Minoru Murai

Analysis of the mating system of the fiddler crab, Uca lactea

Abstract Claw-waving male Uca lactea sometimes attracted wandering females into their burrows for underground mating, but more often mated with nearby resident females on the surface. Female U. lactea mated at all stages of reproduction and experienced one or more copulations on the surface before wandering. Thus wandering females had sufficient spermatozoa to fertilize their eggs before copulating in a males burrow. After refusing to mate with neighbouring or burrow-less males, females were displaced from their burrows by the males. Displacement by males was a major cause of female wandering. Females did not leave their burrows in order to copulate, as do American fiddlers; rather, some mating occurred as an indirect result of wandering. Females who were not displaced by males ovulated and incubated clutches in their own burrows. Thus, although courting by waving a claw to attract a female into the males burrow is preserved in this species, the surface mating system of the Indo-Pacific subgenera is also employed. The sperm of the last male to mate with a female prior to egg extrusion is most probably used to fertilize the eggs. Thus a male that entices a female into his burrow gains a reproductive advantage.

A conspicuous courtship signal in the fiddler crab Uca perplexa: female choice based on display structure

The fiddler crab Uca perplexa has a conspicuous male courtship signal that is directed at females to attract them to the male’s burrow for mating. The signal involves waving the unflexed large claw up and down. To determine whether the spatiotemporal structure of the wave is under selection by female choice, we examined whether females had a preference for any particular features of the wave. Females respond to a waving display by either visiting the male’s burrow entrance or by electing to pass without visiting the burrow. We filmed mate-searching females and the waving males that they visited or passed. We documented the wave structure of these males using frame-by-frame analysis. Males produce a two-part wave with component A preceding component B. Both components have an upstroke, a pause at the apex and a downstroke. The tip of the claw was raised much higher in B than in A. Visited males had a shorter delay between the two wave components than did males that the females passed without visiting. Visited males also produced component B waves that had a slower upstroke than those of passed males. There was a significant correlation between the relative height of the raised claw and the duration of the upstroke of component B. Females were selecting males that raised their major claw to the highest position (two to three times as high as the carapace width). Passed males brought down their major claw earlier and from a lower position than did visited males. The data suggests that wave structure has evolved through female choice. Male display rate and body size were not female choice cues. An earlier study showed that display duration was also not used by females in selecting mates.

Some beavioral characteristics related to food supply and soil texture of burrowing habitats observed on Uca vocans vocans and U. lactea perplexa

From the observations on two species of fiddler crabs, droving is conspicuous in Uca vocans vocans but does not occur in U. lactea perplexa. U. vocans come down to feed at the waters edge with a richer food supply and fine substrate particles. Droving is advantageous to increase feeding efficiency in this species which holds burrows on a relatively sandy habitat where the food supply is less rich. In U. lactea, which burrows at higher levels, on the other hand, the food supply and soil texture of their burrow area are not significantly different from those at the waters edge. Home range relationships of both species are consistent with the occurrence or not of droving behavior and depend on food productivity of burrowing habitats.

Mate acceptance and guarding by male fiddler crabs Uca tetragonon (Herbst)

Abstract The fiddler crab, Uca tetragonon (Herbst) mates both on the surface near female-defended burrows and underground in male-defended burrows. In underground mating, which requires relatively high male investment, males attract both non-ovigerous and ovigerous females into their burrows by claw waving. Males aggressively expel some females soon after they enter their burrows and others after pair formation, but before females spawn. Finally males guard some females in their burrows until they spawn, which presumably ensures paternity. Males do not select mates of a particular body size, but they do differentially accept females with late-stage eggs, those about to release larvae and spawn another clutch. Except at the beginning of the reproductive season, few ripe non-ovigerous females are available because females spawn successively and only in moderate synchrony. By differentially accepting late stage ovigerous females, males may increase their fertilization rates because they minimize the time they spend guarding each of their mates to ensure their paternity. A male-biased operational sex ratio and a high last male advantage in sperm competition are two conditions that may have favored male choice based on female guarding time in this species.

Adaptive droving behavior observed in the fiddler crab Uca vocans vocans

Droving is conspicuous in Uca vocans vocans in summer. The crabs burrowing on the upper habitat have a higher tendency to wander compared to the ones burrowing on the lower habitat. Most of the wandering crabs captured on the low tide levels are relatively large and male. Larger crabs and males prefer to burrow on the upper zones of U. vocans habitat, but the smaller ones and females prefer the lower habitat. The upper level of the U. vocans habitat has relatively low N-content compared to the waters edge. Therefore droving is advantageous in crabs that have burrows on the upper level. Female and smaller resident crabs have faster feeding motions than male and larger ones, and can satisfy their feeding demands more rapidly. Therefore, for males and larger crabs it is advantageous to move away from the burrow area and forage in areas of higher food content near the low tide level where the number of feeding motions increases.

Studies on the interference among larvae of the citrus leaf miner,Phyllocnistis citrella stainton (lepidoptera: Phyllocnistidae)

Observation of the citrus leaf miner,Phyllocnistis citrella Stainton suggested that contacts among larvae frequently occur and either of two larvae encountered is sometimes killed by the interference. A model to describe the competitive interactions in the process of interference was constracted with this species. The distribution of hatchling on an area was considered in the model. The model was based on that of encounters in randomly moving particles in two dimensions. First the time when first death occurs was calculated andn hatchlings were reduced ton−1. Further calculation was made to obtain the time that next death occurs. Such calculation was repeated until the number of larvae was reduced to 1 or the area of mine attained to the completed size. By the simulation based on the data observed on the citrus leaf miner, the numbers of survivors were calculated at different hatchling densities and on different leaves in size. Based on this simulation analysis it was suggested that the following characteristics observed in the present studies enable the maximul number of individuals to become adults. (1) When two hatchlings occur on a leaf, they tend to be separated by the midrib. This implies that female moths tend to reject to lay eggs on the area which has already contained the egg. (2) The larva sometimes avoids the contact immediately before it occurs. (3) The larval mortality caused by a contact is reduced with the larval growth. Since the frequency of contacts increases as the mine is enlarged, the change to high survival prevents the larva from high mortality caused by the increase of contacts. (4) The larval survival does not decrease in spite of the reduction of the area available for mining. When two hatchlings occur on a leaf, they tend to be separated by the midrib. This implies that female moths tend to reject to lay eggs on the area which has already contained the egg. The larva sometimes avoids the contact immediately before it occurs. The larval mortality caused by a contact is reduced with the larval growth. Since the frequency of contacts increases as the mine is enlarged, the change to high survival prevents the larva from high mortality caused by the increase of contacts. The larval survival does not decrease in spite of the reduction of the area available for mining.

Pair Formation in the Burrows of the Fiddler Crab Uca rosea (Decapoda: Ocypodidae)

ABSTRACT Male Uca rosea copulate on the surface of the ground. The vertical type of waving is typical for this species. The claw was raised, stopped at the highest position, and lowered. Jerks were present for the upstroke and the downstroke. The display, however, is nonrequisite before surface mating. In this study, we found that some male U. rosea attracted females into their burrows for underground mating by waving. When a male U. rosea approached close enough to a wandering female, he waved toward her and walked back to his burrow. The male did not enter the burrow, but continued waving while passing his burrow, and, if she entered it, he then followed suit. When a male executed vertical waving toward a female, this behavior led the female closer to his burrow. Male leading of the female was present, but took different forms from that in the subgenus Celuca. The initiation of wandering of most females was associated with their eviction by wandering males. Wandering did not occur spontaneously, because most wandering females had stored sperm supplied from surface matings in their spermathecae. Female U. rosea sometimes descended into burrows during wandering. Pairing underground with a wandering female also occurred sporadically without her responding to waving. They occupied an empty burrow in which they formed pairs. If wandering females did not enter empty burrows, they entered male burrows, where no waving occurred. They need no sperm, but enter burrows probably for replenishing water. The male elicits copulation from the female in exchange for access to his burrow or a near empty burrow. Many pairs did not involve waving. The ovulation rate by female mates was similar between waving and nonwaving males.

THE COST OF RELIABLE SIGNALING: EXPERIMENTAL EVIDENCE FOR PREDICTABLE VARIATION AMONG MALES IN A COST-BENEFIT TRADE-OFF BETWEEN SEXUALLY SELECTED TRAITS

Claw size of male fiddler crabs, Uca perplexa appears to be a target of female choice that increases the likelihood a female will initially approach a male. Here we show that a behavioral display trait, the maximum height that the tip of the claw reaches during a courtship wave, is a strong correlate of the subsequent likelihood that a female will visit a males burrow (which is a prerequisite for a burrow mating). We experimentally manipulated claw mass, to test whether there is a trade-off between claw mass and wave height. Males with a metal weight added to their claw showed a large reduction in wave height, whereas control males (plastic added) showed no net change in wave height. There is therefore a trade-off between these two sexually selected traits (claw size and wave display). More importantly, the greater the initial wave height the smaller the subsequent decline in wave height. Assuming that variation in wave height is an index of quality, this variation in the cost-benefit trade-off is consistent with the requirements of a signaling system that conforms to the handicap principle when fitness is the multiplicative product of different fitness components. We conclude by discussing the ongoing difficulties in testing the handicap principle.

Asymmetry in Male Fiddler Crabs is Related to the Basic Pattern of Claw-waving Display

Morphological asymmetry was correlated with the pattern of claw-waving display in males from five species of fiddler crabs: three vertical wavers (Uca urvillei, U. dussumieri, U. vocans), a lateral waver (U. annulipes), and an intermediate waver (U. tetragonon). On the first, second and third ambulatory legs of male lateral waver crabs, the distance between the inner edge of the basis and the outer edge of the merus was larger on the side bearing the major cheliped than it was on the side with the minor cheliped. A similar asymmetry was observed in male intermediate waver crabs, but only the first ambulatory leg was involved. This morphological asymmetry is clearly related to the style of waving adopted by these crabs. When lateral wavers display, the weight of the major cheliped (which forms about one-third of the total body weight) is carried largely by the anterior ambulatory legs on the same side of the body, but the imbalance of weight during display is less in the intermediate waver. In the vertical waver crab horizontal motion of the major cheliped occurs relatively rarely; thus there is hardly any additional load on the ambulatory legs, which showed no asymmetry. However, the total length of the five sterna bearing thoracic legs tended to be larger on vertical waver males than on the female crabs. Thus the sterna of male crabs bulge outwards more than those of female crabs, and the angle between the sternum bearing the cheliped and the ground surface is larger in male crabs than in females. This may be an adaption enabling the cheliped of the male to be raised higher during the waving display.

What are the consequences of being left-clawed in a predominantly right-clawed fiddler crab?

Male fiddler crabs (genus Uca) have an enlarged major claw that is used during fights. In most species, 50% of males have a major claw on the left and 50% on the right. In Uca vocans vomeris, however, less than 1.4% of males are left-clawed. Fights between opponents with claws on the same or opposite side result in different physical alignment of claws, which affects fighting tactics. Left-clawed males mainly fight opposite-clawed opponents, so we predicted that they would be better fighters due to their relatively greater experience in fighting opposite-clawed opponents. We found, however, that (i) a left-clawed male retains a burrow for a significantly shorter period than a size-matched right-clawed male, (ii) when experimentally displaced from their burrow, there is no difference in the tactics used by left- and right-clawed males to obtain a new burrow; however, right-clawed males are significantly more likely to initiate fights with resident males, and (iii) right-clawed residents engage in significantly more fights than left-clawed residents. It appears that left-clawed males are actually less likely to fight, and when they do fight they are less likely to win, than right-clawed males. The low-level persistence of left-clawed males is therefore unlikely to involve a frequency-dependent advantage associated with fighting experience.