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The paired claws in Gazami crabs, Portunus trituberculatus, are bilaterally asymmetrical, and asymmetry is remarkable on the distal two segments of the first pereiopod, that is, the dactylus and propodus. Shells are exclusively cracked by use of the right chela, representing handedness. In Gazami crabs, handedness is reversed after autotomy of the right chela. Our study focused on the ontogeny of handedness and the mechanism of handedness reversal. Morphologically, asymmetry was first detected in megalopa larvae where the right propodus was significantly larger than the left, as was the canine at the base of the right dactylus. Presumably, the rate of chelagenesis differed between the left and right chelae. With these morphological features, the right chela functioned as a crusher. The crusher exerted a closing force two to three times that of the cutter. With loss of the right crusher, the left chela was bigger than the regenerated right chela and was converted to the crusher. In contrast, the performance of the regenerated right chela deteriorated compared to that of the original right crusher, and exertion of full closing force was inhibited by the more active left chela. Furthermore, crabs with two crusher chelae did not clearly show handedness. A decrease in size and performance of the regenerated right chela can be explained by a default program hypothesis. In conclusion, a difference in the chelagenesis rate results in bilateral asymmetry of the two chelipeds, and then handedness is generated by neural regulation in the thoracic ganglion innervating these claws. Since handedness is reversed after autotomy, the thoracic ganglion would not be lateralized in Gazami crabs. A default program hypothesis is proposed to explain the ontogeny of bilateral chela asymmetry and handedness reversal.

Observations of behavior and direct measurements of force indicated that the cancrid crab Cancer productus could directly crush only the smallest specimens of Protothaca staminea, a venerid bivalve. Crabs opened larger P. staminea by repeatedly loading the same region of the bivalve's shell with a chela; we hypothesized that this repeated loading caused fatigue of the shell material. To test whether significant fatigue damage would accumulate in the number of cycles a crab was likely to exert, live bivalves and cleaned valves were cyclically loaded in a mechanical testing machine to loads of a constant maximum amplitude of 70-100% of the bivalves' predicted static strength. Failure frequently occurred in fewer than 200 cycles. Recordings from strain gauges attached to the chelae of crabs showed that during an attack on a bivalve a crab would squeeze more than 200 times and that failure of the bivalve could occur during a force pulse which was weaker than previous force pulses. We conclude that repeated loading enables crabs to open larger bivalves than could be crushed outright; by greatly increasing the maximum size of prey vulnerability this expands the size range of molluscan prey available to crabs.

Asymmetries in thoracic and leg muscles that move the single huge major cheliped of male fiddler crabs are compared to their contralateral homologues in nine species of Uca. In Minuca (U. longisignalis, U. minax, U. pugnax, and U. rapax) and Boboruca (U. thayeri) species, the major side body muscles are twice as large as the minor muscles, with more muscle fibers. Additionally, the individual sarcomeres are exceptionally long, reaching over 20 µm in length. In contrast, in species of the sub-genus Celuca (U. crenulata, U. panacea, U. pugilator, and U. speciosa), while these muscles may be up to six times larger than their contralateral homologues, with fibers nearly twice as long, the sarcomeres are not so hypertrophied.Major muscles in the chela can be 20 times larger than their contralateral homologues. The largest chela muscles also show exceptionally large sarcomeres among Minuca, but not in Celuca.It is shown that sarcomere lengths are inversely correlated with the speed and duration of the courtship...

The form and function of the paired asymmetric chelae were examined in blue crabs with the normal (right crusher, left cutter) and reverse (left crusher, right cutter) chela laterality or handedness. The force generated by the crusher chela in intact blue crabs was significantly greater than that of its counterpart cutter chela. The relative strength of the crusher is due to its greater mechanical advantage and larger muscle volume. The closer muscles of both chelae were capable of exerting similar forces. For a comparable frequency of stimulation, the fast excitor axon was more effective in the cutter than in the crusher chela. The paired closer muscles were composed of fibers with long sarcomere lengths (>6 sm) characteristic of slow fibers with no significant difference in their mean value or in their distribution between crusher and cutter chelae. This was corroborated by finding that the myofibrillar ATPase activity and oxidative capacity was uniformly similar over most of the muscle in both chelae except for some differentiation in the most proximal and distal regions of the muscle. Such homogeneity in sarcomere lengths and enzymatic properties of the paired closer muscle occurred in blue crabs with the normal and reverse chela laterality. Thus functional differences brought about by motoneuron activation between the asymmetric chelae are probably due to muscle fiber properties other than their sarcomere lengths and enzymatic profiles, such as their cable and contractile properties as well as to neuronal and neuromuscular properties.

Effects of the parasitic isopod, Probopyrus pandalicola, on the sex characters of its host shrimp, Palaemonetes paludosus, are described, and possible mechanisms for these effects are discussed. P. pandalicola sterilize the female host by preventing ovarian maturation but do not affect the external sex characters. In contrast, the growth of the external sex characters of the male host was either acelerated (chela) or retarded (pleopod structures), but there was no parasitic effect on the testes. The mechanism for these castration effects appears to be some combination of a nutritional drain and/or a hormonal interference by the parasite.

During the summer of 1909 while at the Tortugas Laboratory of the Carnegie Institution I undertook a further analysis of the reversal phenomenon in regenerating specimen.@ of the genera Aipheus and Synaipheus. These small crustacea commonly called snapping shrimp, on account of their habit of snapping the large chehe with such force as to produce a surprisingly loud noise, are abundantly found in the “? logger-head― sponge and in the holes of disintegrating coral rock on the Tortugas reefs. There are a number of species five of which, Aipheus formosus and armillatus and Synaipheus minus and two other unidentified species of Synaipheus, were employed in these experiments. The several species differ in size and body color but are es sentially similar in general structure. The first pair of appen dages is decidedly asymmetrical in both sexes. One member of the pair, either the right or the left, is extremely large, in some cases being more than half the size of the body itself. The general type of this chela in the five species follows more or less closely the description given by Wilson,' for the great chela of Aipheus heterochelis. It is greatly rounded or swollen with transverse grooves on either side of the propodus, varying in depth with the species, and presents characteristic color patterns being tipped with a lively rose color in Synaipheus minus while in the other species it is bluish, dark or brown. On the concave side of the dactylus is a swollen knob forming the “? hammer― which fits into a corresponding socket on the outer side of the propodus claw. By extending the dactylus and then suddenly snapping the claws together the “? hammer― is forced into the socket with the surprisingly loud sound. Wilson's description further applies in that the large chela has essentially the same structure in both sexes, while the small

1. The snapping mechanism of snapping shrimps of the genera Alpheus (Crangon) and Synalpheus has been studied and the "crackle" produced by populations of these shrimp surrounding the Bermuda Islands has been analyzed.2. Differences in chela structure between the two genera are enumerated. Earlier beliefs that the sound is produced primarily by contact of the calcified tips upon closure of the dactylopodite on the propodite are corroborated. It is concluded that the "suckers" in the articulation of the joint in Alpheus species do not serve as a cocking mechanism, and is suggested that suction develops in the living animal by depression of the membrane between the segments as the dactylus is raised. Overlapping exoskeletal knobs in the joint probably serve to hold the dactylus in place.3. The structure and physiology of the chela musculature are as in the Crustacea generally. Both fast and slow contractions and an inhibitory effect are demonstrated. The functional significance of a doubly excitable system ...

It is a fact well known to students of regeneration that one part of an organism may exert a measurable influence over the growth and development of another. This has been demon strated for many of the Metazoa under varying forms. The removal of the head of a planarian liberates, as it were, the post jacent tissue, out of which a new head is fashioned. Among macruran crustaceans, the loss of the larger chela of an asym metrical pair has been shown many times to be succeeded by an accelerated growth of the smaller chela and a subsequent retarda tion in the regeneration of the lost chela so that, in the presence of the small chela grown large, it remains the smaller of the two. Finally—not to multiply instances needlessly—when a short length of the column, with hydranth, is cut away from the hydroid Corymorpha, no development beyond closure of the wound occurs proximally until the hydranth is removed from the distal end. In this respect, the behavior of Corymorpha may be contrasted with the behavior of the planarian, since in the latter the presence of the original head on the anterior piece does not inhibit the development of a tail posteriorly. The hydranth in Corymorpha appears somehow to inhibit, in short pieces, even the development normally to be expected at the aboral end. Among the Protozoa, instances of this last sort seem so gen erally to have escaped record, that we have thought it desirable to describe a similar domination in Vorticella sp. of one part over another. When Vorticella divides, the fission plane passes approxi mately through the center of the organism from oral disk to a point immediately to one side of the contractile stalk. Of the zoA¶ids thus formed, one remains attached to the original stalk, while the other swims away by means of cilia which, during the