Effects of fish chemical cues on the interactions between tadpoles and crayfish

We studied the effects of predatory crayfish ( Pacifastacus leniusculus ), the non-lethal effects of fish chemical cues ( Oncorhynchus mykiss ), and the combined effects of crayfish and fish chemical cues on the performance of tadpoles of two co-existing anuran species, Rana temporaria and Bufo bufo , in experimental pools. We also examined grazing effects on periphyton, the main food source for the tadpoles. Crayfish significantly reduced tadpole survival, particularly by feeding on Bufo . Rana benefited from reduced numbers of competitors, resulting from crayfish predation, by increased growth rate, whereas the growth rate of Bufo was unaffected by crayfish. The proportion of Rana in refuges (in relation to the number of survivors at the end of the experiment) was unaffected by crayfish, whereas proportionally more Bufo stayed in refuges in the presence of crayfish, relative to controls. Fish cues had no effect on tadpole survival of either species. During the entire larval period, Rana responded to fish cues by increasing the use of refuges relative to controls, whereas Bufo did not show any significant behavioural response to fish cues. In accordance with these observations, the proportion of Rana in refuges at the end of the experiment was high in the presence of fish cues, whereas the use of refuges by Bufo was not affected by fish cues. Predatory crayfish and fish chemical cues had additive effects on tadpole survival, growth and refuge use. Tadpoles in all treatments reduced periphyton biomass. Both crayfish and fish cues had positive indirect effects on periphyton biomass. The positive indirect effect of fish cues on periphyton was likely an effect of reduced grazing from Rana . Thus lethal, as well as non-lethal, predator effects on prey populations can influence lower trophic levels.     

Metabolic responses of tadpoles to chemical predation cues

Life-history theory predicts that predator-induced defences should incur fitness costs. In larval amphibians, the most frequently reported cost is reduced energy intake due to lower foraging rates; however, recent reports suggest that this trade-off may be uncoupled through morphological and physiological adaptations. Metabolism is a measure of energy expenditure and plasticity in respiration may potentially offset other costs of predator-induction. The aim of this study was to measure the indirect effects of dragonfly larvae on the respiration rate of tadpoles of the Arabian toad (Bufo arabicus) over time and at different predator concentrations. We performed two experiments. In the first experiment (time–response), we exposed tadpoles either to the indirect presence of odonate larvae or predator-free conditions and measured respiration rates 3, 5, 8, 11, 13, 15, 19 and 26 days after the start of the experiment. In the second experiment (dose–response), we used three levels of predator chemicals, equivalent to 1 predator per 10, 100 or 1,000 l plus controls. The respiration rate of predator-exposed tadpoles varied initially, but was 56.3% of controls after 26 days. In the dose–response experiment, the respiration rate of all predator-exposed tadpole groups was reduced by between 19.1 and 27.2% after 21 days. The study demonstrates a mechanism by which tadpoles may be able to adjust their physiology to partially offset the costs of lower energy intake due to predator avoidance.     

Activity of Sphaerotheca breviceps tadpoles in response to chemical cues of the predaceous tadpoles Hoplobatrachus tigerinus

Tadpoles of Sphaerotheca breviceps raised in the laboratory from the egg stage, and hence lacking prior experience of a predator or its odors, were tested to examine their responses to a predator’s (tadpoles of Hoplobatrachus tigerinus) water-borne chemical cues. The stimulus solution was obtained following 24 h of rearing tadpoles of H. tigerinus (one tadpole per 200 mL water) that were not fed during this period. Upon exposure to the stimulus solution the activity of S. breviceps tadpoles decreased by about 90% within 5 min. Their resting period increased significantly over baseline activity, whereas the swimming period, distance traversed, and swimming spurts declined. However, whenever a test tadpole moved, its swimming velocity was high in response to stimulus solution. The antipredatory responses declined with increase in time of storage of the stimulus solution, indicating decay of the predator’s chemical cues. The findings suggest that (1) antipredator defense strategies of S. breviceps do not require prior experience of predators, (2) the predator’s chemical cues are labile in nature, and (3) the response of prey tadpoles to such cues is similar to reported behavior of anuran tadpoles in response to real predators and alarm cues.     

Trade‐offs between constitutive and induced defences drive geographical and climatic clines in pine chemical defences

There is increasing evidence that geographic and climatic clines drive the patterns of plant defence allocation and defensive strategies. We quantified early growth rate and both constitutive and inducible chemical defences of 18 Pinaceae species in a common greenhouse environment and assessed their defensive allocation with respect to each species' range across climatic gradients spanning 31^o latitude and 2300 m elevation. Constitutive defences traded‐off with induced defences, and these defensive strategies were associated with growth rate such that slow‐growing species invested more in constitutive defence, whereas fast‐growing species invested more in inducible defence. The position of each pine species along this trade‐off axis was in turn associated with geography; moving poleward and to higher elevations, growth rate and inducible defences decreased, while constitutive defence increased. These geographic patterns in plant defence were most strongly associated with variation in temperature. Climatic and geographical clines thus act as drivers of defence profiles by mediating the constraints imposed by trade‐offs, and this dynamic underlays global patterns of defence allocation.     

Seeing is believing: information content and behavioural response to visual and chemical cues

Predator avoidance and foraging often pose conflicting demands. Animals can decrease mortality risk searching for predators, but searching decreases foraging time and hence intake. We used this principle to investigate how prey should use information to detect, assess and respond to predation risk from an optimal foraging perspective. A mathematical model showed that solitary bees should increase flower examination time in response to predator cues and that the rate of false alarms should be negatively correlated with the relative value of the flower explored. The predatory ant, Oecophylla smaragdina, and the harmless ant, Polyrhachis dives, differ in the profile of volatiles they emit and in their visual appearance. As predicted, the solitary bee Nomia strigata spent more time examining virgin flowers in presence of predator cues than in their absence. Furthermore, the proportion of flowers rejected decreased from morning to noon, as the relative value of virgin flowers increased. In addition, bees responded differently to visual and chemical cues. While chemical cues induced bees to search around flowers, bees detecting visual cues hovered in front of them. These strategies may allow prey to identify the nature of visual cues and to locate the source of chemical cues.     

Behavioural responses to predator chemical cues and local variation in antipredator performance in Rana temporaria tadpoles

Antipredator behaviour is an important factor influencing survival probability of prey animals, and it may evolve rapidly as a response to changes in predator regime. I studied antipredator behaviour of common frog ( Rana temporaria ) tadpoles from three populations that differ in predator regimes. In the first experiment, tadpoles obtained from four natural matings in each population were subjected to chemical cues from either European perch ( Perca fluviatilis ) or from larvae of the dragonfly Aeshna juncea . Tadpoles decreased their activity in response to both predators, but the spatial behaviour of tadpoles differed between the two predator treatments. In general, there were no differences in behaviours among the populations, but in three out of four studied behaviours there were differences between parentages within the populations suggesting that these behaviours may be genetically determined. The lack of a significant Predator×Population interaction suggests no differences in plastic antipredator behaviour among the populations, while the lack of significant Predator×Parentage interaction suggests no genetic variance within the populations for plastic antipredator behaviour. In the second experiment, tadpoles from the three populations were exposed to predation by a free-ranging A. juncea . In line with the first experiment, there were no differences in survival rate between the populations. R. temporaria tadpoles seem to rely heavily on plastic antipredator behaviour as their main response to predator chemical cues. There was very little indication of local behavioural differentiation and the possible reasons for the lack of divergence among populations are discussed.     

Maternally derived chemical defences are an effective deterrent against some predators of poison frog tadpoles (Oophaga pumilio)

Parents defend their young in many ways, including provisioning chemical defences. Recent work in a poison frog system offers the first example of an animal that provisions its young with alkaloids after hatching or birth rather than before. But it is not yet known whether maternally derived alkaloids are an effective defence against offspring predators. We identified the predators of Oophaga pumilio tadpoles and conducted laboratory and field choice tests to determine whether predators are deterred by alkaloids in tadpoles. We found that snakes, spiders and beetle larvae are common predators of O. pumilio tadpoles. Snakes were not deterred by alkaloids in tadpoles. However, spiders were less likely to consume mother-fed O. pumilio tadpoles than either alkaloid-free tadpoles of the red-eyed treefrog, Agalychnis callidryas, or alkaloid-free O. pumilio tadpoles that had been hand-fed with A. callidryas eggs. Thus, maternally derived alkaloids reduce the risk of predation for tadpoles, but only against some predators.     

Symmetry and information content of chemical structures

A method, based on symmetry, is suggested for determining the information content of systems. A comparison has been made between the information for symmetry, topology, and chemical composition. The new information measure increases when the asymmetry of the molecules and the number of atoms in the latter increases. It can distinguish between different molecular conformations, and give a linear correlation with the absolute entropy for homologous series of chemical compounds.     

Urinary chemical cues affect light avoidance behaviour in male laboratory mice, Mus musculus

Chemical signals from conspecifics can influence the behaviour and neuroendocrine axis of mice. Several different molecules are excreted with urine, depending on hormonal level, and can indicate the sex of the emitter. In male mice, these chemicals are the major urinary proteins (MUPs) and some small volatile odorant molecues that are found bound to them. We tested adult males for light avoidance behaviour in a two-chamber apparatus, with one light and one dark side, in the presence or absence of male urinary substances. The presence of chemical cues on either side of the cage was expected to modify light avoidance behaviour. The volatiles released from purified MUPs had the same effect as whole adult male urine, in that they induced a faster onset of exploration of the light compartment. The results show that mice can use the information carried by the odorant molecules released by MUPs to recognize the urine of male mice, and respond appropriately. Copyright 1999 The Association for the Study of Animal Behaviour.     

Cortisol influences the antipredator behavior induced by chemical alarm cues in the Frillfin goby

We evaluated the effect of increased plasma cortisol levels on fish antipredator behavior induced by conspecific chemical alarm cues. The experimental model for the study was the Frillfin goby Bathygobius soporator. We first confirmed that the alarm substance induces typical defensive antipredator responses in Frillfin gobies and described their alarm substance cells (epidermal ‘club’ cells). Second, we confirmed that intraperitoneal cortisol implants increase plasma cortisol levels in this species. We then demonstrated that exogenous cortisol administration and subsequent exposure to an alarm substance decreased swimming activity to a greater extent than the activity prompted by either stimulus alone. In addition, cortisol did not abolish the sheltering response to the alarm chemical cue even though it decreased activity. As predators use prey movements to guide their first contact with the prey, a factor that decreases swimming activity clearly increases the probability of survival. Consequently, this observation indicates that cortisol helps improve the antipredator response in fish.     

Circadian rhythm does not affect responses to chemical cues in the red swamp crayfish,Procambarus clarkii

Studies of crayfish chemical ecology have been conducted in both day and night conditions. This variation may hinder the comparison of data among studies, if the responses by crayfish to chemical cues are dependent upon the time at which the cues are encountered. We tested the hypothesis that responses to chemical cues are dependent on observation time using the red swamp crayfish, Procambarus clarkii. Procambarus clarkii is known to exhibit a light-regulated circadian rhythm, with nocturnal activity peaks. Habitat use differed significantly between non-stimulated periods and periods of exposure to a food stimulus, but no effects of photoperiod (normal vs. reversed) or laboratory conditions (dark vs. light) were observed. The results suggest that, all else being equal, (1) studies of crayfish chemical ecology can be successfully conducted in a variety of experimental conditions, and (2) previous studies conducted at various times of the day should have comparable results.     

Interaction between chemical and tactile cues in mayfly detection of stoneflies

• 1 Sediments and eleven species of rooted submerged aquatic macrophytes were sampled from twenty-five lakes in southern Quebec to examine species patterns in metal content.
• 2 Linear regression models relating the content of metals in the macrophytes to their sediments demonstrated that the isoetoid (Eleocharis acicitlaris, Eriacaulon septangulare, Isoetes sp. and Sagittaria graminae) and the non-isoetoid species had different plant-sediment relationships. Canopy-forming (Myriophyllum spicatum, Potamogeton amplifalius, P. crispus and P. perfoliatus) and understory species (Elodea canadensis, P. robbinsii and Vallisneria americana) had similar linear regression model slopes, but the understory species had significantly higher intercepts, indicating higher plant metal contents for a given sediment metal concentration.
• 3 We grouped species by their biomass density (biomass (kg)/height (m)), a measure of the packing of the plant canopy, and averaged values for each metal, thereby producing one model per group. Comparison of the models by analysis of covariance revealed no significant differences in metal content. However, the canopy-forming species that occurred in monospecific and mixed species stands (M. spicatum and P. amplifolius) had significantly higher metal content for a given sediment metal content when an understory was present.
• 4 These results show that sediment metal content explains the largest proportion of variation in plant metal content across all sites and species. Deviations from what would be predicted by the most general model are not related to large-scale differences in sediment geochemistry, but to the physical structure of the weedbed within geochemically similar sediments.