Amphipod Density Research Articles

On the diversity of amphipods inhabiting Sargassum as well as clear areas in Kuwait coastal waters, with an assessment of the effect of turbidity and notes on their abundance, composition, and distribution: a preliminary study

Abstract The distribution and abundance patterns of amphipods associated with Sargassum species were studied on a spatial scale along Kuwait’s coastal waters. Macroalgae, in particular Sargassum spp., are known to host a high biodiversity of marine fauna, particularly amphipods, when compared to non-Sargassum (“clear”) areas. Amphipods are an important component in the food web, being consumed by different stages of fish. Sargassum was sampled from four different stations in Kuwait’s waters to determine the amphipod species composition and density. Physicochemical parameters were measured at the time of sampling at each station, focusing on turbidity, as this can promote variation in the faunal distribution between areas, and it was the only non-biological factor showing a distinctive pattern between stations in this study. The objectives of this study thus were to describe the amphipod biodiversity in Sargassum communities and to measure the difference in diversity between Sargassum and clear areas, while concurrently taking into account the effect of turbidity. The most abundant amphipods represented were Pontocrates sp. (cf. arenarius (Spence Bate, 1858)), Ericthonius sp. (cf. forbesii Hughes & Lowry, 2006), Maxillipius rectitelson Ledoyer, 1973, and Podocerus mamlahensis Myers & Nithyanandan, 2016. Among the measured physicochemical parameters, only turbidity had a significant effect on the biodiversity of amphipods (, ) with a negative relationship based on the general linear model test. The effect of Sargassum and non-Sargassum areas on amphipod diversity was statistically not significant (, ). Also, there was no interaction between turbidity and Sargassum/clear areas (, ). However, the diversity was observed to be higher in Sargassum areas of all stations as compared to their clear counterparts, except in Khairan during January 2016, where the situation was the opposite. The abundance was highest in low turbidity, particularly at Salmyia station, compared to the high turbidity conditions at the stations at Failaka and Green Island. The species composition differed as well, the number of species found being the highest in the Sargassum area of Salmyia Station and the lowest at Failaka Island (highest turbidity). Only Salmyia Station (low turbidity) contained 27 species in its Sargassum area, of which only few were present at other stations in both areas. Turbidity affected the diversity of amphipods, and diversity was higher in Sargassum areas at all stations except Khairan in January 2016, but that difference was not significant.

Predatory behaviour of an invasive amphipod in response to varying conspecific densities under higher-order predation risk

Behavioural responses of invasive animals to biotic interactions can inform predictions of their consumptive impacts; however, such biotic contexts are often overlooked. Here, we assessed the interacting effect of conspecific and higher-order predation risk on the per capita consumption and behaviours of the invasive freshwater amphipod Gammarus pulex, using field microcosm and video-recorded lab experiments in Northern Ireland. Gammarus pulex exhibited higher per capita consumption in the presence of conspecifics, owing to reduced handling time of prey, regardless of fish presence and despite reduced swimming time and increased time spent physically interacting with each other. Consumption was lower in the presence of fish in the field, and handling time decreased with greater amphipod densities in the presence of fish cue in the lab. Our results show that impacts of G. pulex are independently influenced by conspecifics and predation risk, whereas handling time revealed an interacting effect of conspecific density and predation risk. Further assessments of the responses of invasive animals to biotic interactions could help explain variability in their impacts at local spatial scales.

Ingestion and fragmentation of plastic carrier bags by the amphipod Orchestia gammarellus: Effects of plastic type and fouling load

Inappropriate disposal of plastic debris has led to the contamination of marine habitats worldwide. This debris can be ingested by organisms; however, the extent to which chewing and gut transit modifies plastic debris is unclear. Detritivores, such as amphipods, ingest and shred natural organic matter and are fundamental to its breakdown. Here we examine ingestion and shredding of plastic carrier bags by Orchestia gammarellus. A laboratory experiment showed these amphipods shredded plastic carrier bags, generating numerous microplastic fragments (average diameter 488.59μm). The presence of a biofilm significantly increased the amount of shredding, but plastic type (conventional, degradable and biodegradable) had no effect. Subsequent field observations confirmed similar shredding occurred on the strandline. Rates of shredding will vary according to amphipod density; however, our data indicates that shredding by organisms could substantially accelerate the formation microplastics in the environment.

Bottom‐up control of parasites

AbstractParasitism is a fundamental ecological interaction. Yet we understand relatively little about the ecological role of parasites compared to the role of free‐living organisms. Bottom‐up theory predicts that resource enhancement will increase the abundance and biomass of free‐living organisms. Similarly, parasite abundance and biomass should increase in an ecosystem with resource enhancement. We tested this hypothesis in a landscape‐level experiment in which salt marshes (60,000 m2 each) received elevated nutrient concentrations via flooding tidal waters for 11 yr to mimic eutrophication. Nutrient enrichment elevated the densities of the talitrid amphipod, Orchestia grillus, and the density and biomass of its trematode parasite, Levinseniella byrdi. Strikingly, L. byrdi prevalence increased over time, up to 13 times higher in nutrient‐enriched marshes (30%) relative to the mean prevalence in reference marshes (2.4%). The biomass density of infected amphipods was, on average, 11 times higher in nutrient‐enriched marshes (1.1 kg/ha) than in reference marshes (0.1 kg/ha), when pooling across all years. Orchestia grillus biomass comprises 67% of the arthropod community biomass; thus, nutrient enrichment elicits a substantial surge in parasitized biomass in the arthropod community. If our results are typical, they suggest that eutrophication can increase parasite abundance and biomass with chronic resource enhancement. Therefore, minimizing aquatic nutrient pollution may prevent outbreaks of parasites with aquatic hosts.

Habitat fragmentation has some impacts on aspects of ecosystem functioning in a sub-tropical seagrass bed

Habitat fragmentation impacts ecosystem functioning in many ways, including reducing the availability of suitable habitat for animals and altering resource dynamics. Fragmentation in seagrass ecosystems caused by propeller scarring is a major source of habitat loss, but little is known about how scars impact ecosystem functioning. Propeller scars were simulated in seagrass beds of Abaco, Bahamas, to explore potential impacts. To determine if plant-herbivore interactions were altered by fragmentation, amphipod grazers were excluded from half the experimental plots, and epiphyte biomass and community composition were compared between grazer control and exclusion plots. We found a shift from light limitation to phosphorus limitation at seagrass patch edges. Fragmentation did not impact top-down control on epiphyte biomass or community composition, despite reduced amphipod density in fragmented habitats. Seagrass and amphipod responses to propeller scarring suggest that severely scarred seagrass beds could be subject to changes in internal nutrient stores and amphipod distribution.

Additive effects of temperature and infection with an acanthocephalan parasite on the shredding activity of Gammarus fossarum (Crustacea: Amphipoda): the importance of aggregative behavior.

Climate change can have critical impacts on the ecological role of keystone species, leading to subsequent alterations within ecosystems. The consequences of climate change may be best predicted by understanding its interaction with the cumulative effects of other stressors, although this approach is rarely adopted. However, whether this interaction is additive or interactive can hardly be predicted from studies examining a single factor at a time. In particular, biotic interactions are known to induce modifications in the functional role of many species. Here, we explored the effect of temperature on leaf consumption by a keystone freshwater shredder, the amphipod Gammarus fossarum. This species is found at high densities in the wild and relies on aggregation as an antipredator behavior. In addition, gammarids regularly harbor acanthocephalan parasites that are known to induce multiple effects on their hosts, including modifications on their functional role. We thus assessed the cumulative effect of both intraspecific interactions and parasitism. Consumption tests were conducted on gammarids, either naturally infected with Pomphorhynchus tereticollis or uninfected, feeding alone or in groups. Our results show that increased temperatures induced a significant increase in consumption, but only to a certain extent. Interestingly, consumption at the highest temperature depended on amphipod density: Whereas a decrease was observed for single individuals, no such effect on feeding was observed for individuals in groups. In addition, infection by acanthocephalan parasites per se significantly negatively impacted the shredding role of gammarids. Overall, the combined effects of parasitism and temperature appeared to be additive. Thus, future studies focusing on the impact of climate change on the functional role of keystone species may benefit from a multimodal approach under realistic conditions to derive accurate predictions.

Methane seepage effects on biodiversity and biological traits of macrofauna inhabiting authigenic carbonates

Authigenic carbonate rocks at methane seeps are recognized as hosting diverse and abundant invertebrate assemblages, with potential forcing from fluid seepage and hydrography. Mensurative studies of carbonate macrofauna (>0.3mm) at Hydrate Ridge, OR revealed little effect of water depth and overlying oxygenation (at 600m and 800m) but a large influence of seepage activity on density, taxonomic composition, diversity, and biological traits (feeding, lifestyle, motility, size and calcification). Rocks exposed to active seepage had 3–4× higher total macrofaunal densities than under inactive conditions. Assemblages exhibited higher species richness and reduced evenness (greater dominance) under active seepage than inactive conditions, but no difference in H′ or rarefaction diversity. Actively seeping sites were characterized by errant (motile), bacterial grazing, small- and medium-sized, heavily calcified species, whereas inactive sites exhibited a greater diversity of feeding modes and more burrowers, sessile, large and lightly calcified species. Active rocks supported more exogonid (Syllidae), ampharetid, and cirratulid polychaetes, provannid snails, pyropeltid limpets, nemerteans, and sponges; whereas inactive rocks supported higher densities of ophiuroids, isopods, gammarid amphipods, hydroids, Typosyllis (Syllidae) and tanaids. Transplant experiments, in which rocks were transferred between active and inactive sites at Hydrate Ridge North (600m), revealed that assemblages respond within 13months to increase or cessation of seepage, taking on the feeding, size and calcification characteristics of the background fauna at the new site. Lifestyles and motility patterns shifted more slowly as the sessile, attached species did not track seepage as quickly. Provannid snails and pyropeltid limpets rapidly colonized rocks transplanted to active sites and disappeared when transplanted to inactive sites. Given the known variability of fluid fluxes and rapid community response, a mosaic of communities changing in space and time is hypothesized to generate the relatively high species diversity at methane seeps.

Effects of fish predation on Posidonia oceanica amphipod assemblages

Amphipod assemblages that inhabit Posidonia oceanica seagrass meadows are potentially relevant trophic resources for ichthyofauna. However, the effects of fish predation on amphipod assemblages in this system have received little attention. To address this gap in knowledge, experimental manipulations of predation intensity (exclu- sion and inclusion cages) were conducted at two sites in a Mediterranean marine protected area, where different lev- els of fish predation were expected to occur. We found that in the absence of predatory fishes (exclusion cages), total amphipod density and biomass were higher than in uncaged areas and partially controlled cages. At the species level, Caprella acanthifera and Iphimedia minuta responded to caging with increased abundance, while in most cases dif- ferent species did not exhibit differences in density or bio- mass between treatments. The presence of one enclosed labrid fish predator (inclusion cages) resulted in a lower density and biomass of Aora spinicornis and a lower bio- mass of Phtisica marina, although total amphipod density

Contrasting patterns of amphipod dispersion in a seagrass meadow between day and night: consistency through a lunar cycle

ABSTRACTChanges in light intensity, typically over the course of a day, affect the dispersion of aquatic organisms at short temporal scales. Amphipods, for example, have strong behavioural responses to light conditions. In this study, we used amphipod assemblages inhabiting a Cymodocea nodosa seagrass meadow on the east coast of Gran Canaria Island (eastern Atlantic) to test whether short-term dispersion of seagrass-associated amphipods differed between day and night, testing the consistency throughout an entire lunar cycle. Replicated artificial seagrass units were deployed, and subsequently retrieved, during the day (from 8:00 am to 18:00 pm) and the night (from 18:00 pm to 8:00 am) on three consecutive days within each of the four moon phases of a complete lunar cycle. We collected 13,467 amphipods corresponding to 32 species and 17 families. Significantly larger abundances of amphipods were collected during the night through the entire moon cycle. The total abundance of amphipods was also affected by the moon phases; under full moon, larger abundances of amphipods dispersed into the artificial seagrass units followed by the third quarter, the new moon and the first quarter. The species density of amphipods per unit followed the same pattern. In conclusion, the short-term dispersion of amphipods living in a seagrass meadow was considerably greater during the night than the day, while dispersion of amphipods was more intense under full moon.

Amphipods as potential prey of the Asian shore crab Hemigrapsus sanguineus: Laboratory and field experiments

The invasive crab Hemigrapsus sanguineus has a generalist diet, consuming primarily macroalgae and sessile and sedentary invertebrate prey. To assess the dietary importance of motile prey, predation by H. sanguineus on the amphipod Hyale plumulosa was examined in laboratory and field experiments. Male and female crabs 7–27mm in carapace width were offered H. plumulosa in laboratory microcosms with and without sediment (sand and rocks) present. Despite sex differences in claw morphology, male and female H. sanguineus consumed similar numbers of amphipods. Large crabs of both sexes consumed fewer H. plumulosa than small crabs, and consumption was lower when sediment was present. Crab abundance was then manipulated in the field in randomized complete block experiments in the lower rocky intertidal zone. In three 14-day experiments, H. sanguineus had no negative effect on amphipod abundance. In fact, excluding crabs from cages led to lower amphipod densities. The results of the 2-species predator–prey laboratory experiments did not transfer to the natural environment, suggesting that complex interactions exist between H. sanguineus and motile amphipod prey in rocky intertidal areas.

Effects of invasive European frogbit and its two physical control methods on macroinvertebrates

European frogbit (Hydrocharis morsus-ranae L.) is an invasive floating plant that negatively affects native plants in freshwater ecosystems. We conducted a field mesocosm experiment in Oneida Lake, New York (USA), to study the effect of frogbit cover on macroinvertebrates and to test the effects of 2 physical control methods, hand pulling and shading, on macroinvertebrate assemblages. We used Hester–Dendy samplers and Ekman grabs to collect surface/phytophilous and benthic macroinvertebrates. Metrics including density of different types of organisms, total abundance, taxon richness, and Simpson’s diversity index were compared between control methods. Frogbit cover had significant positive effects on mollusk density, amphipod density, taxon richness, and Simpson’s diversity in the Hester–Dendy surface samples. Sites with frogbit had significantly fewer benthic worms and more chironomids (Hester–Dendy bottom samples) and possibly higher diversity of benthic macroinvertebrates (Ekman samples). These data suggested that frogbit had positive effects on surface and phytophilous macroinvertebrates and may change population density and increase diversity of benthic macroinvertebrates. Hand pulling and shading did not affect macroinvertebrates except that amphipod density in surface water was significantly lower in shading sites. Therefore, from the perspective of minimizing effects on the macroinvertebrate communities in the lakes, hand pulling is preferable over shading for controlling frogbit.

Temporal variation of the gammaridean fauna (Crustacea, Amphipoda) associated with the sponge Mycale angulosa (Porifera, Demospongiae) in southeastern Brazil

Marine sponges are advantageous microhabitats because of their complex architecture. The system of internal canals provides circulation of water and deposition of particulate organic matter, ensuring availability of food and shelter. Diminutive amphipods have little difficulty penetrating the spaces of sponges and remain in their aquiferous systems as one of the most abundant taxa in this association. This study evaluated the temporal variation of the gammaridean amphipod species associated with the sponge Mycale angulosa. Sponge samples were collected every three months over one year at Pontal da Cruz Beach, Sao Sebastiao Channel, southeastern Brazil. The amphipod assembly varied over time, while the amphipod density and sponge biomass remained approximately constant. Six species contributed to the temporal variation infaunal composition, highlighting the importance of the natural history of each species.

The Influence of Stickleback on the Accumulation of Primary Production: a Comparison of Field and Experimental Data

Humans are currently altering aquatic ecosystems through overfishing and nutrient loading. This is altering ecosystems through top-down and bottom-up processes. In the Baltic Sea, the removal of top predators has increased the density of mesopredators, while eutrophication has boosted primary production. Using a mesocosm experiment, we show that a high density of a mesopredator—the threespine stickleback Gasterosteus aculeatus—increases the biomass of primary production not transferred to higher trophic levels under simplified ecosystem conditions. However, no effect of stickleback density on algae biomass is detected in the field, although stickleback gut inspection reveals that stickleback are feeding on amphipods, and stickleback density tends to correlate negatively with amphipod density in the field. This suggests that trophic cascades induced by the mesopredator release are attenuated in the field and do not reach primary producers. This is probably caused by the complexity of the ecosystem where many processes regulate the food web, such as bottom-up effects, the presence of alternative prey and density-dependent predation and distribution. However, the degree to which the ecosystem will be able to buffer further changes, if human disturbances continue, is unknown. Trophic cascades that reach primary producers could have drastic consequences for the ecosystem by promoting the accumulation of drifting algal mats that alter the benthic habitat.

Species richness and distributions of boreal waterbird broods in relation to nesting and brood‐rearing habitats

ABSTRACTIdentification of ecological factors that drive animal distributions allows us to understand why distributions vary temporally and spatially, and to develop models to predict future changes to populations–vital tools for effective wildlife management and conservation. For waterbird broods in the boreal forest, distributions are likely driven by factors affecting quality of nesting and brood‐rearing habitats, and the influence of these factors may extend beyond singles species, affecting the entire waterbird community. We used occupancy models to assess factors influencing species richness of waterbird broods on 72 boreal lakes, along with brood distributions of 3 species of conservation concern: lesser scaup (Aythya affinis), white‐winged scoters (Melanitta fusca), and horned grebe (Podiceps auritus). Factors examined included abundance of invertebrate foods (Amphipoda, Diptera, Gastropoda, Hemiptera, Odonata), physical lake attributes (lake area, emergent vegetation), water chemistry (nitrogen, phosphorus, chlorophyll a concentrations), and nesting habitats (water edge, non‐forest cover). Of the 5 invertebrates, only amphipod density was related to richness and occupancy, consistently having a large and positive relationship. Despite this importance to waterbirds, amphipods were the most patchily distributed invertebrate, with 17% of the study lakes containing 70% of collected amphipods. Lake area was the only other covariate that strongly and positively influenced species richness and occupancy of scaup, scoters, and grebes. All 3 water chemistry covariates, which provided alternative measures of lake productivity, were positively related to species richness but had little effect on scaup, scoter, and grebe occupancy. Conversely, emergent vegetation was negatively related to richness, reflecting avoidance of overgrown lakes by broods. Finally, nesting habitats had no influence on richness and occupancy, indicating that, at a broad spatial scale, brood distributions are largely driven by the presence of quality brood‐rearing lakes, not nesting habitats. Our findings are relevant to generating conservation plans or management goals; specifically, boreal lakes with abundant amphipods and surface areas >25 ha are important habitat for waterbird broods and merit conservation, especially given the patchy distribution of amphipods. Moreover, these high quality brood‐rearing lakes are much rarer, and thus more constraining, than are quality nesting habitats, which are likely abundant in the boreal. © 2015 The Wildlife Society.

Disturbance-related patterns in unstable rocky benthic habitats of the north-eastern Baltic coast

Distinct patterns of benthic flora and fauna are produced when unstable rocky substrata are disturbed by water motion. This study investigated occurrence of disturbance-related patterns in benthic boulder habitats on the north-eastern Baltic coast. Sessile assemblages, mostly algae on tops of boulders and barnacles and bryozoans underneath, were found to differ between small (50–150 cm2 upperside surface area) and large (200–800 cm2) boulders. Densities of motile gammaridean amphipods were negatively correlated with boulder size. These patterns may be due to small boulders being displaced by water motion more frequently than large ones. Most of the barnacle shells/tests were remnant empty ones, and the proportion of empty tests and living barnacles was similar underneath small and large boulders, suggesting that degradation/removal of empty tests was not influenced by disturbance associated with boulder size. There was no consistent evidence of algae being affected by boulder size, but the tops of boulders had less algae than the edges, a pattern that is typically associated with high rates of overturning. Confirmation of disturbance as having caused these patterns and thus being an important process for structuring benthic biota in this region could be achieved by further manipulative experimentation.

The role of epiphytic algae and different species of Sargassum in the distribution and feeding of herbivorous amphipods

The presence of particular algal species can directly influence the distribution and abundance of mesoherbivores. The aim of this study was to answer the following questions: Do Sargassum beds of different localities show variation in the load of epiphytic algae? Do Sargassum beds of different localities show variation in the density of herbivore amphipods? Does the density of these amphipods in the same algal bed vary with epiphyte load? Are species of Sargassum and their epiphytes equally consumed by amphipods? The collections were carried out in three infralitoral algal beds in southeastern Brazil. Fronds of Sargassum were collected at each algal bed and the associated ampithoid and hyalid amphipods were separated, identified and counted. Sargassum fronds and epiphytes were identified, dryed and weighted. Experiments were carried out to determine the consumption of Sargassum species and epiphytes by amphipods. The weight of the epiphytes Hypnea musciformis and Dyctiopteris delicatula and the density of amphipods were different among sampling sites. There was a relationship between amphipod density and epiphyte weight at two sampling sites. The four amphipod species tested consumed algae, but showed different patterns of consumption. Despite consuming the epiphytes, the ampithoids preferentially feed on the three species of Sargassum , while Hyale nigra had a preference for Hypnea musciformis .

Some life-history parameters of the non-native amphipod Platorchestia platensis (Talitridae) in a warm temperate South African estuary

Density, population structure, growth, mortality and aspects of reproduction of the non-native amphipod Platorchestia platensis were studied for 20 consecutive months (October 2008 to May 2010) in the supralittoral wrack of the Knysna Estuary, South Africa. Amphipod density varied over the sampling period with the lowest numbers recorded in summer (January and February). Ovigerous females with embryos, and juveniles were found in most months with peaks in recruitment found in both April and October, suggesting that in Knysna this species is a biannual breeder. Average female size was significantly greater in winter, with larger females tending to brood more embryos. The largest adults (13.5 mm total length) were always male although the monthly sex ratio was nearly always female biased. Growth rate estimated from a cohort analysis was about 1 mm per month and monthly survival about 69%. The ability to reproduce all year round may be one reason why this introduced species has become established within this warm temperate estuary.

Amphipods (Amphipoda, Gammaridea) at the Gorevoy Utes oil and methane seep, Lake Baikal

We examined amphipods collected from aboard the deepwater manned submersibles “Mir-1” and “Mir-2” at oil and methane seeps near Cape Gorevoy Utes (Central Baikal). Most of the 23 taxa registered inhabit the entire Lake Baikal within a wide depth range. Two species of the genus Echiuropus are probably new. Leptostenus leptocerus (Dybowsky, 1874), that, until now, was regarded as a rare species, is likely to be an indicator for methane and oil discharge areas. Additional data on the morphology of this species were obtained; cuticular sensory (lateral line organs) and nonsensory microstructures were examined by scanning electron microscopy. The density of nectobenthic amphipods was 5-6 times higher on bitumen mounds than that at neighbouring sites with flat bottom. Maximum density of benthic amphipods was recorded on bacterial mats. No amphipods were observed at the bottom with large oil patches. High density of amphipods at the oil-methane seep is attributed to highly structured habitats and food abundance.

Amphipod control of epiphyte load and its concomitant effects on shoalgrass Halodule wrightii biomass

We investigated the interaction between epiphyte-grazer abundance and eutrophi- cation to assess the relative importance of top-down and bottom-up effects in subtropical seagrass meadows. In field experiments using a cageless technique to control amphipod abundance, we measured the effects of grazing and nutrient supply on the growth and productivity of shoalgrass Halodule wrightii and its epiphytes at both protected and wave exposed sites. Amphipod removal at the protected site resulted in 70% greater epiphyte loads on shoalgrass leaves and a 36% reduction in leaf biomass after 10 wk. At the wave-exposed site, where amphipod abundance was consistently low, we found no significant effects of grazer presence or nutrients on epiphyte load or leaf biomass. Average leaf length, however, was significantly reduced in nitrogen-enriched plots. Our results indicate that natural densities of amphipods can reduce seagrass loss by control- ling epiphytes.

Influence of Intra- and Interspecific Interactions on Short-term Movement of the Amphipod Corophium volutator in Varying Environmental Conditions

Movement of animals on intertidal mudflats can have profound consequences for population and community dynamics. Such movement is often influenced by biotic interactions, but the abiotic environment can also be important, affecting the strength of species’ interactions and even changing their outcome. Here, we focused on two biotic interactions and examined their effect on movement of the burrow-dwelling amphipod Corophium volutator: intraspecific interactions (specifically, the effect of conspecific density) and interspecific interactions with a competitor–predator, the mud snail Ilyanassa obsoleta (=Nassarius obsoletus). We assessed the magnitude of the temporal variation and examined the influence of pertinent abiotic variables (lunar cycle, presence of tide pools). Despite substantial temporal variation in movement levels, density of conspecifics did not influence emigration or immigration rates of C. volutator. Mud snails negatively affected immigration of C. volutator, and this was consistent across all lunar phases. However, mud snails influenced mortality and emigration of C. volutator in a contrasting way relative to lunar phases; sometimes amphipods evaded snails and survived, but at other times did not emigrate and were killed. The presence of tide pools did not change the nature or strength of the snail–amphipod interaction. Our results further clarify the complex interactions between C. volutator and I. obsoleta and suggest that resources on the mudflat are sufficient to sustain high densities of amphipods in a density-independent manner. Further, they highlight the importance of considering multiple environmental variables when examining interspecific interactions.