Michael Labarbera

Taphonomic Bias in Analyses of Drilling Predation: Effects of Gastropod Drill Holes on Bivalve Shell Strength

The proportion of bivalve shells with drill holes in fossil assemblages is commonly used as a measure of the intensity of predation by drilling gastropods. Previous studies have assumed that drilled and undrilled bivalve shells have equal preservation potentials. We tested this assumption by measuring the mechanical strength of drilled and undrilled valves of the Recent bivalve Mulinia lateralis. Under compressive loads, drilled valves are significantly weaker than undrilled valves, a difference we attribute to local stress concentrations produced by the presence of the drill hole. Our results suggest that drilled values may break preferentially and hence inferred patterns of predation may reflect taphonomic as well as biological processes

Brachiopod orientation to water movement 1. Theory, laboratory behavior, and field orientations

Hydrodynamic principles and experiments with empty shells predict that pres- sure distributions around brachiopod shells generated by ambient currents should, depending on the orientation of the shell relative to the current, either augment or oppose the ciliary-driven flow of water through the lophophore. For living articulate brachiopods with plectolophes or spirolophes, orientations where the anterior-posterior axis of the shell is parallel to the cur- rent direction should result in pressure distributions which oppose active pumping. This ef- fect should be strongest when the excurrent region of the shell faces into the current. Orienta- tions where the anterior-posterior axis is perpendicular to the current direction should result in pressure distributions which act in concert with active pumping, most strongly when one of the incurrent regions is directed into the current. These effects are independent of specific shell shape. (2) Laqueus californianus and Terebratulina unguicula actively reorient to currents in the laboratory, preferring orientations where the anterior-posterior axis of the shell is perpendicu- lar to the current and the right-left axis is parallel to the current. Both species may tra- verse an arc as great as 1200 to achieve their final orientation. Hemithyris psittacea also will actively reorient to currents, moving towards orientations where the anterior-posterior axis is perpendicular to the current. The maximum rotation observed for H. psittacea was 450. Terebratalia transversa never reoriented in the laboratory. (3) Using epifaunal hydroid colonies as indicators of current direction, both Hemithyris psittacea and Terebratalia transversa are oriented in nature with the anterior-posterior axis of the shell perpendicular to the prevailing currents. While scuba diving, I confirmed this orientation phenomenon for T. transversa by direct measurement of the orientation of the brachiopods relative to prevailing currents. (4) Larval Terebratalia transversa avoid areas with current speeds greater than about 0.2 cm/s during metamorphosis and show no orientation to the ambient currents immediately after metamorphosis. Post-metamorphic T. transversa can actively reorient on the pedicle. The orientation observed in adults is probably achieved by active reorientation to local cur- rents of post-zygolophe juveniles. (5) Threshold current speeds for reorientation in Laqueus californianus and Terebratulina unguicula are low and approximately equal to the excurrent pumping speeds of each species; dynamic pressure rather than viscous entrainment is probably the relevant factor determining reorientation behavior.

Ammonites as Cartesian divers

Many constructional and taphonomic incongruencies dissolve if ammonites are modeled as active Cartesian divers, whose last septum remained a rubber-like, uncalcified membrane until the next chamber cycle began. If the prochoanitic invagination of the septal mantle served as a gas gland able to produce gas at or above ambient pressure, the chamber behind could be operated like a swim bladder by sutural muscles spanning across the septal mantle. Other septal functions probably served as evolutionary stepping stones on the way to this unique design.

The 1996 Lindberg Award : Calcium antagonists alter cell shape and induce procollagenase synthesis in keloid and normal human dermal fibroblasts

Fibroblast cytomorphology is tightly coupled to phenotypic expression, particularly as it relates to extracellular matrix protein synthesis and degradation. We have observed that calcium antagonists, such as verapamil and trifluoperazine, depolymerize actin filaments and alter fibroblast cell shape from bipolar to spherical. Characteristically, the depolymerization of actin filaments, which mediates the cell shape change, turns on procollagenase gene expression in normal human skin fibroblasts. We have found the same effects of calcium antagonists on cell shape, cytoskeletal components, and induction of procollagenase in the keloid fibroblasts of three cell lines, CB792, CW792, and WT949. Rounded cells were seen in 74.8% of verapamil-treated and 86.7% of trifluoperazine-treated cells, whereas only 1.1% of the control cells were spherical. The percentage of cells that synthesized collagenase in the control, verapamil-treated, and trifluoperazine-treated groups was 3.8%, 42.8%, and 53.4%, respectively. Approximately 60% of rounded cells exhibited increased collagenase synthesis when the cells were treated with a calcium antagonist. These results indicate considerable heterogeneity in the phenotypic response to morphologic change. The amount of procollagenase synthesized in a cell was estimated by the fluorescence intensity of the fluorescein-labeled antibody. The normalized fluorescence intensity of procollagenase in the control cells was about 2 to 2.6 times that of background. In contrast, the normalized fluorescence intensity of procollagenase in the calcium antagonist-treated cells was about 2.4 to 12 times that of background. This high intensity level indicates an increase in procollagenase production in the calcium antagonist-treated cells. Calcium green dye used to study cytosolic calcium revealed that after cells were treated with verapamil, the cytosolic calcium ion concentration first increased and then decreased. The change of cytosolic calcium ion concentration may be related to the depolymerization of actin filaments and the alteration of cell shape.

Modes of Feeding in Aggregations of Barnacles and the Shape of Aggregations

The interactions between the form of a barnacle aggregation, its flow environment, and the feeding behavior of each individual was determined in unidirectional flows; both models of barnacle aggregations and live barnacles were used. Hill-shaped aggregations of model barnacles captured significantly more particles than flat aggregations. In general, rows upstream of, and at the peak of, all hill-shaped profiles captured significantly more particles than downstream rows. Living barnacles located at, or upstream of, the peak of natural clusters captured significantly more food particles than did barnacles located downstream. Living barnacles located at, or upstream of, the highest point in a natural cluster fed passively, whereas barnacles downstream of the peak actively swept their cirral net against the flow. Flow was laminar up to the highest point in natural clusters, whereas flow was both reduced and turbulent over the downstream portions. Individual barnacles within a cluster differ in their feeding rates and net energy gains, and therefore differ in their growth such that, in unidirectional flow, the peak of a cluster will shift upstream over time; in oscillating flows, the clusters will develop a symmetrical profile.

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.

Effects of size and behavior on aerial performance of two species of flying snakes (Chrysopelea).

SUMMARY Aerial locomotion in snakes (genus Chrysopelea) is kinematically distinct from any other type of gliding or powered flight, with prominent, high amplitude body undulations visually dominating the behavior. Because it is not known how flying snakes produce aerodynamic forces in flight, the factors that determine snake flight performance are not clear. In this study, the effects of size and behavior on aerial performance were examined both within a species (C. paradisi) and between two species (C. paradisi and C. ornata), using stepwise multiple regressions to identify relevant variables. Smaller C. paradisi traveled farther than larger snakes at lower sinking speeds, with trajectories that shallowed more quickly and reached lower minimum glide angles. Although wing loading increased faster than expected for isometric size increase, wing loading per se was not responsible for performance differences between large and small snakes. Snakes with higher interactions between relative undulation amplitude and body size transitioned out of the initial acceleration phase at higher airspeeds and sinking speeds, and attained higher maximum airspeeds and horizontal speeds; snakes that used higher average relative amplitudes transitioned out of the initial acceleration phase at higher horizontal speeds. Undulation frequency was not significantly related to any performance variable within C. paradisi and was not significantly different between the two species, suggesting that this variable (in contrast to relative undulation amplitude) may have a minor influence on the aerodynamic mechanism of force production in snake flight. C. paradisi and C. ornata differed significantly in most performance comparisons. C. ornata were more massive than C. paradisi at any given body length and in general exhibited poorer gliding performance than C. paradisi. This study contributes towards understanding how an unconventional body form and kinematics can produce a novel mode of aerial locomotion in a vertebrate glider.

Effects of constant mechanical tension on the healing of rabbit flexor tendons

The biomechanical effects of constant mechanical load on tendon repair in vitro were determined for rabbit flexor tendons. Tendons were removed from Zone II, transected, reapproximated with four simple sutures, and cultured in standard medium. Tendons from the right forelimbs were loaded with 3.1-g weights; tendons from the contralateral forelimbs served as unloaded tendons. Tenorrhaphies were disrupted at zero, one, three, and six weeks postsuturing by fixed-speed tensiometry. True maximum stress (strength), normalized energy absorbed, and tangent modulus steadily increased over time, becoming significantly greater than unincubated controls in the loaded and unloaded groups at six weeks. True strain at maximum stress increased with duration for unloaded tendons; after six weeks it was significantly greater than unincubated control tendons. This study demonstrates a method for quantifying the biomechanics of tendon after intrinsic tendon segment healing and presents the first biomechanical evaluation of constant tension applied across the laceration site during an in vitro healing phase.

Endogenous versus Toxin-induced Diabetes in Rats: A Mechanical Comparison of Two Skin Wound-healing Models

This study was designed to compare skin wound healing in three groups of Wistar rats: normal, genetically diabetic, and streptozotocin-induced diabetic. All diabetic animals received daily insulin. Full-thickness mid-line dorsal skin wounds were analyzed biomechanically for strength, toughness, and elasticity (Youngs modulus) at 1 and 3 weeks after wounding. Wounds from normal controls were the strongest, toughest, and least compliant. Genetically diabetic rat wounds were the weakest and had the lowest elastic modulus. Wounds from the streptozotocin-induced rats were intermediate for all parameters measured (ANOVA, p = 0.001). Toxin-induced diabetes is less detrimental to skin wound healing than diabetes of endogenous origin. Whether this is due to basic differences in the models or to differences in duration of diabetes is unknown.

FLEXIBILITY: A MECHANISM FOR CONTROL OF LOCAL VELOCITIES IN HYDROID COLONIES

A design conflict exists in passive suspension feeding colonies between maximizing surface area for feeding and minimizing drag-related forces on the colony. The importance of colony flexibility as a homeostatic mechanism was demonstrated experimentally on the scale of both the entire colony and the polyp. On the colony scale, flexibility reduces the relationship between drag and water velocity from a square to a first power dependence. This finding is consistent with the discovery that flexion in trees also reduces drag to a linear function of velocity. On the polyp scale, colony flexibility strongly damps flow velocity changes at the polyp over at least an order of magnitude change in ambient velocity. This previously unappreciated consequence of flexibility may be an important selective force affecting the evolution of colony form. The separate consequences of flexion at the polyp and whole colony level are considered in a simple conceptual model incorporating polyp feeding success and colony detachment probability over a range of flow velocities. Inspection of the model reveals that the lower velocity limit at which a colony can survive is likely to be constrained by polyp feeding success, while the upper velocity limit may be constrained by either polyp feeding success or the probability ofcolony detachment.