Flocking and feeding in the fiddler crab (UCA tangeri): prey availability as risk-taking behaviour

For a full understanding of prey availability, it is necessary to study risk-taking behaviour of the prey. Fiddler crabs are ideally suited for such a study, as they have to leave their safe burrow to feed on the surface of the intertidal flats during low tide, thereby exposing themselves to avian predators. A study in an intertidal area along the coast of Mauritania showed that small crabs always stayed in the vicinity of their burrow, but large crabs wandered in large flocks (also referred to as droves) to feed on sea-grass beds downshore. Transplanting downshore feeding substrate to the burrowing zone of the small crabs proved that they too preferred to feed on it. Since small crabs can be preyed upon by more species of birds, this suggests that the decision not to leave the burrowing zone might be related to the risk of being fed upon by birds. We calculated predation risk from measurements on the density and feeding activity of the crabs, as well as the feeding density, the intake rate and the size selection of the avian predators. Per hour on the surface, crabs in a flock were more at risk than crabs feeding near their burrow. Thus, though flocking crabs may have benefited from ‘swamping the predator’ by emerging in maximum numbers during some tides only, this did not reduce their risk of predation below that of non-flocking crabs. Furthermore we found that irrespective of activity, large crabs suffered a higher mortality per tide from avian predators than small crabs. This suggests that large crabs could not sufficiently reduce their foraging time to compensate for the increased risk while foraging in a flock, even though they probably experienced better feeding conditions than small crabs staying near their burrow. The greater energy demands of large crabs were reflected in a greater surface area grazed. Thus, with increasing size a fiddler crab has to feed further away from its burrow and so may derive less protection from staying near to it. It seems that growing big does not reduce the risk of predation for fiddler crabs, as it does in many other species with indeterminate growth. As in such species, the most probable advantage of growing big is increased mating success. Ultimately, therefore, prey availability must be understood from the life-history decisions of the prey species.

Habitat selection and energetics of the fiddler crab (Uca tangeri)

We tried to unravel the possible links between the skewed predation risk in Uca tangeri (where large individuals are more at risk from avian predators) and size-dependent changes in the physiology and habitat choice of this fiddler crab species. Over a transect running from low to high in the tidal zone of a beach in Mauritania, the temperature profile at various depths in the substrate, the water-table level of seep water, salt concentration of seep water, depth of the aerobic level, operative temperatures on the surface, and size distribution of crabs were assessed. In addition, resting metabolic rates, Q10 and thermal and starvation tolerances were estimated. Going from low to high in the tidal zone, crab size and burrow depth increased. At the preferred burrowing depth, microclimatological conditions appeared to be equally favourable at all sites. At the surface, conditions were more favourable low in the tidal zone, where also food availability is sufficient to enable small crabs to forage in the vicinity of their burrows. Large crabs have higher energy requirements and are thereby forced to forage in flocks low in the tidal zone where food is probably more abundant. Low in the tidal zone, digging deeply is impossible as the aerobic layer is rather thin. Large crabs prefer living high in the tidal zone as (1) deep burrows ensure better protection against predators, (2) more time is available for digging holes and (3) the substrate is better suited for reproduction. Energy reserves in late summer ensured an average of 34 days of survival. It is argued that the allotment of energy to growth must be considerable even in reproducing animals; the rewards of growth being the disproportional increase in reproductive output with size.

Why do Gull-billed Terns Gelochelidon nilotica feed on fiddler crabs Uca tangeri in Guinea-Bissau?

Gull-billed Terns Gelochelidon nilotica wintering in Guinea Bissau mainly fed on fiddler crabs Uca tangeri and were occasionally seen feeding on fish and locusts. As fiddler crabs have a low energy content, terns need a large gross intake to meet daily energy demands. Fiddler crabs also have a low ratio of digestible flesh to exoskeleton, and therefore tern food intake may be limited by gut capacity. Activity budgets of Gullbilled Terns feeding on fiddler crabs showed that a considerable part of the time was spent resting. The duration of resting intervals increased with energy intake and was positively correlated with the metabolisable energy content of the crab eaten, suggesting that resting periods were required for a proper digestion. The poor quality of fiddler crabs was offset by high capture rates. So daily energy expenditure of the terns could easily be met by feeding on fiddler crabs. Even when resting pauses were included in foraging time, foraging for only 1.5 hours on fiddler crabs satisfied the terns’ daily energy demands. Instead, feeding on energy-rich fish would require about 2.5 hours to satisfy daily energy demands. Compared to the more specialised piscivorous Little Tern Sternula albifrons and Sandwich Tern Sterna sandvicensis, capture rate of fish was poor in Gull-billed Terns. From an energetic point of view, wintering Gull-billed Terns feeding on fiddler crabs seem to have an easy living in Guinea Bissau.

Cadmium, copper, mercury, and zinc concentrations in tissues of the King Crab (Pseudocarcinus gigas) from southeast Australian waters

The concentrations of cadmium, copper, mercury, and zinc were determined in muscle (body, claw, and leg), hepatopancreas, and gill tissues of Pseudocarcinus gigas, an exceptionally large, long-lived, and deep-dwelling crab species. The accumulation patterns observed are discussed in terms of both intra- and interspecies variations, with particular attention to the possible consequences of the extreme size and depth range of P. gigas. Metal concentrations did not depend significantly on sex of the crab. Significant differences between tissues were detected for all metals, and the distribution of metal between the tissues was different for each metal. Significant correlations were found between metal concentrations in the various tissues and crab size, and these are discussed and rationalised. The concentrations of mercury and zinc in muscle tissue increased with crab size and were high compared to other crab species. The concentrations of cadmium and copper present in edible tissues were not especially high compared to other crab species, but the concentration of cadmium in the hepatopancreas is of dietary concern.

Habitat use by the hymenosomatid crab Amarinus lacustris (Chilton) in two south-eastern Australian rivers

The hymenosomatid crab Amarinus lacustris is abundant in some south-eastern Australian rivers; however, little is known of its ecology. Patterns of habitat use by crabs in rivers may be affected by seasonal changes in river discharge. This study investigates population characteristics, timing of reproduction and patterns of habitat use by A. lacustris in five riffle and pool habitats from each of the Hopkins and Merri Rivers in south-west Victoria, Australia, sampled over a twelve-month period. Distribution of Amarinus lacustris was similar between the two rivers, but log-linear modelling showed that there was a strong association between crab sex, habitat occupied and time of year because female A. lacustris showed a shift from riffle to pool habitats during March and April, coinciding with the non-gravid period of the year. Male crabs also showed a change in relative occurrence, occurring most often in riffles during winter–spring (July–November) but being equally common in both habitats in summer–autumn (January–May). These patterns are probably the result of the reproductive cycle of A. lacustris, which appears to show both ontogenetic and sex-related changes in habitat use during its life cycle, taking advantage of seasonal fluctuations in flow regime that may assist egg/larval development and dispersal.

Excretory and storage purines in the Anomuran land crab Birgus latro; guanine and uric acid

Birgus latro excretes nitrogenous waste as a mixture of urate and guanine and not predominantly urate as believed previously. The presence of guanine in faeces was confirmed by enzymatic derivatisation of guanine to xanthine with guanase. This is the first report of significant excretion of guanine outside the Chelicerata. The ratios of urate to guanine within the excreta of animals in field situations (natural diets) and in the laboratory (a range of artificial diets) were 3:2. Rates of excretion of both urate and guanine increased when experimental crabs were fed an artificial diet high in nitrogen. Significant amounts of guanine were also measured in tissues of B. latro, but only urate was present in equivalent tissues of the closely related species Coenobita brevimanus. Coenobita brevimanus did not excrete any significant amount of purines with the faeces.

Dietary assimilation and the digestive strategy of the omnivorous anomuran land crab Birgus latro (Coenobitidae).

On Christmas Island, Indian Ocean, the diet of robber crabs, Birgus latro (Linnaeus) was generally high in fat, storage polysaccharides or protein and largely comprised fruits, seeds, nuts and animal material. The plant items also contained significant amounts of hemicellulose and cellulose. In laboratory feeding trials, crabs had similar intakes of dry matter when fed artificial diets high in either fat or storage polysaccharide, but intake was lower on a high protein diet. Assimilation coefficients of dry matter (69–74%), carbon (72–81%), nitrogen (76–100%), lipid (71–96%) and storage polysaccharide (89–99%) were high on all three diets. B. latro also assimilated significant amounts of the chitin ingested in the high protein diet ( 93%) and hemicellulose (49.6–65%) and cellulose (16–53%) from the high carbohydrate and high fat diets. This is consistent with the presence of chitinase, hemicellulase and cellulase enzymes in the digestive tract of B. latro. The mean retention time (27.2 h) for a dietary particle marker (⁵⁷Co-labelled microspheres) was longer than measured in leaf-eating land crabs. The feeding strategy of B. latro involves the selection of highly digestible and nutrient-rich plant and animal material and retention of the digesta for a period long enough to allow extensive exploitation of storage carbohydrates, lipids, protein and significant amounts of structural carbohydrates (hemicellulose, cellulose and chitin).

Purification and characterisation of endo-β-1,4-glucanase and laminarinase enzymes from the gecarcinid land crab Gecarcoidea natalis and the aquatic crayfish Cherax destructor

Laminarinase and endo-β-1,4-glucanase were purified and characterised from the midgut gland of the herbivorous land crab Gecarcoidea natalis and the crayfish Cherax destructor. The laminarinase isolated from G. natalis was estimated to have a molecular mass of 41 kDa by SDS-PAGE and 71 kDa by gel filtration chromatography. A similar discrepancy was noted for C. destructor. Possible reasons for this are discussed. Laminarinase (EC 3.2.1.6) from G. natalis had a V_max of 42.0 µmol reducing sugars produced min–¹ mg protein–¹, a K_m of 0.126% (w/v) and an optimum pH range of 5.5–7, and hydrolysed mainly β-1,3-glycosidic bonds. In addition to the hydrolysis of β-1,3-glycosidic bonds, laminarinase (EC 3.2.1.39) from C. destructor was capable of significant hydrolysis of β-1,4-glycosidic bonds. It had a V_max of 19.6 µmol reducing sugars produced min^–1 mg protein^–1, a K_m of 0.059% (w/v) and an optimum pH of 5.5. Laminarinase from both species produced glucose and other short oligomers from the hydrolysis of laminarin. Endo-β-1,4-glucanase (EC 3.2.1.4) from G. natalis had a molecular mass of 52 kDa and an optimum pH of 4–7. It mainly hydrolysed β-1,4-glycosidic bonds, but was also capable of significant hydrolysis of β-1,3-glycosidic bonds. Two endo-β-1,4-glucanases, termed 1 and 2, with respective molecular masses of 53±3 and 52 kDa, were purified from C. destructor. Endo-β-1,4-glucanase 1 was only capable of hydrolysing β-1,4-glycosidic bonds and had an optimum pH of 5.5. Endo-β-1,4-glucanases from both species produced some glucose, cellobiose and other short oligomers from the hydrolysis of carboxymethyl cellulose.

Potential endocrine disruption of ovary synthesis in the Christmas Island red crab Gecarcoidea natalis by the insecticide pyriproxyfen

The effect of the insecticide, pyriproxyfen on early ovary synthesis was examined in the Gecarcinid land crab, Gecarcoidea natalis. Crabs were fed a mixture of either leaf litter and bait containing 0.5% (wt/wt) pyriproxyfen (experimental groups), or a mixture of leaf litter and a control bait containing no pyriproxyfen (control groups), at simulated baiting doses of 2 kg ha− 1 and 4 kg ha− 1, during the period in which G. natalis synthesises its ovaries. A third group of crabs were fed ad libitum either the bait containing 0.5% Pypriproxyfen or the control bait. Pyriproxyfen affected early ovary development in G. natalis. The ovaries from crabs in the experimental groups at all baiting levels had a higher total nitrogen content and dry mass than the ovaries from crabs in the control groups. Pyriproxyfen affected the histology of the ovaries. Ovaries from animals in the experimental groups were more mature, containing more previtellogenic and early vitellogenic oocytes, of a larger diameter, than the ovaries from crabs in the control groups. Significant amounts of pyriproxyfen accumulated within the midgut gland and ovary, the hypothesised target tissues, while minor amounts of pyriproxyfen was accumulated in the muscle, a hypothesised non target tissue. Pyriproxyfen may have stimulated early ovary development and induced synthesis of yolk protein by mimicking methyl farnesoate and thus causing endocrine disruption. Given this, pyriproxyfen should not be used to control invasive insects in environments where gecarcinid and other land crab species are present.

A life history model for the giant crab Pseudocarcinus gigas

This study examined P. gigas populations from Cape Naturaliste (Western Australia) to Flinders Island (Tasmania), investigated reproduction, developed a unique tag to study their movement and growth, and collected new environmental information that allowed the development of a model of the life history for this iconic endemic Australian species.

Spermatogenesis in the blue swimming crab, Portunus pelagicus, and evidence for histones in mature sperm nuclei

Spermatogenesis in the blue swimming crab, Portunus pelagicus, is described by light and electron microscopy. The testis is composed of anterior (AT) and posterior (PT) lobes, that are partitioned into lobules by connective tissue trabecula, and further divided into zones (germinal, transformation and evacuation), each with various stages of cellular differentiation. The vas deferens is classified into three distinct regions: anterior (AVD), median (MVD), and posterior (PVD), on the presence of spermatophores and two secretions, termed substance I and II. Based on the degree and patterns of heterochromatin, spermatogenesis is classified into 13 stages: two spermatogonia (SgA and SgB), six primary spermatocytes (leptotene, zygotene, pachytene, diplotene, diakinesis, and metaphase), a secondary spermatocyte (SSc), three spermatids (St 1–3), and a mature spermatozoon. Spermatid stages are differentiated by chromatin decondensation and the formation of an acrosomal complex, which is unique to brachyurans. Mature spermatozoa are aflagellated, and have a nuclear projection and a spherical acrosome. AUT-PAGE and Western blots show that, during chromatin decondensation, there is a reduction of most histones, with only small amounts of H2B and H3 remaining in mature spermatozoa.

The last piece in the cellulase puzzle : the characterisation of β-glucosidase from the herbivorous gecarcinid land crab Gecarcoidea natalis

A 160 kDa enzyme with β-glucosidase activity was purified from the midgut Gland of the land crab Gecarcoidea natalis. The enzyme was capable of releasing glucose progressively from cellobiose, cellotriose or cellotetraose. Although β-glucosidases (EC 3.2.1.21) have some activity towards substrates longer than cellobiose, the enzyme was classified as a glucohydrolase (EC 3.2.1.74) as it had a preference for larger substrates (cellobiose<cellotriose=cellotetraose). It was able to synthesise some cellotetraose by the transglycosylation of smaller substrates – another common feature of glucohydrolases. The interaction between the glucohydrolase described here and the endo-β-1,4-glucanases described previously for G. natalis provides a complete model for cellulose hydrolysis in crustaceans and possibly in other invertebrates. After mechanical fragmentation by the gastric mill, multiple endo-β-1,4-glucanases would initially cleave β-1,4-glycosidic bonds within native cellulose, releasing small oligomers, including cellobiose, cellotriose and cellotetraose. The glucohydrolase would then attach to these oligomers, progressively releasing glucose. The glucohydrolase might also attach directly to crystalline cellulose to release glucose from free chain ends. This two-enzyme system differs from the traditional model, which suggests that total cellulose hydrolysis requires the presence an endo-β-1,4-glucanse, a cellobiohydrolase and a β-glucosidase