Amy S. Johnson

Thick vs. thin : thallus morphology and tissue mechanics influence differential drag and dislodgement of two co-dominant seaweeds

Abstract The lower rocky intertidal zone of many moderately exposed and exposed shores in the Gulf of Maine, USA, is co-dominated by two species of red macroalgae, Chondrus crispus Stackhouse and Mastocarpus stellatus (Stack. in With.) Guiry. These species are anatomically, morphologically, ecologically and phylogenetically similar. We quantified: 1. (1) mechanical properties of the stipe; and 2. (2) flow forces on the stipe relative to thallus area and biomass of these species, to determine mechanical and morphological characteristics that could explain the greater winter dislodgement of C. crispus thalli in mixed stands. Although stipes of both species broke at the same mean force, C. crispus stipes were relatively thick, weak and extensible compared to the relatively thin, strong and stiff stipes of M. stellatus . Risk of breakage increased with size in both species because: 1. (1) their stipes weakened with increasing cross-sectional area; and 2. (2) cross-sectional area of the stipe failed to increase in linear proportion to frond area. Drag on C. crispus thalli of > 3 g fresh weight was greater than on M. stellatus of the same biomass, whereas drag on smaller fronds ( C. crispus thalli was greater mainly because they exhibit greater surface area for a given biomass than do large M. stellatus . Dislodgement by hydrodynamic forces has more severe ecological consequences for M. stellatus because regeneration from holdfasts is slower than in C. crispus . A reduced surface area:biomass ratio coupled with greater strength may lessen wave-induced disturbance and be important to maintaining a high abundance of M. stellatus in the low intertidal zone of wave-swept shores.

Drag, Drafting, and Mechanical Interactions in Canopies of the Red Alga Chondrus crispus

Dense algal canopies, which are common in the lower intertidal and shallow subtidal along rocky coastlines, can alter flow-induced forces in their vicinity. Alteration of flow-induced forces on algal thalli may ameliorate risk of dislodgement and will affect important physiological processes, such as rates of photosynthesis. This study found that the force experienced by a thallus of the red alga Chondrus crispus (Stackhouse) at a given flow speed within a flow tank depended upon (1) the density of the canopy surrounding the thallus, (2) the position of the thallus within the canopy, and (3) the length of the stipe of the thallus relative to the height of the canopy. At all flow speeds, a solitary thallus experienced higher forces than a thallus with neighbors. A greater than 65% reduction in force occurred when the thallus drafted in the region of slower velocities that occurs in the wake region of even a single upstream neighbor, similar to the way racing bicyclists draft one behind the other. Mechanical interactions between thalli were important to forces experienced within canopies. A thallus on the upstream edge of a canopy experienced 6% less force than it did when solitary, because the canopy physically supported it. A thallus in the middle of a canopy experienced up to 83% less force than a solitary thallus, and forces decreased with increasing canopy density. Thus, a bushy morphology that increases drag on a solitary thallus may function to decrease forces experienced by that thallus when it is surrounded by a canopy, because that morphology increases physical support provided by neighbors.

Strength, Drag, and Dislodgment of Two Competing Intertidal Algae from Two Wave Exposures and Four Seasons

Abstract Intertidal macroalgae often experience greater risk of dislodgment with increasing size because of underscaling of breaking force of their stipes relative to drag on their thalli. This ratio (breaking force/drag) indicates safety from breakage at a given flow speed, with values greater than one indicating safety from breakage and values lower than one indicating danger of breakage. We examined this force ratio for the largest thalli of two species of co-dominant, red algae, Chondrus crispus Stackhouse and Mastocarpus stellatus Stack. In With. (Guiry), in four seasons at two wave exposures. During fall and winter, the largest thalli in both populations were dislodged resulting in a decrease in mass of the largest thalli found. This decrease was greater for Chondrus than for Mastocarpus , but their mass-specific force ratios (at 0.55 m s −1 ) were equal indicating similar size-specific risk of dislodgment. The equality of force ratios was underlain by two similarities: (1) breaking force was independent of mass and not different between species; (2) mass-specific drag was not different between species. These similarities were underlain by dissimilar causes: (i) similarity in breaking force (the product of cross-sectional area and material strength) occurred because greater material strength of Mastocarpus compensated for greater mass-specific cross-sectional area of Chondrus ; (ii) similarity in mass-specific drag (a function of planform area and the coefficient of drag) occurred because greater drag coefficients for Mastocarpus compensated for greater mass-specific planform areas of Chondrus . The similarity in force ratio, if it held at season- and site-relevant flow speeds, would suggest that during seasons of minimal growth and high wave exposure, the mass of the largest thalli of both species should be the same. Chondrus , however, had a greater mass at both sites in all seasons. Chondrus experienced greater decreases in mass probably because it grew larger and larger thalli are less safe. Extrapolation of a site-relevant force ratio for Chondrus in the fall revealed (1) that the site-relevant force ratio did not differ between exposures even though the mass-specific force ratio was greater at the protected site, and (2) a paradoxical result that all Chondrus thalli studied ought to have dislodged, but had not. This paradox may be resolved by consideration of the protection conferred by canopies of Chondrus : a canopy may effectively raise its site-relevant force ratio. Perhaps differences in protection conferred by different canopies explain why larger Chondrus persist with Mastocarpus even given a similarity in mass-specific force ratio.

Are melanized feather barbs stronger

SUMMARY Melanin has been associated with increased resistance to abrasion, decreased wear and lowered barb breakage in feathers. But, this association was inferred without considering barb position along the rachis as a potentially confounding variable. We examined the cross-sectional area, breaking force, breaking stress, breaking strain and toughness of melanized and unmelanized barbs along the entire rachis of a primary feather from an osprey (Pandion haliaetus). Although breaking force was higher for melanized barbs, breaking stress (force divided by cross-sectional area) was greater for unmelanized barbs. But when position was considered, all mechanical differences between melanized and unmelanized barbs disappeared. Barb breaking stress, breaking strain and toughness decreased, and breaking stiffness increased, distally along the rachis. These proximal-distal material property changes are small and seem unlikely to affect flight performance of barbs. Our observations of barb bending, breaking and morphology, however, lead us to propose a design principle for barbs. We propose that, by being thicker-walled dorso-ventrally, the barbs flexural stiffness is increased during flight; but, by allowing for twisting when loaded with dangerously high forces, barbs firstly avoid failure by bending and secondly avoid complete failure by buckling rather than rupturing.

Seasonal Acclimatization of Antioxidants and Photosynthesis in Chondrus crispus and Mastocarpus stellatus, Two Co-Occurring Red Algae With Differing Stress Tolerances

Mastocarpus stellatus and Chondrus crispus are red macroalgae that co-dominate the lower rocky intertidal zones of the northern Atlantic coast. M. stellatus is more tolerant than C. crispus of environmental stresses, particularly those experienced during winter. This difference in tolerance has been attributed, in part, to greater contents or activities of certain antioxidants in M. stellatus. We compared the photosynthetic capacities and activities of three antioxidant enzymes—superoxide dismutase (SOD), ascorbate peroxidase (APX), and glutathione reductase (GR)—as well as the contents of ascorbate from fronds of M. stellatus and C. crispus collected over a year. Photosynthetic capacity increased in winter, but did not differ between species in any season. The activities of the three antioxidant enzymes and the contents of ascorbate were significantly greater in tissues collected during months with mean air and water temperatures below 7.5 °C (“cold” months; December, February, March, April) than in months with mean air temperatures above 11 °C (“warm” months; June, July, August, October). Overall, C. crispus had significantly greater SOD and APX activities, while M. stellatus had higher ascorbate contents. Species-specific differences in GR activity depended upon mean monthly temperatures at the time of tissue collection; C. crispus had higher activities during cold months, whereas M. stellatus had higher activities during warm months. Taken together, these data indicate that increased ROS scavenging capacity is a part of winter acclimatization; however, only trends in ascorbate content support the hypothesis that greater levels of antioxidants underlie the relatively greater winter tolerance of M. stellatus in comparison to C. crispus.

Structural Strengthening of Urchin Skeletons by Collagenous Sutural Ligaments

Sea urchin skeletons are strengthened by flexible collagenous ligaments that bind together rigid calcite plates at sutures. Whole skeletons without ligaments (removed by bleaching) broke at lower apically applied forces than did intact, fresh skeletons. In addition, in three-point bending tests on excised plate combinations, sutural ligaments strengthened sutures but not plates. The degree of sutural strengthening by ligaments depended on sutural position; in tensile tests, ambital and adapical sutures were strengthened more than adoral sutures. Adapical sutures, which grow fastest, were also the loosest, suggesting that strengthening by ligaments is associated with growth. In fed, growing urchins, sutures overall were looser than in unfed urchins. Looseness was demonstrated visually and by vibration analysis: bleached skeletons of unfed urchins rang at characteristic frequencies, indicating that sound traveled across tightly fitting sutures; skeletons of fed urchins damped vibrations, indicating loss of vibrational energy across looser sutures. Furthermore, bleached skeletons of fed urchins broke at lower apically applied forces than bleached skeletons of unfed urchins, indicating that the sutures of fed urchins had been held together relatively loosely by sutural ligaments. Thus, the apparently rigid dome-like skeleton of urchins sometimes transforms into a flexible, jointed membrane as sutures loosen and become flexible during growth.

Sutural loosening and skeletal flexibility during growth: determination of drop-like shapes in sea urchins.

The shape of sea urchins may be determined mechanically by patterns of force analogous to those that determine the shape of a water droplet. This mechanical analogy implies skeletal flexibility at the time of growth. Although comprised of many rigid calcite plates, sutural collagenous ligaments could confer such flexibility if the sutures between plates loosened and acted as joints at the time of growth. We present experimental evidence of such flexibility associated with weight gain and growth. Over 13–, 4–, and 2–week periods, fed urchins (Strongylocentrotus droebachiensis) gained weight and developed looser sutures than unfed urchins that maintained or lost weight. Further, skeletons of fed urchins force–relaxed more than did those of unfed urchins and urchins with loose sutures force–relaxed more than those with tight sutures. Urchins (Strongylocentrotus franciscanus) fed for two and a half weeks, gained weight, also had looser skeletons and deposited calcite at sutural margins, whereas unfed ones did not. In field populations of S. droebachiensis the percentage having loose sutures varied with urchin diameter and reflected their size–specific growth rate. The association between feeding, weight gain, calcite deposition, force relaxation and sutural looseness supports the hypothesis that urchins deform flexibly while growing, thus determining their drop-like shapes.

Walking versus breathing: mechanical differentiation of sea urchin podia corresponds to functional specialization

The podia of sea urchins function in locomotion, adhesion, feeding, and respiration; but different podia on a single urchin are often specialized to one or more of these tasks. We examined the morphology and material properties of podia of the green sea urchin, Strongylocentrotus droebachiensis, to determine whether, despite apparent similarities, they achieve functional specialization along the oral-aboral axis through the differentiation of distinct mechanical properties. We found that oral podia, which are used primarily for locomotion and adhesion, are stronger and thicker than aboral podia, which are used primarily for capturing drift material and as a respiratory surface. The functional role of ambital podia is more ambiguous; however, they are longer and are extended at a lower strain rate than other podial types. They are also stronger and stiffer than aboral podia. In addition, all podia become stronger and stiffer when extended at faster strain rates, in some cases by nearly an order of magnitude for an order of magnitude change in strain rate. This strain-rate dependence implies that resistance to rapid loading such as that imposed by waves is high compared to resistance to slower, self-imposed loads. Thus, the serially arranged podia of S. droebachiensis are functionally specialized along an oral-aboral axis by differences in their morphology and mechanical properties.

Sag-Mediated Modulated Tension in Terebellid Tentacles Exposed to Flow

The long, compliant feeding tentacles of the terebellid polychaete Eupolymnia heterobranchia not only stretch out over a sandflat substratum but also extend into flow. Tentacles suspended perpendicular to flow responded to increasing velocity by increasing their sag. An analysis of tension in these tentacles, mathematically analogous to that applicable to suspension bridges, shows that sagging permits the tentacles to avoid increases in tension that would otherwise occur as flow increases. Force modulation was achieved by active muscular control rather than by passive material properties. Although these tentacles would certainly break in the experimental flows if they did not sag, the low tension achieved suggests that some other reason, such as limitations on the adherence of cilia and mucus, accounts for the level of tension observed. Because drag is maximum on tentacles oriented perpendicular to flow, reorientation of tentacles, either by sagging or by dangling parallel to flow, additionally reduces tension by reducing drag. Theoretical estimates of drag on tentacles oriented parallel to flow show that they are never in danger of being broken. Drag is sufficient, however, to assist in passive extension of tentacles. While reorientation is a common mode of drag reduction among marine organisms, sagging represents a novel mechanism of mediating structural forces resulting from flow.

Quantification of dendritic and axonal growth after injury to the auditory system of the adult cricket Gryllus bimaculatus

Dendrite and axon growth and branching during development are regulated by a complex set of intracellular and external signals. However, the cues that maintain or influence adult neuronal morphology are less well understood. Injury and deafferentation tend to have negative effects on adult nervous systems. An interesting example of injury-induced compensatory growth is seen in the cricket, Gryllus bimaculatus. After unilateral loss of an ear in the adult cricket, auditory neurons within the central nervous system (CNS) sprout to compensate for the injury. Specifically, after being deafferented, ascending neurons (AN-1 and AN-2) send dendrites across the midline of the prothoracic ganglion where they receive input from auditory afferents that project through the contralateral auditory nerve (N5). Deafferentation also triggers contralateral N5 axonal growth. In this study, we quantified AN dendritic and N5 axonal growth at 30 h, as well as at 3, 5, 7, 14, and 20 days after deafferentation in adult crickets. Significant differences in the rates of dendritic growth between males and females were noted. In females, dendritic growth rates were non-linear; a rapid burst of dendritic extension in the first few days was followed by a plateau reached at 3 days after deafferentation. In males, however, dendritic growth rates were linear, with dendrites growing steadily over time and reaching lengths, on average, twice as long as in females. On the other hand, rates of N5 axonal growth showed no significant sexual dimorphism and were linear. Within each animal, the growth rates of dendrites and axons were not correlated, indicating that independent factors likely influence dendritic and axonal growth in response to injury in this system. Our findings provide a basis for future study of the cellular features that allow differing dendrite and axon growth patterns as well as sexually dimorphic dendritic growth in response to deafferentation.