To understand their function and ontogeny better, we conducted a morphometric analysis of claw size and shape variation in the strikingly heterochelous, north-eastern Pacific ghost shrimp, Neotrypaea (formerly Callianassa) californiensis. Master claws approached 25% of total body weight in mature males, but rarely exceeded 10% in females. Minor claws were less than 3% of body weight in both sexes. The proportions of right and left master claws did not differ significantly from 50:50. Males exhibited a greater positive allometry than females in both master and minor claw size, though master claws differed more than minor claws. Sexual dimorphism was also observed in master but not minor claw shape: compared to females, mature male master claws: a) were proportionally higher relative to their length; b) exhibited a deeper propodal notch and consequently a larger gape; c) developed a more slender and more distally hooked dactyl; and d) exhibited more well-developed teeth about the periphery of the claw gape.
The shape of the conspicuous gape in mature male master claws bore a close resemblance to the cross-section of similar-sized master claws. The shape of this gape, and the presence of fine teeth about its periphery, strongly suggests that master claws function in a highly stereotyped form of grappling during agonistic encounters or perhaps during mating between similar-sized conspecifics. In addition, a landmark morphometric analysis of relative growth suggested that the pronounced propodal notch develops via localized deformations near the base of the fixed finger rather than via a more generalized contraction of the ventral manus region. Finally, a preliminary survey suggests that the distinctive propodal notch, which may be diagnostic of the hypothesized grappling function, has evolved at least twice in the Callianassidae, once in the Callianassinae and once in the Callichirinae. Sexual selection may have significantly influenced the evolution of these unusual master claws.