Evolutionary patterns of naticid gastropods of the Chesapeake Group: an example of coevolution?

The naticid gastropod predator–prey system has been cited as a possible example of coevolution. Previous work indicated that Miocene bivalves from the Chesapeake Group responded to naticid drilling with a predation-avoidance strategy of gradual thickness increase over evolutionary time. The present study examined whether the predators, Euspira heros (Say) and Neverita duplicata (Say), coevolved by increasing their efficiency in response to prey adaptation.Neither predator showed significant trends in most characters thought to affect predator efficiency, including measures of globosity and aperture size relative to shell height. Stasis occurred for all such characters except Euspira aperture height, which showed a statistically significant decrease of 9 percent through the section. Morphologic trends occurred in two additional characters, shell thickness relative to height, and shell height. Euspira increased in thickness by 63 percent through the section; Neverita decreased in thickness by 37 percent (through slightly less section) and simultaneously increased mean height by 60 percent.The size increase exhibited by Neverita could be interpreted as a coevolutionary response to increased prey defenses. Naticids are subject to predation by other naticids, however; the trends in size and thickness may be viewed instead as an evolutionary strategy to escape naticid predation, including cannibalism. Although the possibility of coevolution is difficult to eliminate, Chesapeake Group naticids appear to have evolve more in response to their own predators than to evolution of their prey.

Effect of Copper on the interaction between the Predator Didinium nasutum and its Prey Paramecium caudatum

Tests to determine acute copper toxicity levels demonstrated that the protozoan predator Didinium nasutum were more susceptible to copper stress than its prey Paramecium caudatum Thus we predicted that Paramecium and Didinium densities, at the equilibrium point of their interaction, would be higher at sublethal copper levels due to a decrease in the predator's efficiency. This situation is likely to produce a decrease in the stability of the system. However, isocline analysis did not support the predictions based on the acute lethality tests. Equilibrium densities of both predator and prey did not change at copper levels between 30 and 180 μg/L. The mathematical analysis suggested that the interaction became less stable with increasing copper concentrations. However, stability decreased due to hormesis in Didinium at sublethal copper levels and not due to a reduction in predator efficiency as expected. At 300 μg Cu/L, densities of both species at equilibrium were higher and the stability of the system decreased. This decrease in stability resulted from a reduction in predator efficiency, as 300 μg Cu/L is not sublethal for Didinium.

Changes in the predator-avoidance behaviour of juvenile guppies (Poecilia reticulata) exposed to pentachlorophenol

The effects of pentachlorophenol on the predator-avoidance behaviour of the guppy (Poecilia reticulate) in response to largemouth bass (Micropterus salmoides) predation was investigated. There were no consistent effects of pentachlorophenol on habitat use or general behaviour of the guppies. In the presence of predators, all guppies occurred significantly more often alone, motionless, and in the top third of the water column in the nonopen areas. Nine variables associated with predator efficiency were monitored to determine which treatment groups of guppies were easiest to capture. The bass had significantly lower capture success, performed more strikes and chases, and spent more time chasing guppies from the untreated and 100 μg/L groups than those from the 500 and 700 μg/L groups. This suggests that the guppies from the two high treatment groups had a slower response to predator attack and could not maintain a prolonged escape burst of speed.

EFFECTS OF PREDATOR AND PREY DENSITIES ON INTERACTIONS BETWEEN GOLDFISH AND DAPHNIA PULEX (DE GEER)

The effects on predator efficiency of varying predator and prey densities in a confined system were tested by use of the goldfish and Daphnia pulex. Within the limitations of the experiment, the per capita consumption of prey was reduced by increasing predator density. Also, as prey density increased, the effect on per capita consumption of increasing predator density became less.