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.     

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.     

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.     

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.     

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.     

Strategies of mate finding in the European field cricket (Gryllus campestris) at different population densities: a field study

Abstract. 1. Members of a field population of Gryllus campestris L. varied in their walking and calling activity. In both sexes, some individuals occupied burrows whereas others walked around in the observation area. Males at burrows could be either silent or calling.
2. In the course of one summer, population density decreased and the initial balanced sex ratio changed to a large surplus of males.
3. At high population density, there were equal numbers of non-calling males at burrows, calling males at burrows and walking males, while walking females predominated over females at burrows. Non-calling males at burrows achieved more encounters with females than did calling and walking males. Females met males by walking through the population and by waiting at burrows. Thus, calling and phonotaxis were not essential for mate finding and calling was less effective than previously thought.
4. At low population density calling males predominated. Calling males at burrows achieved the most encounters with females. Females met males only by walking around in the population area. Calling was thus more important in mate finding than at high population density.
5. Changes in sex ratio and population density may cause the flexibility in mate finding behaviour of individual crickets.     

Semi-monthly reproductive cycles in male and female fiddler crabs, Uca pugnax

In the fiddler crab, Uca pugnax, numbers of displaying males, numbers of burrows with half-dome superstructures (hoods), numbers of females with decalcified vulvar opercula, and numbers of females hatching their eggs, all follow a semi-monthly cycle with peaks near the time of the spring tides. These are all aspects of reproduction in Uca. Decalcification of female opercula, although necessary for mating and egg deposition, is neither necessary nor sufficient for behavioural response to courting males. In fact, behavioural responsiveness to males probably precedes decalcification in most cases. The different aspects of the reproductive cycle are coordinated so that females and males are ready to mate at about the same time near one spring tide, and the resulting eggs will be ready to hatch on the next spring tide. Two decalcified females which were kept isolated from males produced fertile clutches, suggesting that stored sperm from a previous mating may fertilize eggs in a later cycle. This may be one reason for the relative rarity of observed matings or precopulatory behaviour in Uca.     

Foraging activity of the sand beach isopod Scyphax ornatus Dana

Abstract

The terrestrial isopod Scyphax ornatus lives on exposed sand beaches. Adult isopods spent the daytime in burrows near the high water mark and made nightly foraging excursions over the uncovered middle beach. Hourly records of their night time distribution showed that Scyphax congregated near the water’s edge during the last 4 h of flood tide, where they fed on carrion. Insects (mostly drowned honey bees) were the main food item, but coelenterates, amphipods, crabs, and goose barnacles were also eaten. During ebb and early flood tides Scyphax occurred at low densities over the uncovered middle beach. Analysis of the distribution of large and small food items on the beach showed that by feeding during the flood tide, Scyphax exploited a rich concentration of carrion that accumulated in the swash zone and that was moved slowly up the beach by the incoming tide.     

Biology and distribution of the littoral rove beetle Psamathobledius punctatissimus (Le Conte) (Coleoptera: Staphylinidae)

The intertidal macrofauna on a small sheltered marine beach at Pawley's Island, South Carolina, was dominated by insects, primarily Psamathobledius punctatissimus, a small beetle which attained densities of up to 2 260 adults m⁻². Both adult and larval beetles remained buried in the sand when covered by the tide, then emerged to form mole-like surface trails and feed upon diatoms when exposed. Eggs and young larvae were maintained within special maternal burrows, while larger larvae and non-breeding adults occupied their own individual burrows. Both adults and larvae became comatose when in direct contact with water, but recovered after as long as six hours submergence. In the field, burrows retained air during tidal coverage, allowing the beetles to avoid direct contact with sea water. Seaward extension of the population is probably limited by time available for feeding, while longshore distribution is restricted by sand texture and diatom concentration, or by sediment mobility in more exposed locations. Although very locally distributed, P. punctatissimus has achieved considerable success in a habitat poorly exploited by competitors of either marine or terrestrial origin. Contribution No. 434 of the Belle W. Baruch Institute for Marine Biology and Coastal Research Contribution No. 434 of the Belle W. Baruch Institute for Marine Biology and Coastal Research     

Thermal biology, torpor, and activity in free-living mulgaras in arid zone Australia during the winter reproductive season

Little is known about the energy conservation strategies of free-ranging marsupials living in resource-poor Australian deserts. We studied activity patterns and torpor of free-living mulgaras (Dasycercus blythi) in arid central Australia during the winter of 2006. Mulgaras are small (approximately 75 g), nocturnal, insectivorous marsupials, with a patchy distribution in hummock grasslands. Mulgaras (six males, three females) were implanted intraperitoneally with temperature-sensitive transmitters and monitored for 6-55 d. Temperature profiles for different microhabitats and the thermal properties of soil and a number of burrows were also measured. Air temperature ranged from -3 degrees C at night to 30 degrees C during the day. Although burrows buffered temperature extremes, the thermal diffusivity of the sandy soil was high, and many burrows were shallow. Hence, soil and burrow temperatures averaged about 15 degrees C. The activity of mulgaras was often restricted to a few hours after sunset, before they retired into their burrows. Mulgaras employed torpor frequently, often entering torpor during the night and arousing around midday, with arousals occurring later on cooler days. Shallow burrows allowed cooling below mean T(soil). Consequently, body temperatures as low as 10.8 degrees C were observed. The longest torpor bout was 20.8 h. Torpor patterns changed seasonally and differed between males and females. From June to August, females entered torpor almost daily despite mating and gestation, but from the end of the gestation period onward, they remained normothermic. In contrast, males showed only shallow and short torpor during the mating season, but from mid-July, a transition to more frequent and deeper torpor resembling that of females was observed. Apparently, in both sexes, the reproductive effort entails energetic costs, but torpor, as an energy-saving mechanism, and reproduction are not exclusive in mulgaras. In a resource-poor environment during the least productive part of the year, frequent torpor seems to provide the means to compensate for the increased energetic costs associated with reproduction.     

Monogamous Mating System and Spawning Cycle in the Gobiid Fish, Amblygobius phalaena (Gobiidae)

Reproductive ecology and mating system of the gobiid fish, Amblygobius phalaena, were studied on the coral reef at Sesoko Island, Okinawa, Japan. This goby usually lives in pairs, and maintains territories with several burrows for shelter and spawning. Although a few paired individuals changed partners, most pairs remained together over successive rounds of reproduction. Mate guarding by females appeared to prevent males from mating with other females. Spawnings were synchronous with semilunar periods. Several expected spawnings failed to occur (12%). These may have been caused by the delays in spawning preparation of the paired females or by the disturbance caused by a typhoon. A pair spawned in one of the several burrows within their home ranges. Eggs were deposited on the ceiling of the burrow, and were tended by the paired male for 3–4 days until embryos hatched. The males tended eggs at the expense of their feeding. Aggression toward fishes approaching their burrows were exhibited by the males as well as the females. Because of its low frequency in females, this behavior did not limit their ability to feed.     

Reproductive ecology of three ocypodid crabs II. Incubation sites and egg mortality

Mortality of eggs during incubation was estimated for three ocypodid crabs,Scopimera globosa, Ilyoplax pusillus andMacrophthalmus japonicus, and the influence of incubation sites was discussed. These crabs all lived in isolated burrows and fed on sediments during day time low tide.S. globosa andI. pusillus inhabited the upper intertidal sandflats, whereasM. japonicus inhabited the lower intertidal mudflats. Females of bothS. globosa andI. pusillus remained in their plugged burrows without feeding throughout incubation and the mortality of eggs was low despite large broods relative to body size. On the other hand, females ofM. japonicus fed actively on surface mud during incubation and the mortality of eggs was high despiite small broods relative to body size. InS. globosa andI. pusillus, the ovaries of ovigerous females were small until egg-hatching, whereas inM. japonicus, the ovaries grew rapidly during incubation and females were able to produce consecutive broods. I conclude that incubation of eggs in burrows may be advantageous in species which inhabit the upper interidal sandflats, even though the crabs cannot forage during incubation, since otherwise their eggs would be exposed to strong heat stress and desication during the summer. Furthermore, such species may produce few large broods because of less frequent interruption of feeding than that associated with production of many small broods.     

Sex Biased State Dependence in Natal Dispersal in Desert Isoponds, Hemilepistus reaumuri

Differences in the emergence, movement, and settling patterns of individuals during natal dispersal can provide testable hypotheses about the costs and functions of movement. Emergence, movement, and settling patterns were studied in desert isopods, Hemilepistus reaumuri. The young of this semelparous, monogamous, crustacean emerge from their natal burrows each spring and search for sites to establish new burrows or gain acceptance as mates in occupied burrows. Dispersal was measured in a long, narrow corridor into which individuals marked after emergence were monitored. Females emerged slightly earlier than males with substantial overlap. Size or condition varied with time before settling differently in males and females. Isopods in good or poor condition did not differ in distance traveled, but males in good condition took more time before settling. Small males were more likely to start new burrows and took less time before settling, suggesting they might be acting in anticipation of losing contests for female-initiated burrows. Larger females and those in higher condition were more likely to start new burrows and took less time before settling. The pattern in females could reflect male choice or constraints or costs associated with burrow establishment in females, which should be tested. Measures of dispersal based on recaptures of traveling or recently settled individuals may differ from the distribution of successful reproduction. In this study travelers were observed at shorter average distances than settlers, but successful settlers traveled less far than unsuccessful ones.     

Burrows of Paragnathia (Crustacea: Isopoda) and bledius (Arthropoda: Staphylinidae) Enhance cliff erosion

In coastal cliffs consisting of Pliocene fine‐grained clastic sediments at El Rompido, Huelva, Spain we discovered around the high water line an extensive zone of about 1.5 m in height burrows that were made by up to 4 mm long isopods Paragnathia formica (Hesse) and small 5 mm long staphylinid beetles (Bledius cf. corniger Rosenhauer). The burrowing zone was crowded with irregular burrows 2–5 mm in diameter and extended some 5 cm deep in the cliff. The base of the cliff showed a clear (bio)erosional notch due to this burrowing activity, comparable to those produced by marine boring and scraping organisms in, for example, calcareous rocks. This indicates that the activity of Paragnathia and Bledius helps in eroding the cliffs, a phenomenon not published earlier as far as we know. A comparable but less extensive burrowed zone was found farther upstream near Cartaya. Their presence around the high tide zone also might prove these burrows to be good markers of former high tide lines in older cliffs.     

Further notes on the natural history of the South American pepper frog, Leptodactylus labyrinthicus (Spix, 1824) (Anura, Leptodactylidae)

Leptodactylus labyrinthicus tadpoles reach a large size in the nest through consumption of trophic eggs. We previously suggested that the trophic eggs are laid just after amplexus has finished, but our new data do not support this hypothesis. We also present further details on the natural history of the species with regard to breeding activity, spawning site, retreats and the ability of tadpoles in preying upon fully-growth heterospecific tadpoles. We also show that the tadpoles are mainly nocturnal and take diurnal refuges. We collected the data in Brazil in three localities within the Cerrado Biome. We examined burrows used by L. labyrinthicus males, verified if females still contained mature eggs just after released from amplexus, and tested the ability of tadpoles in preying fully-growth heterospecific tadpoles. Field observations and experiments were conducted on tadpole activity time, hiding behaviour and level of susceptibility to predation by the bird leaf-scrapers in four sheltering situations. Reproduction could start before the first rains; this may be advantageous by allowing the tadpoles to exploit eggs of other frogs. We found one floating nest built in a temporary pool. The nest of the species is normally circumscribed in an excavated basin beside the water body. Adult males were found during the day with their head-out of the entrance of underwater burrows, which were perforations through dense root mats beside calling/spawning sites. Probably, these burrows in permanently water-filled soil are actively excavated by males. Females released all their eggs during the amplexus, so trophic eggs are not produced by the currently-accepted mechanism. Fully-grown heterospecific tadpoles were not preyed upon by L. labyrinthicus tadpoles, which can prey only slow-moving newly hatched ones. Field tadpoles took shelter under mud/dead leaves during daylight and became exposed on the bottom at night. Free-ranging leaf-scrapers removed dead leaves from a pool with their beaks and preyed upon tadpoles. In the experiments, the tadpoles sheltered under gravel/leaves during daylight, but they were exposed at night. Leaf-scrapers ate all exposed tadpoles, but no tadpole of the gravel/leaves trays was consumed. Hence the nocturnal habits and use of diurnal refuges may protect the tadpoles from visual predators, such as the leaf-scrapers.     

Behavior of Four Species of Fiddler Crabs,Genus Uca,in Southeast Sulawesi,Indonesia

We studied the behaviors of four species of sympatric fiddler crabs on Kaledupa Island, Indonesia. Species differences in activity level, grooming, burrowing and feeding were related to their habitat and food. Uca chlorophthalmus, living in muddy mangrove areas, were inactive and spent most of the time feeding in place. Females fed 50% faster than males and spent more time feeding. U. vocans was the dominant species at the beach in silty sand and was very active. Its feeding rate was about twice that of the former species, females fed more rapidly than males, and many crabs of both sexes fed in droves at the water’s edge during ebb tides. During ebb tides, they spent most of their time feeding, while at flood tide they engaged in a greater variety of activities, including burrow maintenance. They frequently walked while feeding and interacted aggressively. U. tetragonon lived in a pebbly band along one edge of the beach, by a quay. Their feeding rate was comparable in both sexes and slower than that of U. vocans; they fed largely on filamentous algae growing on the quay, which provides better food, and fed faster during flood tide than ebb tide. They spent more time in waving and other sex-related activities, and were seldom aggressive, except during the week of the full moon. Burrowing activities included placing excavated mud balls some distance away from their burrows and re-arranging them. U. dussumieri inhabited the other end of the beach in muddier substrate. They did not have sex differences in feeding rates and their rate of scooping food into their mouths was slow, but feeding claws made multiple pinches of the substrate, thus accumulating more material in each clawful of food.     

Non-breeding behaviour of Magenta Petrels Pterodroma magentae at Chatham Island, New Zealand

Magenta Petrels Pterodroma magentae were caught at light-attraction stations on southwest Chatham Island, New Zealand, and most were fitted with transmitters. Of 52 captured since 1993, 71% were males, and all 36 tracked adequately proved to be non-breeders in the breeding season of capture. Our data indicated no sex bias in their probability of being captured at lights. Males provided 86% of trackings, and 87% of trackings of birds flying over the breeding area were males. Males landed 118 times; females 13 times. Only males were found on the ground, by night and day, apparently unassociated with burrows (three with and ten without transmitters), but subsequently digging burrows ( n  = 8). Of 19 birds banded as fledglings up to 2000, males were first recaptured nearing 4 years old (at lights and on the ground) and a female nearing 6 years old (in burrow). Among 37 fledglings, the sex ratio was even. Nine tracked males occupied burrows, as did two females, but the latter were older recaptures (10+ and 25+ years old). It appears that only males claimed existing, or dug new, burrows. They then attracted a mate to the burrow by means unknown, but from among females frequenting an inshore courtship area near the colony, or occasionally flying over the colony, at night. Females established in burrows, but then losing their mate, were able to re-mate there, by calling from near the burrow or by attracting a mate in flight or from the postulated inshore courtship area. Both sexes sometimes took years to pair or re-mate, possibly reflecting the dearth of available mates.     

Use of local cues in the night-time navigation of the wandering desert spider <Emphasis Type="Italic">Leucorchestris arenicola</Emphasis> (Araneae,Sparassidae)

Adult male Leucorchestris arenicola can walk round-trips of several tens of meters in search of females. Most excursions end with the spiders returning to their burrow. For small animals homing over distances of several meters is theoretically impossible without the aid of external cues. It was investigated, whether the spiders use local cues or they rely solely on global cues. Individually marked male spiders were captured during their excursions and displaced several meters inside an opaque box. Ten out of twelve displaced spiders returned to their burrows. This shows that the male L. arenicola are using local cues during their homing, as the comparatively small displacement distances could not be detected by means of global, e.g. celestial cues. In order to test whether the spiders could be using olfactory guidance, the burrows were displaced by 2 m while the spiders were out on their journeys. In 12 out of 15 experiments, the spiders did not find their burrows. These results show that the burrows do not function as olfactory beacons for the homing spiders.     

Environmental factors affecting the spawning burrow selection by the gobiid Valenciennea longipinnis

Spawning burrow selection by the longfinned goby Valenciennea longipinnis was studied in the near-shore moat on coral reefs, Okinawa, Japan. The gobies make several burrows within their home range, and spawn in one of them. To examine the factors important for spawning burrow selection three characteristics were investigated: current strength, burrow length and effect of underground water on the burrow. Among the burrows, pairs tended to spawn in a larger burrow irrespective of their body sizes. Most of the other non-spawning burrows were too small for a pair to stay together, because hard substrata may prevent the fish from excavating and shaping the burrow as they like. Moreover, pairs preferred to spawn in burrows where the underground water was oozing out, probably because the male's parental burden will decrease due to the higher dissolved oxygen concentration in such burrows. Although current strength may affect a water-exchange in a V. longipinnis burrow in relation to water-exchange function of a mound, it did not affect the spawning burrow selection because of the smaller velocity difference among the burrows relative to the daily fluctuation of tidal current.     

Microhabitat and rhythmic behavior of tiger beetle Callytron yuasai okinawense larvae in a mangrove forest in Japan

Mangrove forests are regularly flooded by tides at intervals of approximately 12.4 h (tidal rhythm). Larvae of the tiger beetle Callytron yuasai okinawense in a mangrove forest made shallow burrows in mounds up to 1 m in height constructed by the mud lobster Thalassina anomala. No larval burrows were observed on the forest floor, which was very muddy even during low tide. Some larvae plugged the burrow openings before they were submerged at high tide. The mean interval between consecutive burrow plugging events was 12.37 h, which is similar to the period of tidal cycles. Nine out of 30 larvae plugged the burrow openings even when the burrows did not become submerged. Plugging behavior may be governed by an endogenous biological clock, or may be a response to exogenous information about tidal level (e.g. moisture seeping through the ground).