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

Linking El Niño, local rainfall, and migration timing in a tropical migratory species

Current climate models project changes in both temperature and precipitation patterns across the globe in the coming years. Migratory species, which move to take advantage of seasonal climate patterns, are likely to be affected by these changes, and indeed, a number of studies have shown a relationship between changing climate and the migration timing of various species. However, these studies have almost exclusively focused on the effects of temperature change on species that inhabit temperate zones. Here, we explore the relationship between rainfall and migration timing in a tropical species, Gecarcoidea natalis (Christmas Island red crab). We find that the timing of the annual crab breeding migration is closely related to the amount of rain that falls during a 'migration window' period prior to potential egg release dates, which is in turn related to the Southern Oscillation Index, an atmospheric El Niño-Southern Oscillation Index. As reproduction in this species is conditional on successful migration, major changes in migration patterns could have detrimental consequences for the survival of the species. This study serves to broaden our understanding of the effects of climate change on migratory species and will hopefully inspire future work on rainfall and tropical migrations.

Migration-related changes in gene expression in leg muscle of the Christmas Island red crab Gecarcoidea natalis: seasonal preparation for long-distance walking

During their annual breeding migration the Christmas Island land crab Gecarcoidea natalis sustains locomotion aerobically for up to 12 h per day compared with just 10 min during the dry season when their muscles quickly become anaerobic. A seasonal transition to an endurance-muscle phenotype would thus seem essential for migrating crabs. The current study employed a gene discovery approach comparing two expressed sequence tag (EST) libraries, one each for leg muscle from dry (non-migrating) and wet season (migrating) crabs. The 14 most abundant transcripts differed in their representation between the two libraries. The abundances of transcripts of genes predicted to code for different proteins forming contractile muscle components, including actin, troponin and tropomyosin, were significantly different between seasons and thus between physiological states. The shift in the isoform composition of the contractile elements provided evidence for a switch from slow phasic (S1) to slow tonic (S2) fatigue-resistant muscle fibres. A tropomyosin (tm) transcript aligned with a tm isoform of lobster (tmS2), and semi-quantitative RT-PCR confirmed this isoform to be more abundant in the migrating crab muscle. Two LIM protein coding genes, a paxillin-like transcript (pax) and a muscle LIM protein (mlp), were relatively up-regulated in muscle of wet season crabs. These proteins have a fundamental role in muscle development and reconstruction, and their comparative up-regulation is consistent with a remodelling of leg muscle for migration in the wet season. Such a transition would result in an increased representation of aerobic endurance-type fibres concomitant with the greater aerobic exercise capacity of the migrating red crabs.

Metabolic Depression is Delayed and Mitochondrial Impairment Averted during Prolonged Anoxia in the ghost shrimp, Lepidophthalmus louisianensis (Schmitt, 1935)

Lepidophthalmus louisianensis burrows deeply into oxygen-limited estuarine sediments and is subjected to extended anoxia at low tides. Large specimens (>2 g) have a lethal time for 50% mortality (LT(50)) of 64 h under anoxia at 25º C. Small specimens (<1 g) have a significantly higher LT(50) of 113 h, which is the longest ever reported for a crustacean. Whole body lactate levels rise dramatically under anoxia and exceed 120 µmol g.f.w.(-1) by 72 h. ATP, ADP, and AMP do not change during 48 h of anoxia, but arginine phosphate declines by over 50%. Thus arginine phosphate may help stabilize the ATP pool. Surprisingly, when compared to the aerobic resting rate, ATP production under anoxia is unchanged during the first 12 h, and drops to only about 50% between 12 and 48 h. Finally, after 48 h of anoxia, a major metabolic depression to less than 5% occurs. Downregulation of metabolism is delayed in L. louisianensis compared to many invertebrates that exhibit facultative anaerobiosis. Bioenergetic constraints as a result of eventual metabolic depression led to ionic disturbances like calcium overload and compromised membrane potential of mitochondria. Because these phenomena trigger apoptosis in mammalian species, we evaluated the susceptibility of ghost shrimp mitochondria to opening of the mitochondrial permeability transition pore (MPTP) and associated damage. Energized mitochondria isolated from hepatopancreas possess a pronounced capacity for calcium uptake. Exogenous calcium does not stimulate opening of the MPTP, which potentially could reduce cell death during prolonged anoxia.

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.

Spatio-temporal variation in serum chemistry of the lobster, Homarus americanus Milne-Edwards

Monthly variations in serum chemistry of the American lobster, Homarus americanus Milne-Edwards, were investigated at one location in Long Island Sound (LIS). Comparisons between three locations within and outside LIS were also made for a single time point. Most serum analytes displayed significant fluctuation over the study period and between locations. Temporal patterns could be classified as: low in cool months/high in warm months, i.e. Na, Cl, Na:K ratio, Ca, albumin:globulin ratio, percentage Fe saturation; high in cool months/low in warm months, i.e. pH, K, urea, total protein, albumin, globulin, alanine aminotransferase (ALT), aspartate aminotransferase (AST), lipaemia; June spike, i.e. glucose, cholesterol, creatine kinase, iron, transferrin iron-binding capacity; other less obvious fluctuations, i.e. Mg, PO4; and no apparent fluctuation, i.e. HCO3, alkaline phosphatase. The proportion of samples correctly classified into month of collection by a subset of 13 analytes using discriminant analysis improved as the months progressed from May (0.75) to October (>0.95). Discriminant analysis also resolved 96.5% of samples by location. The significant depression of serum calcium at the eastern LIS site correlates with excretory calcinosis, a calcium storage disease described from lobsters at this site, but contrasts with a seasonal elevation in serum calcium recorded in the temporal component of the study. Serum proteins, the electrolytes Ca and K and the enzymes ALT and AST proved to have the strongest spatio-temporal patterns of variation. Serum chemistry is a useful research tool for lobster populations, but the dearth of information on the homology of analyte functions in this species with those in vertebrate species makes interpretation of the results challenging. Late summer/autumn water conditions appear to cause stress for lobsters in LIS.

Experimental Evidence of Food‐Independent Larval Development in Endemic Jamaican Freshwater‐Breeding Crabs

In an experimental study, we compared reproductive and developmental traits of endemic sesarmid crabs from Jamaica living in landlocked limnic or terrestrial habitats. Laboratory rearing and behavioral observations showed that the larval development of Sesarma windsor, Sesarma dolphinum (both from freshwater brooks), and Metopaulias depressus (the bromeliad crab) invariably consists of two nonfeeding zoeal stages and a facultatively lecithotrophic megalopa. In a quantitative study of life-history processes characterizing this developmental mode, we provide for S. windsor first data of biomass and elemental composition (dry mass, W; carbon, hydrogen, nitrogen, collectively, CHN) during development from the egg through successive larval stages. These data show that larval independence of food is based on an enhanced female energy allocation in reproduction, reflected in unusually large egg size (1.45-1.70 mm), as well as high contents of C and H (about 60% and 9%, respectively) and high C : N ratios (7.6-8.4) in eggs and early larvae. During zoeal development, about 6% of initial W and 9% of N but 13% each of C and H were lost; similar losses occurred during megalopal development in continued absence of food. These patterns reflect the metabolic utilization of stored organic matter, with preferential degradation of lipid reserves. Fed megalopae gained greater amounts of W and N as compared with C and H (increments of 37% and 38% vs. 25% and 19%, respectively), indicating preferential investment of nutritional energy in proteins required for the formation of new tissues and organs, while generally decreasing proportions of CHN within total W suggested an increasing mineralization of the exoskeleton. Although survival and molt cycle duration of the megalopa stage were not affected by absence of food, significant effects were found in the size of first-stage juvenile crabs, indicating a trade-off between nutritional flexibility in the last larval stage and postmetamorphic fitness. Similar patterns of development and biomass in M. depressus as well as preliminary data obtained for S. dolphinum and Sesarma fossarum suggest that reproductive and developmental traits may be similar in all endemic Jamaican sesarmids. These traits are interpreted as life-history adaptations to development in landlocked habitats, probably playing a key role during adaptive radiation.

Regulation of urine reprocessing in the maintenance of sodium and water balance in the terrestrial Christmas Island red crab Gecarcoidea natalis investigated under field conditions

Land crabs produce isosmotic urine but reduce salt loss by reabsorbing salt via the gills to produce a dilute excretory fluid (P). This branchial salt reclamation is regulated in response to changes in dietary salt availability. The regulation of branchial Na reabsorption and osmotic status was investigated in the terrestrial crab Gecarcoidea natalis on Christmas Island. Confinement within field enclosures had no general effect on salt and water balance compared with crabs free in the rainforest but there were seasonal effects. Extracellular fluid volume was 27.9% body mass during the wet season but only 22.7% in the dry season. Urine production was 53 ml kg(-1) day(-1) in the dry season but 111 ml kg(-1) day(-1) in the wet season, while water flux rates were 140 ml kg(-1) day(-1) and 280 ml kg(-1) day(-1), respectively. Serotonin but not dopamine increased urine production by at least 16% but only during the dry season when rates were seasonally lowered. Crabs acclimated to drinking 50% seawater increased haemolymph osmotic pressure and downregulated branchial reabsorption of salt. Net Na flux (J(net)) and unidirectional Na influx (J(in)) were investigated in branchial perfusion experiments. In red crabs acclimated to drinking freshwater, J(in), J(net) and the activity of the Na(+)/K(+)-ATPase were increased by serotonin, indicating that the increase of sodium absorption was due to a stimulation of the ATPase. Red crabs drinking 50% seawater reduced J(net) primarily due to increased passive loss (J(out)), since both J(in) and Na(+)/K(+)-ATPase were unchanged. Serotonin and dopamine abolished the increased diffusive loss and re-established J(net) with no change in J(in). G. natalis exhibits different regulatory systems. Branchial salt uptake can be adjusted via the leak component when adequate salt is available but also by stimulated active uptake under diluting conditions. The gills are important sites of ion pumping in euryhaline aquatic crabs, and the upregulation of J(net) in red crabs is reminiscent of that in marine crabs. Serotonergic stimulation of branchial uptake, independent of cAMP, and hormonally modulated ion leakage are presently unique to terrestrial species.

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.

Ecology and behavior of Gecarcoidea natalis, the Christmas Island red crab, during the annual breeding migration

The terrestrial crab Gecarcoidea natalis is endemic to the forests of Christmas Island but must migrate each year to the coast to breed. During 1993 and 1995, radio-tracking, mark and recapture, and counting methods were used to establish the routes, walking speeds, direction of travel, and destinations of migrating crabs, as well as crab numbers and distribution. The density of crabs ranged from 0.09 to 0.57 crabs per square meter, which gave a population estimate of 43.7 million adult crabs on the island. During the dry season the crabs were relatively inactive but on arrival of the wet season immediately began their migration. The crabs generally walked in straight lines, and most crabs from around the Island traveled toward the northwest shore instead of simply walking toward the nearest shore. The maximum recorded distance walked by a red crab in one day was 1460 m, but the mean was 680 m per day in 1993 and 330 m in 1995. Comparing the 1993 and 1995 study seasons, there was a 3-week difference in the timing of the start of the migration, but the spawning date was fixed by the lunar phase and took place 17 to 18 days after mating. In 1993, late rain prompted a "rushed" migration and crabs walked directly to their shore destinations; in contrast, in 1995 most crabs made stops of 1 to 7 days during the downward migration. By giving the crabs a chance to feed along the way and minimizing the time that the population was concentrated near the shore, these stops may be important in ensuring that the animals have enough food after the long dry season. Furthermore, this behavior implies that the crabs are able to judge how far away they are from the shore during the migration.