Mate Choice by Chemical Cues in Western Redback (Plethodon vehiculum) and Dunn's (P. dunni) Salamanders

Western redback and Dunn's salamanders (Plethodon vehiculum and Plethodon dunni, respectively) can distinguish between potential mates by using chemical cues. In laboratory choice tests, adult males of both species showed significant discrimination between chemical cues of gravid females over non-gravid females of equal body size. Furthermore, males of both species differentiated the odour of paired gravid females that differed by ? 5 mm snout-vent length (SVL). Given that clutch size is related to female body size in these species, adult males may be able to distinguish between females via cues that signal potentially high female reproductive success. In choice tests, P. vehiculum females did not discriminate between two relatively large males that differed by ? 5 mm SVL. However, females of P. vehiculum did discriminate between two relatively small males that differed by the same amount. Apparently, P. vehiculum females ranked males by both absolute and relative body size using chemical cues. This pattern could reflect a female preference for large males or that females avoid mating with the smallest males.     

Which Factors Affect Sex Ratio of Spined Loach (genus Cobitis) in Lake Müggelsee?

The distribution and extent of chemical alarm signaling systems among some families of fishes, including the Cottidae, remains unclear. In laboratory experiments, we tested whether reticulate sculpins, Cottus perplexus, respond to chemical alarm signals released by injured conspecifics. Sculpins decreased movement following exposure to skin extracts from conspecifics, but did not respond to cues of syntopic speckled dace, Rhinichthyes osculus, or allotopic swordtails, Xiphophorous helleri. Additional tests demonstrated that the responses of sculpins to alarm cues were dependent on the hunger level of the test fish. Sculpins deprived of food for 2 days failed to respond to conspecific alarm cues, however, the same individuals fed to satiation did respond to alarm cues.     

Underwater Video Reveals Strong Avoidance of Chemical Alarm Cues by Prey Fishes

A diversity of fishes release chemical cues upon being attacked by a predator. These cues, commonly termed alarm cues, act as sources of public information warning conspecifics of predation risk. Species which are members of the same prey guild (i.e. syntopic and share predators) often respond to one another's alarm cues. The purpose of this study was to discriminate avoidance responses of fishes to conspecific alarm cues and cues of other prey guild members from responses to unknown damaged fish odours and novel odours. We used underwater video to measure avoidance responses of freshwater littoral species, namely fathead minnows (Pimephales promelas), finescale dace (Chrosomus neogaeus), and brook stickleback (Culaea inconstans), to both injured fish cues and novel non‐fish odours. The cyprinids (minnows and dace) showed significant avoidance of minnow cues over swordtail cues, morpholine, and the control of distilled water and tended to avoid fathead cues over cues of known prey guild members (stickleback). Cyprinids also significantly avoided cues of stickleback over unknown heterospecific cues (swordtail) and tended to avoid stickleback cues over morpholine and the distilled water control. Stickleback significantly avoided fathead minnow extract over the distilled water and tended to avoid stickleback and swordtail over distilled water. We conclude that fishes in their natural environment can show dramatic changes in behaviour upon exposure to alarm cues from conspecifics and prey guild members. These responses do not represent avoidance of cues of any injured fish or any novel odour.     

Faecal pellets as burrow markers: intra- and interspecific odour recognition by western plethodontid salamanders

Chemical communication is widespread among plethodontid salamanders (Caudata: Plethodontidae), but little information exists on the role of odours in interspecific competition in general and among species in western North America in particular. The present study examined the potential of faecal odours for intra- and interspecific communication among sympatric plethodontids in two-choice tests in the laboratory. In experiment 1, Plethodon vehiculum and Aneides ferrus from Vancouver Island, B.C., Canada, were presented with two burrows marked with (1) a faecal pellet of male P. vehiculum versus a control clay pellet, and (2) a faecal pellet of male A. ferreus versus a control pellet. In experiment 2, responses of P. vehiculum, P. dunni and P. vandykei from Washington, U.S.A., were tested towards faecal pellets of conspecific males and females, and towards pellets of congeneric males. Both P. vehiculum and P. dunni distinguished odours of conspecific males and females, based on their behavioural responses towards faecal and control pellets. Only P. dunni, however, avoided burrows marked with faeces of conspecific individuals. In contrast, neither A. ferreus nor P. vandykei appeared to distinguish faecal odours of conspecific salamanders from control pellets. In tests with odours of heterospecific individuals, P. dunni and P. vehiculum distinguished odours of each other, and P. vandykei distinguished odours of P. vehiculum. The data from these two experiments suggest that P. dunni use faecal pellets as territorial markers both in intraspecific communication and in interspecific encounters with P. vehiculum.     

Tests of aggregative preferences of wandering salamanders (Aneides vagrans)

Plethodontid salamanders usually exhibit territorial spacing patterns and behaviors. Previous works suggest that plethodontids from Vancouver Island, Canada lack strong territoriality. In one Vancouver Island species, Aneides vagrans, salamanders may even be found in the field in small aggregations. We investigated preferences of A. vagrans for occupied vs unoccupied cover objects and for cover objects occupied by conspecifics of different age/size classes. We also tested A. vagrans preferences for conspecific vs heterospecific salamanders in an attempt to determine if the lack of territoriality by A. vagrans is species-specific and perhaps related to habitat segregation. A. vagrans did not avoid occupied cover objects or exhibit a preference for occupied vs unoccupied sites. Age class did not affect cover object choice. A. vagrans did not avoid heterospecifics that exhibit the highest degree of resource overlap (Plethodon vehiculum), which we believe might be attributable to a benign and resource-rich habitat, although alternative explanations exist. A. vagrans did avoid syntopic newts (Taricha granulosa), though this is probably due to toxic newt skin secretions.     

Learned Recognition of Intraspecific Predators in Larval Long-Toed Salamanders Ambystoma macrodactylum

The ability of prey to detect predators and respond accordingly is critical to their survival. The use of chemical cues by animals in predator detection has been widely documented. In many cases, predator recognition is facilitated by the release of alarm cues from conspecific victims. Alarm cues elicit anti‐predator behavior in many species, which can reduce their risk of being attacked. It has been previously demonstrated that adult long‐toed salamanders, Ambystoma macrodactylum, exhibit an alarm response to chemical cues from injured conspecifics. However, whether this response exists in the larval stage of this species and whether it is an innate or a learned condition is unknown. In the current study, we examined the alarm response of naïve (i.e. lab‐reared) larval long‐toed salamanders. We conducted a series of behavioral trials during which we quantified the level of activity and spatial avoidance of hungry and satiated focal larvae to water conditioned by an injured conspecific, a cannibal that had recently been fed a conspecific or a non‐cannibal that was recently fed a diet of Tubifex worms. Focal larvae neither reduced their activity nor spatially avoided the area of the stimulus in either treatment when satiated, and exhibited increased activity towards the cannibal stimulus when hungry. We regard this latter behavior as a feeding response. Together these results suggest that an anti‐predator response to injured conspecifics and to cannibalistic conspecifics is absent in naïve larvae. Previous studies have shown that experienced wild captured salamanders do show a response to cannibalistic conspecifics. Therefore, we conducted an additional experiment examining whether larvae can learn to exhibit anti‐predator behavior in response to cues from cannibalized conspecifics. We exposed larvae to visual, chemical and tactile cues of stimulus animals that were actively foraging on conspecifics (experienced) or a diet of Tubifex (naïve treatment). In subsequent behavioral treatments, experienced larvae significantly reduced their activity compared to naive larvae in response to chemical cues of cannibals that had recently consumed conspecifics. We suggest that this behavior is a response to alarm cues released by consumed conspecifics that may have labeled the cannibal. Furthermore, over time, interactions with cannibals may cause potential prey larvae to learn to avoid cannibals regardless of their recent diet.     

Evidence for the Predator Attraction Hypothesis in an amphibian predator–prey system

Many species possess damage-released chemical alarm cues that function in alerting nearby individuals to a predator attack. One hypothesis for the evolution and/or maintenance of such cues is the Predator Attraction Hypothesis, where predators, rather than prey, are the “intended” recipients of these cues. If a predator attack attracts additional predators, these secondary predators might interfere with the predation event, providing the prey with a better chance to escape. In this study, we conducted two experiments to explore this hypothesis in an amphibian predator/prey system. In Experiment 1, we found that tiger salamanders (Ambystoma mavortium) showed a foraging attraction to chemical cues from wood frog (Lithobates sylvaticus) tadpoles. Salamanders that were experienced with tadpole prey, in particular, were strongly attracted to tadpole alarm cues. In Experiment 2, we observed experimental encounters between a tadpole and either one or two salamanders. The presence of the second predator caused salamanders to increase attack speed at the cost of decreased attack accuracy (i.e., increasing the probability that the tadpole would escape attacks). We also found that the mere presence of visual and chemical cues from a second predator did not affect this speed/accuracy trade-off but did cause enough of a distraction to increase tadpole survival. Thus, our findings are consistent with the Predator Attraction Hypothesis for the evolution and/or maintenance of alarm cues.     

Avoidance of Chemical Alarm Cues Released from Autotomized Tails of Ravine Salamanders (Plethodon richmondi)

The ability of animals to detect and avoid areas containing chemical alarm cues from conspecifics is well documented in aquatic species. The ability to detect chemical alarm cues in terrestrial organisms has not been tested until recently. In this study, we tested the ability of the ravine salamander ( Plethodon richmondi ) to detect and avoid areas containing chemicals released from the autotomized tails of conspecifics and sympatric zigzag salamanders ( P. dorsalis ). We also ascertained whether any avoidance response could be attributed to the size or sex of the animal. Our results suggest that ravine salamanders avoid substrates containing odors released from the autotomized tails of conspecifics, but not of heterospecific salamanders and that this response occurs independent of the size and/or sex of the animal. By avoiding areas where a conspecific has recently been injured, an organism could reduce its chances of encountering a predator and thus increase its chances of survival.     

Interspecific competition promotes habitat and morphological divergence in a secondary contact zone between two hybridizing songbirds

Interspecific competition is assumed to play an important role in the ecological differentiation of species and speciation. However, empirical evidence for competition's role in speciation remains surprisingly scarce. Here, we studied the role of interspecific competition in the ecological differentiation and speciation of two closely related songbird species, the Common Nightingale (Luscinia megarhynchos) and the Thrush Nightingale (Luscinia luscinia). Both species are insectivorous and ecologically very similar. They hybridize in a secondary contact zone, which is a mosaic of sites where both species co‐occur (syntopy) and sites where only one species is present (allotopy). We analysed fine‐scale habitat data for both species in both syntopic and allotopic sites and looked for associations between habitat use and bill morphology, which have been previously shown to be more divergent in sympatry than in allopatry. We found that the two nightingale species differ in habitat use in allotopic sites, where L. megarhynchos occurred in drier habitats and at slightly higher elevations, but not in syntopic sites. Birds from allotopic sites also showed higher interspecific divergence in relative bill size compared to birds from syntopic sites. Finally, we found an association between bill morphology and elevation. Our results are consistent with the view that interspecific competition in nightingales has resulted in partial habitat segregation in sympatry and that the habitat‐specific food supply has in turn very likely led to bill size divergence. Such ecological divergence may enhance prezygotic as well as extrinsic postzygotic isolation and thus accelerate the completion of the speciation process.     

Behavioral interactions between salamanders and centipedes: competition in divergent taxa

Red-backed salamanders, Plethodon cinereus, use territorialadvertisement in the form of agonistic displays and pheromonalscent marking as a mechanism for intraspecific interferencecompetition. Although ecological and behavioral interactionsamong species of salamanders have been well studied, littleis known about the interactions between territorial P. cinereusand other ecologically similar species, such as large predatoryinvertebrates. Our field data indicate that P. cinereus anda large syntopic centipede, Scolopocryptops sexspinosus, exhibitnegative spatial associations in natural habitats, possiblyindicating interspecific territoriality. Only seven instancesof salamander/centipede co-occurrence were recorded from a fieldsample of 247 occupied cover objects. Cover object size waspositively correlated with salamander SVL (tip of the snoutto the anterior end of the cloaca), but there was no correlationof cover object size to centipede length. Data on the abilityof P. cinereus to differentiate among chemicals on the substratesuggest that visual cues are not necessary to elicit a territorialresponse from intruding salamanders. Although in laboratorytrials salamanders behaved similarly toward intruders of bothspecies, biting was directed only toward centipedes. Salamandersspent significantly more time approaching centipedes than theydid approaching other salamanders. Approach behavior was oftenassociated with nose tapping and may be an investigative, ratherthan aggressive, behavior. We suggest that territorial P. cinereusrespond similarly to intruding salamanders and centipedes, butthat they escalate more readily to biting centipedes becauseS. sexspinosus is sightless and thus unable to respond to visualsignals.     

Predation Risk Avoidance by Terrestrial Amphibians: The Role of Prey Experience and Vulnerability to Native and Exotic Predators

We studied avoidance, by four amphibian prey species (Rana luteiventris, Ambystoma macrodactylum, Pseudacris regilla, Tarichia granulosa), of chemical cues associated with native garter snake (Thamnophis elegans) or exotic bullfrog (R. catesbeiana) predators. We predicted that avoidance of native predators would be most pronounced, and that prey species would differ in the intensity of their avoidance based on relative levels of vulnerability to predators in the wild. Adult R. luteiventris (presumably high vulnerability to predation) showed significant avoidance of chemical cues from both predators, A. macrodactylum (intermediate vulnerability to predation) avoided T. elegans only, while P. regilla (intermediate vulnerability to predation) and T. granulosa (low vulnerability to predation) showed no avoidance of either predator. We assessed if predator avoidance was innate and/or learned by testing responses of prey having disparate levels of prior exposure to predators. Wild‐caught (presumably predator‐exposed) post‐metamorphic juvenile R. luteiventris and P. regilla avoided T. elegans cues, while laboratory‐reared (predator‐naive) conspecifics did not; prior exposure to R. catesbeiana was not related to behavioural avoidance among adult or post‐metamorphic juvenile wild‐reared A. macrodactylum and P. regilla. These results imply that (i) some but not all species of amphibian prey avoid perceived risk from garter snake and bullfrog predators, (ii) the magnitude of this response probably differs according to prey vulnerability to predation in the wild, and (iii) avoidance tends to be largely learned rather than innate. Yet, the limited prevalence and intensity of amphibian responses to predation risk observed herein may be indicative of either a relatively weak predator–prey relationship and/or the limited importance of predator chemical cues in this particular system.     

Fathead Minnows, Pimephales promelas, Learn to Recognize Chemical Alarm Cues of Introduced Brook Stickleback, Culaea inconstans

In four experiments conducted over a 6-year period, we investigated whether fathead minnows, Pimephales promelas, could acquire the ability to recognize chemical alarm cues of introduced brook stickleback, Culaea inconstans. A laboratory experiment documented that stickleback-naïve minnows did not exhibit an anti-predator response when exposed to the chemical alarm cues of stickleback. In a laboratory experiment conducted 5 years after the introduction of stickleback to the pond, minnows exhibited an antipredator response to stickleback cues. Moreover, in a field experiment the minnows exhibited avoidance of areas labelled with stickleback alarm cues. Minnows raised from eggs taken from the test pond did not exhibit an anti-predator response to stickleback cues while minnows from the test pond that had experience with stickleback cues did respond to stickleback cues. Our results provide clear evidence that cross-species responses to chemical alarm cues of fishes can be learned. Learned recognition of alarm cues has important implications for predator/prey interactions.     

Damage released prey alarm substances or predator odours? Risk assessment by an aquatic oligochaete

Although the abilities of prey to detect and respond to chemical substances associated with a predator have been widely reported, the factors promoting the evolution of responses to prey alarm cues vs. predator odours are still vague. In this article, we combined field research with laboratory experiments to explore which chemical substance associated with predator activity (predator odour, conspecific or heterospecific alarm substances) induces defence responses in the aquatic oligochaete Stylaria lacustris, which is vulnerable to common littoral predators. The field results indicated that predators injure the oligochaetes and a great proportion, up to 45% of individuals in the population, were found to be damaged. The results of the laboratory experiments revealed that chemical odours from damselfly larvae feeding on S. lacustris did not induce the defence response in the oligochaetes. On the contrary, oligochaetes detected and responded to alarm substances from damaged conspecifics alone and substances from damaged cladoceran Daphnia magna. We discussed conditions favouring the responses to damage released prey alarm cues instead of predator odours in Stylaria lacustris. Our data suggest that the selection of responses to alarm cues from damaged prey vs. predator odours may be dependent on three factors: (1) non-species-specific predation, (2) divergence of food niche of the different stages of the predator and (3) complex food web with multiple predators. Handling editor: S. Declerk     

Friends in Low Places: Responses of a Benthic Stream Fish to Intra‐Prey‐Guild Alarm Cues

Many aquatic species produce chemical alarm cues that serve as a warning to nearby conspecifics. In mixed‐species aggregations, individuals may also benefit by ‘eavesdropping’ on the chemical alarm cues of other species that are in the same prey guild. Rainbow Darters (Etheostoma caeruleum) are benthic fish that co‐occur with native Ozark Minnows (Notropis nubilus), recently introduced Western Mosquitofish (Gambusia affinis), and native Oklahoma Salamanders (Eurycea tyrnerensis), all of whom are vulnerable to the same predators. We tested the responses of darters to the damage‐released alarm cues of conspecifics (positive control), minnows, and mosquitofish; alarm cues from Bumblebee Gobies (Brachygobius doriae) served as a negative (allopatric) control. We also tested the response of sympatric and allopatric darters to the damage‐released alarm cues of Oklahoma Salamander. Darters exhibited a fright response to conspecific and minnow alarm cues, but not to cues from mosquitofish or gobies. Lack of response to mosquitofish cues could be because they are introduced or because they typically occur higher in the water column than darters. Darters that were sympatric with the salamander exhibited a fright response to the alarm cues of the salamander, while allopatric darters did not. Rainbow Darters can develop responses to the alarm cues of syntopic species (minnows and Oklahoma Salamander) within their prey guild.     

Influence of Body Size on the Responses of Fathead Minnows, Pimephales promelas, to Damselfly Alarm Cues

A wide diversity of aquatic organisms release chemical alarm cues upon encountering or being attacked by a predator. These alarm cues can be used by nearby individuals to assess local predation risk. Receivers warned by chemical alarm cues gain a survival benefit when encountering predators. Animals that are in the same prey guild (i.e. that co‐occur and share the same predators) may learn to recognize each others’ chemical alarm cues. This ability may confer an adaptive advantage if the prey animals are vulnerable to the same predators. However, if the prey grow to different sizes and as a consequence are no longer vulnerable to the same suite of predators, then there should no longer be an advantage for the prey to respond to each others’ alarm cues. In this study, we exposed small and large fathead minnows (Pimephales promelas) to cues from syntopic injured damselfly larvae (Enallagma boreale), cues from injured mealworm larvae (Tenebrio molitor) and to distilled water. Small minnows exhibited antipredatory behaviour and increased shelter use in response to injured damselfly cues but not to the controls of injured mealworm or distilled water. On the contrary, large minnows exhibited no significant change in shelter use in response to any of the injured cues. These data demonstrate that fathead minnows exhibit an antipredator response to damselfly alarm cues, but only when minnows are small and members of the same prey guild as damselfly larvae. These results demonstrate the considerable flexibility in the responses to heterospecific alarm cues.     

Earthworms, as ecosystem engineers, influence multiple aspects of a salamander��s ecology

Ecosystem engineers create habitat that can be used by other species in multiple ways, such as refuges from predators, places to breed, or areas with increased prey resources. I conducted a series of enclosure experiments to: (1) determine if salamanders use earthworm burrows, and (2) examine the potential influence of earthworm burrow use and indirect effects on salamander intra- and interspecific competition, predator avoidance, and seasonal performance. I found that one species of woodland salamander, Plethodon cinereus, used earthworm burrows 50% of the time when burrows were present. Neither adults nor juveniles of the congeneric P. glutinosus used earthworm burrows. Intraspecific, but not interspecific, competition by P. cinereus affected salamander behavior when earthworms were absent, with P. cinereus found under cover objects >70% of the time when alone or with a P. glutinosus, but only 40% of the time when with another P. cinereus. When earthworms were present, the behavior of P. cinereus was similar across salamander treatments. Earthworms decreased the amount of leaf litter and microinvertebrates, although this did not affect salamander mass. In subsequent experiments using only P. cinereus, the refuge provided by earthworm burrows increased the survival of P. cinereus over the winter and allowed P. cinereus to avoid being consumed by the common garter snake (Thamnophis sirtalis). Because earthworm burrows provide a refuge for P. cinereus during intraspecific encounters, in the presence of a predator and over the winter, they may serve as an important belowground?Caboveground linkage in eastern forests where salamanders are common.     

Evolution mediates the effects of apex predation on aquatic food webs

Ecological and evolutionary mechanisms are increasingly thought to shape local community dynamics. Here, I evaluate if the local adaptation of a meso-predator to an apex predator alters local food webs. The marbled salamander (Ambystoma opacum) is an apex predator that consumes both the spotted salamander (Ambystoma maculatum) and shared zooplankton prey. Common garden experiments reveal that spotted salamander populations which co-occur with marbled salamanders forage more intensely than those that face other predator species. These foraging differences, in turn, alter the diversity, abundance and composition of zooplankton communities in common garden experiments and natural ponds. Locally adapted spotted salamanders exacerbate prey biomass declines associated with apex predation, but dampen the top-down effects of apex predation on prey diversity. Countergradient selection on foraging explains why locally adapted spotted salamanders exacerbate prey biomass declines. The two salamander species prefer different prey species, which explains why adapted spotted salamanders buffer changes in prey composition owing to apex predation. Results suggest that local adaptation can strongly mediate effects from apex predation on local food webs. Community ecologists might often need to consider the evolutionary history of populations to understand local diversity patterns, food web dynamics, resource gradients and their responses to disturbance.     

Are behavioural responses to predation cues linked across life cycle stages?

1. Prey organisms can perceive cues to predation hazard and adopt low‐risk behaviours to increase survival. Animals with complex life cycles, such as insects, can exhibit such anti‐predatory behaviours in multiple life stages. 2. Cues to predation risk may induce ovipositing females to choose habitats with low predation risk. Cues to predation risk may also induce larvae to adopt facultative behaviours that reduce risk of predation. 3. One hypothesis postulates that anti‐predation behaviours across adult and larval stages may be negatively associated because selection for effective anti‐predator behaviour in one stage leads to reduced selection for avoidance of predators in other stages. An alternative hypothesis suggests that selection by predation favours multi‐component defences, with both avoidance of oviposition and facultative adoption of low‐risk behaviours by larvae. 4. Laboratory and field experiments were used to determine whether defensive responses of adult and larval mosquitoes are positively or negatively associated. The study tested effects of waterborne cues from predatory Toxorhynchites theobaldi on oviposition choices and larval behaviours of three of its common prey: Culex mollis, Limatus durhamii and Aedes albopictus. 5. Culex mollis shows strong anti‐predator responses in both life stages, consistent with the hypothesis of a multi‐component behavioural defence. The other two species showed no detectable responses to waterborne predator cues in either adult or larval stages. Larvae of these unresponsive species were significantly more vulnerable to this predator than was C. mollis. 6. For these mosquitoes, species appear either to have been selected for multi‐component defences against predation or to act in ways that could be called predator‐naïve.     

Niche partitioning and the effect of interspecific competition on microhabitat use by two sympatric galaxiid stream fishes

1. Numerous interacting abiotic and biotic factors influence niche use and assemblage structure of freshwater fishes, but the strength of each factor changes with spatial scale. Few studies have examined the role of interspecific competition in structuring stream fish assemblages across spatial scales. We used field and laboratory approaches to examine microhabitat partitioning and the effect of interspecific competition on microhabitat use in two sympatric stream fishes (Galaxias‘southern’ and Galaxias gollumoides) at large (among streams and among sites within streams) and small (within artificial stream channels) spatial scales. 2. Diurnal microhabitat partitioning and interspecific competition at large spatial scales were analysed among three sympatry streams (streams with allotopic and syntopic sites; three separate catchments) and four allopatry streams (streams with only allotopic sites; two separate catchments). Electro‐fishing was used to sample habitat use of fishes at 30 random points within each site by quantifying four variables for each individual: water velocity, depth, distance to nearest cover and substratum size. Habitat availability was then quantified for each site by measuring those variables at each of 50 random points. Diet and stable isotope partitioning was analysed from syntopic sites only. Diel cycles of microhabitat use and interspecific competition at small spatial scales were examined by monitoring water velocity use over 48 h in artificial stream channels for three treatments: (i) allopatric G. ‘southern’ (10 G. ‘southern’); (ii) allopatric G. gollumoides (10 G. gollumoides) and (iii) sympatry (five individuals of each species). 3. One hundred and ninety‐four G. ‘southern’ and 239 G. gollumoides were sampled across all seven streams, and habitat availability between the two species was similar among all sites. Galaxias‘southern’ utilised faster water velocities than G. gollumoides in both the field and in channel experiments. Both species utilised faster water velocities in channels at night than during the day. Diet differences were observed and were supported by isotopic differences (two of three sites). No interspecific differences were observed for the other three microhabitat variables in the field, and multivariate habitat selection did not differ between species. Interspecific competition had no effect on microhabitat use of either species against any variable either in the field (large scale) or in channels (small scale). 4. The results suggest that niche partitioning occurs along a subset of microhabitat variables (water velocity use and diet). Interspecific competition does not appear to be a major biotic factor controlling microhabitat use by these sympatric taxa at any spatial scale. The results further suggest that stream fish assemblages are not primarily structured by biotic factors, reinforcing other studies de‐emphasising interspecific competition.     

Trade‐offs between salinity preference and antipredator behaviour in the euryhaline sailfin molly Poecilia latipinna

Salinity preference and responses to predatory chemical cues were examined both separately and simultaneously in freshwater (FW) and saltwater (SW)‐acclimated sailfin mollies Poecilia latipinna, a euryhaline species. It was hypothesized that P. latipinna would prefer FW over SW, move away from chemical cues from a crayfish predator, and favour predator avoidance over osmoregulation when presented with both demands. Both FW and SW‐acclimated P. latipinna preferred FW and actively avoided predator cues. When presented with FW plus predator cues v. SW with no cues, P. latipinna were more often found in FW plus predator cues. These results raise questions pertaining to the potential osmoregulatory stress of salinity transitions in euryhaline fishes relative to the potential fitness benefits and whether euryhalinity is utilized for predator avoidance. This study sheds light on the potential benefits and consequences of being salt tolerant or intolerant and complicates the understanding of the selection pressures that have favoured the different osmoregulatory mechanisms among fishes.