Seiji Goshima

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.

COMPETITION FOR SHELTER OF PREFERRED SIZES BETWEEN THE NATIVE CRAYFISH SPECIES CAMBAROIDES JAPONICUS AND THE ALIEN CRAYFISH SPECIES PACIFASTACUS LENIUSCULUS IN JAPAN IN RELATION TO PRIOR RESIDENCE, SEX DIFFERENCE, AND BODY SIZE

Abstract Intra- and inter-specific competition for shelter, in relation to prior residence, sex differences, and body-size effects, was studied in the laboratory to assess the potential impact of the alien crayfish species Pacifastacus leniusculus on the endangered Japanese endemic crayfish species Cambaroides japonicus. The resident was placed in an aquarium, which contained a single shelter of preferred size for both C. japonicus and P. leniusculus. After 24 h, the intruder was introduced into the aquarium. In the experiments using different sex pairs of crayfish of approximately equal size, male residents had a significant prior residence effect against female intruders in both crayfish species in the intraspecific contests. However, residents of P. leniusculus dominated C. japonicus regardless of the sex in interspecific contests. In experiments using male crayfish pairs of different sizes, larger individuals had more successful possessions of the shelter against smaller individuals regardless of crayfish species. The body-size advantage strongly influenced the outcome of the both intra- and inter-specific contests regardless of residents or intruders and overcame the prior residence effect in both species. These results suggest that P. leniusculus is superior to C. japonicus in shelter competition if P. leniusculus invades the habitat of C. japonicus.

Mate choice by males of the hermit crab Pagurus filholi : Do males assess ripeness and/or fecundity of females?

Males of the hermit crab, Pagurusfilholi, often grasp the edges of shells occupied by females and drag them during the mating season. This behavior was experimentally confirmed to be a precopulatory guarding behavior displayed by males for ripe females, and males were found to recognize females which were within about 5 days of spawning. Most theoretical models for mating preference assume the choosing sex (the male in the present case) has complete reproductive information about potential mates, and predict that males will preferably choose more fecund females and/or females that will require less guarding time (i.e. that will spawn sooner) as partners. Several male-choice experiments between two ripe females, both previously guarded by other males, were carried out to examine the above predictions. Males did not prefer females of larger size, higher fecundity or with less time remaining until spawning. These results suggest that males may not have complete information about potential partners, rather that male hermit crabs may adopt a mating strategy of pairing with the first ripe female they encounter. Even with such incomplete mate assessment, males may enhance their reproductive success by recognizing ripe females that will spawn within a given time (about 5 days in the present case).

Precopulatory mate guarding in the hermit crab Pagurus middendorffii (Brandt) (Decapoda: Paguridae): effects of population parameters on male guarding duration

Abstract The effects of sex ratio, competitor size, and the encounter rate with females on the duration of male precopulatory guarding were tested in the hermit crab Pagurus middendorffii (Brandt, 1851) to examine how guarding duration varies in relation to social factors. Size and owner advantages in male–male contest competition were also examined. Larger and owner males were stronger competitors than smaller and challenger males, respectively. Males guarded females earlier and, consequently, longer when the sex ratio was more male biased, when size differences between competitor sizes were small, and when encounter rates were low, which are consistent with theoretical predictions. However, contrary to these predictions, small males did not guard earlier than large males when size differences were large. These results suggest that males might assess not only the operational sex ratio based on the encounter rate with females but also their competitive ability relative to other males and the possibility of successful guarding.

THE DESIGN AND PRODUCTION OF A SEXUAL SIGNAL: HOODS AND HOOD BUILDING BY MALE FIDDLER CRABS UCA MUSICA

Courting male fiddler crabs Uca musica sometimes build hoods at the entrances of their burrows to which females come for mating. Females differentially orient to burrows with hoods and thereby show a mate preference for hood builders. Here we describe how this mode of sexual selection may affect hood design and building. Larger males built generally larger but not higher hoods. Small males may build relatively high hoods so that they will be conspicuous to females of all sizes and construction or other costs may limit hood height. Most males built only one hood each biweekly reproductive cycle, typically on a day that many females chose mates, and they finished construction before females began mate searching. Both patterns fit predictions based on applying ideal free theory to the timing of sexual signaling. Sexual selection may favor more frequent hood building but the timing of hood building appears to be optimal.

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.

Shell-size preference of hermit crabs depends on their growth rate

The hypothesis that hermit crabs modify their shell-size preference according to their prospective growth rate was examined in the hermit crab Pagurus middendorffiiA simple model predicts that individuals should select larger shells when (1) shell availability is limited, (2) they approach moulting and (3) they will have a large increase in size at the next moult. In experiments, selected shell size decreased with the duration of the moult, and increased with increasing growth, supporting predictions 2 and 3.

Asymmetry in Mutual Predation between the Endangered Japanese Native Crayfish Cambaroides Japonicus and the North American Invasive Crayfish Pacifastacus Leniusculus: A Possible Reason for Species Replacement

Abstract Asymmetry in mutual predation can be important in species replacement between native and invasive species. Mutual predation between the endangered Japanese native crayfish species Cambaroides japonicus and the North American invasive crayfish Pacifastacus leniusculus was studied to assess the potential impact of P. leniusculus on C. japonicus. We made laboratory experiments to examine mutual predation (i) between individuals in pairs of the same and different species and (ii) among a group comprising single and mixed species. We also made field surveys to clarify the frequency of cannibalism of the two species. In laboratory experiments, the frequency of cannibalism was low by both two species, but predation by P. leniusculus on C. japonicus was severe and most P. leniusculus survived both in different species pairs and in mixed species groups. In field surveys, remains of the two crayfish species, which may have been caused by predation by the same species, i.e., cannibalism, were scanty. The results suggest that asymmetry in mutual predation can be an important cause of species replacement of the native C. japonicus by the invasive P. leniusculus in the field.

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.

Seasonal Horizontal and Vertical Distribution Patterns of the Supralittoral Amphipod Trinorchestia Trinitatis in Relation to Environmental Variables

The seasonal distribution of the talitrid amphipod Trinorchestia trinitatis on an exposed sandy beach was examined in relation to moisture content and temperature profiles of the sand. Sand in which animals burrowed always had moisture levels >1.7% of the wet sediment weight throughout the year. Monthly mean moisture values for talitrids ranged from 3.0-5.0%. During winter, talitrids were found high up on the beach far from the reach of the surf and burrowed deeply to avoid freezing. After the thaw in early spring, animals reduced their burrow depth and moved downshore. Large animals began migration earlier than smaller animals. From late spring to summer, the population concentrated near the strandline left by wave action during the preceding high tide, burrowing shallowly. During this period the distribution patterns varied with animal size and reproductive condition. Small animals, especially juveniles, were located shoreward of and burrowed at shallower depths than large animals and ovigerous females, which were located higher on the beach. In late summer, burrowing depth of talitrids increased as temperatures in the surface layers rose. Behavioral responses, such as burrowing and surface migration to cope with the wide fluctuation of environmental factors, mainly sand moisture content and sand temperature, are suggested to be important mechanisms enabling T trinitatis to survive in the physically severe sandy beach environment.