Effect of microclimatic factors and tide on vertical migrations of the mangrove crab Sesarma leptosoma (Decapoda: Grapsidae)

The tree crab Sesarma leptosoma Hilgendorf migrates up the mangrove trunks twice a day, both in the morning and in the evening, leaving its refuge near the tree base to feed on fresh leaves. In spite of the 150 cm of tide excursion at the base of the trees, the timing of the migrations seems largely controlled by nontidal factors. The timing of the morning upward migration is controlled by the light level alone. The morning downward migration takes place as soon as the temperature rises and relative humidity falls, with no relationship to tidal cycle or light level. Only the evening upward flow seems primarily controlled by the tide, while the evening downward flow, which ends at twilight, seems to be related to the timing of the evening upward migration, to the tide and again to light levels. During the day, crabs abandon the canopy to avoid low relative humidity, usually coupled with the higher wind speeds, which would quickly dehydrate them. At night, crabs stop migrating, probably because their movements and orientation capability along the trunk and in the canopy are largely vision dependent. Nontidal factors have never previously been shown to influence the migratory behaviour of an intertidal crustacean species so extensively. Received: 14 June 1997 / Accepted: 17 July 1997     

Infestation dynamics of gnathiid isopod juveniles parasitic on the coral-reef fish Hemigymnus melapterus (Labridae)

The dynamics of infestation by parasitic juveniles of gnathiid isopods were investigated at Lizard Island in the summer of 1997/1998 to determine when, and at what rate, they infest fishes. Variation in gnathiid abundance on wild-caught fish (Hemigymnus melapterus) between dawn and sunset was examined, and unparasitized H. melapterus in cages were exposed to gnathiids in the field for 4 h (8 h for fish sampled at 06:00 h) at five different times of the day and night (10:00, 14:00, 18:00, 22:00, 06:00 hrs) on three reefs. To control for any potential effect of the cleaner-fish Labroides dimidiatus, which consumes large numbers of gnathiids each day, sampling was carried out on reefs from which all cleaner-fish had been removed. Gnathiid abundance decreased during the day. Standardized abundance per wild-caught fish was 1.9 times higher at dawn than at sunset. Gnathiids successfully infested fish in cages. Sixty-one percent of the fish in cages were infested with ≥1 gnathiids, with 51% of the fish having between 1 and 4 gnathiids after 4 h exposure. Gnathiids infested fish both day and night. The log₁₀ (abundance +1) of gnathiids per caged fish varied significantly between time periods, with higher gnathiid numbers on fish sampled at 18:00, 22:00, and 06:00 hrs than on fish sampled at 10:00 and 14:00 hrs. In contrast to gnathiid abundance on wild-caught fish, the number of gnathiids on caged fish sampled at sunset was as high as that at dawn. The estimated mean (SE) cumulative number of gnathiids per caged fish [mean size = 13.7 cm (±0.25)] per day was 7.8 (1.1); this is similar to the estimated mean number of 7.3 gnathiids on similar-sized wild-caught fish at 6:00 hrs. The high infestation rate of gnathiids on caged fish in the late afternoon contrasted with the low numbers on wild-caught fish at this time, suggesting that factors other than infestation behaviour may be responsible for the low numbers of gnathiids on wild-caught fish in the afternoon. Received: 19 August 1998 / Accepted: 1 June 1999     

Use of time, space and food resources in the mangrove climbing crab Selatium elongatum (Grapsidae: Sesarminae)

The activity patterns and use of space of the mangrove dweller Selatium elongatum were studied in the field, in combination with gut content analyses for evaluation of the crab's natural diet. This sesarmine crab proves to be one of the most adapted to climbing and living on mangroves. Moreover, S. elongatum showed a strong preference for being active at high tide, both during the day and night, always along the trunks a few centimetres above the water level, feeding on floating algae and mangrove leaves. Larger males were always recorded within activity areas into which they allow females and smaller males but exclude males of the same size, which probably compete for females. These results confirm that, in spite of the homogeneous herbivorous diet, little niche overlap exists among the several sesarmine crabs inhabiting the East African mangroves, all being leaf consumers segregated on the basis of their horizontal and vertical zonation, and/or of their activity rhythm. Received: 28 September 1998 / Accepted: 14 June 1999     

Population ecology of the mud crab <Emphasis Type="Italic">Scylla paramamosain</Emphasis> (Estampador) in an estuarine mangrove system; a mark-recapture study

Mud crabs of the genus Scylla are commercially important mangrove residents that are extensively fished throughout their range in the Indo-West Pacific. Despite this high level of exploitation very little is known about the population dynamics of any of the Scylla species. The present study concentrated on an exploited population of Scylla paramamosain in a natural estuarine mangrove on Can Coc Island in the mouth of the Hau River, Mekong Delta, Vietnam. A total of 6,114 juvenile crabs captured on the seaward mangrove fringe were internally tagged by injection of coded microwire tags and released over a period of 29 days. Recaptures were monitored over the following 144 days. There was little migratory movement within the estuary; of 285 recaptured crabs, 93% were recovered within the island mangrove study area and only two individuals were recovered from mangroves lining the opposite mainland riverbanks. A von Bertalanffy growth function was fitted to growth increment data from recaptured crabs, with L _max = 150 mm to derive a growth constant, k = 2.39 year⁻¹. The theoretical age at which carapace width is zero (t ₀) was derived from known size at recruitment at instar 1, giving a value of −0.0095 years. Previous studies of the same population have shown that female S. paramamosain reach maturity at a mean size of 102 mm carapace width. The present study indicates that they attain this size at around 160 days from first settlement in the mangrove fringe. Abundance of juvenile crabs in the study area was estimated by the Petersen method as 1,101,500 (95% CI 4,17,300–1,785,800, representing 1,102 crabs ha⁻¹ of mangrove. Estimation of mortality from tag returns and from the age-catch curves during a period of constant recruitment were comparable (Z = 1.11 and 1.04 month⁻¹, respectively). Fishing only accounted for 14% of total mortality, suggesting that at the time exploitation was not at a critical level despite the apparent high level of fishing activity.     

Fraction distribution and migration of heavy metals in mangrove-sediment system under sulphur and phosphorus amendment

In this study, mangrove seedlings (Kandelia obovata (S. L.)) were cultivated in rhizo-boxes, which contain sediments collected from natural mangrove forest and modified with different rates of sulphate and phosphate. The fraction distributions of Zn, Cd, Cu, Ni and Pb in rhizosphere and non-rhizosphere sediments were studied by using a sequential extraction method. Metal concentrations in plant tissues and iron plaque on root surface were also determined to reveal migration variation of heavy metals in the plant-sediment system. The results showed that the activities of K. obovata roots enhance the reducible metals while reducing acid-extractable and oxidisable metals; sulphur amendment benefits the combination of metal ions with S^2– and therefore reduces the bioavailability of metal pollutants; addition of sulphur also improves the content of iron plaque on the root surface, which plays an important role in metal accumulation by K. obovata root tissue; the addition of sulphur markedly reduces the concentration of Cd in roots, but significantly enhanced the concentrations of Cu, Zn, Ni and Pb in roots. The results indicate that sulphur and phosphorus content in mangrove sediment, and the growth of mangrove plant can significantly influence the migration of heavy metals in the mangrove wetland ecosystem.     

Role of grapsid crabs, Parasesarma erythrodactyla, in entry of mangrove leaves into an estuarine food web: a mesocosm study

The influence of the crab Parasesarma erythrodactyla on the entry of the organic matter derived from Avicennia marina mangrove leaves in a sub-tropical mangrove ecosystem of southeast Queensland, Australia, was simulated using tidal mesocosms. Degradation of mangrove leaf organic matter was followed by analysing the fatty acid composition, carbon, and nitrogen isotopic signatures of the surface sediment and suspended particulate organic matter (SPOM) with and without the presence of crabs. Assimilation of mangrove organic matter by P. erythrodactyla was also assessed by stable isotope and fatty acid analyses in tissues and faeces. Results of the chemical tracer analyses question the adaptability of P. erythrodactyla to a diet comprised exclusively of mangrove leaves, and suggest that these organisms were dependent on additional food sources in their natural environment. Crab processing of senescent leaves significantly accelerated the transfer of mangrove organic matter to the surface sediments, as shown by a higher C/N ratio, a higher contribution of long-chain fatty acids and a more depleted C isotopic signature of sediment samples in the mesocosms with crabs compared to those without crabs.     

Competition and interaction between Neosarmatium smithi (Crustacea: Grapsidae) and Terebralia palustris (Mollusca: Gastropoda) in a Kenyan mangrove

 In a Kenyan mangrove, we studied the interactions between the gastropod Terebralia palustris and the crab Neosarmatium smithi when foraging on decaying mangrove leaves. Interactions are considerable on account of their wide overlap in zonation (Rhizophora mucronata belt), food items (mangrove leaves) and activity window (diurnal low water). The snails find a leaf by a systematic transecting of the platform and eat the leaf after crawling on to it, usually en masse. The crabs rarely venture beyond 80 cm from their burrows, and once they find a leaf they quickly drag it back into their burrow, probably to reduce the strong intra-specific competition. A crab is able to drag a leaf away even if a number of snails are already feeding on it, by strongly pulling the leaf away or by pushing the snails off the leaf. The success of the “thief ” depends on both the crab's size and the number of snails on the leaf. Received: 25 July 1999 / Accepted: 27 April 2000     

Feeding and burrowing ecology of two East African mangrove crabs

The behavior and ecology of two mangrove crabs,Sesarma meinerti De Man, 1887 andCardisoma carnifex (Herbst, 1794) were investigated at the beginning of the rainy season (October–November 1988) at Mida Creek, Kenya. Both species occupy upper intertidal levels, above mean high-water neap, and completely overlap in their zonation. Each burrow lasts ca. 3 wk, with no significant difference between the species. Both are more active around dusk and dawn and also follow a similar trend in their foraging activity, but differ in that burrowing is mainly diurnal inC. carnifex and nocturnal inS. meinerti. A hierarchy of food preference, established by offeringC. carnifex andS. meinerti leaves of five mangrove species, proved similar for both, withBruguiera gymnorhiza ranking first andAvicennia marina last, but significant only forC. carnifex. A rough estimate of the amount of litter consumed by these two species and of the soil mixed up by their burrowing activity indicates that they play a role of primary importance in the ecology of East African mangroves.     

Catch rate and emergence of male and female spanner crabs (Ranina ranina) in Australia

Catches of the spanner crab, Ranina ranina (Linnaeus, 1788), in southern Queensland, Australia in 1982 and 1983 showed a seasonal cycle, with peak catches from August to September. In most catches, males outnumbered females. Catches of both male and female spanner crabs declined in November and December, when a high proportion of females caught were ovigerous. Fish, echinoderms and polychaetes were the main food items. Closed-circuit time-lapse video recording was used to study the emergence behaviour of R. ranina in a 6 m-diameter tank. Spanner crabs in this tank were buried most of the time, emerging on average for only 1.7 h d^-1. They emerged mainly between 16.00 and 24.00 hrs and, less often, between 01.00 and 15.00 hrs. Males remained emerged significantly longer than females. The duration of emergence of the females peaked before the spawning season, becoming shorter during the period when they were ovigerous. Temperature and emergence were negatively correlated. It is concluded that reproductive behaviour has a strong influence on catch composition of spanner crabs.     

Significance of mangrove–mudflat boundaries as nursery grounds for the mud crab, Scylla paramamosain

Fisheries managers frequently try to protect juveniles in order to preserve stocks. Juveniles can be protected by either implementing changes designed to avoid catching immature animals (e.g. increasing mesh size or altering fishing techniques) or protecting nursery grounds. To prevent the capture of immature animals, an estimate of size at maturity is required as well as a knowledge of both fishing methods and the exact location of the nursery grounds. Strong demand for juvenile mud crabs to stock aquaculture ponds has resulted in development of fisheries targeting crabs of all sizes from instar 1 to mature individuals. Using five different fishing methods, different stages in the life cycle of Scylla paramamosain were followed for a period of 16 months in an estuarine population in the Mekong Delta, Vietnam. Mangrove habitat utilisation begins when crabs settle out from the plankton at instar 1 [modal internal carapace width (ICW), 0.5 cm] amongst the pneumatophores at the mangrove fringe. Increasingly larger crabs were found deeper into the mangrove but they were still living on the surface (modal ICW size class, 1.5 cm). As their size increases, the crabs either dig burrows (modal ICW size class, 4.5 cm) or they live in the sub-tidal zone, migrating into the mangrove with each tide to feed (modal ICW size class, 4.5 cm). Larger crabs were caught offshore (modal ICW size class, 12.5 cm) where females accounted for 60% of the catch although of these, only 63% were mature. Recruitment of early instars was continuous but peaked in December to February. Subsequent peaks in the catch rates of larger size classes indicated the development of a single cohort with an estimated growth rate of 2.0 cm ICW per month. On the basis of abdominal width, females were estimated to mature at 10.2 cm ICW although at 9.7 cm ICW, 50% of females had disengaged abdomens. Abdominal disengagement occurred in males at the slightly smaller size of 9.1 cm ICW. Allometric relationships between chela height and carapace width suggested 50% of males acquire mature chelae at 10.2 cm ICW. These results demonstrate the close linkage between early life stages of S. paramamosain and certain specific niches within mangrove habitats, with the main adult population found to be living sub-tidally at some distance from the mouth of the estuary. The study also highlights the special importance of the mangrove fringe, the border between the mangrove forest and the sea, an area which is particularly vulnerable to physical and anthropogenic impacts.     

Mechanisms to initiate a Heterosigma akashiwo red tide in Osaka bay

The ecological role of diel vertical migration of Heterosigma akashiwo Hada to initiate red tide was investigated in Tanigawa Fishing Port and Sano Harbor, Osaka Bay, Japan during red tide seasons in 1979 and 1980. This species migrated toward the surface early in the morning at a velocity of 1.0 to 1.3 m h^-1. Downward migration was found in the afternoon, and more than 2.0×10³ cells ml^-1 aggregated in the bottom layer at night. The upward migration started before sunrise and downward shifting occurred prior to sunset. The movement is presumably associated with circadian rhythm, which is known as one of the biological periodicities. H. akashiwo crossed steep temperature and salinity gradients (6.5°C and 5.7 S) during the diel vertical migration. High values of particulate organic carbon and nitrogen concentrations were obtained in dialysis bags suspended in situ at identical layers with high cell concentration, while the values for surface and bottom bags were comparatively low. The results reveal that H. akashiwo migrates toward the sea surface to carry out photosynthesis effectively, and to the bottom to utilize nutrients efficiently.     

福建洛阳江口红树林湿地恢复对蟹类密度的影响

底栖动物尤其是蟹类对红树林湿地生态系统的生态功能恢复有重要意义,研究检验不同红树林种类及林龄和郁闭度是否对蟹类分布情况存在显著影响。从福建洛阳江口红树林湿地动态变化对蟹类密度的影响可以看出：不同植被类型对蟹类密度的影响达到极显著水平。光滩与成熟的秋茄（Kandelia candel）,桐花树（Aegiceras corniculatum）和白骨壤（Avicennia marina）之间有显著性差异（P〈0.05）,而桐花树、秋茄、白骨壤之间无显著性差异（P〉0.05）。在不同红树林树种和林龄2因素对蟹类密度的影响的双因素分析中：树种、林龄、树种和林龄间互作均对蟹类密度的影响达到极显著水平;在不同红树林树种和郁闭度2因素对蟹类密度的影响的双因素方差分析中：树种对蟹类密度的影响达到极显著水平,郁闭度对蟹类密度的影响达到显著水平;树种和郁闭度之间互作对蟹类密度的影响不显著。     

Feeding ecology of the mangrove crab <Emphasis Type="Italic">Ucides cordatus</Emphasis> (Ocypodidae): food choice,food quality and assimilation efficiency

Food preferences, consumption rates and dietary assimilation related to food quality were investigated for the large semi-terrestrial and litter-consuming mangrove crab Ucides cordatus cordatus (Ocypodidae, L. 1763) in northern Brazil. Stomach contents were composed of mangrove leaves (61.2%), unidentified plant material and detritus (28.0%), roots (4.9%), sediment (3.3%), bark (2.5%), and animal material (0.1%). U. cordatus prefers Rhizophora mangle over Avicennia germinans leaves despite a higher nitrogen content, lower carbon to nitrogen (C/N) ratio and lower tannin content of the latter. Also, assimilation rates for senescent R. mangle leaves (C: 79.3%, N: 45.4%) were higher than for A. germinans leaves (C: 40.6%, N: 9.1%). Faeces composition indicates that A. germinans leaves were more difficult to masticate and digest mechanically. The leaf-ageing hypothesis, according to which crabs let leaves age in burrows to gain a more palatable and nutritive food, was rejected for U. cordatus since N content, C/N ratio and the abundance of microorganisms did not differ significantly between senescent leaves and leaves taken from burrows. The low microbial biomass on leaf surfaces and in the sediment indicates its minor importance for the nutrition of U. cordatus. It is concluded that high ingestion and assimilation rates of a R. mangle diet together with the consumption of algae allow for a high intake of C, N, and energy. The data suggest that the digestibility of mangrove leaves by U. cordatus is not hampered by tannins. This may have provided a competitive advantage over other leaf-consuming invertebrates unable to digest mangrove litter with high tannin concentrations. Due to the large stock biomass of U. cordatus in the study area, a great amount of finely fragmented faeces is produced (about 7.1 ton dry matter ha⁻¹ year⁻¹ in a R. mangle forest) which is enriched in C, N and bacterial biomass compared to the sediment. The decomposition of mangrove litter, and thus nutrient and energy transfer into the sediment, is greatly enhanced due to litter processing by U. cordatus.     

Leaf-litter processing by invertebrates in a mangrove forest in Queensland

The way leaf shredders handle and shred leaves under laboratory conditions and in the mangrove forest at Myora Springs, Queensland, Australia, was investigated during the period 1980 to 1984. Field observations on the behaviour of the crab Sesarma erythrodactyla during low tide revealed that this species spends most of its time foraging over mud. S. erythrodactyla eats mangrove leaves where they fall or drags them into burrows or hollow logs. Through their feeding activities, crabs (S. erythrodactyla, Metopograpsus frontalis, Helice leachii, Clistocoeloma merguiensis, Leptograpsus variegatus, Paragrapsus laevis, Ilyograpsus paludicula), isopods (Exospaeroma alata, Campaecopia sp.) amphipods (Orchestia sp., Melita sp.), and a capitellid polychaete (Capitellides sp.), break down whole mangrove leaves into small particles. The way in which a leaf is broken down by leaf-shredders influences the size and composition of the particulate organic matter (POM) in the environment. The POM egested by leaf-shredders varies from 32 to 1171 m. POM in this size range occurs in the guts of 38 invertebrate species which feed on mud, suggesting that they depend on the plant detritus produced by leafshredders as a source of food. Leaf-shredders therefore constitute a primary link in the marine food web of mangrove forests.     

Feeding and mouthpart morphology in the semaphore crabHeloecius cordiformis (Decapoda: Brachyura: Ocypodidae)

The mouthparts of the Australian semaphore crabHeloecius cordiformis (H. Milne-Edwards, 1837) are described. Male and female semaphore crabs were observed and collected from mangrove swamps around the south side of Botany Bay, Australia between 1983 and 1987. Semaphore crabs inhabit mangrove mud/sand flats between the midlittoral intertidal zone and the supralittoral fringe of bordering saltflats. Scanning electron micrographs provide detailed pictures of mouthpart morphology and the setae present on the third, second, and first maxillipeds; the second and first maxillae; and the mandibles. Amongst other setal types, spoon-tipped, corn, and bottle-brush setae are described. While semaphore crabs are primarily deposit feeders (microphagous), they have well developed mandibles and are, therefore, also able to eat larger plant and animal material (macrophagous). A brief comparison is made between the mouthparts of semaphore crabs and fiddler crabs (Uca spp.), since the two genera are closely related, occupy similar niches, and resemble each other in appearance. This comparison reveals that while the second maxilliped setal morphology differs between the two genera (Heloecius possesses corn setae,Uca possesses spoontipped setae) these setae may have similar functions. The feeding behaviour ofH. cordiformis is related to substrate composition, burrow construction and various mouthpart specialisations. It is conlcuded that, within the context of the present study, certain structural e.g. mouthpart modifications allowing both scavenging and deposit feeding, and behavioural modifications e.g. mound building in wet muddy habitats, enable semaphore crabs to occupy a wide range of habitat types within the mangrove.Please address all correspondence and requests for reprints to Dr Maitland at his present address at the University of the Witwatersrand     

Diel vertical migration and timing of metamorphosis of larvae of the Dungeness crab Cancer magister

Joint USA/USSR ichthyoplankton surveys off the coasts of Washington, Oregon and northern California during the years 1981 to 1985 sampled more than 120 stations each year, from 5 to 360 km offshore and between Latitude 40° and 48° N, providing information on ontogeny and diel migration of larvae of the Dungeness crab Cancer magister on a scale not studied previously. We developed a maximum likelihood method for estimating abundance and fraction in the neuston at each station from a neuston tow and an oblique bongo tow. Latestage megalopae migrate vertically on a diel basis, with the fraction in the neuston being (on average) 62% at night (19.00 to 08.00 hrs Pacific Standard Time, PST) and 8% during the day (08.00 to 19.00 hrs PST). The hourly pattern of this migration includes a peak in the early evening, possibly another in the early morning, and an intermediate level in the late afternoon. We detected no dependence of vertical migration on cloud cover or sea state. Early-stage megalopae were present in much lower fractions in the neuston, but weakly displayed the same diel pattern of migration. Zoeae appeared to be below the neuston at all times, except for 2 or 3 h in the evening. From an abrupt change in larval stage in samples from a north-south cruise, we concluded that the majority of the larvae metamorphose from zoeae to megalopae over a fairly short time span (2 to 4 wk) at a given latitude. In later cruises, 95% of the larvae were megalopae, indicating that metamorphosis over the study area either occurs at the same time or proceeds from south to north over a time span of less than a month in early spring.     

Activity,track and speed of movement of the crab Scylla serrata in an estuary

Ultrasonic transmitters were used to track the movements of the crab Scylla serrata (Forskal) over 24 h periods in the Kowie estuary, South Africa. Laboratory experiments using infra-red time-lapse photography to record activity indicated that the transmitters did not affect duration of emergence, amount of movement or feeding. In the estuary, S. serrata was active on average for 13 h. out of 24 h, most activity was at night. The distance moved per night by continuously tracked crabs averaged 461 m, but ranged between 219 and 910 m. Most movement was slow, modal speed was 10 to 19 m h^-1. Slow movements were independent of direction of current and are assumed to be related to use of contact chemoreception for location of prey. About one-seventh of movements were faster than 70 m h^-1; these were most frequently against the current and may be related to olfactory location of food. The crabs did not occupy a distinct territory, but tended to remain in the same general area although they were capable of moving at least 800 m along the length of the estuary at night.     

Fiddler crab burrowing affects growth and production of the white mangrove (<Emphasis Type="Italic">Laguncularia racemosa</Emphasis>) in a restored Florida coastal marsh

Positive plant–animal interactions are important in community ecology, but relatively little attention has been paid to their effect on the production of mangroves, dominant halophytic trees in tropical coastal marshes. Here, the role of fiddler crab (Uca spp.) burrowing on the growth and production of the white mangrove, Laguncularia racemosa (<2 years old), was examined in a restored marsh in Tampa Bay, Florida (27°41.65 N, 82°30.34 W) with manipulative experiments from June 2006 to May 2007. Fiddler crab burrowing significantly increased mangrove height by 27%, trunk diameter by 25%, and leaf production by 15%, compared to mangroves in crab exclusion enclosures. Additionally, the exclusion of fiddler crabs significantly increased interstitial water salinity from 32.4 to 44.2, and decreased the oxidation–reduction potential of the low organic sediments, but did not affect soil pH or sulfide concentration. Mangrove height, trunk diameter, and leaf production along a transect that varied in crab burrow density were positively associated with the number of crab burrows. Further, the density of sympatric Spartina alterniflora shoots was positively correlated with crab burrow density along the transect. As in temperate marshes, fiddler crabs can have significant ecological effects on mangrove communities, serving as ecological engineers by modulating the amount of resources available to marsh plants, and by altering the physical, chemical, and biological state of these soft sediment communities. In restored coastal systems that typically have very poor sediment quality, techniques such as soil amendment could be used to facilitate a more natural interaction between crabs and mangroves in ecosystem development.     

Diel variability of feeding activity in haddock (Melanogrammus aeglifinus) larvae in the East Shetland area, North Sea

Investigations of factors affecting feeding success in fish larvae require knowledge of the scales of variability of the feeding process itself and the indices used to assess this variability. In this study, we measured short-term (diel) variability in feeding rates of wild haddock (Melanogrammus aeglifinus) larvae four times per day during a 10-d cruise in the northern North Sea. Feeding activity was evaluated using indices of gut fullness, prey digestive state and biochemical measurements (tryptic enzyme activity). The gut fullness and the enzyme activity indices indicated moderate to high rates of food consumption throughout the cruise. Time series analysis of the three indices showed significant diel variability in all indices and enabled identification of significant lags between food uptake and peak digestive enzyme activity. The typical pattern of food consumption and digestion was characterized by maximal ingestion of prey early in the evening (19:00 hrs) and peak digestive enzyme activity at 01:00  hrs. The time scale over which enzyme activities reacted to prey ingestion was ca. 6 h, and is consistent with expectations from controlled laboratory experiments with other larval fish species. Significant diel variability in tryptic enzyme activity suggests that attempts to relate this measure of feeding success to other variables (e.g. food concentrations) should take care to accommodate natural cycles in feeding activity before making statistical comparisons. Received: 29 October 1998 / Accepted: 18 June 1999     

Compression of home ranges in ghost crabs on sandy beaches impacted by vehicle traffic

Animal movement is a pivotal element of many ecological processes, and on ocean-exposed sandy shores, ghost crabs (genus Ocypode) undertake extensive nocturnal forays on the beach surface. Because crab populations are also threatened by vehicle traffic, indicators that can detect sublethal effects before population declines are manifest are important. To this end, we tested on a barrier island in Eastern Australia whether movement patterns of crabs respond predictably to disturbance by vehicles; this was done by tracking (using the spool-and-line technique) crabs at night in beach sections open and closed to traffic. Beach traffic not only halved population densities of crabs on the unvegetated beach seawards of the dunes, but it also fundamentally changed crab behaviour and movement: individuals from beach areas rutted by tyre tracks travelled shorter distances in a more erratic zigzag pattern, and they had significantly compressed home ranges. Such behavioural changes linked to human pressures could be well suited as an early warning signal for wider negative ecological impacts (as demonstrated by reduced abundances). They also emphasize the need to incorporate sublethal effects into the assessment and management of ecological changes resulting from beach recreation.