus bidentatus

The effects of 2 organophosphorous insecticides, temephos (Abate®) and chlorpyrifos (Dursban®), on populations of the salt-marsh snail Melampus bidentatus Say (Basommatophora: Melampidae) were investigated in a series of field experiments. These toxicants were aerially applied to mosquito breeding areas in New Jersey salt marshes in the manner normally used in biting-fly control operations. No significant change in the population density of M. bidentatus was observed in response to multiple treatments of granular formulations of either temephos or chlorpyrifos. However, there was a highly significant (P<0.01) depression in population density observed in the plot exposed to multiple treatments of a temephos emulsion. This depression was followed by a resurgence in density levels, beginning 3 weeks after the last treatment. The specific mechanism for thisperturbation is not known. However, it is hypothesized that it might involve some combination of indirect insecticide effects coupled with age-specific effects. Tests of age structure of the affected population using the chi-square distribution supported this hypothesis.

Predation by the salt marsh killifish Fundulus heteroclitus (L.) in relation to prey size and habitat structure: Consequences for prey distribution and abundance

Laboratory feeding preference experiments show that the maximum size of prey eaten (the snail Melampus bidentatus (Say) and the amphipod Orchestia grillus (Bosc)) increases with increasing size of the predator, Fundulus heteroclitus (L.). Melampus > 7 mm in shell height escape predation by even the largest killifish. In the laboratory, consumption of prey is reduced in high marsh habitat relative to low marsh, particularly in the case of larger fish. Low marsh has few grass stems per unit area, while high marsh is considerably more complex, with dense small stems providing cover for prey and reducing successful fish hunting. The population of Melampus in low marsh within Great Sippewissett salt marsh consists mainly of large snails but this size is rare in high marsh. The size-distribution is inverse for Orchestia, with large amphipods more abundant in high marsh. The construction of fences excluding Fundulus from the marsh surface led to low marsh size-distributions of Melampus and Orchestia resembling those of high marsh, in agreement with the laboratory results. Killifish predation seems to be an important factor regulating the abundance and size-distribution of the two prey species in the two marsh habitats.