Size-dependent locomotory performance creates a behaviorally mediated prey size refuge in the marine snail Olivella semistriata: a study in the natural habitat

The effects of the variability of individual prey locomotory performance on the vulnerability to predation are poorly understood, partly because individual performance is difficult to determine in natural habitats. To gain insights into the role(s) of individual variation in predatory relationships, we study a convenient model system, the neotropical sandy beach gastropod Olivella semistriata and its main predator, the carnivorous snail Agaronia propatula. The largest size class of O. semistriata is known to be missing from A. propatula’s spectrum of subdued prey, although the predator regularly captures much larger individuals of other taxa. To resolve this conundrum, we analyzed predation attempts in the wild. While A. propatula attacked O. semistriata of all sizes, large prey specimens usually escaped by ‘sculling’, an accelerated, stepping mode of locomotion. Olivella semistriata performed sculling locomotion regardless of size, but sculling velocities determined in the natural environment increased strongly with size. Thus, growth in size as such does not establish a prey size refuge in which O. semistriata is safe from predation. Rather, a behaviorally mediated size refuge is created through the size-dependence of sculling performance. Taken together, this work presents a rare quantitative characterization in the natural habitat of the causal sequence from the size-dependence of individual performance, to the prey size-dependent outcome of predation attempts, to the size bias in the predator’s prey spectrum.

Differential Interaction Strengths and Prey Preferences Across Larval Mosquito Ontogeny by a Cohabiting Predatory Midge

Understandings of natural enemy efficacy are reliant on robust quantifications of interaction strengths under context-dependencies. For medically important mosquitoes, rapid growth during aquatic larval stages could impede natural enemy impacts through size refuge effects. The identification of biocontrol agents which are unimpeded by ontogenic size variability of prey is therefore vital. We use functional response and prey preference experiments to examine the interaction strengths and selectivity traits of larvae of the cohabiting predatory midge Chaoborus flavicans (Meigen 1830) (Diptera: Chaoboridae) towards larval stages of the Culex pipiens (Diptera: Culicidae) mosquito complex. Moreover, we examine the influence of search area variation on selectivity traits, given its importance in consumer-resource interactions. Chaoborids were able to capture and consume mosquito prey across their larval ontogeny. When prey types were available individually, a destabilizing Type II functional response was exhibited towards late instar mosquito prey, whereas a more stabilizing Type III functional response was displayed towards early instars. Accordingly, search efficiencies were lowest towards early instar prey, whereas, conversely, maximum feeding rates were highest towards this smaller prey type. However, when the prey types were present simultaneously, C. flavicans exhibited a significant positive preference for late instar prey, irrespective of water volume. Our results identify larval chaoborids as efficacious natural enemies of mosquito prey, with which they frequently coexist in aquatic environments. In particular, an ability to prey on mosquitoes across their larval stages, coupled with a preference for late instar prey, could enable high population-level offtake rates and negate compensatory reductions in intraspecific competition through size refuge.

Resistance to and repair of shell breakage induced by durophages in Late Ordovician brachiopods

Repaired shell breakage in Late Ordovician brachiopods from the Cincinnatian Series in the tri-state area of Indiana-Kentucky-Ohio may be described in increasing order of severity as scalloped, divoted, cleft and embayed. Concavo-convex brachiopod taxa display disproportionately higher frequencies of shell repair assigned to each category, whereas inflated, biconvex, plicate, sulcate taxa display disproportionately lower frequencies of shell repair. Certain plicate biconvex taxa lack examples of cleft and embayed valves. Plano-convex and dorsi-biconvex, costate taxa showed intermediate frequencies of shell repair, but lack representatives of embayed valves. Selective pressure for evolution of morphologic characters resistant to shell breakage may have favored phyletic trends of increasing size, geniculation and progressive development of a commissural ridge around the lophophore platform of the interior of the concave brachial valve of Leptaena and Rafinesquina. Size-frequency distributions for repaired and undamaged valves provide equivocal evidence of a size refuge from predator-induced shell breakage in Rafinesquina. Among the contemporaneous, potentially durophagous predators, nautiloids probably inflicted the sublethal injuries sustained by the brachiopods. The incriminating evidence includes a fragment of a crushing element imbedded in a valve of Rafinesquina that bears a very striking resemblance to calcified rhyncholites of Mesozoic to Recent nautiloids.

Some aspects of the foraging ecology of migrant juvenile sandpipers in the outer Bay of Fundy

Thirty-seven sandpipers (29 semipalmated and 8 least) were collected in early September 1983 on an intertidal flat on the outer Bay of Fundy. Analysis of esophagus and gizzard contents revealed a variety of benthic invertebrates but the diets of both species were dominated by the amphipod Corophium volutator (Pallas). Females of both species ate larger Corophium than did males of the same species. Both least and semipalmated sandpipers were underutilizing a cohort of large (7–10 mm) Corophium. It is possible that the large amphipods have reached a size refuge from predation such that the energy required to handle and ingest these large prey offsets too great a portion of the energy obtained.