Movement patterns of the limpet Cellana grata (Gould) observed over a continuous period through a changing tidal regime

Time-lapse videography of limpets mounted with light-emitting diodes was used to monitor the movements of a population of the non-homing Cellana grata on a vertical gully wall in Hong Kong. Animals were monitored for >7 days to examine spatial and temporal variation in their behaviour as the tides transited from a semi-diurnal to an almost diurnal pattern. Movement was synchronised with the tides, irrespective of the day–night cycles. Limpets rested low on the shore and were stimulated to move by the rising tide. Individuals moved up shore with the flooding tide, maintaining themselves in the awash zone, and then down shore on the ebbing tide, until they reached a resting height when the tide then fell beneath them, exhibiting ‘zonal shuttling’. A tight coupling of limpet position to tide height persisted through the changing tidal pattern, and almost all animals displayed the same organisation of activity over all tides. Initiation of activity and maximum height reached were probably controlled by the tides, but the cessation of activity may have been controlled by an internal clock. The pattern observed is consistent with the threat of attack from aquatic predators coupled with the need to minimise physical stresses while exposed. It is also consistent with the avoidance of grazing lower on the shore where interspecific competition may be more intense. Limpets showed two peaks of activity per tide, corresponding to when the rate of change of tide height was the greatest, except when tides became much reduced during the transit to a diurnal pattern. Movement was triphasic: an initial rapid phase, followed by a slower phase in the high shore and then a rapid phase before the limpets stopped moving. This structure is common in limpets and in this case is likely to be a consequence of animals maintaining themselves within the awash zone. Tide height appears to determine foraging activity, but with modifications in the behaviour in response to factors operating at more local temporal and spatial scales.     

Foraging and homing behaviour in the high-shore, crevice-dwelling limpet Helcion pectunculus (Prosobranchia: Patellidae)

The foraging activity of the high-shore, crevice-dwelling limpet Helcion pectunculus (Gmelin, 1791) (Prosobranchia: Patellidae) from an exposed shore on the eastern Cape coast of South Africa was monitored. Activity was compared during spring and neap tides and between spring, summer, autumn and winter. Rhythms of activity in this limpet varied depending upon microhabitat; individuals inhabiting both east- and west-facing rock surfaces were active during nocturnal low tides, but limpets on west-facing rock surfaces were also active during daytime low tides, whilst in the shade. Individuals travelled further during foraging excursions in winter (xˉ=85.53 cm) than in either spring (xˉ=55.7 cm) or summer (xˉ=48.8 cm) and also during spring low tides (xˉ=89.8 cm) compared with neaps (xˉ=40.9 cm). H. pectunculus exhibited rigid homing to a fixed scar within a crevice, and feeding excursions were found to consist of three distinct phases: a relatively rapid outward phase, a much slower foraging phase and a rapid homeward phase. Whether or not these limpets graze throughout an excursion is not known. Foraging was always highly directional, with a mean vector which took limpets onto an area of the rock face with the highest microalgal biomass. The fact that H. pectunculus took advantage of optimal feeding areas and memorized their location to enable return visits suggests a learning component in herbivorous gastropod foraging behaviour. Received: 29 December 1997 / Accepted: 8 May 1998     

Monitoring spatio-temporal variation in molluscan grazing pressure in seasonal, tropical rock pools

In Hong Kong, low-shore rock pools support a patchy mosaic of different successional stages of crustose coralline algae. Echinoid and molluscan grazers are abundant in these low-shore rock pools and spatial and temporal variation in grazing pressure is thought to play an important role in structuring these assemblages. Direct records of graze marks using a modified wax-disc method were used to monitor spatial (different pools) and temporal (periods, 3 or 4 days each; spring vs. neap tides; summer vs. winter) variations in grazing pressure. Radula marks of Chlorostoma argyrostoma, Lunella coronata and Nerita albicilla were common in all pools. Grazing pressure was highly variable with tidal state and season but also within these two factors. Lunella and Nerita exerted highest grazing pressure at neap tides during summer, whilst Chlorostoma exerted highest grazing pressure in winter. Although overall patterns of temporal variation were similar and consistent in all pools, there was between pool variation in grazing pressure and individual species showed different patterns. Within the seasonal and tidal variations, large period-to-period differences were found for all species, illustrating small-scale temporal variation. Fine-scale spatial and temporal patterns of variation in grazing pressure of different species, therefore, contribute to the highly dynamic nature of these low-shore rock pools.     

How hot for how long? The potential role of heat intensity and duration in moderating the beneficial effects of an ecosystem engineer on rocky shores

Studies of the importance of ecosystem engineers have focused on their benefit to biodiversity through ameliorating environmental stress, without understanding the exact benefits gained by associated organisms. On monsoonal tropical shores, species experience strong seasonality in environmental conditions from almost temperate winters to tropical summers when mass mortalities can occur during daytime emersion. The limpet, Cellana grata, associates with an ecosystem engineer, the barnacle Tetraclita japonica, in summer, but not in winter. To understand the benefits of this association, physiological responses (heart rates and osmotic responses) of the limpet either amongst Tetraclita or on open rock surfaces were investigated under three environmental conditions in a laboratory mesocosm: awash (non-stressed), low (30 °C) and high thermal stress (40 °C) of varying durations (3 or 6 h). In general, at 30 °C, limpets showed similar physiological responses under all conditions for both exposure durations. After 6 h at 40 °C, however, all limpets on open rock surfaces died, whereas those associated with barnacles survived. The surviving limpets experienced similar levels of stress as those exposed to 40 °C on open rock surfaces for half the time (3 h), showing that both the level and duration of stress were important. Limpets, therefore, gain benefits from engineering species when conditions are extreme or stress is prolonged. Under low temperatures or for short durations (e.g. winter), associating with barnacles does not provide physiological benefits. Understanding how, and to what extent, associates benefit from ecosystem engineers highlights how close the margin between survival and mortality can be when conditions are extreme.     

Spatial and temporal strategies in the surface activity of some sandy beach arthropods living along the French Atlantic Coast

During a field study on the eco-ethology of sandy beach arthropods conducted along the French Atlantic Coast it emerged thatTylos europaeus, Phaleria cadaverina andLabidura riparia were among the most abundant species that lived in the eulittoral. Tetradirectional pitfall traps, placed from the base of the dune to the tidal limits, were used. The traps intercepted surface-active arthropods, and the number of traps varied according to the levels of the tides. Data were recorded hourly during two periods corresponding to a spring tide and the following neap tide. At the saine time the most important environmental parameters were registered each hour. The results indicate that all three species show prevalently nocturnal surface activities. ForT. europaeus andPh. cadaverina an influence of the synodic and tidal phases was found on the zonation of the surface activity and on the extent of movement. The latter was due to the greater or smaller distances that the isopods and tenebrionids had to move from the diurnal burrowing sites to the foraging zones. ForL. riparia no variations in the total mean zonations were found between the two synodic phases, and only at neap tide were seaward, followed by landward movements found. Multiple regression analysis showed that the environmental parameters registered in this period were never limiting, as threshold values in this case probably were not reached.     

Life-history features of the marine intertidal limpet Notoacmea scutum (Gastropoda) in central California

Growth of individually marked specimens of Notoacmea scutum (Rathke) was followed for 15 months (April, 1978 to June, 1979) at a field site in central California, USA. Growth rates were highly seasonal, being highest from late spring through early summer and lowest during winter. Growth rate also varied significantly with intertidal height: limpets lower in the intertidal zone grew faster. Both relationships may have been related to the relative abundance of food or to the amount of time available for foraging. Recently settled limpets, which were present throughout the year, grew to approximately 17 mm in length by the end of their first year; by then, they had also reached the size at which mature gonads were consistently found (16 mm). Limpets were almost 26 mm long by the end of the second year. Substantial mortality occurred throughout the year, but it was particularly severe during winter, when a large portion of the reproductive population was lost. This pronounced winter mortality was probably not caused by invertebrate predators such as seastars. Most limpets at the study site were less than 2 yr old.     

Abundance of estuarine crab larvae is associated with tidal hydrologic variables

Abundances of brachyuran megalopae and juveniles were measured throughout consecutive tidal cycles during six 2 to 3 d sampling periods in summer 1992, and associated with rates of change of tidal hydrologic variables in the Newport River Estuary. Current speeds and rates of pressure change fitted sinusoidal (tidal) models well; however, rates of salinity and temperature change did not. Analysis of plankton samples taken during spring and neap tides showed peak abundances during nighttime rising tides for all taxonomic groups: Callinectes sapidus, Uca spp., Xanthidae, and Pinnixa spp. megalopae, and Pinnotheres spp. juveniles. Megalopal and juvenile abundances from time-intensive sampling were related to rates of changes in the hydrologic variables using stepwise logistic regression. No hydrologic variable accounted well for the presence of Uca spp. megalopae. Megalopal presence was best predicted by current speed for Pinnixa spp. megalopae, and rates of changes in pressure for xanthid megalopae and Pinnotheres spp. juveniles, and salinity for C. sapidus megalopae. These variables might act as cues causing megalopae to ascend into the water column at a particular point in the flooding tide, and subsequently descend to or near the bottom prior to ebb flow. In this way, larvae which develop on the continental shelf or lower estuary undergo transport up the estuary by behaviorally altering their swimming activity and depth concurrent with tidal changes.     

Chlorophyll blooms and the spring/neap tidal cycle: observations at two stations on the coast of Connemara,Ireland

Water temperature, chlorophyll a and phaeophytin were recorded every 2 to 4 d, in order to describe short-term variability in phytoplankton blooms. Data were collected from May to September 1984 and March to September 1985 at shore stations on the coast of Connemara, western Ireland. Comparisons with offshore data suggest that the shore data is representative of coastal water. Chlorophyll a was highest at neap tides in spring, and at spring tides in early summer. In late summer, transient chlorophyll blooms occurred at neap tides, possibly due to a localized accumulation of flagellates in stable conditions.     

Day length,latitude and behavioural (in)flexibility in baboons<Emphasis Type="Italic"> (Papio cynocephalus ursinus</Emphasis>)

Annual cycles in day length are an important consideration in any analysis of seasonal behaviour patterns, since they determine the period within which obligate diurnal or nocturnal animals must conduct all of their essential activities. As a consequence, seasonal variation in day length may represent an ecological constraint on behaviour, since short winter days restrict the length of the time available for foraging in diurnal species (with long summer days, and thus short nights, a potential constraint for nocturnal species). This paper examines monthly variation in activity patterns over a 4-year study of chacma baboons (Papio cynocephalus ursinus) at De Hoop Nature Reserve, South Africa. Time spent feeding, moving, grooming and resting are all significant positive functions of day length, even before chance events such as disease epidemics and climatically mediated home range shifts have been accounted for. These results provide strong support for the idea that day length acts as an ecological constraint by limiting the number of daylight hours and thus restricting the active period at certain times of year. Day length variation also has important implications across populations. Interpopulation variation in resting time, and non-foraging activity in general, is a positive function of latitude, with long summer days at temperate latitudes apparently producing an excess of time that cannot profitably be devoted to additional foraging or social activity. However, it is the short winter days that are probably of greatest importance, since diurnal animals must still fulfil their foraging requirements despite the restricted number of daylight hours and elevated thermoregulatory requirements at this time of year. Ultimately this serves to restrict the maximum ecologically tolerable group sizes of baboon populations with increasing distance from the equator. Seasonal variation in day length is thus an important ecological constraint on animal behaviour that has important implications both within and between populations, and future studies at non-equatorial latitudes must clearly be mindful of its importance.     

Intra-specific variability in the temporal organisation of foraging activity in the limpet<Emphasis Type="Italic"> Patella vulgata</Emphasis>

Patella vulgata is one of the most common limpet species on rocky shores throughout the north-eastern Atlantic. Foraging activity of this species has been described during various different diel and tidal phases and several factors have been suggested that might influence the timing of its activity. Among these, inclination of rock surfaces seems to have a clear role, with limpets on horizontal rocks being active during daylight at high water, while limpets on vertical rocks were active during night-time low water. However, by comparing the results of previous investigations it is difficult to separate clearly the effect of any single factor from the effects of their interactions. In the present paper we investigated the simultaneous influence of height on the shore, slope of rock surface and date of sampling (August–September and March–April) on the behaviour of a population of P. vulgata on the Isle of Man (UK). The study was carried out using modern telemetric methods, allowing the activity patterns of limpets to be recorded for long periods of time. The results showed that limpets concentrated their activity during two distinct temporal windows—daytime tide-in and night-time tide-out—while no significant activity was observed during daytime tide-out or night-time tide-in. The observed behaviour appeared to be driven by complex interactions among the three factors examined, and the influence of rock slope alone was not so strong as to override the effect of other factors completely. In addition, a key finding of this study was the conspicuous variability in the individual behaviour of P. vulgata. The greatest level of variability was observed in limpets on vertical surfaces on the upper shore, which showed an extreme switch in behavioural patterns, according to date of sampling (daylight/tide-in foraging during the spring and night-time/tide-out foraging during late summer). In addition, individuals apparently sharing the same set of conditions (inclination, height on the shore and sampling date) differed in their behaviour, and a given behavioural pattern was not always shared by all individuals from a given group. Finally, variability was also observed at the intra-individual level since the same individual could be active during different tidal and diel phases. In conclusion, although the existence of endogenous rhythms has previously been demonstrated in this species, our data clearly show that P. vulgata was able to modulate its behavioural patterns in relation to a variety of exogenous determinants.Communicated by R. Cattaneo-Vietti, Genova     

The effect of microbial activity upon the sedimentary sulphur cycle

The sulphate content of an intertidal sediment at Anglesey (UK) was shown to vary during the year, the sulphate-reducing activity of the microbial population being limited by low temperature during winter, and by the low numbers of sulphate-reducing bacteria during May, 1969. A corresponding annual variation of the sedimentary sulphide could not be demonstrated, and the sulphide which was formed biologically during the course of the year was almost entirely lost from the sediment. This loss of sulphide was probably due to subsurface oxidation; and flushing, by water entering at the base of the sediment. It is suggested that sulphide was only precipitated within the sediment during summer when sulphate reduction was active, and even then only during part of the tidal cycle when flushing did not occur.     

Biological and physical aspects of migration in the estuarine amphipod Gammarus zaddachi

The amphipod Gammarus zaddachi (Sexton) conducts extensive migrations along estuaries from near the limit of tidal influence in winter to more downstream reaches (where reproduction occurs) in spring. A return migration then takes place, primarily by juveniles, until the seaward areas are depopulated in winter. The present study was conducted between 1988 and 1990 in the Conwy Estuary, North Wales. This represents the first investigation on this species in a strongly tidal estuary, where the amphipods appear to migrate vertically into the water column on flood or ebb tides to control horizontal transport and to maintain preferred distributions. The timing of vertical migration seems to be largely controlled by an endogenous circatidal swimming rhythm. Phasing of peak activity relative to the time of expected high tide varies with season; upstream migrants in the autumn showed peak activity at the time of expected high tide, while in the spring at the time of downstream migration the rhythm was phase-delayed, with peak activity during the expected ebb tide. Together with the season, position along the estuary also affected the timing of peak endogenous activity; downstream migrants, originally active on the ebb tide and experimentally displaced seawards, showed a phase-advance of the rhythm relative to the time of high tide. Salinity-preference behaviour also varied between different developmental stages, with ovigerous females (downstream migrants) showing no preference between fresh and saline water, and juveniles (upstream migrants) showing a significant preference for freshwater. The interactions of endogenous rhythmicity and salinity-preference behaviour are discussed as controlling factors of migration in this species.     

Factors affecting variability of foraging excursions in a population of the limpet Patella vulgata (Mollusca,Gastropoda)

Paths moved by the limpet Patella vulgata L. were monitored on a sheltered vertical rocky shore in North Wales using time-lapse photography throughout nocturnal low tides in April and November 1992, roughly corresponding to periods of minimum and maximum gonad ripeness, respectively. Various motion parameters, including total duration, total length, and maximum distance reached from home were computed from 124 complete foraging routes obtained from 18 higher-zoned and 18 lower-zoned limpets. P. vulgata typically performed a single loop per night (average total length, ca. 70 cm), moving from the individual home scar to graze the surrounding algal grounds (average maximum distance, ca. 25 cm), then following its own trail back home. On the average, limpets moved from the home scar during three of four available nocturnal low tides and exploited about half the emersion time. Evidence for size-related variability in behaviour was found, as a positive correlation was assessed between both total duration and legth of the excursions, and shell length. In addition, zonation was proved to affect the limpets' foraging behaviour, since low-shore limpets moved faster and covered greater distances than high-shore ones. Moreover, consistent seasonal variation in foraging behaviour emerged, since in April the excursions were longer and longer-lasting than in November, but limpets exploited a larger fraction of potential activity phases in November than in April. The within-population variability in the temporal and spatial characteristics of the foraging excursions is discussed in relation to the available data on zonal and seasonal variation of food resources and in relation to physiological changes due to reproductive cycle.     

Estuarine epifauna recruit despite periodic hypoxia stress

In some estuaries, the recruitment of epifaunal benthic invertebrates coincides with a significant environmental stress, low water-column dissolved oxygen, termed hypoxia (̀ mg O₂ l^-1). Recruitment of epifaunal species was measured in the lower York River, a subestuary of the Chesapeake Bay, USA, which experiences predictable, periodic hypoxia associated with neap/spring tidal cycling during summer. Recruitment substrata were exposed during 48-h deployments in two areas with differing levels of hypoxia, and epifauna were allowed to recruit during periods of low oxygen (neap tides) and high oxygen (spring tides) in 1996 and 1997. Recruitment was often high during neap tides, even when severe oxygen depletion (<0.5 mg O₂ l^-1, <0.71 ml O₂ l^-1) occurred during deployments; indeed, peak recruitment episodes of several dominant epifaunal taxa, and of total epifauna, coincided with hypoxic events during both summers. Increased recruitment during neap tides suggests that factors besides hypoxia influenced recruitment in the York River; these factors may have included changes in larval availability and lower current speeds. This study illustrates how the relationship between recruitment and large-scale stresses, such as hypoxia, may be difficult to predict, since large-scale stresses are often correlated with numerous other factors. Short-term hypoxia appears to have little effect on recruitment in the field for many epifaunal species in this ecosystem, which may explain, in part, why substrata exposed for longer durations (1 month) in this system showed few effects of hypoxia on community composition or diversity. High larval tolerance of hypoxic stress may allow communities to persist even though the summer hypoxia season coincides with the recruitment of many epifaunal species. Electronic Supplementary Material is available if you access this article at http://dx.doi.org/10.1007/s00227-002-0930-6. On that page (frame on the left side), a link takes you directly to the supplementary material.     

Field observations on the rhythmic behaviour of Nerita textilis (Gastropoda: Prosobranchia)

Field observations on Nerita textilis Dillwyn inhabiting rocky cliffs at Sar Uanle (Southern Somalia) revealed that during the resting phase some of the snails are scattered on the upper cliff and some are clustered at lower levels. During feeding they all migrate downward, but to different extents. Maximum activity of both groups occurs during low water (LW) between midday and midnight. During spring tides (ST), the scattered snails reside higher up and make smaller feeding excursions than during neap tides (NT). During ST, some clustered snails join the belt of scattered individuals, while the trend reverses during NT. A diurnal and a tidal component appear in their periodicity.Publication of the Centro di Studio per la Faunistica ed Ecologia Tropicali del C.N.R. Researches on the coast of Somalia. The shore and the dune of Sar Uanle, No. 14.     

Size-assortative mating and non-reciprocal copulation in a hermaphroditic intertidal limpet: test of the mate availability hypothesis

Mating behaviour and mating patterns are affected to a large extent by body size in both hermaphrodites and gonochorists. Detailed research on mating patterns, mate choice, pre- and post-copulatory sexual selection in marine pulmonates is wanting, thus warranting more attention as a study system in the future. The simultaneous hermaphroditic limpet Siphonaria capensis (Pulmonata: Basommatophora) shows size-dependent fecundity, and acts as a suitable organism to test the effects of body size on mating patterns, mating success, gender expression and reciprocity in hermaphroditic marine gastropods. We mainly used bootstrap resampling techniques to estimate the effects of different factors on mating patterns. In the populations studied, a strong size-assortative mating pattern was observed, where small-scale spatial distribution of potential mating partners (the mate availability hypothesis) could explain 65% of this pattern, while mate choice and mating constraints explaining the rest. No significant difference in mating success between limpets with different body size was found, even though in one population, the sperm recipients were larger than the non-copulating limpets. Interestingly, we found that intromission was non-reciprocal during copulations. This may mean that this species does not conform to the common rule of reciprocity predicted for hermaphrodites, unless there is sex-role alternation between individuals in a mating pair. The mating partners consisted of similar sized, acting males and females, thus without any indication of the body size determining the gender expression. The matings took place in early mornings only at spring tides and the animals were observed laying egg masses only during neap tides. Since siphonariid limpets possess both a spermatheca and a seminal vesicle, which may have either a sperm storing and/or digestive function, post-copulatory sexual selection (e.g., sperm competition) cannot be dismissed.     

Sexual and asexual reproduction of <Emphasis Type="Italic">Coscinasterias</Emphasis><Emphasis Type="Italic">tenuispina</Emphasis> (Echinodermata: Asteroidea) from Rio de Janeiro,Brazil

An intertidal population of the fissiparous sea star Coscinasterias tenuispina (Lamarck, 1816) was sampled over a 1 year period at Ponta de Itaipu, Niterói, Rio de Janeiro. Gonadal indices revealed an annual reproductive cycle. Spawning occurred in the late winter and early spring between August and November. Pyloric caecal indices did not show a clear annual cycle or a reciprocal relationship with gonadal indices. The population appears to be sustained only by fission, as only males were found. Fission occurred throughout the year, but was most frequent in the winter (July, August and September). Incidence of fission was correlated with the occurrence of the extremely low tides during daytime that exposes many specimens to air. Austral winter tides are low by day and high by night, while the austral summer tides are low by night and high by day. The most extreme annual deviation in seawater temperature is about 5°C. Larval recruitment seems to be negligible, since very small individuals were not found.     

Feeding preferences,seasonal gut repletion indices,and diel feeding patterns of the sea urchin<Emphasis Type="Italic"> Tripneustes gratilla</Emphasis> (Echinodermata: Echinoidea) on a coastal habitat off Toliara (Madagascar)

The feeding biology of the echinoid Tripneustes gratilla from a seagrass bed off Toliara, on the south-west coast of Madagascar, was investigated during the summer and winter seasons. Repletion indices of the gut of T. gratilla varied seasonally, in such a way that higher amounts of food occurred in the gut in winter than in other seasons. Conversely, a more detailed analysis of one summer and one winter sample of the gut tissue dry weights revealed higher weights in summer (February) than in winter (August). This was interpreted as a varying capability of nutrient storage and closely related to the feeding activity of the echinoid. This inverse relationship between feeding activity and nutrient storage was suggested to be dependent on gonadal growth. Indeed, higher food consumption was observed during the post-spawning period (August), that is, when most of the energy accumulated was used up for gonadal growth. Low feeding activity occurred when a sufficient amount of energy was accumulated in gut tissues (February), the time corresponding to the initiation of gonadal growth. Investigations on the feeding habits, using Ivlev's electivity index, revealed preferential feeding of T. gratilla on Syringodium isoetifolium, the dominant seagrass species found in the gut. This was partly related to the higher availability of that particular seagrass species in the field. However, its soft and terete leaf blade's morphology also makes it easier to manipulate and ingest. Studies on the feeding behaviour indicated the presence of a diel feeding cycle in T. gratilla individuals in both summer and winter seasons. Yet, its pattern varied between the two studied months (one summer and one winter) as the cycle was less contrasted during period of active feeding, i.e. in winter compared to summer. This study provides novel information on how intrinsic factors, such as physiological state, can govern the feeding activity of T. gratilla living in a seagrass habitat.     

Cardiac responses to abiotic factors in two tropical limpets, occurring at different levels of the shore

Laboratory experiments tested whether the cardiac activity of two species of limpets (Cellana spp.), co-occurring on the rocky shores of Hong Kong but having different zonal distributions, was differently affected by variations in physical environmental characteristics. Experiments reproduced exposure of limpets to dehydration, high water temperature and reduced salinity, all conditions that can occasionally be faced by animals during low tides. Cardiographic traces were obtained using a non-invasive method based on infrared light sensors glued onto the limpet shell in a position directly over the heart. The signals were filtered and amplified, viewed on portable oscilloscopes and automatically transferred to computer. When tested under acclimation temperatures, the lower shore C. toreuma had faster heart rates than the upper shore C. grata, both in air and when submerged in seawater. Regular alternation of heartbeating and acardia were observed in C. grata exposed to both dry and saturated air, whilst C. toreuma showed transient acardia only during exposure to dry air. Both species showed a similar increase in heart rate with increasing temperature (Q₁₀~1.8 in the range 25-34°C). When submerged in freshwater, C. grata regularly alternated acardia with beating at a rate slightly lower than in seawater, whilst C. toreuma showed strong bradycardia with reduction of beat amplitude, followed by steady acardia. Recovery of heartbeating after return to initial salinity was faster in C. grata, whilst C. toreuma, initially bradycardic, showed a consistent return to tachycardia. Overall, this study shows that the two congeneric species exhibit different cardiac responses to variations in environmental characteristics, which may be related to their differing vertical distribution.     

Temporal constraints and female preference for burrow width in the fiddler crab, <Emphasis Type="Italic">Uca mjoebergi</Emphasis>

We studied sampling behaviour and mate choice in the fiddler crab Uca mjoebergi. Once a female selects a mate, she copulates in his burrow and remains there until releasing her aquatic larvae. U. mjoebergi occurs in habitats that are inundated only by the highest amplitude spring tides. Females can only release their larvae during these tides, and release before or after will result in complete failure of reproductive effort. Matings occur over a 5-day period near the end of neap tides. Our results suggest that within the mating period, females adjust their larval developmental rates by selecting specific burrows in which to incubate their clutches. We found that at the start of the mating period, females chose larger males as mates. Since male size was positively correlated to burrow width, females were selecting wide burrows and effectively incubating at lower temperatures. This would slow down the developmental rates of larvae. In contrast, females that mated late in the mating period selectively chose small males. By incubating in narrower, warmer burrows, these females may increase the developmental rates of larvae. We propose that females are selecting burrows to influence incubation rate and ensure timely release of their larvae. Female U. mjoebergi appear to adjust their preference for the direct benefits of mate choice to increase their reproductive success.