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Levinton &Al - 1995 - Functional Differences Between Major and Minor Claws of Uca

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Journal ELSEVIER of Experimental Marine Biology 193 (1995) 147-160 and Ecology JOURNAL OF EXPERIMENTAL MARINE BIOLOGY AND ECOLOGY Functional differences between the major and minor claws of fiddler crabs (Uca, family Ocypodidae, Order Decapoda, Subphylum Crustacea): A result of selection or developmental constraint? Jeffrey S. Levintona**, Michael L. Judgeb, Josepha P. Kurdziel” “Department of Ecology and Evolution, State University of New York, Stony Brook, New York, NY 11794, USA bDepartm
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  ELSEVIER Journal of Experimental Marine Biology and Ecology193 (1995) 147-160 JOURNAL OFEXPERIMENTALMARINE BIOLOGYAND ECOLOGY Functional differences between the major and minorclaws of fiddler crabs (Uca, family Ocypodidae, OrderDecapoda, Subphylum Crustacea): A result of selectionor developmental constraint? Jeffrey S. Levintona**, Michael L. Judgeb, Josepha P. Kurdziel” “Department of Ecology and Evolution, State University of New York, Stony Brook, New York,NY 11794, USAbDepartment of Biology, Manhattan College, Riverdale, New York, NY 10471, USA Abstract We studied the functional differentiation between the major and minor claws of fiddlercrabs (Uca, family Ocypodidae) by means of a strain gauge apPARATUS and bymorphometric estimates of claw function. In Uca pugnux, (Smith 1870) closing force is anapproximately log-linear function of claw length, but the slope for minor claws is greaterthan that for the major claws, and the y-intercept for major claws is displaced upwardsfrom that of the minor claw. The pattern of closure also differs: major claws impartrhythmic short pulls, whereas minor claws squeeze far more irregularly. The short pulls ofthe major claw correspond to rapid squeezes observed in videotapes of combat, whereasthe irregular pulls of the minor claw match expectations for gathering particles andtransferring them to the mouth parts. The mechanical advantage declines steadily withincreasing major claw length. These results suggest that the evolution of form of theextremely large major claws in fiddler crabs involved functional shifts, as opposed to adevelopmentally constrained extrapolation in size of the minor claw. Keywords: Biomechanics; Fiddler crab; Ocypodidae; Sexual selection; Uca pugnax 1. Introduction It is the purpose of this paper to consider the evolution of a novel structure, themajor claw of male fiddler crabs, and to assess the degree to which the rise of such * Corresponding author.0022-0981/9.5/$09.50 @ 1995 Elsevier Science B.V. All rights reserved SSDI 0022-0981(95)00115-S  148 J.S. Levinton et al. I .I. Exp. Mar. Bid. Ed. 19.3 (1995) 147-160 a structure was under the control of a developmental constraint, as opposed tobeing under the control of natural selection for efficient function as a closingdevice. It has been argued that the evolution of structures may be guided bydevelopmental programs, which might be an internal structural force as importantin guiding evolutionary direction as natural selection (Maynard Smith et al.,1985). In deer for example, the now extinct Irish elk had enormous antlers and ithas been suggested that these antlers were used in display only, and not forcombat (Gould, 1974). It has been further suggested that a developmentalprogram might be responsible for constraining antler size to a certain proportionof body size, which would strongly modify the role of natural selection incontrolling antler form (Gould, 1982). This claim would explain why the Irish elkused its enormous antlers for display only: By its size, its antlers would have beenconstrained to be very large. Display might be the only option, assuming that thelarge antlers are inefficient in combat. Has display dominated closing function inthe evolution of the fiddler crab’s giant major claw?As we shall show below, fiddler crabs are an excellent model system to examinethese issues because both claws grow with precisely the same morphology, untilsome undetermined signal causes one of the two chelae, apparently at random, tocontinue to enlarge to nearly half of the male’s body weight. This major claw isused exclusively for display and combat, but damage in combat is slight tononexistent. The evolution of the major claw, moreover, must have been causedby sexual selection in an ancestral species, which might have put emphasis on thedisplay value of the claw, at the expense of function, i.e. generation of a closingforce. So the question arises: Has the developmental connection between minorand major claws constrained the major claw to continue to grow along the sameallometric growth trajectory, simply to evolve a large display structure? If not, arethere differences between the two claw types that can be ascribed to function andnatural selection? The fiddler crab system is especially valuable as there iscomplete separation of feeding and sexual function between the two claw types inthe male, which allows explicit studies of differentiation of function that are not asapproachable in other decapods.Decapods have chelae that range widely in size, function, and closing strength(Warner & Jones, 1976; Vermeij, 1977). Deposit feeding crabs of the familyOcypodidae typically have small and slender claws that are used to manipulateand carry sedimentary grains (Crane,1975). By contrast, carnivorous species,including some members of the genus Ocypode, have larger more robust claws,sometimes capable of crushing rather thick shells (Vermeij, 1977). In manycarnivorous species, the two chelae are differentiated into a cutter and crusher,whose mechanical advantage and closing speed are differentiated. In the lobster Homarus americanus, the handedness of this differentiation is determined byenvironmental factors (Govind & Pearce, 1986). Closing force and biomechanical-ly significant dimensions of a crab chela can also be altered by diet (Smith &Palmer, 1994).Fiddler crabs of the genus Uca show among the strongest degree of cheliped  J.S. Levinton et al. I J. Exp. Mar. Biol. Ecol. 193 (1995) 147-160149 sexual dimorphism of all the decapods. In all species, female claws are isomorphic,but in the male one claw is extremely large-nearly half of the body weight of thecrab (Crane, 1975). The male’s major claw grows with strong positive allometry(Morgan, 1923) but the minor claw is similar in size to that of the female(Levinton, unpubl. data) and, as in the female, is used for feeding. The majorclaw, however, is not used for feeding but is employed in complex waving displaysand in combat with other males (Crane, 1975). The lack of two feeding claws putsthe male at a disadvantage in food acquisition and some compensations in termsof feeding rate and time spent feeding have been observed (Valiela et al., 1974;Caravello & Cameron, 1987). Uca pugnax males can never gain energy as fast asfemales and respond differently to food abundance, in accordance with expecta-tions of optimal foraging theory (Weissburg, 1993).Populations of most species of Uca have an equal abundance of males withright and left handed major claws, suggesting that the determination of handed-ness is facultative, as in the cutter-crusher dimorphism of the lobster (Govind &Pearce, 1986). Most likely, a hormonal signal induces the differentiation, and thissignal is, in effect, random with respect to side. In small males the removal of onecheliped results in the development of the other into a major claw. After thedevelopmental determination occurs, however, a major claw, even if removed, willdevelop from the same limb primordium (Yamaguchi, 1977; Ahmed, 1978). In thesubgenus Thalassuca, however, two species are nearly all right handed (Barnwell,1982; Jones & George, 1982). Although Thalassuca does not appear to be in anancestral position in the Uca clade (C. Sturmbauer, J. Levinton, and J. Christy,unpubl. data), it is not clear whether or not the ancestor of Uca was indeterminatein handedness.The extreme dimorphism of males, accompanied by complete differentiation offunction, gives us an excellent opportunity to ask questions about the morphologi-cal and functional changes that accompanied the evolution of the major cheliped.Superficially, the major and minor claws are similar in overall dimensions,although the feeding claws have spatulate, setae-laden tips and the major clawshave pointed tips and may not occlude in many species. In this paper, we wish toaddress the question of whether the evolution of the large major claw involved animportant biomechanical change in morphology. Owing to the developmentalconnection between the claws, a null model might suggest that the evolution ofthe major claw was constrained developmentally along a growth track that wasentrenched in the developmental program of the minor claw. In other words, adevelopmental constraint hypothesis would argue that the dimensions and closingfunction of the major claw is constrained by a growth pattern that was establishedduring the evolution of the minor claw, whose current form we assume to be theancestral monomorphic state of Uca. This hypothesis cannot be dismissed lightlybecause the major claw is used for display and generally non-injurious combat. Analternative hypothesis would argue that the function of combat and display causeda bout of natural selection, resulting in an alteration of the major claw’smorphology and function.  150 J.S. Levinton et al. I J. Exp. Mar. Bid. Ecol. 19.3 (199-f) 147-160 2. Materials and methods Adult male U. pugnax (Smith 1870) were collected from field populations at theFlax Pond Spartina salt marsh (Old Field, New York, USA) in the summers of1993 and 1994. Crabs were kept in a recirculating sea water system at roomtemperature, with a tidal and light cycle matching that of Flax Pond, under thecontrol of an electronic cycling device (Chrontrol, Lindberg Enterprises, SanDiego, California, USA).We employed an in vivo technique using the whole animal (as opposed toindividual claw preparations), whose claw was inserted with the pollex tip into afixed brass ring and the dactyl tip into a brass ring connected to a flexible Plexiglasbeam. Most of our measurements employed the apparatus described in Levinton& Judge (1993) which employed two strain gauges on the beam, connected to aWheatstone Bridge and interfaced to a microcomputer via an analog-to-digitalconnecting board. The Wheatstone Bridge/amplifier was powered by a regulatedDC power supply of %15 V. Gape angle between the dactyl and pollex was keptconstant at ca. 35-37”. Because claw gape angle was kept constant for major andminor claws in our study, we assume that the degree of muscle shortening wasapproximately constant.We recorded maximum closing force of major and minor claws, using animals ofcarapace length with a range of 5.2-15.3 mm. For the major claws we measuredclosing force in 120 individuals (length range was 5.9-40.4 mm), using theapparatus described in Levinton and Judge (1993) but with a regulated DC powersupply. For minor claws we used the above apparatus for 39 specimens, but wealso employed another flexible beam apparatus with a more slender beam foranother 109 crabs, in order to measure the rather smaller closing forces of thesesmaller claws (length range was 4.0-9.5 mm).The dimensions of this cantilever were 76.6 mm long X 11.2 mm wide X 1.6 mmthick. Both types of apparatus were calibrated by hanging a range of standardweights on the end of the cantilever and the voltage output was measured bymeans of the A/D board. The calibration with the D.C. power source was linearand repeatable and the deflected cantilever quickly returned to zero voltage ineither apparatus when the weights were removed. In both apparatus types thecalibration was linear throughout our range of deflections.The slender-beam apparatus was cross-calibrated by measuring closing force of17 crabs on both apparatus types. We discovered that measurements of estimatedclosing force between the slender-beam apparatus and our old apparatus differedby an average factor of 2.21 but with statistically indistinguishable slope (n = 17, SE = 0.211), despite the linear response in calibration by weights. We thusmultiplied the measurements obtained from the slender beam apparatus by the2.21 correction factor in all analyses that are presented below. Taking the range ofclaw lengths for which we have data for the two pieces of apparatus, it waspossible to perform an analysis of covariance to compare the data. After the 2.21correction factor was applied there was no significant difference between size-related trends in closing force, either in slope or intercept (F = 2.37, p = 0.126).

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