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

Terrestrial crustaceans (Arthropoda, Crustacea): taxonomic diversity, terrestrial adaptations, and ecological functions

Terrestrial crustaceans are represented by approximately 4,900 species from six main lineages. The diversity of terrestrial taxa ranges from a few genera in Cladocera and Ostracoda to about a third of the known species in Isopoda. Crustaceans are among the smallest as well as the largest terrestrial arthropods. Tiny microcrustaceans (Branchiopoda, Ostracoda, Copepoda) are always associated with water films, while adult stages of macrocrustaceans (Isopoda, Amphipoda, Decapoda) spend most of their lives in terrestrial habitats, being independent of liquid water. Various adaptations in morphology, physiology, reproduction, and behavior allow them to thrive in virtually all geographic areas, including extremely arid habitats. The most derived terrestrial crustaceans have acquired highly developed visual and olfactory systems. The density of soil copepods is sometimes comparable to that of mites and springtails, while the total biomass of decapods on tropical islands can exceed that of mammals in tropical rainforests. During migrations, land crabs create record-breaking aggregations and biomass flows for terrestrial invertebrates. The ecological role of terrestrial microcrustaceans remains poorly studied, while omnivorous macrocrustaceans are important litter transformers and soil bioturbators, occasionally occupying the position of the top predators. Notably, crustaceans are the only group among terrestrial saprotrophic animals widely used by humans as food. Despite the great diversity and ecological impact, terrestrial crustaceans, except for woodlice, are often neglected by terrestrial ecologists. This review aims to narrow this gap discussing the diversity, abundance, adaptations to terrestrial lifestyle, trophic relationships and ecological functions, as well as the main methods used for sampling terrestrial crustaceans.

Making the grade: Physiological adaptations to terrestrial environments in decapod crabs

All extant macroscopic terrestrial diversity has evolved from the ancestors of a small group of successful terrestrial colonizers, but in a few lineages this transition has independently occurred multiple times in spite of the significant functional challenges it presents. Decapod crabs have transitioned from marine to terrestrial environments at least ten times, occupy diverse habitats, and display varying degrees of terrestriality. Previous attempts to categorize land crab diversity relied on single traits, did not explicitly distinguish between brachyuran and anomuran lineages, and did not separate lineages that colonized land via freshwater or marine environments. As a result, critical phylogenetic and ecological constraints were missing from these earlier classifications. In this paper, I reclassify terrestriality in the land crabs by designating four transition pathways that reflect deep phylogenetic relationships between the two decapod crab infraorders and the route-specific nature of this transition. I then describe the adaptive traits that evolved in response to six primary terrestrial selective challenges. I conclude by proposing six grades of terrestriality in this system that describe observable trait-by-environment associations, and propose studies that can test the hypothetical sequence of trait evolution and the nature of convergence in the land crabs using phylogenomic and transcriptomic tools.

Effects of emersion on acid-base regulation, osmoregulation, and nitrogen physiology in the semi-terrestrial mangrove crab, Helice formosensis

Emersion limits water availability and impairs the gill function of water-breathing animals resulting in a reduced capacity to regulate respiratory gas exchange, acid-base balance, and nitrogenous waste excretion. Semi-terrestrial crustaceans such as Helice formosensis mitigate these physiological consequences by modifying and recycling urine and branchial water shifting some branchial workload to the antennal glands. To investigate how this process occurs, Helice formosensis were emersed for up to 160 h and their hemolymph and urinary acid-base, nitrogenous waste, free amino acids, and osmoregulatory parameters were investigated. Upon emersion, crabs experienced a respiratory acidosis that is restored by bicarbonate accumulation and ammonia reduction within the hemolymph and urine after 24 h. Prolonged emersion caused an overcompensatory metabolic alkalosis potentially limiting the crab's ability to remain emersed. During the alkalosis, hemolymph ammonia was maintained at control levels while urinary ammonia remained reduced by 60% of control values. During emersion, ammonia may be temporarily converted to alanine as part of the Cahill cycle until re-immersion where crabs can revert alanine to ammonia for excretion coinciding with the crabs' observed delayed ammonia excretion response. The presence of high hemolymph alanine concentrations even when immersed may indicate this cycle's use outside of emersion or in preparation for emersion. Furthermore, H. formosensis appears to be uniquely capable of actively suppressing its rate of desiccation in absence of behavioral changes, in part by creating hyperosmotic urine that mitigates evaporative water loss.

Exploring the versatility of the perfused crustacean gill as a model for transbranchial transport processes

The study of transbranchial ion and gas transport of water-breathing animals has long been a useful means of modeling transport processes of higher vertebrate organs through comparative physiology. The molecular era of biological research has brought forward valuable information detailing shifts in gene expression related to environmental stress and the sub-cellular localization of transporters; however, purely molecular studies can cause hypothetical transport mechanisms and hypotheses to be accepted without any direct physiological proof. Isolated perfused gill experiments are useful for testing most of these hypotheses and can sometimes be used outright to develop a well-supported working model for transport processes relating to an animal's osmoregulation, acid-base balance, nitrogen excretion, and respiratory gas exchange as well as their sensitivity to pollutants and environmental stress. The technique allows full control of internal hemolymph-like saline as well as the ambient environmental fluid compositions and can measure the electrophysiological properties of the gill as well as the transport rates of ions and gases as they traverse the gill epithelium. Additives such as pharmaceuticals or peptides as well as the exclusion of ions from the media are commonly used to identify the importance of specific transporters to transport mechanisms. The technique can also be used to identify the penetrance, retention, and localization of pollutants within the gill epithelium or to explore the uptake and metabolism of nutrients directly from the ambient environment. While this technique can be applied to virtually any isolatable organ, the anatomy and rigidity of the decapod crustacean gill make it an ideal candidate for most experimental designs.

High temperature acclimation alters the emersion behavior in the crab Neohelice granulata

An increase in environmental temperature can deleteriously affect organisms. This study investigated whether the semiterrestrial estuarine crab Neohelice granulata uses emersion behavior as a resource to avoid thermal stress and survive higher aquatic temperatures. We also examined whether this behavior is modulated by exposure to high temperature; whether, during the period of emersion, the animal loses heat from the carapace to the medium; and whether this behavior is altered by the temperature at which the animal has been acclimated. The lethal temperature for 50% of the population (LT₅₀) was determined through 96-h mortality curves in animals acclimated at 20 °C and 30 °C. The behavioral profile of N. granulata during thermal stress was based on monitoring crab movement in aerial, intermediary, and aquatic zones. Acclimation at a higher temperature and the possibility of emersion increased the thermotolerance of the crabs and the synergistic effect of acclimation temperature. The possibility of leaving the hot water further increased the resistance of these animals to thermal stress. We observed that when the crab was subjected to thermal stress conditions, it spent more time in the aerial environment, unlike under control conditions. Under the experimental conditions, it made small incursions into the aquatic environment and stayed in the aerial environment for a longer time in order to cool its body temperature. The animals acclimated at 20 °C and placed into water at 35 °C remained in the aerial zone. The animals acclimated and maintained at 30 °C (control) that were placed in water at 35 °C with the possibility of emerging into hot air transited more frequently between the aquatic and aerial zones than did the animals that were put in water at 35 °C with the possibility of emerging into a cooler air environment. We conclude that emergence behavior allows N. granulata to survive high temperatures and that this behavior is influenced by acclimation temperature.

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.

Here We Are, But Where Do We Go? A Systematic Review of Crustacean Transcriptomic Studies from 2014-2015

Despite their economic, ecological, and experimental importance, genomic resources remain scarce for crustaceans. In lieu of genomes, many researchers have taken advantage of technological advancements to instead sequence and assemble crustacean transcriptomes de novo However, there is little consensus on what standard operating procedures are, or should be, for the field. Here, we systematically reviewed 53 studies published during 2014-2015 that utilized transcriptomic resources from this taxonomic group in an effort to identify commonalities as well as potential weaknesses that have applicability beyond just crustaceans. In general, these studies utilized RNA-Seq data, both novel and publicly available, to characterize transcriptomes and/or identify differentially expressed genes (DEGs) between treatments. Although the software suite Trinity was popular in assembly pipelines and other programs were also commonly employed, many studies failed to report crucial details regarding bioinformatic methodologies, including read mappers and the utilized parameters in identifying and characterizing DEGs. Annotation percentages for assembled transcriptomic contigs were low, averaging 32% overall. While other metrics, such as numbers of contigs and DEGs reported, correlated with the number of sequence reads utilized per sample, these did reach apparent saturation with increasing sequencing depth. Most disturbingly, a number of studies (55%) reported DEGs based on non-replicated experimental designs and single biological replicates for each treatment. Given this, we suggest future RNA-Seq experiments targeting transcriptome characterization conduct deeper (i.e., 50-100 M reads) sequencing while those examining differential expression instead focus more on increased biological replicates at shallower (i.e., ∼10-20 M reads/sample) sequencing depths. Moreover, the community must avoid submitting for review, or accepting for publication, non-replicated differential expression studies. Finally, mining the ever growing publicly available transcriptomic data from crustaceans will allow future studies to focus on hypothesis-driven research instead of continuing to simply characterize transcriptomes. As an example of this, we utilized neurotoxin sequences from the recently described remipede venom gland transcriptome in conjunction with publicly available crustacean transcriptomic data to derive preliminary results and hypotheses regarding the evolution of venom in crustaceans.

The branchiostegal lung of Uca vocans (Decapoda: Ocypodidae): Unreported complexity revealed by corrosion casting and MicroCT techniques

The study of adaptation to terrestrial life in crabs poses several physiological questions. One of the major challenges the crabs have to face is respiration of air: most of the time, gills are unsuitable to perform oxygen exchange out of the water. Fiddler crabs, like other representatives of the Ocypodidae, have developed an additional mechanism of respiration by improving the circulation that lines the branchiostegal chamber, thus developing a branchiostegal lung. In the present study we describe the hitherto unreported complex morphology of the branchiostegal lung of the fiddler crab Uca vocans by means of corrosion casting techniques and 3D reconstruction. This complexity leads us to reconsider the degree of terrestriality of U. vocans and its evolutionary pathway towards land.

Evolution of air breathing: oxygen homeostasis and the transitions from water to land and sky

Life originated in anoxia, but many organisms came to depend upon oxygen for survival, independently evolving diverse respiratory systems for acquiring oxygen from the environment. Ambient oxygen tension (PO2) fluctuated through the ages in correlation with biodiversity and body size, enabling organisms to migrate from water to land and air and sometimes in the opposite direction. Habitat expansion compels the use of different gas exchangers, for example, skin, gills, tracheae, lungs, and their intermediate stages, that may coexist within the same species; coexistence may be temporally disjunct (e.g., larval gills vs. adult lungs) or simultaneous (e.g., skin, gills, and lungs in some salamanders). Disparate systems exhibit similar directions of adaptation: toward larger diffusion interfaces, thinner barriers, finer dynamic regulation, and reduced cost of breathing. Efficient respiratory gas exchange, coupled to downstream convective and diffusive resistances, comprise the "oxygen cascade"-step-down of PO2 that balances supply against toxicity. Here, we review the origin of oxygen homeostasis, a primal selection factor for all respiratory systems, which in turn function as gatekeepers of the cascade. Within an organism's lifespan, the respiratory apparatus adapts in various ways to upregulate oxygen uptake in hypoxia and restrict uptake in hyperoxia. In an evolutionary context, certain species also become adapted to environmental conditions or habitual organismic demands. We, therefore, survey the comparative anatomy and physiology of respiratory systems from invertebrates to vertebrates, water to air breathers, and terrestrial to aerial inhabitants. Through the evolutionary directions and variety of gas exchangers, their shared features and individual compromises may be appreciated.

Brain architecture of the largest living land arthropod, the Giant Robber Crab Birgus latro (Crustacea, Anomura, Coenobitidae): evidence for a prominent central olfactory pathway?

BACKGROUND

Several lineages within the Crustacea conquered land independently during evolution, thereby requiring physiological adaptations for a semi-terrestrial or even a fully terrestrial lifestyle. Birgus latro Linnaeus, 1767, the giant robber crab or coconut crab (Anomura, Coenobitidae), is the largest land-living arthropod and inhabits Indo-Pacific islands such as Christmas Island. B. latro has served as a model in numerous studies of physiological aspects related to the conquest of land by crustaceans. From an olfactory point of view, a transition from sea to land means that molecules need to be detected in gas phase instead of in water solution. Previous studies have provided physiological evidence that terrestrial hermit crabs (Coenobitidae) such as B. latro have a sensitive and well differentiated sense of smell. Here we analyze the brain, in particular the olfactory processing areas of B. latro, by morphological analysis followed by 3 D reconstruction and immunocytochemical studies of synaptic proteins and a neuropeptide.

RESULTS

The primary and secondary olfactory centers dominate the brain of B. latro and together account for ca. 40% of the neuropil volume in its brain. The paired olfactory neuropils are tripartite and composed of more than 1,000 columnar olfactory glomeruli, which are radially arranged around the periphery of the olfactory neuropils. The glomeruli are innervated ca. 90,000 local interneurons and ca. 160,000 projection neurons per side. The secondary olfactory centers, the paired hemiellipsoid neuropils, are targeted by the axons of these olfactory projection neurons. The projection neuron axonal branches make contact to ca. 250.000 interneurons (per side) associated with the hemiellipsoid neuropils. The hemiellipsoid body neuropil is organized into parallel neuropil lamellae, a design that is quite unusual for decapod crustaceans. The architecture of the optic neuropils and areas associated with antenna two suggest that B. latro has visual and mechanosensory skills that are comparable to those of marine Crustacea.

CONCLUSIONS

In parallel to previous behavioral findings that B. latro has aerial olfaction, our results indicate that their central olfactory pathway is indeed most prominent. Similar findings from the closely related terrestrial hermit crab Coenobita clypeatus suggest that in Coenobitidae, olfaction is a major sensory modality processed by the brain, and that for these animals, exploring the olfactory landscape is vital for survival in their terrestrial habitat. Future studies on terrestrial members of other crustacean taxa such as Isopoda, Amphipoda, Astacida, and Brachyura will shed light on how frequently the establishment of an aerial sense of olfaction evolved in Crustacea during the transition from sea to land. Amounting to ca. 1,000,000, the numbers of interneurons that analyse the olfactory input in B. latro brains surpasses that in other terrestrial arthropods, as e.g. the honeybee Apis mellifera or the moth Manduca sexta, by two orders of magnitude suggesting that B. latro in fact is a land-living arthropod that has devoted a substantial amount of nervous tissue to the sense of smell.

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.

Ammonia excretion in aquatic and terrestrial crabs

The excretory transport of toxic ammonia across epithelia is not fully understood. This review presents data combined with models of ammonia excretion derived from studies on decapod crabs, with a view to providing new impetus to investigation of this essential issue. The majority of crabs preserve ammonotely regardless of their habitat, which varies from extreme hypersaline to freshwater aquatic environments, and ranges from transient air exposure to obligate air breathing. Important components in the excretory process are the Na+/K+(NH4+)-ATPase and other membrane-bound transport proteins identified in many species, an exocytotic ammonia excretion mechanism thought to function in gills of aquatic crabs such as Carcinus maenas, and gaseous ammonia release found in terrestrial crabs, such as Geograpsus grayi and Ocypode quadrata. In addition, this review presents evidence for a crustacean Rhesus-like protein that shows high homology to the human Rhesus-like ammonia transporter both in its amino acid sequence and in its predicted secondary structure.

Hypoxic tolerance in air-breathing invertebrates

Terrestrial invertebrates experience hypoxia in many habitats and under a variety of physiological conditions. Some groups (at least insects) are much more capable of recovery from anoxia than most vertebrates, but there is still a tremendous unexplained variation in hypoxia/anoxia tolerance among terrestrial invertebrates. Crustaceans and arachnids may be less often confronted with hypoxic environments than insects and myriapods and also seem to be less hypoxia/anoxia tolerant. Tracheated groups, especially those that are able to ventilate their tracheal system like many insects, cope with lower critical PO2 than nontracheated groups. Modulation of oxygen carrier proteins is normally not important in hypoxia resistance. Recent application of genetic and cellular tools are revealing that many of the same pathways documented for mammals (e.g. HIF, nitric oxide) function to regulate morphological and biochemical responses to hypoxia/anoxia in insects.

Oxygen and carbon dioxide sensitivity of ventilation in amphibious crabs, Cardisoma guanhumi, breathing air and water

Amphibious crabs, Cardisoma guanhumi, were acclimated to breathing either air or water and exposed to altered levels of oxygen and/or carbon dioxide in the medium. Hypercapnia (22, 36 and 73 torr CO(2)) stimulated a significant hypercapnic ventilatory response (HCVR) in both groups of crabs, with a much greater effect on scaphognathite frequency (Deltaf(SC)=+700%) in air-breathing crabs than water-breathing crabs (Deltaf(SC)=+100%). In contrast, hyperoxia induced significant hypoventilation in both sets of crabs. However, simultaneous hyperoxia and hypercapnia triggered a greater than 10-fold increase in f(SC) in air-breathing crabs but no change in water-breathing crabs. For water-breathing crabs hypoxia simultaneous with hypercapnia triggered the same response as hypoxia alone-bradycardia (-50%), and a significant increase in f(SC) at moderate exposures but not at the more extreme levels. The response of air-breathing crabs to hypoxia concurrent with hypercapnia was proportionally closer to the response to hypercapnia alone than to hypoxia. Thus, C. guanhumi were more sensitive to ambient CO(2) than O(2) when breathing air, characteristic of fully terrestrial species, and more sensitive to ambient O(2) when breathing water, characteristic of fully aquatic species. C. guanhumi possesses both an O(2)- and a CO(2)-based ventilatory drive whether breathing air or water, but the relative importance switches when the respiratory medium is altered.

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.