Summary

1. General accounts of the natural history and behaviour of fiddler crabs suggest there exist two broad mating patterns in the genus. Most western and Indo-Pacific species mate on the surface of intertidal substrates near burrows females defend. The sexes associate only briefly during courtship and mating. In contrast, males of many American species court from and defend burrows to which females come for mating. Copulation occurs underground in burrows plugged at the surface; the sexes usually remain together for at least several hours.



Here we summarize and contrast recent detailed field studies of the mating systems of U. pugilator, an American species, and U. vocans, a species widely distributed in the western and Indo-Pacific. We indicate how differences in the breeding ecology of these two species may account for basic differences in modes of sexual selection leading to the two broad mating patterns in the genus.



2. U. pugilator burrows in protected sandy substrates in the upper intertidal and supratidal zone. During ebb tide, nonbreeding crabs leave burrows they occupy during high tide to forage on food-rich substrates in the lower intertidal zone. Reproductively active males remain in the burrow zone where they fight for and defend burrows from which they court. Large males win most fights for burrows and tend to defend burrows high on the elevation gradient, especially during periods with relatively high tides. Females usually approach and descend the burrows of several males before choosing their mates by remaining in males' burrows. Males remain underground with their mates for 1–3 days until after they oviposit their eggs. Some males then emerge and leave their burrows while others sequester their mates in the chambers where mating and oviposition has occurred, dig new chambers and resume courtship, perhaps attracting additional females. In either case, females remain underground for approximately 2 weeks, finally emerging to release their planktonic larvae. Burrows that do not collapse due to tidal inundation or flooding by groundwater are best for breeding and usually are located relatively high on the elevation gradient. Females choose mates indirectly by preferring to breed in burrows that will remain intact while they oviposit and incubate their eggs. Large males mate more often than small males because they are better able to defend burrows at locations females prefer to breed. The mating system of U. pugilator may be classified as resource-defence polygyny.



3. U. vocans burrows in open muddy substrates in the mid- to lower intertidal zone. At a site near Chunda Bay, Australia, where the reproductive behaviour of this species has been studied in depth, both sexes feed near burrows they defend. Females tend to occupy their burrows for longer periods and move shorter distances than do males. Mating occurs on the surface near the burrows that females defend. Females accept both resident and wandering males as mates. They show no preference for mating with larger males. Female choice may be based on other male morphological or behavioural characteristics. Females oviposit their eggs either while on the surface or in their burrows. They produce relatively small clutches and are active on the surface throughout their breeding periods. Males fight both their neighbours and wandering males. Large males tend to win fights and defend burrows in areas where large females, which produce relatively many eggs, are most dense. Such areas may offer greater protection from predators than areas occupied by smaller females. Small males mate about as often as large males but may father fewer larvae. The mating system of U. vocans is resource-free and promiscuous.



4. The mating systems of U. pugilator and U. vocans differ fundamentally in that female U. pugilator require access to a specific microenvironment to breed successfully, while female U. vocans do not. We suggest this difference occurs because of contrasts in clutch sizes and the mobility and movement patterns of feeding females. Female U. pugilator produce relatively large clutches and probably experience more intense selection from factors that can cause egg loss and mortality than do U. oocans, which produce clutches of sufficiently small volume to be protected by their abdominal flaps. Hence, the range of suitable breeding environments for U. pugilator is small compared to that for U. vocans. In addition, U. pugilator burrows in areas that are relatively food-poor, leading to daily migrations to and from food-rich substrates in the lower intertidal zone, preventing female defence of an area suitable for both breeding and feeding. U. vocans, however, burrows in areas sufficiently rich to support feeding, leading to relatively low female mobility and defence of burrows that are also suitable breeding sites.



5. Adaptive radiation of the genus Uca in the Americas is manifest by trends toward smaller adult size, higher population densities, more frequent microgeographic sympatry and increased terrestriality, compared to species in the western and Indo-Pacific regions. We outline the general features of the selection mechanisms tying each of these trends to the evolution of resource—defence mating systems. Intraspecific variation in the courtship behaviour and site of mating in U. lactea and U. vocans supports our contention that resourse—defence behaviour tends to occur at high population densities. Additional data are needed to evaluate the other hypotheses critically.

Ecology and evolution of mating systems of fiddler crabs (genus uca)

The hypothesis that semilunar timing of larval release by fiddler crabs (genus Uca) results in transport of the final larval stage (megalopa) by spring tide currents to substrates in the upper estuary occupied by adults was tested and rejected. Water temperatures in the North Inlet estuary, South Carolina, increased from approximately 20° C to 28° C and the length of larval life decreased during the May-September breeding season. Nevertheless, ovigerous female U. pugilator, U. pugnax, and U. minax, collected bimonthly and maintained in the laboratory, released larvae ± 1.5 d of the full and new moons throughout most of the breeding season. Megalopae of Uca spp. were most abundant in a small tidal creek in the upper estuary during nocturnal flood tides and near the bottom about 5 d before and after the spring tides in September. Uca spp. and several other estuarine crabs appear to release larvae near the times of the high tides that are followed by the nocturnal ebb tides of greatest amplitude during the s...

Adaptive significance of semilunar cycles of larval release in fiddler crabs (genus uca): test of an hypothesis

Mating preferences of individual females and mating patterns among marked crabs were studied during the summers of 1973–1976 on sand beaches in mangrove habitat on the west coast of Florida in order to determine if females choose mates indirectly on the basis of the quality of a resource males defend and females use during breeding, directly on the basis of male size, or both.

Female choice in the resource-defense mating system of the sand fiddler crab, Uca pugilator

Do intertidal organisms simply respond to the rise and fall of tides, or do they possess biological timing and navigation mechanisms that allow them to anticipate when conditions are most favourable? How are the patterns of growth, development and reproduction of some marine plants and animals related to changes in day-length or to phases of the moon? The author describes how marine organisms, from single cells to vertebrates, on sea shores, in estuaries and in the open ocean, have evolved inbuilt biological clockwork and synchronisation mechanisms which control rhythmic processes and navigational behaviour, permitting successful exploitation of highly variable and often hostile environments. Adopting a hypothesis-testing and experimental approach, the book is intended for undergraduate and postgraduate students of marine biology, marine ecology, animal behaviour, oceanography and other biological sciences and also as an introduction for researchers, including physiologists, biochemists and molecular biologists entering the field of chronobiology.

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Chronobiology of Marine Organisms

The courtship, aggression and mating system of a “primitive” fiddler crab (Uca vocans: Ocypodidae)

Gravid female Uca pugilator (Bosc) maintained in conditions of 14h L: 10h D light regime and constant temperature without simulated tides showed a significant positive correlation between time of hatch and time of the nighttime high tide. Light: Dark experiments showed no entrainment by LD cues. Larval hatching was delayed until dark when high tide occurred between 1900 and 2130 hrs. Hatch time was a positive linear function of the time of the nighttime high tide when high tide occurred after 2130 hrs. Time of hatch was delayed by the light period on days lacking a nighttime high tide. Experiments conducted under constant light (LL) and temperature, without simulated tides, indicated a persistent free-running rhythm of approximately 25 h 03 min, suggesting an endogenous timer. Rhythmic larval release on the nighttime high tide may be adaptive as a predator-avoidance mechanism for adult gravid females.

M. E. Bergin

Papers

Hatching rhythms in Uca pugilator (Decapoda:Brachyura)