No abstract

No abstract

No abstract

Plants and animals that inhabit the intertidal zone of wave-swept shores are generally small relative to terrestrial or subtidal organisms. Various biological mechanisms have been proposed to account for this observation (competition, size-specific predation, food-limitation, etc.). However, these biological mechanisms are constrained to operate within the mechanical limitations imposed by the physical environment, and these limitations have never been thoroughly explored. We investigated the possibility that the observed limits to size in wave-swept organisms are due solely or in part to mechanical, rather than biological, factors.The total force imposed on an organism by breaking waves and postbreaking flows is due to both the water's velocity and its acceleration. The force due to velocity (a combined effect of drag and lift) increases in strict proportion to the organism's structural strength as the organism increases in size, and therefore cannot act as a mechanical limit to size. In contrast, the force due to the water's acceleration increases faster than the organism's structural strength as the organism grows, and thus constitutes a potential mechanical limit to its size. We incorporated this fact into a model that predicts the probability that an organism will be destroyed (by breakage or dislodgement) as a function of five parameters that can be measured empirically: (!) the organism's size, (2) the organism's structural strength, (3) the maximum water acceleration in each wave, (4) the maximum water velocity at the time of maximum acceleration in each wave, and (5) the probability of encountering waves with given flow parameters.The model was tested using a variety of organisms. For each, parameters 1-4 were measured or calculated; the probability of destruction, and the size-specific increment in this probability, were then predicted. For the limpets Collisella pelta and Notoacmaea scutum, the urchin Strongylocentrotus purpuratus, the mussel Mytilus californianus (when solitary), and the hydrocoral Millepora complanata, both the probability of destruction and the size-specific increase in the risk of destruction were determined to be substantial. It is conjectured that the size of individuals of these species may be limited as a result of mechanical factors, though the case of M. complanata is complicated by the possibility that breakage may act as a dispersal mechanism. In other cases (the snails Thais canaliculata, T. emarginata, and Littorina scutulata; the barnacle Semibalanus cariosus), the size-specific increment in the risk of destruction is small and the size limits imposed on these organisms are conjectured to be due to biological factors.Our model also provides an approach to examining many potential effects of environmental stress caused by flowing water. For example, these methods may be applied to studies of: (!) life-history parameters (e.g., size at first reproduction, age at first reproduction, timing of reproductive cycles, length of possible reproductive lifetime), (2) the effects of...

No abstract

For the past 30 yr wave—swept shores have served as a model system for experimentation in community ecology. Due in large part to the severity of the physical environment, individual plants and animals are frequently disturbed, turnover in the community is rapid, and experiments can be conducted in months which in other habitats would require years. However, the experimental advantage of rapid turnover must be weighed against our ability to account for its causes. Only if we can predict the rate of turnover can we predict the dynamics of the community. On wave—swept shores where disturbance is dominated by environmental effects, the ultimate ability to predict community structure rests on the the proximal ability to predict the physical environment and to understand its consequences. Some important aspects of the wave—swept environment (such as the tides) are well understood, but the effects of wave—induced hydrodynamic forces, perhaps the predominant environmental stress on shoreline organisms, has been thought to be unpredictable. Indeed, the stochastic nature of ocean waves precludes the short—term prediction of wave forces. However, as with the random motion of molecules in a gas, the short—term unpredictability of the ocean's surface can form the basis for a robust statistical approach to the prediction of long—term events. This study employs the statistics of the random sea to predict the largest wave to which a littoral site will be subjected in a year (≈5.9 times the yearly average significant wave height), and uses hydrodynamic theory to predict the force that this large wave exerts on individual organisms. The result is a quantitative measure of wave exposure, a mechanistic link between local wave climate and species—specific survivorship that can be used as a tool for exploring the relationship between environmental severity and community ecology. The proposed method is tested by predicting the rate at which patches of bare substratum are formed in beds of the mussel Mytilus californianus, a dominant competitor for space on rocky shores in the Pacific Northwest. Predicted rates are very similar to those measured in the field, suggesting that this method can provide useful input into models of intertidal patch dynamics. Data from several sites around the world suggest that the yearly average waviness of the ocean at any particular site can (over the course of decades) vary by as much as 80% of the long—term mean. The methodology proposed here allows this decade—to—decade variation in wave climate to be translated into the resulting variation in survivorship; predicting, for example, that an increase of 1 m in yearly average significant wave height results in a fourfold increase in the rate of patch formation in a mussel bed. Such a shift would have substantial consequences for community dynamics. M. californianus is unusual in that the expected wave—induced stress is near the species' modal strength, and it will be of interest to determine if this characteristic is common among dominant competitors for s...

The intertidal zone of wave-swept rocky shores is characterized by high velocities and exceedingly rapid accelerations. The resulting hydrodynamic forces (drag, lift, and the accelerational force) have been hypothesized both to set an upper limit to the size to which wave-swept organisms can grow and to establish an optimal size at which reproductive output is maximized. This proposition has been applied previously to intertidal animals that grow isometrically, in which case the accelerational force is the primary scaling factor that constrains size. In contrast, it has been thought that the size of waveswept algae is limited by the interaction of drag alone with these plants' allometric pattern of growth.Here we report on empirical measurements of drag and accelerational force in three common species of intertidal algae (Gigartina /eptorhynchos, Pelvetiopsis limitata, and Iridaea j/accida). The drag coefficients for these species decrease with increased water velocity, as is typical for flexible organisms. For two of these species, this decline in drag coefficient is dramatic, leading to small drag forces with concomitant low drag-induced mortality at plant sizes near those observed in the field. However, all three species have surprisingly large inertia coefficients, suggesting that these plants experience large accelerational forces in surf-zone flows. Preliminary calculations show that these accelerational forces combine with drag to act as a size-dependent agent of mortality, constraining the size of these algae.This study further models the interplay between size-dependent survivorship and reproductive ability to predict the size at which reproductive output peaks. This "optimal size" depends on the strength distribution and morphology of the algal species and on the flow regime characteristic of a particular site. This study shows that the optimal size predicted for G. /eptorhynchos, calculated using velocities and accelerations typical of the moderately protected location where this species was collected, closely matches its observed mean size. Similarly, the predicted optimal sizes of P.limitata and l.f/accida at the exposed site where these plants were sampled also match their mean observed sizes. These data, although preliminary, suggest that mechanical factors (in particular the accelerational force) may be important in limiting the size of intertidal macroalgae and that attention solely to biological constraints may be inappropriate.

Lignified cell walls are widely considered to be key innovations in the evolution of terrestrial plants from aquatic ancestors some 475 million years ago. Lignins, complex aromatic heteropolymers, stiffen and fortify secondary cell walls within xylem tissues, creating a dense matrix that binds cellulose microfibrils and crosslinks other wall components, thereby preventing the collapse of conductive vessels, lending biomechanical support to stems, and allowing plants to adopt an erect-growth habit in air. Although "lignin-like" compounds have been identified in primitive green algae, the presence of true lignins in nonvascular organisms, such as aquatic algae, has not been confirmed. Here, we report the discovery of secondary walls and lignin within cells of the intertidal red alga Calliarthron cheilosporioides. Until now, such developmentally specialized cell walls have been described only in vascular plants. The finding of secondary walls and lignin in red algae raises many questions about the convergent or deeply conserved evolutionary history of these traits, given that red algae and vascular plants probably diverged more than 1 billion years ago.