Mark W. Denny

The structure and properties of spider silk

Abstract Silks from the spiders orb-web are amongst the very best structural materials produced by nature: spiders can produce a large aerial filter from a minimal amount of material. By studying how the structure and properties of silks are matched to their mechanical function, we can increase our general understanding of structure-property relationships in fibrous polymers.

Mechanical Limits to Size in Wave-Swept Organisms

Hydrodynamic forces imposed by ocean waves are thought to limit the size of nearshore plants and animals, but it has proved difficult to determine the mechanism. Explanations based on the scaling mismatch between hydrodynamic accelerational forces and the strength of organisms do not work. Mechanisms that incorporate the allometry of drag and strength accurately predict the maximal size of intertidal algae but not of animals, and internally imposed inertial forces may explain the limits to size in large kelps. The general question of size in wave-swept organisms remains open and intriguing.

The Largest, Smallest, Highest, Lowest, Longest, and Shortest: Extremes in Ecology

Biostatistics channels ecologists into thinking primarily about the mean and variance of a probability distribution. But many problems of biological interest concern the extremes in a variable (e.g., highest temperature, largest force, longest drought, maximum lifespan) rather than its central tendency. Such extremes are not adequately addressed by standard biostatistics. In these cases an alternative approach–the statistics of extremes–can be of value. In the limit of a large number of measurements, the probability structure of extreme values conforms to a generalized distribution described by three parameters. In practice these parameters are estimated using maximum likelihood techniques. Using this estimate of the probability distribution of extreme values, one can predict the expected time between the imposition of extremes of a given magnitude (a return time) and can place confidence limits on this prediction. Using data regarding sea—surface temperature, wave—induced hydrodynamic forces, wind speeds, and human life—spans we show that accurate long—term predictions can at times be made from a surprisingly small number of measurements if appropriate care is taken in the application of the statistics. For example, accurate long—term prediction of sea—surface temperatures can be derived from short—term data that are anomalous in that they contain the effects of an extreme EL Nino. In the cases of wave—induced forces and wind speeds, the probability distribution of extreme values is similar among years and diverse sites, indicating the possible existence of unifying principles governing these phenomena. Limitations and possible misuse of the method are discussed.

Locomotion: the cost of gastropod crawling.

The power of locomotion of a terrestrial slug rises linearly with crawling speed. The metabolic cost of movement is 904 joules per kilogram per meter, considerably more than that reported for other forms of locomotion. This high cost is primarily attributable to the production of the pedal mucus by which the slug adheres to the substratum.

Quantifying “wave exposure”: a simple device for recording maximum velocity and results of its use at several field sites

Abstract Numerous studies have established that exposure to wave action can affect all aspects of the life history of a marine organism, and can thereby have an important role in the structuring of marine populations and communities. However, relatively few studies have attempted to quantify the aspect of water motion that is likely to be of interest. In many cases, an important source of disturbance or mortality is due to the hydrodynamic forces generated by moving water. As a result, the maximum water velocity to which an organism is exposed will determine whether it can survive in a given environment. Here we describe a maximum velocity recorder that is a modification of the described dynamometer. The device is simple and inexpensive to build and easily deployed in large numbers. A mathematical model is presented that describes the conditions under which the device provides accurate estimates of maximal water velocity, and when it should be used with caution. A number of recorders were used to quantify maximal daily water velocities at three intertidal habitats on the central coast of California. The correlation of these measurements with qualitative descriptions of each site is discussed.

Surviving hydrodynamic forces in a wave-swept environment: Consequences of morphology in the feather boa kelp, Egregia menziesii (Turner)

Organisms on wave-swept intertidal shores often must withstand water velocities of 10 to 20 m/s and accompanying accelerations of 400 m/s2. Because drag and accelerational forces increase with area and volume respectively, the bigger an individual is, the larger the forces on it become. This size-force relationship suggests that large intertidal algae must be particularly well designed if they are to survive. The feather boa kelp, Egregia menziesii (Turner), grows to a size that is unusually large for its wave-exposed habitat, leading us to inquire into the mechanisms that allow it to attain this stature. A hydro-mechanical computer model is used to predict the magnitude of the forces on E. menziesii in various wave conditions and for various plant sizes. Based on measurements of the strength of E. menziesii, an intact, average-sized plants survivorship in the range of predicted wave forces is estimated to be nearly 100%. However, entanglement with other plants and damage from herbivory can substantially lower an individuals chances of surviving a severe storm. We speculate that E. menziesiis straplike shape may be an adaptation for accommodating the unpredictable distribution of tensions along the plants stipe and for reducing drag.

Lift as a mechanism of patch initiation in mussel beds

Abstract The mussel Mytilus californianus is the dominant competitor for space in the mid-intertidal zone of wave-swept rocky shores in the Pacific Northwest where it forms extensive tightly packed beds. The rate at which patches are formed in these beds, can play an important role in community ecology by controlling the establishment and persistence of fugitive species. Despite the biological importance of physical disturbance, the mechanism of patch initiation has not been adequately explained. Battering by logs can create patches, but is the predominant mechanism only on shores near active logging sites. In other areas, it has been speculated that the hydrodynamic forces associated with storm waves somehow cause patches to form. However, the forces acting along the direction of flow — drag and the acceleration reaction — are unlikely to initiate patch formation. Here, it is suggested that fluid-dynamic lift forces imposed on mussel beds by breaking waves are sufficient to dislodge individual mussels and trigger patch formation. Arguments are presented suggesting that the likelihood of dislodgment by lift is consistent with the observed rate of patch formation in the absence of log battering.

Hot limpets: predicting body temperature in a conductance-mediated thermal system

SUMMARY Living at the interface between the marine and terrestrial environments, intertidal organisms may serve as a bellwether for environmental change and a test of our ability to predict its biological consequences. However, current models do not allow us to predict the body temperature of intertidal organisms whose heat budgets are strongly affected by conduction to and from the substratum. Here, we propose a simple heat-budget model of one such animal, the limpet Lottia gigantea, and test the model against measurements made in the field. Working solely from easily measured physical and meteorological inputs, the model predicts the daily maximal body temperatures of live limpets within a fraction of a degree, suggesting that it may be a useful tool for exploring the thermal biology of limpets and for predicting effects of climate change. The model can easily be adapted to predict the temperatures of chitons, acorn barnacles, keyhole limpets, and encrusting animals and plants.

The Effects of Hydrodynamic Shear Stress on Fertilization and Early Development of the Purple Sea Urchin Strongylocentrotus purpuratus

Life in the highly turbulent surf zone poses a severe challenge to reproduction in free-spawning animals. Not only can breaking waves quickly dilute the gametes shed by spawning organisms, but turbulence-induced shear stresses may limit fertilization and interfere with normal development. A Couette cell was used to re-create some of the effects of turbulent water motion to study effects of environmentally relevant shear stresses on fertilization in the purple sea urchin (Strongylocentrotus purpuratus). Although low shear stresses improved fertilization success (presumably by increasing mixing), exposure to high shear stresses (of the magnitude found in the surf zone) substantially decreased fertilization success, probably by interfering with contact between egg and sperm. Furthermore, eggs fertilized at high shear stresses often showed abnormal development and low survival of eggs through the blastula stage.

Thermal stress on intertidal limpets : long-term hindcasts and lethal limits

SUMMARY When coupled with long-term meteorological records, a heat-budget model for the limpet, Lottia gigantea, provides a wealth of information regarding environmental and topographic controls of body temperature in this ecologically important species. (1) The maximum body temperature predicted for any site (37.5°C) is insufficient to kill all limpets, suggesting that acute thermal stress does not set an absolute upper limit to the elevation of L. gigantea on the shore. Therefore, the upper limit must be set by behavioral responses, sublethal effects or ecological interactions. (2) Temperatures sufficient to kill limpets are reached at only a small fraction of substratum orientations and elevations and on only three occasions in 5 years. These rare predicted lethal temperatures could easily be missed in field measurements, thereby influencing the interpretation of thermal stress. (3) Body temperature is typically higher than air temperature, but maximum air temperature can nonetheless be used as an accurate predictor of maximum body temperature. Warmer air temperatures in the future may thus cause increased mortality in this intertidal species. Interpretation of the ecological effects of elevated body temperature depends strongly on laboratory measurements of thermal stress, highlighting the need for additional research on the temporal and spatial variability of thermal limits and sublethal stress. The lengthy time series of body temperatures calculated from the heat-budget model provides insight into how these physiological measurements should be conducted.