Strategies for migration in the terrestrial christmas island red crab gecarcoidea natalis: intermittent versus continuous locomotion

The systematics of land crabs of the genus Gecarcoidea and recognition of a pseudocryptic species, G. humei, from the eastern Indian Ocean (Crustacea : Decapoda : Gecarcinidae)

The gecarcinid genus Gecarcoidea H. Milne Edwards, 1837 is currently represented by two species from the Indo-West Pacific – the widely distributed purple land crab, G. lalandii H. Milne Edwards, 1837, and the red crab endemic to Christmas Island, G. natalis (Pocock, 1889). One species, G. humei (Wood-Mason, 1874), described from the Nicobar Islands, has had a confused taxonomic history, but was treated as a junior synonym of G. lalandii by Türkay (1974) in his revision of the family. In this study, using molecular as well as morphological characters, we show that G. humei is a valid species. Gecarcoidea lalandii and G. humei have distinct non-overlapping distributions throughout much of their range, with G. lalandii occurring in most of South-east Asia and the West Pacific, while G. humei is known only from the eastern Indian Ocean. On Christmas Island, in the eastern Indian Ocean, however, all three species are present. As a result, Christmas Island is the only locality where all extant species of Gecarcoidea are found. The three species can also be separated by differences in live colours and patterns, as well as proportions of the carapace, male abdomen, ambulatory legs and third maxillipeds, and details of the orbits and male first gonopods.

Reproduction and management of the mangrove crab Ucides cordatus (Crustacea, Brachyura, Ucididae) at Iguape, São Paulo, Brazil

The mangrove crab Ucides cordatus is one of the most exploited crustaceans in Brazil. The present study investigated the breeding season of this species and the period of the "andada" phenomenon, when the crabs are active outside their burrows and perform agonistic behaviors. Furthermore related them to environmental factors, in a mangrove on the coast of Brazil, with inferences about management of this crab. The crabs reproduced from October through February. The "andada" occurred from November through February, with typical characteristics in the first two months, during the full and new moon periods, and was influenced positively by the air and soil temperature, luminosity, and tidal amplitude, and negatively by salinity. Based on the results of the present study, the period when the "andada" occurs in other parts of Brazil could be defined, which will aid in managing this fishery. For Iguape, state of São Paulo, Brazil, we suggest a legal off-season for both sexes in December or during the full and new moon, including January.

Respiratory and acid-base responses during migration and to exercise by the terrestrial crab Discoplax (Cardisoma) hirtipes, with regard to season, humidity and behaviour

The terrestrial crab Discoplax hirtipes (formerly Cardisoma hirtipes) exhibits a seasonally dichotomous activity pattern governed by the seasonal rainfall on Christmas Island, with a breeding migration in the wet season. Greater activity in the wet season reflects a release of constraints on ion and water balance independent of changes in exercise physiology. The respiratory responses to walking exercise by D. hirtipes were assessed with regard to humidity and season. In the laboratory, crabs walked 38% faster when humidity was high, while the O(2) uptake across the lung became diffusion limited despite a Ca(2+)-induced increase in O(2) affinity of the haemocyanin. Crabs walking in 90% relative humidity exhibited a larger metabolic acidosis while re-oxidation of l-lactate was only 0.49 mmol l(-1) h(-1). The wet-season crabs were more active and exhibited a respiratory acidosis compared with the quiescent dry-season crabs. The migration was close to the limit of the aerobic scope of the crabs, and the pulmonary oxygen partial pressure (P(O(2))) was depressed, the O(2) uptake diffusion limited and the arterial-venous P(O(2)) difference diminished. Additional, enforced exercise induced a metabolic acidosis. The optimum strategy for migration would be walking continuously but at a speed within the aerobic scope. D. hirtipes is influenced by seasonal rain and responds to lower ambient humidity by limiting exercise. The behavioural response is paramount since the changes in respiratory status were determined primarily by the differences in commitment to, and investment in, walking and not by direct effects of humidity on respiratory physiology.

Utilisation of glycogen, ATP and arginine phosphate in exercise and recovery in terrestrial red crabs, Gecarcoidea natalis

Intermittent locomotion by terrestrial crustaceans may under specific circumstances increase walking distance and may allow partial re-oxidization of anaerobic products, and replenishment of ATP and arginine phosphate. The Christmas Island red crab G. natalis undertakes a substantial breeding migration each year. The leg muscles of G. natalis subjected to bouts of 2.5 min walking and 2.5 min rest were severely anaerobic. Adenylate energy charge and the large arginine phosphate stores were greatly reduced. Walking for 4 min with pauses of only 1 min exacerbated the anaerobiosis and utilised 50% of the endogenous muscle glycogen. Post-exercise, the adenylate energy charge recovered before the arginine phosphate charge and a large and persistent hyperglycaemia accompanied the restoration of glycogen. Arginine phosphate functioned as a large, longer term, energy reservoir-almost as part of the adenylate pool. Gluconeogenesis is yet to be generally substantiated in decapod crustaceans but G. natalis appears to remove lactate slowly and to reincorporate exogenous glucose into muscle glycogen in the same time frame as lactate removal from the haemolymph. The 4:1 exercise/pause regimen facilitated access to energy stores and increased walking distance, and it allowed L-lactate and H(+) efflux from the muscle during pausing. These responses are similar to those of G. natalis in the field, except during the migration when walking was entirely aerobic. Determinations of adenylate, fuel and arginine phosphate reserves and usage during the migration are required together with more detailed behavioral analysis to resolve the dichotomy in metabolic response.

The ecophysiology of air-breathing in crabs with special reference to Gecarcoidea natalis

To succeed on land rather than in water, crabs require a suite of physiological and morphological changes, and ultimately the ability to reproduce without access open water. Some species have modified gills to assist in gas exchange but accessory gas exchange organs, usually lungs, occur in many species. In accomplished air-breathers the lung becomes larger and more vascularised with pulmonary vessels directing oxygenated haemolymph to the heart. The relative abundance of O(2) in air promotes relative hypoventilation and thus an internal hypercapnia to drive CO(2) excretion. Land crabs have a dual circulation via either lungs or gills and shunting between the two may depend on respiratory media or exercise state. During their breeding migration on Christmas Island Gecarcoidea natalis maintained arterial Po(2) by branchial O(2) uptake, while pulmonary O(2) pressure was reduced; partly because exercise doubled relative haemolymph flow through the gills. Related species rely on elevated haemocyanin concentration and affinity for O(2) to assist uptake but this compromises unloading at the tissues and thus the aerobic scope of tissues. Aquatic crabs exchange salt and ammonia with water via the gills but in land crabs this is not possible. Birgus latro has adopted uricotelism but other species excrete ammonia in either the urine or as gas. Land crabs minimise urinary salt loss using a filtration-reabsorption system analogous to the kidney. Urine is redirected across the gills where salt reabsorption occurs in systems under hormonal control, although in G. natalis this is stimulatory and in B. latro inhibitory. While crabs occupy a range of habitats from aquatic to terrestrial, these species do not comprise a physiological continuum but across the crab taxa individual species possess appropriate and specific physiological features to survive in their individual habitat.

Metabolic status and respiratory physiology of Gecarcoidea natalis, the Christmas Island red crab, during the annual breeding migration

With the arrival of the monsoonal rains and after months of inactivity during the dry season, the terrestrial crab Gecarcoidea natalis embarks on its annual breeding migration to the coast. The physiological demands of the migration were assessed by determining respiratory gases in the hemolymph, key metabolites, and energy stores in G. natalis during two migratory seasons. At the end of each day of migration the pulmonary hemolymph PO2 decreased by 1-2.5 kPa, but the hemocyanin remained saturated with O2 and the venous reserve was largely unchanged (O2 > 0.4 mmol x l(-1)). The breeding migration of red crabs was accomplished without recourse to anaerobiosis, even though at times walking speeds (up to 6.2 +/- 0.5 m x min(-1)) exceeded those that promoted anaerobiosis in non-migrating crabs and in crabs exercised in the laboratory. In contrast to all previous studies, at the end of each day of migrating, red crabs experienced an alkalosis (up to 0.1 pH units) rather than any acidosis. This alkalosis was removed overnight when the crabs were inactive. Although there were seasonal fluctuations in the glycogen, glucose, and triglyceride stores, crabs engaging in the migration did not draw on these stores and must have fed along the way. In contrast, crabs returning from breeding activities on the shore terraces had significantly depleted glycogen stores. Additionally, in 1993, the male crabs returning from the breeding activities on the terraces were dehydrated and experienced a decrease in muscle tissue water of 11%. In contrast to the breeding migration per se, fighting for burrows in which breeding occurs produced severe anaerobiosis in males, especially the victors: after 135 s of combat, the maximum L-lactate concentration in the hemolymph was 35 mmol x l(-1). It appears that burrowing, courtship, and mating are more demanding than the migration itself. Furthermore, the data provide evidence that the metabolic responses of migrating individuals of G. natalis might be different from those at other times of the year.

Locomotion, respiratory physiology, and energetics of amphibious and terrestrial crabs

The transition from breathing air to breathing water requires physiological and morphological adaptations. The study of crustaceans in transitional habitats provides important information as to the nature of these adaptations. This article addresses the physiology of air breathing in amphibious and terrestrial crabs and their relative locomotor abilities. Potamonautes warreni is an apparently amphibious freshwater crab from southern Africa, Cardisoma hirtipes is an air-breathing gecarcinid crab with some dependency on freshwater, and Gecarcoidea natalis is an obligate air-breathing gecarcinid endemic to Christmas Island in the Indian Ocean. All three species have well-developed lungs but retain gills and show seasonally different activity patterns that, in the gercarcinids, especially G. natalis, include long-distance breeding migrations. The three species were better at breathing air than water, but P. warreni was the best at breathing water. Cardisoma hirtipes is essentially an obligate air breather and appears to experience facultative hypometabolism during immersion. Cardisoma hirtipes has a haemocyanin with a high affinity for O(2) that facilitates loading from air but makes 30% of the Hc bound O(2) inaccessible. The gecarcinids but not P. warreni show increased diffusion limitation for O(2) over the lung during exercise. Gecarcoidea natalis outperforms C. hirtipes by virtue of a unique haemolymph shunt from the lung into the gills. Paradoxically, it is modifications of the gills for aerial O(2) uptake in G. natalis that allow for relatively greater haemolymph oxygenation. Despite showing decreased arterial-venous DeltaPo(2), P. warreni increased the arterial-venous Delta[O(2)] with no recourse to anaerobiosis during 5 min exercise. In the short term, P. warreni is more adept at walking than C. hirtipes. The breeding migrations of C. hirtipes and G. natalis were completely aerobic, but G. natalis walk farther and probably faster. Seasonal changes in underlying metabolism of G. natalis are strongly implied, including variations in hyperglycaemic hormone, variable basal metabolic rates, and a diel alkalosis present only in migrating crabs. The persistent dependence on water for reproduction is a determining factor in the biology of air-breathing crabs. The annual migrations include costs other than locomotion, for example, burrow construction and intermale competition. Estimates of costs that consider walking alone will underestimate the metabolic and stored fuel requirements for successful reproduction.

Respiratory gas transport, metabolic status, and locomotor capacity of the Christmas Island red crab Gecarcoidea natalis assessed in the field with respect to dichotomous seasonal activity levels

Red crabs, Gecarcoidea natalis, exhibit seasonal activity patterns: low activity during the dry season when they shelter in burrows to avoid dehydration, and high activity during the wet season. Red crabs were examined in situ in the rainforest of Christmas Island to determine if there were underlying seasonal differences in the capacity for exercise and associated metabolism. During both seasons, free-ranging (FR) crabs engaged in their normal activities and, together with crabs induced to exercise for 5 min, were sampled for haemolymph and muscle tissue. Respiratory gases in the haemolymph and key metabolites were measured to assess differences in metabolic status of FR and exercised crabs. Actively foraging FR crabs during the wet season exhibited a relative haemolymph hypoxia (2.9 kPa) and accumulated an extra 3 mmol. litre(-1) of CO(2) compared to the relatively inactive FR crabs during the dry season. Wet-season crabs appeared to be in a state of relative respiratory acidosis compared to dry-season animals. This hypercapnia may arise as a consequence of a relative hypoventilation in animals with a relatively higher metabolic rate during the wet season. Oxygenation of pulmonary and arterial haemolymph was similar and remained high after 5 min of exercise, indicating that the gills and lungs functioned similarly in gas exchange in both FR and exercised crabs. During exercise, venous O(2) reserves decreased and red crabs experienced a mixed respiratory/metabolic acidosis. Similar changes, after 5 min of enforced exercise, in metabolite concentrations, pH and respiratory gas status in the haemolymph during both sampling seasons suggest that the crabs maintain similar capacity to increase exercise during the wet and the dry seasons, despite the differences in underlying physiological status. This is important since after prolonged inactivity during the dry season, with the arrival of moonsoonal rains, red crabs must engage in their annual breeding migration.