Forces generated during stretch in the heart of the lobster Homarus americanus are anisotropic and are altered by neuromodulators.

Mechanical and neurophysiological anisotropies mediate three-dimensional responses of the heart of ITALIC! Homarus americanus Although hearts ITALIC! in vivoare loaded multi-axially by pressure, studies of invertebrate cardiac function typically use uniaxial tests. To generate whole-heart length-tension curves, stretch pyramids at constant lengthening and shortening rates were imposed uniaxially and biaxially along longitudinal and transverse axes of the beating whole heart. To determine whether neuropeptides that are known to modulate cardiac activity in ITALIC! H. americanusaffect the active or passive components of these length-tension curves, we also performed these tests in the presence of SGRNFLRFamide (SGRN) and GYSNRNYLRFamide (GYS). In uniaxial and biaxial tests, both passive and active forces increased with stretch along both measurement axes. The increase in passive forces was anisotropic, with greater increases along the longitudinal axis. Passive forces showed hysteresis and active forces were higher during lengthening than shortening phases of the stretch pyramid. Active forces at a given length were increased by both neuropeptides. To exert these effects, neuropeptides might have acted indirectly on the muscle via their effects on the cardiac ganglion, directly on the neuromuscular junction, or directly on the muscles. Because increases in response to stretch were also seen in stimulated motor nerve-muscle preparations, at least some of the effects of the peptides are likely peripheral. Taken together, these findings suggest that flexibility in rhythmic cardiac contractions results from the amplified effects of neuropeptides interacting with the length-tension characteristics of the heart.

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.

Form and Motion of Donax variabilis in Flow.

The coquina clam, Donax variabilis, rides flow from waves, migrating shoreward during rising tides and seaward during falling tides. This method of locomotion, swash-riding, is controlled not only behaviorally but also morphologically. The shape of this clam causes it to orient passively; a clam rotates in flow, usually in backwash, until its anterior end is upstream. Rotation is about a vertical axis through a pivotal point where the shell touches the sand. The density, weight distribution, and wedge-like shape are all important in effecting orientation. Such orientation is significant because it contributes to stability of motion. On an unoriented clam, upward lift can be higher than its underwater weight--a circumstance that results in uncontrollable tumbling. In contrast, once oriented with its anterior end upstream, a clam experiences downward lift that contributes to its stability while sliding in backwash. Furthermore, when the anterior end is upstream, drag is reduced relative to when the ventral, dorsal, or posterior ends are upstream. Since orientation occurs only above a minimum velocity, it has the effect of slowing a clam's motion over the substratum in rapid flows. Stability, drag, and speed reduction enhance a clam's ability to gain a foothold and dig in after a swashride, before wave flows can wash it off the beach and out to sea.

Behavioral Control of Swash-Riding in the Clam Donax variabilis.

Clams of the species Donax variabilis migrate shoreward during rising tides and seaward during falling tides. These clams spend most of the time in the sand, emerging several times per tidal cycle to ride waves. Migration is not merely a passive result of waves eroding clams out of the sand; rather clams actively jump out of the sand and ride specific waves. Such active migration is experimentally demonstrated during a falling tide by comparing the motion of dead and live clams; live clams emerge from the sand and move seaward even when dead ones do not. As low tide approaches, live clams become progressively less active. They cease migrating for 2 hours around low tide and resume jumping to migrate shoreward after the tide has turned. During the rising tide, far from being passive, the clams jump out to ride only the largest 20% of waves. Specifically, they choose swash that have the largest excursion, i.e., those swash that move furthest on the beach.

Discrimination Among Wave-Generated Sounds by a Swash-Riding Clam.

Clams, Donax variabilis, responded to sound stimuli presented to them in a laboratory aquarium by jumping out of the sand, lying on the sand for several seconds, and digging in again. On a beach, clams jump out of the sand and ride waves, migrating shoreward with the rising tide and seaward with the falling tide. Parallels between clam behavior on a beach and that elicited in the laboratory suggest that clams cue on wave sounds to jump out of the sand. Three aspects of the response to sound were parallel. (i) Clams were most responsive to low-frequency sounds similar to those produced on a beach by waves rolling onto shore. (ii) Clams were also more responsive to louder sounds; on a beach, clams jump preferentially for the largest (loudest) 20% of waves, (iii) Responsiveness in the laboratory had an endogenous tidal rhythm, with highest activity occurring at high tide and no activity occurring at low tide; this rhythm corresponds to the activity of clams on the beach from which they were collected. By using sounds to identify large waves, clams can ride selected waves and continuously maintain position at the sea's edge as the tide floods and ebbs.

Causes and Consequences of Fluctuating Coelomic Pressure in Sea Urchins.

We measured coelomic pressure in sea urchins to determine whether it was high enough to support a pneu hypothesis of growth. In Strongylocentrotus purpuratus the pressure was found to fluctuate rhythmically about a mean of -8 Pa, and was negative for 70% of the time. This is at variance with the theoretically required positive pressures of the pneu hypothesis. Furthermore, there were no sustained significant differences between the pressure patterns of fed and starved urchins, presumed to be growing and not growing, respectively. The rhythmical fluctuations in pressure were caused by movements of the lantern which changed the curvature and tension of the peristomial membrane. We developed a mathematical and morphological model relating lantern movements, membrane tension, and pressure, that correctly predicts the magnitude of the fluctuations. Pressures predicted by the model depend also on coelomic volume changes. In Lytechinus variegatus simultaneous retraction of the podia, which causes expansion of the ampullae, resulted in an 8.8 Pa increase in coelomic pressure, relative to the pressure during simultaneous podial protraction.