Behavioral Responses and Interactions of Estuarine Animals With an Invasive Marsh Plant: A Laboratory Analysis

Invasion by Phragmites australis into tidal marshes previously dominated by Spartina alterniflora is viewed as a serious environmental threat along the Atlantic coast of the US, but little is known about the relative habitat value of the two plants for most estuarine species. This study was designed to investigate behavioral responses, in the laboratory, of three species to the two plants. Fiddler crabs, Uca pugnax, grass shrimp, Palaemonetes pugio, and larval mummichogs (killifish), Fundulus heteroclitus were introduced into aquaria with a bare area, an area with dead Phragmites stems, and an area with dead Spartina stems. All species distributed themselves equally between the Spartina and the Phragmites. The behavior of larval mummichogs in the tanks with predators was observed. In the presence and absence of stems, they utilized the surface of the water as a refuge, as well as the stems, when present. This behavior was equally as effective as being among the stems in promoting larval survival. In microcosms with blocks of marsh with living plants, fiddler crabs and grass shrimp again did not show a preference for either species of plant, while juvenile and adult mummichogs were not consistent. Small fish chose Spartina when in the small microcosm and had no preference in the large one. Large fish chose Spartina in the small microcosm and Phragmites in the large one. Predation by adult mummichogs on grass shrimp was comparable in Spartina and Phragmites microcosms, and predation on tethered shrimp was equivalent in adjacent Spartina and Phragmites marshes in the field.     

Coarse woody debris as a refuge from predation in aquatic communities

This study demonstrates experimentally that coarse woody debris (CWD) can provide refuge from predation in aquatic habitats. In the Rhode River subestuary of Chesapeake Bay, Maryland, (USA), we (1) measured the abundance of CWD, (2) examined the utilization of CWD by mobile epibenthic fish and crustaceans, and (3) tested experimentally the value of CWD as a refuge from predation. CWD was the dominant above-bottom physical structure in shallow water, ranging in size from small branches (<2 cm diameter) to fallen trees (>50 cm diameter). In response to experimental additions of CWD, densities of common epibenthic cpecies (Callinectes sapidus, Fundulus heteroclitus, Fundulus majalis, Gobiosoma bosc, Gobiesox strumosus, Palaemonetes pugio, and Rithropanopeus harrisii) increased significantly compared to control sites without CWD. In laboratory experiments, grass shrimp (P. pugio) responded to predatory fish (F. heteroclitus and Micropogonias undulatus) by utilizing shelter at CWD more frequently than in absence of fish. Access to CWD increased survivorship of grass shrimp in laboratory and field experiments. These experimental results (1) support the hypothesis, commonly proposed but untested for freshwater habitats, that CWD can provide a refuge from predation for epibenthic fish and invertebrates and (2) extend the recognized functional importance of CWD in freshwater to estuarine and marine communities. We hypothesize that CWD is an especially important refuge habitat in the many estuarine and freshwater systems for which alternative physical structure (e.g., vegetation or oyster reefs) are absent or in low abundance.     

Diel variation in predator abundance, predation risk and prey distribution in shallow-water estuarine habitats

Predation by visual predators is often affected by light conditions and may therefore exhibit strong diel variation. The dominant predators on grass shrimp, Palaemonetes pugio, are finfish predators that are thought to locate their prey by visual cues. We examined the response of grass shrimp to diel variation in predation risk in the nearshore shallow waters of the Chesapeake Bay. We used diel shoreline seines to assess the relative abundance of predators. We assessed the relative risk of predation with shrimp tethered at refuge (30 cm) and nonrefuge (60 cm) depths. To measure grass shrimp response to predation risk, we used dipnets to monitor habitat use. Four predominantly visual predators dominated the shoreline seine catches, Fundulus heteroclitus, Micropogonias undulatus, Morone americana and Morone saxatilis. Total predator abundance had a diel component, with dramatic nighttime decreases in total abundance, whereas guild composition and relative abundance remained unchanged. Relative predation risk for tethered shrimp exhibited significant time by habitat interaction. During the day, depth negatively affected survivorship of tethered shrimp while at night overall survivorship increased and there was no effect of depth. Shrimp habitats use reflected diel predation risks. Abundances in the near shore were highest during the day with decreased abundances at night. Together, the seine and tethering data highlight the importance for a refuge (e.g., shallow water) from predation during the daytime and a relaxation of predation pressure at night.     

Stage-structured interactions between seasonal and permanent residents of an estuarine nekton community

Estuarine assemblages of fishes and natant decapod crustaceans (i.e. nekton) comprise both permanent resident species and juveniles of coastal marine species that use estuaries primarily as nurseries. In an attempt to understand how the young of marine species successfully invade communities of permanent estuarine residents we studied potential interactions between two of the most abundant decapod crustaceans in nekton assemblages of the southeastern United States. Three years of quantitative samples from an intertidal marsh on Sapelo Island, Georgia showed that densities of the resident daggerblade grass shrimp Palaemonetes pugio were reduced during the time that juvenile white shrimp Penaeus setiferus used the estuary as a nursery. Results of a field enclosure experiment showed that white shrimp had no significant lethal or sublethal effects on adult grass shrimp. However, they did reduce survival of both juvenile and larval grass shrimp in laboratory experiments, suggesting the potential importance of a stage-dependent predatorprey interaction between the two shrimp species. The mortality rate of young grass shrimp in the presence of white shrimp was unaffected by grass shrimp density, but larvae (2.6–3.0 mm) suffered higher mortalities than did juveniles (5.0–15.0 mm). We suggest that the vulnerability of grass shrimp to predation by white shrimp is related to their molting cycle. The window of vulnerability opens more often for younger grass shrimp because they molt more frequently. When combined with losses due to other predators and competitors, the impact of early white shrimp cohorts on grass shrimp larvae and juveniles may prevent the resident species from maintaining its population at high densities, thereby freeing resources in the nursery for subsequent cohorts of juvenile white shrimp.     

Implications of a fluctuating fish predator guild on behavior, distribution, and abundance of a shared prey species: the grass shrimp Palaemonetes pugio

In some systems, the identity of a prey species' dominant predator(s) may not be constant over time. In cases in which a prey species exhibits different responses to various predator species, such changes in predator identity may have population-wide consequences. Our goals were to determine (1) whether mortality of and refuge use by the grass shrimp, Palaemonetes pugio, were predator-specific, and (2) how effects of prey size and habitat interacted with predator type. Striped bass (Morone saxatilis) exerted twice as much predation pressure as mummichog (Fundulus heteroclitus), although not equally as great on large (female) and small (male) shrimp. Mummichog, which fed preferentially on large shrimp, forced a partitioning of habitat between the two shrimp size classes. In contrast, large and small shrimp occupied similar habitats when subjected to striped bass, which fed on both size classes equally. Refuge use of grass shrimp depended on predator type. In the presence of mummichog, which occupied shallower depths in the water column than striped bass, shrimp stayed deep and close to structural habitat. Striped bass, which were deeper, caused shrimp to move high in the water column away from structural habitat. When both predators were present, shrimp distribution was similar to that when only striped bass were present, striped bass predation rate was enhanced, and overall mortality was higher than with either predator alone. Results suggest that at times when mummichogs are the dominant predators, large (female) shrimp experience higher predation than small (male) shrimp and are physically separated from their potential mates. When striped bass are more abundant, male and female shrimp may share a similar, shallow, less structure-oriented distribution and be subjected to higher mortality. When both predators are present, mortality rates may be higher still. This predator-, size-, and habitat-specificity of grass shrimp behavior suggests significant population and distribution consequences of fluctuating predator guilds and fluctuating cover of structural habitats in the field.     

Taxonomic diversity, origin, and conservation status of Bermuda killifishes (Fundulus) based on mitochondrial cytochrome b phylogenies

Sequence variation in the mitochondrialCytochrome b (Cytb) gene was assayed infour of six extant Bermuda killifishpopulations, representing two endemic species,to test taxonomic and phylogenetic hypothesesand reconstruct colonization history. Twodivergent (4.6%) haplotypes were detected; oneis identical to the Georgia 2c haplotype ofF. heteroclitus and is fixed in threeeastern populations: Lover's Lake (F.relictus), Mangrove Lake (F. bermudae),and Walsingham Pond. The second is fixed andrestricted to a western population, Evan'sPond. Likelihood and parsimony cytochrome btrees recognize a Bermuda Fundulus / F. heteroclitus clade in which the Evan's Pondhaplotype is basal. Phylogenies and haplotypedivergence indicate at least two Bermudacolonizations, the more recent involvingtransfer of the Georgia 2c haplotype. Enforcing Bermuda killifish monophyly, aspredicted from a single colonization event,does not increase tree length significantly;i.e., the trees also are consistent with asingle colonization. However, divergencebetween the Evan's Pond haplotype and the F. bermudae / F. relictus / Georgia 2chaplotype (4.6%) far exceeds the maximumdivergence among all F. heteroclitus,F. bermudae, and F. heteroclitushaplotypes (1.2%) and argues for independentcolonizations. Alternatively, recentintroduction of F. heteroclitus couldaccount for occurrence of the GA2c haplotype inBermuda but does not explain the presence ofthe genetically divergent Evans' Pondhaplotype. Cytb sequences areuninformative of the taxonomic status ofBermuda endemics, F. bermudae and F.relictus, but support recognition of theEvan's Pond population as an evolutionarilysignificant unit within the F. heteroclitus group.     

Fundulus as the premier teleost model in environmental biology: opportunities for new insights using genomics

A strong foundation of basic and applied research documents that the estuarine fish Fundulus heteroclitus and related species are unique laboratory and field models for understanding how individuals and populations interact with their environment. In this paper we summarize an extensive body of work examining the adaptive responses of Fundulus species to environmental conditions, and describe how this research has contributed importantly to our understanding of physiology, gene regulation, toxicology, and ecological and evolutionary genetics of teleosts and other vertebrates. These explorations have reached a critical juncture at which advancement is hindered by the lack of genomic resources for these species. We suggest that a more complete genomics toolbox for F. heteroclitus and related species will permit researchers to exploit the power of this model organism to rapidly advance our understanding of fundamental biological and pathological mechanisms among vertebrates, as well as ecological strategies and evolutionary processes common to all living organisms.     

Increased abundance of snails and trematode parasites of <Emphasis Type="Italic">Fundulus heteroclitus</Emphasis> (L.) in restored New Jersey wetlands

The Hackensack Meadowlands District is a large heavily degraded, brackish marsh system in the urbanized northeastern region of New Jersey, USA. Six study sites were used, three of which were restored (Mill Creek, Skeetkill Creek and Vince Lombardi), and three others were unrestored (Richard DeKorte Park, Cedar Creek and Kingsland Creek). Highly significant differences were found with respect to snail abundance and gill parasite abundance. In the three restored sites, significantly more Littoridinops tenuipes were found, and Fundulus heteroclitus had significantly more digenean trematode metacercariae gill infections than at unrestored sites. As habitat quality improves following restoration, the number of suitable digenean trematode parasite hosts multiplies as substrate for benthic invertebrates (first intermediate host) increases and usage by other species, such as Fundulus spp. (second intermediate host), is encouraged, which then attracts more wading birds (definitive host). Though the restoration process enhances trophic complexity, including primary consumers (gastropods), secondary consumers (fish) and tertiary consumers (wading birds), and ultimately parasite diversity, restoration also helps facilitate parasite life cycles.     

Interactions between the grass shrimp <Emphasis Type="Italic">Palaemonetes pugio</Emphasis> and the salt marsh grasses <Emphasis Type="Italic">Phragmites australis</Emphasis> and <Emphasis Type="Italic">Spartina alterniflora</Emphasis>

The grass shrimp Palaemonetes pugio, a species common to Spartina alterniflora-dominated marshes, may be sensitive to the invasion of the common reed Phragmites australis in northeastern US salt marshes. We examined two questions: (1) Do grass shrimp have a preference for the native plant over the non-native plant? (2) Are grass shrimp more effective foragers on P. australis? We tested the first hypothesis by comparing the amount of time shrimp spend in physical contact with the plant types over a 1-h period. Shrimp were observed under different arrangements of vegetation to control for differences in conspicuous structural features. Additionally, the amount of time shrimp spent foraging on S. alterniflora and P. australis shoots was compared to determine if shrimp graze more often on S. alterniflora. Shrimp spent significantly more time in contact with S. alterniflora only when plant types were grouped at opposite ends of aquaria, but did not exhibit a foraging preference for this plant type. To address our second question, we investigated the effects of shrimp foraging on stem epifauna, an assemblage of semi-aquatic invertebrates associated with macrophyte shoots. Previous research showed that P. australis supports a lower density of stem-dwelling epifauna relative to S. alterniflora. We hypothesized that the primary grazer of this community, P. pugio, can forage on P. australis stems more effectively due to structural differences between the two plants, causing the lower abundance of epifauna through top-down effects. We exposed individual shoots inhabited by epifauna to shrimp and compared faunal densities on exposed shoots to densities on control shoots after 18 h. The reduction of epifauna by predation was proportional on the two plant types. Therefore, top-down effects can be ruled out as an explanation for the patchy distribution of epifauna observed in P. australis–S. alterniflora marshes.     

Relative effects of turbidity and light intensity on reactive distance and feeding of an estuarine fish

Synopsis Gulf killifish Fundulus grandis were allowed to prey on daggerblade grass shrimp Palaemonetes pugio in clear water with bright light, turbid water containing bentonite clay, and clear water treatments where the light intensity was adjusted to match that in the bottom of the turbid tanks. Significantly fewer shrimp were consumed in the turbid tanks relative to the clear and shade treatments where predation rates did not differ significantly. The results suggested that the influence of suspended particles on predation rates was a consequence of light scattering and was not related to a decrease in light intensity. Reactive distances were subsequently determined for human observers viewing a small target in elongated tanks containing turbid water (7.3–60.5 NTU) under conditions of both low (8-10 E m^–2 s^–1) and high illumination (153–1249 E M^–2 s^–1). Reactive distance was primarily governed by turbidity while light intensity had little influence except at low turbidities. The shape of the relationship between reactive distance and turbidity for humans resembled curves reported for a variety of fish species.     

Temperature and kairomone induced life history plasticity in coexisting Daphnia

We investigated the life history alterations of coexisting Daphnia species responding to environmental temperature and predator cues. In a laboratory experiment, we measured Daphnia life history plasticity under different predation risk and temperature treatments that simulate changing environmental conditions. Daphnia pulicaria abundance and size at first reproduction (SFR) declined, while ephippia (resting egg) formation increased at high temperatures. Daphnia mendotae abundance and clutch size increased with predation risk at high temperatures, but produced few ephippia. Thus, each species exhibited phenotypic plasticity, but responded in sharply different ways to the same environmental cues. In Glen Elder reservoir, Kansas USA, D. pulicaria dominance shifted to D. mendotae dominance as temperature and predation risk increased from March to June in both 1999 and 2000. Field estimates of life history shifts mirrored the laboratory experiment results, suggesting that similar phenotypic responses to seasonal cues contribute to seasonal Daphnia population trends. These results illustrate species-specific differences in life history plasticity among coexisting zooplankton taxa.     

Morphological and behavioral antipredatory adaptations of decapod zoeae

Summary Zoeae of some species of estuarine decapods are retained in the estuary throughout development while others are exported into nearshore coastal waters. The horizontal migrations of decapod zoeae to coastal waters may have evolved to reduce the probability of encountering planktivorous fishes which are most abundant in the estuary. If so, then the morphological vulnerability of zoeae to fish predation should be inversely related to the number of predators occurring where they develop. Six species of estuarine decapod zoeae were offered to Menidia menidia and Fundulus heteroclitus. The behavioral interactions were observed to determine the prey's vulnerability to predation, and the mode of operation and relative effectiveness of their defenses. Feeding trials and behavioral observations both demonstrated that M. menidia 6–16 mm long preferred Uca minax and Callinectes sapidus zoeae, which are exported from the estuary, to Rhithropanopeus harrisii, Sesarma reticulatum and Palaemonetes pugio, which are retained within estuaries. Pinnotheres ostreum zoeae develop in the lower estuary and fish demonstrated an intermediate preference for the zoeae. Menidia menidia 20–40 mm long showed similar preferences for R. harrisii S. reticulatum, P. ostreum and U. minax as did small silversides. Large-mouthed demersal fish, Fundulus heteroclitus 6–10 mm long, also preferred U. minax to R. harrisii, but more readily preyed on zoeae than did M. menidia. The exported species of zoeae have shorter spines and smaller bodies than do retained zoeae, except P. ostreum which is small, spineless and passively sinks when attacked by fish. Other retained species of zoeae also have postcontact behavioral defenses which enhance the effectiveness of their morphological defenses. Zoeae do not evade attacks by fishes, but fishes quickly learned to avoid zoeae, which increases the effectiveness of the zoeae's antipredatory adaptations.     

Changes in Diet and Food Consumption of Largemouth Bass Following Large-scale Hydrilla Reduction in Lake Seminole, Georgia

Largemouth bass Micropterus salmoides Lacepède growth (in length) increased an average of 14% and bioenergetics modeling predicted a 38% increase in total annual food consumption following a large-scale reduction of hydrilla Hydrilla verticillata L.f. Royle in Spring Creek, a 2,343-ha embayment of Lake Seminole, Georgia. Coverage of submersed aquatic vegetation (SAV) declined from 76% to 22% in 1 year due to a drip-delivery fluridone treatment. In contrast, largemouth bass growth only increased an average of 4% and bioenegetics modeling predicted a 13% increase in total food consumption over the same time period in the Chattahoochee River embyament, where SAV coverage naturally declined from 26% to 15%. Diets were collected from a total of 4,409 largemouth bass over a 2.5-year period in the two embayments; the primary diet item (by weight) for largemouth bass in both embayments was sunfish (mostly Lepomis spp.). Diets before and after SAV reduction were generally similar for fish greater than stock-size (≥203 mm) in the Spring Creek arm; however, fewer invertebrates were consumed after SAV reduction. Low diet similarity was observed in smaller fish, caused by a decline in consumption of grass shrimp and sunfishes and an increase in use of damselflies, shiners Notropis spp., and topminnows Fundulus spp. after SAV reduction. Diets were similar between the same time periods for all sizes of fish in the Chattahoochee River arm. These results agreed with many laboratory results describing the effects of aquatic plant density on largemouth bass food consumption and growth, and demonstrated that increased predation efficiency resulting from decreased plant abundance was likely a stronger factor determining growth rates than any potential diet shift that may occur as a result in vegetation decline.     

Chaoborus and fish-mediated influences on Daphnia longispina population structure,dynamics and life history strategies

Summary This study examined the long term effects of predation by larvae of the midge Chaoborus and simulated fish predation on experimental Daphnia longispina populations. Chaoborus predation, relative to fish predation, led to populations composed of larger individuals as a whole, larger egg-bearing individuals, and a larger primiparous instar. Daphnia retained helmets beyond the first instar in response to the presence of Chaoborus. Both types of predation, relative to predator-free controls, reduced prey population size and rates of increase, but increased population death rates. The reduction in population size due to predation led to increased resource availability for individuals remaining in the populations and increased individual fecundity in the predation treatments. The differences noted between the Chaoborus, fish, and control treatments increased with predation intensity.     

Caridina nilotica in Lake Victoria: abundance, biomass, and diel vertical migration

Caridina nilotica (Decapoda: Atyidae) in offshore waters of Lake Victoria were investigated with both day and night sampling over a period of two years. Offshore populations are mainly planktonic rather than benthic, and the animals exhibit diel vertical migrations into near-surface waters at night. These changes in diel abundance as well as the size-frequency distribution of the migrating shrimp suggest that the migratory behavior is in response to visual planktivory, because only the very smallest individuals (2–4 mm) remain in surface waters during the day. During October 1992, abundances were estimated both by vertical net sampling and by underwater video transect methods. Concordance was established between abundances estimated by the two methods. Only about 9% (night) to 14% (day) of the Caridina population appeared to be epibenthic. We suggest that the behavior of the animal is consistent with the hypothesis that it is not a strict detritivore as previously reported; rather it may engage in facultative planktivory, especially at night.     

The performance of the leaf mining microlepidopteran Bucculatrix maritima (Stt.) on the salt marsh halophyte,Aster tripolium (L.), exposed to different salinity conditions

Summary The performance of phytophagous insects is influenced by the nutritional quality of the food plant, which may vary with environmental conditions. Hardly any information exists on food-plant mediated effects of variable soil salinity on the performance of phytophagous insects. Conspicuous differences in salinity levels, however, are found in soils of intertidal wetlands such as salt marshes and mangroves. The growth of larvae of Bucculatrix maritima, a leaf miner of the salt marsh halophyte Aster tripolium, was studied on the host plant along the salinity gradient of the Westerschelde estuary (S.W. Netherlands). In addition, its performance on A. tripolium grown on low or high salinity culture medium was investigated experimentally. Although salinity conditions significantly influenced the chemistry of the host plants, insect performance seemed almost unaffected, although near the mouth of the estuary high environmental salinities may have caused some inhibition of larval growth. The results contrast with our previous studies on the stem-borer Agapanthia villosoviridescens, which showed that growth and development was conspicuously influenced by the changing characteristics of Aster tripolium along the estuarine salinity gradient. The location-dependent qualities of halophytes in an estuary thus appear to have species-specific effects on insect performance. We hypothesize that this phenomenon contributes to the existence of non-identical distribution patterns of phytophagous insects associated with the same halophyte in an estuary.     

Predator/prey interactions: A link between the individual level and both higher and lower level effects of toxicants in aquatic ecosystems

Behavior can demonstrate linkages ofcontaminant effects at different levels oforganization from the biochemical/cellular to theorganism, population, and community levels.Mummichogs, Fundulus heteroclitus, from acontaminated area were previously found to havereduced condition, growth, and longevity, comparedwith conspecifics from clean sites. Thispopulation-level observation may be due to theirimpaired predator/prey behavior, which is associatedwith altered levels of serotonin in their brains. Theyare slow, less able to capture live food, and eat muchnon-nutritious detritus, which may contribute to theirdecreased growth and condition. They are also lessable to avoid being captured by blue crabs, which canalso contribute to their reduced life span andsize-structure. Grass shrimp, Palaemonetespugio, a major prey species, are not behaviorallyimpaired at this polluted site, but are more numerousand larger in size than at the reference site,probably due to reduced mummichog predation.     

Microsatellite primers for the Atlantic coastal killifish,Fundulus heteroclitus,with applicability to related Fundulus species

The mummichog (Fundulus heteroclitus), a common Atlantic coastal killifish, is a model vertebrate species for the study of molecular genetic variation in natural populations and of environmental toxicology. We report the development of a set of 20 microsatellite loci in this species. Average expected heterozygosity across all loci was 0.84 (range: 0.60–0.97), revealing a high level of variability at most loci. A survey of seven additional Fundulus species yielded one or two robust amplification products in over half (63%) of the species–primer combinations tested. Therefore, many of these loci will also prove useful in studies of other members of the genus Fundulus.     

Spatiotemporal variation in predispersal seed predation intensity

The effect of predispersal seed predation by Bruchus atomarius (Bruchidae, Coleoptera) on individual performance and population dynamics of the perennial forest herb, Lathyrus vernus (Leguminosae), was investigated in 11 permanent plots over 4 years. Seed predation and parameters describing intra-specific neighbour distance, plant size, inflorescence size, flowering phenology and current and previous herbivore damage were measured on all plants. In addition, demographic information from all plots was analysed using transition matrix population models in order to estimate the influence of seed predation on population growth rates. Predispersal seed predation rates differed significantly among years. Plot averages ranged from 0 to 83.7%. However, most of the variation occurred among individuals. Within individuals there was no consistency in predation rates among years. Exposure to herbivory, plant size and flowering phenology did not affect predation rates but individuals with larger inflorescences suffered from significantly higher predation. Seed predation in L. vernus was not influenced by neighbour distances of individual plants but it was positively correlated with the average density of seeds within plots, suggesting that seed predation is density dependent at the patch level. The reduction in population growth rate due to seed predation ranged from 0 to 7.6%. The sensitivity of population growth rate to reductions in seed production varied considerably among years and plots. This variation was mainly due to differences in the reproductive value of seeds and seedlings. The intensity of seed predation over the range found was not correlated with changes in population growth rate. The results of this study suggest that the influence of external factors, like seed predation, on population growth rate largely depends on the demographic transition rates in the investigated population.     

Induced defences in an endangered amphibian in response to an introduced snake predator

Introduced species have contributed significantly to the extinction of endemic species on islands. They also create new selection pressures on their prey that may result in modified life history strategies. Introduced viperine snakes (Natrix maura) have been implicated in the decline of the endemic midwife toad of Mallorca (Alytes muletensis). A comparison of A. muletensis tadpoles in natural pools with and without snakes showed that those populations subject to snake predation possessed longer tails with narrower tail fins but deeper tail muscles. Field and laboratory experiments showed that these changes in tail morphology could be induced by chemical and tactile cues from snakes. Populations of tadpoles that were subject to snake predation also displayed clear bimodal size-frequency distributions, with intermediate-sized tadpoles missing from the pools completely. Tadpoles in pools frequented by snakes developed faster in relation to their body size than those in pools without snakes. Variation in morphology between toad populations may therefore be caused by a combination of size-selective predation and tadpole plasticity. The results of this study indicate that the introduction of alien species can result in selection for induced defences, which may facilitate coexistence between predator and prey under certain conditions.