Rachel Ann Merz

Flow mediates predator-prey interactions between triclad flatworms and larval black flies

The velocity preferences of suspension-feeding larval black flies (Simulium vittatum) and predatory triclads (Dugesia dorotocephala), both alone and together, were determined in a laboratory flow chamber in which a gradient of current velocities was maintained and manipulated (...) These results clearly demonstrate the potential importance of flow in mediating predator-prey interactions in benthic stream communities

Predator Prey Interactions In A Benthic Stream Community: A Field Test Of Flow-Mediated Refuges

Abstract Ecological theory suggests that the impact of predation can be strongly modified by the existence of regions of the environment in which prey are less accessible to predators, which underscores the need for empirical studies examining the factors influencing the availability and importance of such prey refuges. Our study tested whether benthic microhabitats with high flows provide suspension-feeding larval black flies (Simulium␣vittatum) with a spatial refuge in which the negative impact of predatory flatworms (Dugesia dorotocephala) is reduced. We conducted a short-term field experiment in Chester Creek (southeastern Pennsylvania, United States) to examine how the number of black fly larvae inhabiting tile substrates responded to manipulated variations in flatworm abundance and current speed. The abundance of flatworms declined with increasing current speed, thereby creating the potential for sites with high flows to provide larvae with a refuge from these predators. Multiple regression analysis revealed that the final abundance of larvae exhibited a significant negative relationship to flatworm abundance and a significant positive relationship to current speed. After adjusting for variations in elapsed time and initial larval abundance, flow and predators explained 38% of the variation in the rate of change in larval abundance. The positive correlation between larval abundance and flow had two components: a positive, direct effect of flow on larvae, which arises because these food-limited consumers prefer to reside within sites with faster flows where they can feed at higher rates; and a negative effect of flow on predators, and of predators on larvae, which combine to yield a positive indirect effect of flow on larvae. This indirect effect demonstrates the existence of flow-mediated refuges (i.e., microhabitats in which the impact of predation is reduced due to high flows), although the effect accounts for a small proportion of total variation in larval abundance. A consideration of biomechanical relationships suggests that microhabitats with high flows are likely to create prey refuges in a wide range of freshwater and marine benthic environments. In particular, predators will often experience greater dislodgement forces than prey because of their larger size and because they project farther above the bed where current speeds are faster. Moreover, the ability to resist a given dislodgement force may be greater for many prey, especially those that are sessile or semi- sessile.

Resilin in dragonfly and damselfly wings and its implications for wing flexibility

Although there is mounting evidence that passive mechanical dynamics of insect wings play an integral role in insect flight, our understanding of the structural details underlying insect wing flexibility remains incomplete. Here, we use comparative morphological and mechanical techniques to illuminate the function and diversity of two mechanisms within Odonata wings presumed to affect dynamic wing deformations: flexible resilin vein‐joints and cuticular spikes. Mechanical tests show that joints with more resilin have lower rotational stiffness and deform more in response to a load applied to an intact wing. Morphological studies of 12 species of Odonata reveal that resilin joints and cuticular spikes are widespread taxonomically, yet both traits display a striking degree of morphological and functional diversity that follows taxonomically distinct patterns. Interestingly, damselfly wings (suborder Zygoptera) are mainly characterized by vein‐joints that are double‐sided (containing resilin both dorsally and ventrally), whereas dragonfly wings (suborder Epiprocta) are largely characterized by single‐sided vein‐joints (containing resilin either ventrally or dorsally, but not both). The functional significance and diversity of resilin joints and cuticular spikes could yield insight into the evolutionary relationship between form and function of wings, as well as revealing basic principles of insect wing mechanical design. J. Morphol., 2011.

Postmortem changes in strength of gastropod shells: evolutionary implications for hermit crabs, snails, and their mutual predators

Calliostoma ligatum shells inhabited by hermit crabs were weaker than shells inhabited by snails collected at the same locality. When shells of C. ligatum were loaded repetitively to 80% of their predicted failure load, hermit crab-inhabited shells showed an immediate drop in shell strength followed by a progressive further loss of shell strength over the next 18 days. Snailinhabited C. ligatum shells exhibited a decrease in strength after 9 days, but returned to initial values within 18 days of loading. Hermit crabs thus bear shells significantly weaker than they were when borne by the gastropods that produced them. Reported similarities in vulnerability of gastropods and hermit crabs to shell-crushing predators may be artifacts of the metric (critical size) used to compare vulnerabilities. Hermit crabs probably were a significant factor in the diversification of durophagous predators in the Mesozoic, supplying a prey base identical in size and shape to gastropods but with significantly lower resistance to crushing. The unpredictability of strength in hermit-crab inhabited shells may maintain the apparently inefficient indiscriminate attacks common among durophagous predators.

Polychaete chaetae: Function, fossils, and phylogeny.

In this article we review the phylogenetic distribution of major chaetal types within the Polychaeta, discuss what has been demonstrated about chaetal function in modern worms, and examine what is known about the evolution of chaete through the fossil record. We conclude with specific cautions about how chaetae are treated in phylogenetic analyses and make suggestions about how they could be used to provide a stronger phylogenetic signal.

Holding on by their hooks: anchors for worms

We examined the hooked setae of a dominant group of tube‐dwellers, the polychaete annelids, and found a pattern of setation that is predictable by tube type, exclusive of worm taxon or orientation; we also demonstrated the mechanical significance of these hooked setae. When tube‐dwelling worms belonging to different lineages are pressurized, they resist differentially as a function of the direction in which hooks face. The results are consistent with the hypothesis that hooks are used primarily to resist removal of worms from their tubes, are polyphyletic in origin, and are active agents of resistance.

Feeding postures of suspension-feeding larval black flies: the conflicting demands of drag and food acquisition

SummaryWe tested whether larval black flies actively control the positioning of their feeding appendages (labral fans), and if so, whether their posture represents a balance between the conflicting demands of drag and feeding. We compared the postures of live larvae with the postures of larvae killed by heat-shock in three different flow regimes in a laboratory experiment; we assumed that the postures of heat-killed larvae approximated a passive response to drag. The average height of the labral fans above the bed declined significantly in faster flows, and was significantly greater in live than dead larvae. There was also a significant interaction effect, since the difference between the fan heights of live and dead larvae was greater in slower flows. Two mechanisms may contribute to this result. Larvae in slower flows have to increase their fan heights more than larvae living in faster flows to achieve comparable increases in velocity and thus particle flux. In addition, muscular strength may limit the feeding postures larvae can assume. The fan heights of live larvae also varied depending on the concentration of food particles: larvae exposed to low food concentrations held their fans higher above the bed than did larvae exposed to high food concentrations in the same flow regime. This change in posture is due neither to an uneven particle concentration in the boundary layer nor to added drag from particles trapped in the labral fans. Collectively, our results indicate that these suspension feeders actively control their feeding posture, and suggest that these varying postures represent a dynamic balance between the conflicting needs of minimizing drag and maximizing feeding.

Jointed setae – their role in locomotion and gait transitions in polychaete worms

Abstract Many families of polychaete worms have jointed setae in which the joint is external to the body and is not directly controlled by muscles or nerves. We assessed the role of these specialized structures in the hesionid polychaete, Ophiodromus pugettensis , by examining speed, step length, stride distance, stride frequency and gait transitions in worms with and without setal joints. Individual worms were videotaped while they moved over sandy surfaces at a range of speeds. The worms were then anaesthetized and all their compound setae were trimmed either distally or proximally to the setal joints. After two days of recovery the worms were videotaped a second time while they again moved over sandy surfaces at a range of speeds. From the video tapes we analyzed their locomotory performance before and after setal ablation. Animals in which the setae were shortened but in which the joint was left intact showed no consistent change in speed, step length, stride distance, stride frequency or gait transitions. Animals in which the joint had been removed both changed gaits at slower speeds (walking to undulatory walking and undulatory walking to swimming) and showed a significant decrease in maximum swimming speeds and stride distance. A subset of data containing only cases where the worms were moving at the same speed in the same gait before and after setal ablation was analyzed. In these instances, after the removal of the joint, the worms had significantly smaller stride distances and compensated for this by increasing stride frequency. In O. pugettensis , the undulatory walking gait is analogous to the trot–gallop transition in quadrupedal mammals because the animal switches from moving the appendages on a relatively rigid body to using a combination of body flexion and appendage movement to achieve propulsion, however, unlike quadrupedal mammals this transition takes place over a wide range of speeds and at different sites on the body as speed increases. These experiments indicate that jointed setae may be important both in allowing a worm to better control setal contact and traction with the substrate as well as in altering the effectiveness of its swimming stroke.

Chaetae and mechanical function: tools no Metazoan class should be without

To address the functional contributions of capillary chaetae in the maldanid polychaete Clymenella torquata, we compared irrigation efficiency and tube structure for animals with intact and trimmed capillary chaetae. We measured pumping rates for worms before and after they were anaesthetized and subjected either to capillary trimming or mock trimming, i.e. handling without trimming. Worms with trimmed chaetae were significantly less effective at moving water through their tubes than those with intact chaetae. There were no significant differences in the ability of control worms to move water within their tubes. No significant changes in rates of peristalsis were observed among experimental or control groups. These data strongly suggest that body musculature and capillary chaetae work in concert to hold worms in position within tubes during peristaltic pumping. When chaetae are shortened, the body musculature must contract to a greater degree, increasing the functional diameter of the worm to achieve the necessary traction with the tube wall, resulting in less efficient irrigation. We also compared the inner diameters of original field tubes to tubes built by control worms or worms after capillary trimming. The inner diameters of new tubes built by worms with shortened chaetae were larger than their original tubes, while those of both control groups were not. One possible explanation is that the chaetae have a sensory role and shortened chaetae send the false message that the nascent tube walls are farther away than they are, the body contracts in compensation and the tube is widened, however this idea has not been tested.