Reproductive ecology of three ocypodid crabs I. The influence of activity differences on reproductive traits

Reproductive traits of three ocypodid crabs,Scopimera globosa, Ilyoplax pusillus andMacrophthalmus japonicus, were compared.S. globosa andI. pusillus, inhabiting the upper-middle intertidal zone, produced 1–2 large broods per year, whereasM. japonicus, inhabiting the lower intertidal zone, produced 4–5 small broods per year. InS. globosa andI. pusillus, ovigerous females remained in their plugged burrows without feeding until their eggs hatched. On the other hand, femaleM. japonicus fed actively on surface mud while incubating. We concluded that few large broods may be advantageous in crab species that incubate in burrows, whereas continuous small broods may be advantageous in species that feed actively while incubating.     

Burrow structure and use in the sand fiddler crab, Uca pugilator (Bosc)

Uca pugilator, the sand fiddler crab, constructs two kinds of burrows in protected, sandy upper-intertidal and supratidal substrates on the west coast of Florida. Temporary burrows are built and used as a refuge by non-breeding crabs during high tide periods and at night when crabs cease feeding in the intertidal zone. Breeding burrows are constructed and defended by courting males and are the site of mating, oviposition and the incubation of eggs by females. Up to three ovigerous females may be accommodated in a single breeding burrow, each female sequestered in a separate terminal chamber. The construction and defence of burrows specialized for breeding may be an adaptive response by males to the preferences females exhibit when selecting a breeding site.     

Underground mating in the fiddler crab Uca tetragonon: the association between female life history traits and male mating tactics

Brood size and other life-history traits of females affect male investment in mating. Female Uca tetragonon, producing relatively small broods, were attracted to the burrows of males for underground mating (UM) while carrying eggs. Most UM females released larvae and ovulated new broods during the pairing, averaging 3.9 days. While a female was incubating one brood, another brood was developing within the ovaries because the females were feeding adequately during incubation. These findings suggest that in U. tetragonon, a small-brood species, females increase the total number of broods produced by breeding continually. In contrast, in large-brood species, feeding by ovigerous females is relatively brief and not enough to prepare the next brood during incubation, inducing temporal separation between incubation and brood production. Unlike females in other ocypodids where females with large broods remain in the breeding burrows of males, most female U. tetragonon left the male after UM. Wandering in female U. tetragonon after the pairs separate may occur because their small broods are adequately protected by an abdominal flap. Relative brood size probably determines the vulnerability of the incubated broods to the females' surface behavior. Hence, male reproductive success in large-brood species may decrease greatly if males expel their mates after ovulation, although this is not necessarily so in small-brood species. Whether the male drives away the female or not may depend on which behavior within either small- or large-brood species yields the greater male reproductive success. In U. tetragonon some females extruded eggs in their own burrows after surface mating as well as in males' burrows after UM. It was unclear whether females chose a male with a larger burrow as an UM mate unlike several large-brood species. Burrows of both UM males and ovigerous females in U. tetragonon were relatively smaller than those in some large-brood species, indicating that incubation of small broods does not require large burrows. Rather than benefits of UM by female choice, wandering resulting from intersexual conflict, and sperm competition may explain why some females mate in males' burrows in this small-brood species.     

A trematode infection with no effect on reproductive success of a sand-bubbler crab

The infection effects of the parasitic digenean trematode on the body weight and reproductive success of the sand-bubbler crab were examined. Gynaecotyla squatarolae (Trematoda: Microphallidae) infects the body cavity of Scopimera globosa (Decapoda: Scopimeridae) and uses the crab as its second intermediate host. The parasites infected all reproductive crabs examined to varying degrees. Larger crabs of both sexes had more parasites than smaller ones, probably because body size reflects age, and older crabs had a longer period of exposure to infection. Males had more parasites than females, probably because of sexual difference in acting time on the surface. Ovigerous females stay in closed burrows and do not act on the surface during incubation, and so have less chance of infection than males. The quantity of infecting parasites did not explain variations in either body weight or reproductive success of individual crabs in a field experiment. The life history of this parasite, relative body size of the crabs, and cost and the possible benefit of manipulation for the parasite may explain these results.     

Size-dependent Mating Behaviours of Male Sand-bubbler Crab,Scopimera globosa: Alternative Tactics in the Life History

Factors leading to the separation of mating behaviours were investigated in the sand-bubbler crab, Scopimera globosa. The crabs mated on the surface (surface copulation, SC) and underground (UC). UC males were large (old) whilst SC males were small (young). Burrowless females bred in the UC males' burrows. These females accepted UC in exchange for access to a burrow. UC occurred much more frequently than SC in the burrow area in which females oviposited. Most SC occurred in the water-saturated area affording a rich diet. SC was accepted by most large and small females in both areas and most UC by small females in the burrow area. SC was an alternative to UC for males in that there was a size dependence between types of copulation. These two mating behaviours involved different degrees of interaction with neighbouring males. Males attempting to carry a female to their burrows for UC were more often disturbed by other males than were males attempting SC. In the interaction for both UC and SC, larger males were likely to resist the disturbances. UC males needed their own burrows, but these burrows were not enlarged before mating. UC males have a higher paternity of eggs than SC males, because SC males' sperm is often displaced by other males. Thus, UC was a behaviour with relatively higher costs and benefits for male crabs than SC behaviour. Alternative mating behaviours in male S. globosa are conditional, and explained by intrasexual interactions and a male life history strategy with a trade-off between growth and reproduction. It is not likely that the size dependence of male mating behaviour is caused by mate preference of females for UC males in the burrow area.     

ECOLOGY AND EVOLUTION OF MATING SYSTEMS OF FIDDLER CRABS (GENUS UCA)

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.     

Mechanisms of cross-shore distribution pattern of the intertidal mud crabMacrophthalmus japonicus

In the intertidal mud crabMacrophthalmus japonicus, most large crabs occurred in the upper sandy areas. Mechanisms of the size-dependent distribution were interpreted experimentally. In the lower muddy area, food was more abundant and crab density was much higher than in the upper area. Under the high density conditions, interaction among large crabs became severe and more large crabs abandoned their burrows. In the experiment on burrowing-site selection, large crabs burrowed only in the muddy side under the low density, but also in the sandy side under high density conditions. The burrowing in the upper sandy area may be advantageous for large crabs, though food availability is lower, because of less competition and higher survivorship under the low crab density.     

A new species of Poly dor a (Polychaeta: Spionidae) from the Indo-West Pacific and first record of host hermit crab egg predation by a commensal polydorid worm

A new spionid polychaete, Polydora robi, is described from intertidal and shallow subtidal areas in the Philippine Islands and Bali, Indonesia. Polydora robi belongs to the Polydora ciliata/ websteri species group and is characterized by a rounded prostomium, triangular occipital tentacle, needlelike posterior notosetae, and a pygidium with digitiform composite cirri surrounding the anus. Adults burrow into empty gastropod shells inhabited by hermit crabs. The burrows of the worms typically extend from an external opening in the apex of the shells to an opening in the central body whorls along the columella. The species was found to ingest the fertilized eggs and developing embryos attached to the pleopods of host hermit crabs. The occurrence of egg predation and the symbiotic relationship between polydorids and hermit crabs is discussed. Known egg predators of hermit crabs are reviewed.     

Anti-predator responses of the intertidal crab Heterozius rotundifrons (Brachyura: Belliidae) in air and water

The intertidal crab Heterozius rotundifrons responds to tactile input, as occurs during a predation attempt, by hyper-extending all of its limbs and remaining in that posture for a variable length of time. We compared the duration of this anti-predator response: (1) in the day versus night (2) in two fluid media (air versus water) (3) after exposure to additional predator cues in one medium (air or water) and testing in the other medium (4) for crabs from different parts of the tidal range and (5) for females with and without eggs on their pleopods.?Crabs showed the posture at night as well as during the day. They also executed the posture when tested in air and extended the duration of the posture in air when they detected an additional predation-risk cue, shadows passing overhead. When crabs experienced input in one medium there was no effect on the duration of behavior shown in the other medium. Crabs from the lower portion of the intertidal showed a markedly longer duration of the limb-extended posture compared to crabs from the higher end of the tidal range of this crab. Berried females responded the same as females without eggs in both air and in water. Thus, crabs show this anti-predator behavior under a wide variety of conditions, but do not appear to transfer information received in one medium to behavior shown in the other media.     

Spatial and temporal aspects of reproduction in North Carolina fiddler crabs (Uca pugilator Bosc)

Male courtship cycles, female cycles of mating, incubation and larval release, and foraging patterns of Ucap ugilator Bosc found on sloping and elevated flat salt marsh beaches in North Carolina were studied to identify correlates between the spatial and temporal distribution of resources and behavioral differences. We also compared temperate and subtropical (Florida) populations. All showed semimonthly cycles of reproduction. However, North Carolina crabs courted, mated, and spawned within the intertidal zone, rather than above it as did Florida crabs, probably because salt marsh substrata provide stable (non-collapsing) incubation chambers for brooding females. Crabs on sloping beaches foraged in herds at food-rich lower zones during both diurnal and nocturnal low tides. Those on elevated flats, where the food supply is uniformly distributed, rarely left their burrows to feed. When herding occurred, it was primarily during new moon nocturnal low tides. Finally, crabs on elevated flats exhibited temporally restricted spawning periods compared to those on sloping beaches where water depths, even during the neap tides, were sufficient for larval release.     

Reproduction and larval development of Polydora robi (Polychaeta: Spionidae), an obligate commensal of hermit crabs from the Philippines

Abstract. The reproduction of a recently described spionid polychaete, Polydora robi , is examined from the Philippines. Adults inhabit a burrow in the apex of gastropod shells occupied by hermit crabs. Females were found to deposit broods of 18–94 egg capsules in the summer (June–August) and winter months (January–March) sampled. Paired or single egg capsules are attached by stalks to the inside wall of the burrow and contain 40–106 eggs which average 97 μm in diameter. The total number of eggs per brood ranges from 941–8761 eggs and is positively correlated with the total number of segments and length of female worms. Adults of P. robi are polytelic, producing ≤9 successive broods over a 3-month period; a mean of 6.7 d was exhibited between broods in the laboratory. Females utilize sperm stored in the seminal receptacles during successive spawnings. Development occurs within egg capsules until the 3-segment stage, at which time the planktotrophic larvae are released. Juveniles of ∼20 segments are competent to settle on gastropod shells inhabited by hermit crabs. Members of P. robi are relatively fecund, semicontinuous breeders; the life-cycle in this species is similar to the only other known obligate polydorid commensal of hermit crabs.     

Small- and large-scale effect of the SW Atlantic burrowing crab Chasmagnathus granulatus on habitat use by migratory shorebirds

Here we address the question of whether the presence of the burrowing crabs Chasmagnathus granulatus affects small- and large-scale habitat use by migrant shorebirds. This crab is the dominant species in soft bare sediments and vegetated intertidal areas along the SW Atlantic estuaries (southern Brazil 28°S to the northern Argentinean Patagonia 42°S). They generate very extensive burrow beds in soft bottom intertidal areas. Our information shows that this burrowing crab affects the small-scale habitat use by shorebirds, given that shorebirds never walk through the funnel-shaped entrances of burrows. Given that crab burrow entrances occupy up to 40% of the intertidal area, there is a large decrease of available shorebird habitat in crab beds, restricting their activity to the spaces between the burrows. The southern migratory shorebird Charadrius falklandicus maximize the use of these areas by foraging closer to the burrows than the other bird species. Neotropical migrants, such as Calidris fuscicollis, Pluvialis squatarola and Tringa melanoleuca, used foraging paths that tended to maximize the distance from burrows, especially the distance to larger burrows. A field experiment showed that this was not necessarily due to a decrease in the availability of polychaetes near the crab burrows. A combination of landscape measurements and satellite images showed that crab beds covered up to 40% of the intertidal area of the Mar Chiquita coastal lagoon (37°40′S, Argentina), and nearly 100% of the intertidal area of the Bahia Blanca estuary (38°48′-39°25′S, Argentina). These two estuaries are located along the migratory flyway of Neotropical migratory shorebirds, but the Bahia Blanca estuary (area∼110,000 ha) shows a much lower shorebird diversity than Mar Chiquita (area∼4500 ha). The most common species in Bahia Blanca is the two-banded plover C. falklandicus, the species least affected by crabs at Mar Chiquita and which prefers to use high-density crab areas as foraging sites. The oystercatcher Haematopus palliatus was also most abundant in high-density crab areas, but they used these areas for resting. The abundances of preys varied during the study period and between the crab density areas, indicating that the use of these areas by birds is independent of crab density. However, burrowing crabs affect the depth distribution of polychaete and thus their availability to shorebirds. We suggest that this shorebirds-burrowing organism interaction could be generalized for other intertidal estuarine habitats.     

Pillar Function in the Fiddler Crab Uca beebei (II): Competitive Courtship Signaling

Male Uca beebei court and attract females into burrows they defend on muddy sand flats in the intertidal zone on the Pacific coast of the tropical Americas. Mating, oviposition and incubation (breeding) occur underground in males' burrows. Some courting males build mud pillars (2 cm high) at the entrance of their burrow. The purpose of this field study was to assess the role of pillars in competitive courtship signaling among males. I studied the effect of pillars on female behavior by recording the responses of wandering females to courtship from males resident at burrows with and without pillars. I also caught females, released them individually in a circular arena with an equal number of empty burrows with and without pillars around its circumference, and chased the females with a simulated avian predator. Females moved to burrows of both types more often when they were courted (82%) than when they were chased (67%). Receptive females were attracted to the burrows of the males that courted them significantly more often (97%) when these burrows had pillars than when they lacked pillars (66%). However, once females entered males' burrows they were equally likely to remain, mate and breed in both types of burrows. Females also more often moved to burrows with pillars (66%) than to burrows without pillars when they ran from the simulated predator. Both male courtship displays and pillars probably provide cues females use to locate males' burrows. The visual similarity between pillars and a display courting males give immediately before they enter their burrows suggests that pillars are icons of the display. The effect of pillars on female behavior, the timing of pillar building relative to when females choose mates, and contrasts in the behavior of males that do and those that do not build pillars suggest that pillar building has evolved due to competition among males to attract females into their burrows.     

The use of open burrows to estimate abundances of intertidal estuarine crabs

Estimation of absolute (or true) abundances of intertidal burrowing crabs is a difficult problem in some estuarine habitats because the nature of the substratum and behaviour of crabs can restrict researchers to the use of sampling methods which at best estimate only apparent, or relative, abundances. One method that has been used is to count open burrows to estimate population densities. This paper discusses the results of a test examining the validity of using burrows to estimate apparent abundances of a temperate ocypodid crab, Heloecius cordiformis. inhabiting mangrove forests of south-eastern Australia. Under appropriate circumstances, this method may provide a quick and reliable estimate of abundance of crabs.     

Elevated predation risk changes mating behaviour and courtship in a fiddler crab

The fiddler crab, Uca beebei, lives in individually defended burrows, in mixed-sex colonies on intertidal mud flats. Avian predation is common, especially of crabs unable to escape into burrows. Mating pairs form in two ways. Females either mate on the surface at their burrow entrance (''surface mating'') or leave their own burrow and sequentially enter and leave (''sample'') courting males'' burrows, before staying in one to mate underground (''burrow mating''). We tested whether perceived predation risk affects the relative frequency of these mating modes. We first observed mating under natural levels of predation during one biweekly, semi-lunar cycle. We then experimentally increased the perceived predation risk by attracting grackles (Quiscalus mexicanus) to each half of the study site in two successive biweekly cycles. In each experimental cycle, crabs were significantly less likely to mate on the side with more birds. Moreover, on the side with elevated predation risk, the number of females leaving burrows to sample was greatly reduced relative to the number of females that surface-mated. Males waved less and built fewer mud pillars, which attract females, when birds were present. We discuss several plausible proximate explanations for these results and the effect of changes in predation regime on sexual selection.     

Why egg-caring males of Isaza (<Emphasis Type="Italic">Gymnogobius isaza</Emphasis>, Gobiidae) refuse additional females: preliminary field observations

Like many other gobies, males of the Isaza goby, Gymnogobius isaza endemic to Lake Biwa, Japan, conduct parental care of eggs at nests, and females are likely to choose mates while visiting nests. The reproductive strategy should induce polygyny, but Isaza males never accept additional females in one breeding cycle. Sampling data of broods indicated that the egg mass areas were much smaller than the nest sizes, suggesting that nest size is not the limiting factor in obtaining further eggs from additional females. The brood size greatly decreased as the duration of care progressed. Few individuals including caring males ate eggs, and heterospecific egg predators were rarely observed. Sixty percent of egg masses at the middle stages of egg development were infected with aquatic fungi, some being covered with a fungus mat that drastically reduced survivorship. Infected egg masses contained more eggs than non-infected ones at the same stage, indicating that large egg masses are prone to be frequently destroyed by fungi. It is likely that the activity of parental males is lowered during the long care periods at low water temperature in early spring. Such lowered activity of caring males might be responsible for infections in large broods that would have needed more care. We propose the hypothesis that male rejection of additional females may be related to optimal brood size, which will be less susceptible to fungus infection and produce more hatching young than otherwise. This hypothesis will explain not only male avoidance of additional females but also some unique reproductive behaviors of this fish such as some females spawning of a portion of of the mature eggs in one nest.     

Costs of incubation and immunocompetence in the collared flycatcher

This paper investigates the costs of incubation in terms of reduced reproductive success and investigates whether incubation competes with immune function for resources. I performed a clutch size manipulation experiment in which two eggs were either removed from or added to the nests of collared flycatchers, Ficedula albicollis, for 1 week during incubation and subsequently returned to their original nests before hatching. To induce immune response, the females were challenged with sheep red blood cells. While the duration of incubation, hatching success and fledgling number did not differ between experimental groups, fledgling condition was significantly lower in broods that had been enlarged during incubation. Neither the females' condition nor their ability to respond to a novel antigen differed between treatments. The relationship between antibody production and female condition was significantly positive, but only among females incubating reduced clutches. I conclude that the costs of incubation in the collared flycatcher are not negligible and are manifested only at the chick-rearing phase.     

Mating and role of seminal receptacles in the reproductive biology of the spider crab Maja squinado (Decapoda, Majidae)

The reproductive biology of the spider crab Maja squinado was analyzed based on monthly samples from an 18-month study carried out in Galicia (NW Spain) and laboratory experiments holding primiparous and multiparous females in captivity with and without males. The seminal receptacles of adult females were analyzed and their relationship with the presence and developmental stage of the eggs and the gonad maturity stage was determined. Gonad maturation in primiparous females began one or two months after the pubertal moult. Females having gonads in an advanced stage of development made their appearance in December and the first spawning took place in mid-winter or early spring. The percentage of ovigerous females from March to September was ∼75%. As the incubation period progressed, the ovaries became mature again in order to carry out the next spawning. Under experimental conditions the breeding cycle started earlier in multiparous females, during their second yearly cycle, than in primiparous ones. After mating, female spider crabs store sperm in seminal receptacles and this sperm is used in the fertilization of eggs immediately prior to spawning. The analyses of seminal receptacles consisted of the estimation of fullness and the number of differentiated sperm masses. The number of masses ranged between 0 and 6 in field samples (median for females with stored sperm=1) and was positively correlated with fullness. Differences in colour and volume of individual masses showed that, at least in some cases, females carried out successive matings with long intervals in between. This storage mechanism allowed females to fertilize successive broods without remating (as was also shown under experimental conditions). Juvenile females from shallow waters did not have developed seminal receptacles which indicated that mating was not possible until the onset of maturity. Postpubertal females in shallow waters (August to October), including animals participating in aggregations, always showed empty receptacles. The seasonality of receptacle fullness showed that mating involved hard-shelled females and occurred in deep water during the autumn migration from juvenile habitats or in the wintering habitats, during the last stages of gonad maturation (November to February). After fertilization ovigerous females continued to store sperm, but the volume was lower than in non-ovigerous females. Mating may occur in ovigerous females, particularly in the final period of incubation, because in females with broods almost ready to hatch, both new and older sperm masses were seen in the receptacles (distinguished by colour and size). The fullness of the receptacles decreased both in ovigerous and non-ovigerous females in the final phase of the annual breeding cycle (August–October), however, some sperm was still available. In the laboratory, mating was observed, and no courtship nor postcopulatory guarding was recorded. The analysis of receptacles from laboratory experiments indicated that primiparous and multiparous females showed differences in the seasonality of mating in the first phase of the breeding cycle (September–January), related to differences in the timing of gonad maturation and hatching. Mating occurred in the final stages of gonad maturation, a short time before hatching, and matings were detected in ovigerous females. Multiple matings were also evident, to a greater extent than in the field, probably due to the higher availability of males. Females underwent over four successive spawnings in the laboratory without having to recopulate, and the incubation lasted on the average from 40 to 58 days (∼18 and 16°C respectively) and the mean duration between hatching and the next spawning was 3.4 days. It is estimated that most females carry out three successive spawnings during the annual cycle.     

Tower construction by the manicure crab <Emphasis Type="Italic">Cleistostoma dilatatum</Emphasis> during dry periods on an intertidal mudflat

Animals living on upper intertidal mudflats experience habitat desiccation during neap tides when water does not flood the habitat. Individuals of the manicure crab Cleistostoma dilatatum construct cone-shaped towers at the entrance of their burrows, in which they remain during neap tides. These towers are the tallest known structures compared to body size built by crabs living on intertidal flats. The frequency of tower construction followed semilunar tidal cycles with most building done prior to neap tides when few crabs were active on the mudflat surface. Bigger crabs tended to make taller and wider towers with a wider pinhole on the top. These towers may regulate the microclimate in burrows.     

Aspects of the biology of the big-handed crab,Heterozius rotundifrons (Decapoda: Brachyura), from Kaikoura,New Zealand

Abstract

From January 1975 to June 1976, samples of Heterozius rotundifrons A. Milne Edwards, 1867 were taken monthly from the intertidal zone at First Bay, Kaikoura Peninsula (42°25′S, 173°42′E). Small crabs (< 8 mm carapace width) were never common; they were either too cryptic to be collected in quantity, or have a different habitat from larger crabs. Ovigerous females were present in all months except February (in both 1975 and 1976), but generally formed less than 50% of the total sample of females each month. The number of newly deposited eggs (0.75×0.81 mm) carried by females increased with increase in carapace width according to the equation y = -1123.56+102.97x (r ² = 0.8213). Egg development lasted 3–5 months, and egg mortality during this period was almost 10%. The overall sex ratio for the 18-month sampling period was 1846 ♀ : 993 ♂, which suggests that females were approximately twice as numerous as males. However, when crabs were sorted into size classes it was evident that the sex ratio was not significantly different from 1:1 in crabs of 6–12 mm carapace width, whereas females significantly (P<0.001) outnumbered males in the 13–21 mm size range. The right and left chela were approximately equal in length in females of all sizes, but the right chela of large males was greatly enlarged compared with the left, and with the chelae of females of comparable carapace width. Increase in the growth rate of the males’ right chela commenced at a carapace width of approximately 11.0 mm. Since the smallest ovigerous female collected also had a carapace width of 11.0 mm, it is concluded that both males and females attain sexual maturity at this size.