Prey selection by molluscivorous cichlids foraging on a schistosomiasis vector snail,Biomphalaria glabrata

Summary This paper considers prey size selection by four molluscivorous cichlids feeding on the intermediate host snail of Schistosoma parasites, Biomphalaria glabrata. Haplochromis ishmaeli obtains its prey by crushing the snails between the pharyngeal jaws, whereas H. xenognathus, H. sauvagei and Macropleurodus bicolor apply both pharyngeal crushing and oral shelling. The fishes crushed significantly more snails with the highest reward in biomass per second of crushing. Oral shelling occurred far less often than pharyngeal crushing. Encounter rates with prey showed significant variations between different size classes of prey. The fish have no overall knowledge of snail availability in a tank. The probability that a snail will be eaten at encounter, calculated from the number encountered and the number eaten, reflects the prey size preference of the fish. Those snails with the highest biomass/crushing-time ratio had the highest probability of being crushed; observed and predicted prey size preferences corresponded well. Although for oral shelling the potential reward in biomass per second is of the same magnitude as for crushing, the probability of successful shelling is very low. Apparently the fish prefer prey with lowest risks.     

The “ricochet effect” and prey capture in colonial spiders

Summary Increased prey capture efficiency in colonial spiders is a consequence of the ricochet effect, as prey are captured after they bounce off several webs in succession. In this study, the prey capture of three species of colonial spiders in the genus Metepeira from Mexico are compared. These species, from different habitats, show varying levels of social organization (group size and withingroup spacing) that affect prey capture from ricochets. Metepeira sp. a (a presumed new species tentatively named atascadero) from desert grassland habitats, occur solitarily or in small groups, and gain little from prey ricochets: prey capture rates are low and variance in prey captured/spider is high. M. spinipes, from mesic agricultural sites, occur in groups of 10–150, and show a ricochet effect resulting in more and larger prey, and reduced variance in capture rate. M. incrassata, from tropical rainforest/agricultural sites, occur in large colonies of hundreds to thousands of individuals, and show a similar ricochet effect. The ricochet effect does not influence taxonomic composition of prey in either M. atascadero or M. spinipes, but does in tropical M. incrassata. This result, however, is primarily due to the capacity of certain taxa (eg., Lepidoptera), more common in the tropics, to escape more easily from spider webs. A comparison of prey capture efficiency of colonial M. incrassata with that of solitary M. atascadero shows that the ricochet effect provides an increase in efficiency across all size classes of prey.     

Stonefly nymphs use hydrodynamic cues to discriminate between prey

Summary Playback experiments conducted in a Rocky Mountain, USA, stream determined whether predatory stonefly nymphs (Kogotus modestus; Plecoptera: PerlodiMae) used hydrodynamic cues to discriminate prey species from nonprey species. In the laboratory we recorded pressure wave patterns associated with swimming escape behavior of Baetis bicaudatus (Baetidae), the favored mayfly prey species, and those of a nonprey mayfly, Ephemerella infrequens (Ephemerellidae). We video taped the responses of 24-h starved Kogotus to Baetis playbacks, Ephemerella playbacks or no playbacks made by oscillating (or not) live mayflies (Ephemerella) or clear plastic models placed within in situ flow-through observation boxes. The probability of attacks per encounter with Baetis playbacks was highest and independent of the model type used, but Kogotus also showed an unexpected high probability of attacks per encounter when Ephemerella playbacks were made through live Ephemerella. Thus, Kogotus discriminated between Baetis and Ephemerella swimming patterns but only when playbacks were made through the plastic model. Kogotus never attacked motionless mayflies or motionless plastic models. We allowed some Kogotus to successfully capture one small Baetis immediately before playbacks, which resulted in a much higher probability of attacks per encounter with Baetis playbacks on either model and a heightened discrimination of prey versus nonprey playbacks. The probability of attacks per encounter by Kogotus with live Baetis swimming under similar experimental conditions was strikingly similar to its response to Baetis playbacks made by oscillating the plastic model after a successful capture. Order of playback presentation (Baetis first or Ephemerella first) did not influence predatory responses to mayfly swimming patterns. This study is the first to document the use of hydrodynamic cues by stream-dwelling predators for discrimination of prey from nonprey and provides a mechanism to explain selective predation by stoneflies on Baetis in nature.     

The evolutionary importance of mechanoreception in three erpobdellid leech species

Summary The ability of an erpobdellid leech, Erpobdella montezuma, to discriminate between two congeneric amphipod prey (Hyalella montezuma and Hyalella azteca) through mechanoreception was examined. Both E. montezuma and H. montezuma, a pelagic filter feeder, are endemic to the near thermally constant environment of Montezuma Well, Arizona, and form a highly specialized predator-prey relationship. In contrast, H. azteca, a benthic detritivore, is widely distributed throughout North America. Erpobdella montezuma was highly responsive to the prey signals of the endemic H. montezuma and showed only a limited response to H. azteca. An inverse relationship occurred between the frequency of attack and size of the leech. The abilities of Erpobdella punctata and Nephelopsis obscura, neither found in Montezuma Well, to detect the signals of both amphipod prey were also examined. These species with non-specialized diets showed a low response to the signals of both prey species. This study demonstrates that very close predator-prey relationships can develop over evolutionary time in isolated aquatic systems through the use of mechanoreception.     

Underwater observations of piranhas in western Brazil

Synopsis The behaviour of three piranha species,Serrasalmus marginatus, S. spilopleura, andPygocentrus nattereri, and their prey fishes was studied underwater in the Pantanal region, Mato Grosso, Brazil. General habits, predatory tactics, feeding behaviour, and social interactions while foraging, as well as defensive tactics of prey fishes were observed.S. marginatus is solitary whereas the other two species live in shoals; their agonistic behaviour varies accordingly, the simplest being displayed by the solitary species. Predatory tactics and feeding behaviour also vary:S. spilopleura shows the most varied diet and highly opportunistic feeding strategy, which includes aggressive mimicry. The solitaryS. marginatus, besides fin and scale-eating, occasionally cleans larger individuals ofP. nattereri. Several cichlid species display defensive tactics clearly related to piranha attacks: tail protecting, watching, and confronting the predator are the most commonly observed behaviours. Piranhas seem to strongly influence use of habitat, social structure, and foraging mode of the fish communities.     

Actual vs perceived profitability: A study of floral choice of honey bees

The relationship between actual and perceived profitability was investigated by quantifying choice behavior of foraging honey bees (Apis mellifera).Rate of net energy gain was used as a measure of profitability. Individual bees repeatedly chose between a yellow and a blue tubular artificial flower which had different associated profitabilities. Handling costs were manipulated by using flowers of different depth;energy gains were manipulated by using different volumes of a 20% (w/w) sucrose solution. Bees' discrimination between the two flowers increased as the absolute difference in profitabilities increased and discrimination decreased as profitabilities of both flowers increased. Therefore, discrimination depended on the relative difference in profitability between flowers, and therefore, perception was a nonlinear function of actual rates of net energy gain. The results also suggested that motivation to choose between flowers depended on the level of profitability. We suggest that nonlinear perceptual scales and motivation should be incorporated into models of food choice as constraints and should be considered in the design of experiments used to test predictions of models and in the interpretation of experimental results.     

A general principle for the allocation of limited resources

Summary A marginal fitness theorem is derived for the allocation of a limited resource among alternative activities that have effects on the fitness of an individual. The marginal advantage theorem states that at the evolutionarily stable strategy (ESS), the marginal gains from increasing each of the allocations (expressed as partial derivatives of the fitness advantage of a rare mutant) are equal. The theorem is true for all proportional allocations (a + b + c + ...=j), regardless of the number of allocations, the nature of the response curves describing the direct effects of the allocations [f(a), etc.], or the way the effects of different allocations combine into fitness. The theorem is extended to size-number compromises and packaging strategies. The marginal advantage theorem is used to derive general theorems about the marginal effects of allocations [f (a), etc.] at the ESS and matching rules concerned with the total fitness to cost ratios of allocations at the ESS. The marginal advantage theorem is applicable to diverse allocation strategies, and provides a method for obtaining ESS allocations for any number of allocations and their components.     

Resource utilization of sympatric and experimentally allopatric cutthroat trout and Dolly Varden charr

Summary Resource utilization by cutthroat trout (CT) and Dolly Varden charr (DV) was studied 8 years after experimental transfers from sympatry had established reproducing allopatric populations in two nearby fishless lakes. Allopatric DV significantly increased their utilization of shallow-dwelling zoobenthos, and increased their vertical distribution in comparison to that in sympatry. In contrast, allopatric CT showed little change in the proportions of major prey types utilized, and, if anything, restricted their vertical distribution in comparison to that in sympatry. The results can be explained by the hypothesis that the resource use of DV is strongly influenced by interspecific competition from CT, whereas CT largely remains unaffected by this interaction. An alternative hypothesis, that lake differences can explain the differences in resource use between sympatry and allopatry, was evaluated by comparing food resource availability and other biotic and abiotic characteristics of the three study lakes. None of these could account for the shift in resource use by DV between sympatry and allopatry, but lake differences may explain why allopatric CT showed a restricted habitat use in comparison with their sympatric donor stock. The results of this whole-lake transfer experiment are consistent with earlier reported field and laboratory studies, and suggest that the aggressive dominance of CT is the most important mechanism by which DV are displaced from littoral and near-surface habitats in sympathy with CT.