Physiological ecology
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  Locomotion & tenacity
  The topic of physiological ecology is divided into a section on locomotion & tenacity considered here, and sections on CHEMORECEPTION, GAS EXCHANGE & METABOLISM, DIEL SEASONAL & TIDAL RHYTHMS, OSMOTIC REGULATION & SALINITY TOLERANCE, and THERMAL STRESSES considered in other sections.
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Research study 1

photograph of walking legs of Dungeness crabs Cancer magister with apodeme locations indicatedWalking in crabs and other crustaceans is accomplished by jointed walking legs powered by muscles.  Each joint of a leg has only 1 degree of freedom of movement, like a hinge, and can only open (extend) or close (flex).  Each motion is effected through muscles pulling on apodemes that extend from the limb segment ahead to the one behind where the muscles are massed.  One muscle, the extensor, attaches to a corresponding extensor apodeme which, when pulled, causes the segment ahead to extend.  When the opposing flexor muscle in the same leg segment contracts the limb segment ahead flexes.  The extent of muscle contraction required to move the limb is regulated by proprioceptors, or tension-receptor nerves, present in the apodemes.  Studies on merus segments of walking legs of Dungeness crabs Cancer magister in Oregon show that these tension receptors lie along the face of the flexor apodeme.  If isometric tension is applied to the walking leg the nerves increase their firing frequency in accordance with the degree of tension. MacMillan & Dando 1972 Mar Behav Physiol 1: 185.

Research study 2

photogaph of the lithodid crab Lopholithodes mandtiiphotograph of red rock crab Cancer productus The widely varying carapace morphology of west-coast brachyurans seems to suggest selection for traits other than hydrodynamic streamlining.  However, a comparison of hydrodynamic properties of 3 crab species, including the west-coast Cancer productus and Lopholithodes mandtii, indicates at least some correlation between design and habit. The species differ in streamlining of carapace and, while both crawl about on rocks, Cancer inhabits shallow water often in the intertidal zone, while Lopholithodes lives in deeper water.  Cancer is highly motile while Lopholithodes is more sedentary. The latter uses its strong walking legs to grip the substratum and is often found perched on rocky subtidal promontories.  Both species crawl sideways.  Measurements in flume-tanks in the laboratory show, as expected, that Cancer has lower drag coefficient and higher lift coefficient than Lopholithodes.  What is of interest in a comparison of these crabs is the ratio between slipping speed (the current speed at which lateral displacement first occurs) and lift-off speed (the current speed at which the animal is lifted off the bottom).  The values are 1.9 for Cancer (similar to that for plaice) and 3.3 for Lopholithodes. The author suggests that the carapace of Cancer is actually adapted to maximise slipping speed (perhaps for quicker getting around its habitat?) and that a tendency for displacement in Lopholithodes is resisted actively by its holding firmly onto the substratum.  Although the author suggests that Lopholithodes may favour areas of quiet water, such as may occur between large rocks, in fact it commonly frequents the tops of boulders in quite strong currents, relying on its strong legs to hold on.  Blake 1985 J Zool Lond 207: 407.

NOTE   the author also includes Callinectes sapidus in the study but, as this is is an Atlantic-coast swimming crab with no close relations on the west coast north of Baja California, it is not included here

NOTE   the comparison of the 2 species is not perfect, as the author chooses to test Cancer with its legs removed and irregularities smoothed over with plasticine, while Lopholithodes is tested intact.  Only one individual of each species is tested

Research study 3

photograph of shore crab Pachygrapsus crassipes courtesy Jackie Soanes, Bodega Marine Laboratory, Californiagraph showing stepping frequency of leading and trailing walking legs in the shore crab Pachygrapsus crassipesIntertidal crabs, such as Pachygrapsus crassipes, must contend with 2 markedly different environments depending on whether the tide is in or out.  Its mass differs by a factor of 6 in the two environments.  In air, it is the vertical force of gravity that destabilises or causes loss of control in a walking crab, while in water it is the horizontal forces of drag that destabilise the crab.  Studies in Mission Bay, California show that when walking, Pachgrapsus keeps its dactyli in contact with the substratum more than 50% of the time when in air, but less than this when in water, indicating a greater need for stability in the former.  Does a crab push or pull itself along when it walks on level terrain?  This seems to depend on whether the crab is walking in air or in water. In air on smooth terrain, the 2 forces seem to be about equal at speeds <5cm . sec-1, but at faster speeds the trailing legs seem to be pulled along by the leading legs (see graph). In water, the trailing legs are in contact with the substratum for a relatively longer period.  The author ascribes this to a greater pushing effort to overcome drag forces.  Hui 1992 J Exp Biol 165: 213. Photo courtesy Jackie Soanes, Bodega Marine Laboratory, California.

NOTE  the 4th pair of legs on either side tend to be used more for tactile sensing than for walking, so these limbs are excluded from the analyses

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photograph of a shore crab Hemigrapsus oregonensis taken from a video

CLICK HERE to see a video of a shore crab Hemigrapsus oregonensis walking on sand in air. Note that it is moving in a forwards direction, unlike subtidal crabs that tend to favour sideways movement.

NOTE  the video replays automatically

Research study 4

schematic showing lift and drag forces on a crab

Water flow benefits marine invertebrates by carrying nutrients, gametes, scents, and larvae, but may be detrimental through forces of breakage and dislodgment, and through interference with motility. In unidirectional flows of water an organism is subject to drag and lift forces, while in wave-swept flows it is subject to additional forces of acceleration1 (see diagram on Left). Intertidal crabs, like other intertidal dwellers, are subject to risk of dislodgement from waves and currents.  A crab will be overturned or sheared from the substratum when drag and acceleration forces exceed its stabilising moment (defined as a combination of mass, tenacity, and lift). These features have to be determined separately for a particular organism in a particular graph showing forces to dislodge crabs walking on different substratahabitat.

Studies at the Bodega Marine Laboratory, California on tenacity or force to dislodge shore crabs2 Pachygrapsus crassipes on different substrata, and in different water velocities and accelerations show that net force to dislodge is an order of magnitude greater on rugose rock substratum than on smooth rock, and 2 orders of magnitude greater than on mud (see graph on Right, note log scale on ordinate axis). The graph3 shows how the forces that Pachygrapsus uses to resist dislodgement vary not just with substratum type (rugose or smooth rock, and mud), but also with extent of employment of legs to grasp the substratum. The authors’ calculations show that under non-storm conditions, a Pachygrapsus will not be dislodged in wave-swept habitats when grasping a rock substratum with maximum tenacity. Tenacity is lower in moving crabs, and the authors show that hydrodynamic forces will restrict motility of large crabs more than that of small crabs on smooth, but not on rugose rock surfaces.  Reduced motility has obvious effects on fitness, such as decreased ability to find food, shelter, and mates, and to escape from predators.  Dislodgement can lead to wounding and death.  Lau & Martinez 2003 J Exp Mar Biol Ecol 295: 1.

NOTE1  acceleration forces act in the same direction as drag forces if the water is speeding up, as in waves that are building, but in the opposite direction if the water is slowing down, as in waves that are dying 

NOTE2 measurements are done on live crabs as well as on models created by filling the exoskeletons with epoxy putty

NOTE3  the log scale actually masks the large differences measured..  Each demarcation on the ordinate axis represents one order of magnitude difference

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