Gecarcinus Lateralis Research Articles

Invader–invader mutualism influences land snail community composition and alters invasion success of alien species in tropical rainforest

Mutualism between invaders may alter a key characteristic of the recipient community, leading to the entry or in situ release of other exotic species. We considered whether mutualism between invasive yellow crazy ant Anoplolepis gracilipes and exotic honeydew-producing scale insects indirectly facilitated land snails (exotic and native) via the removal of a native omnivore, the red land crab Gecarcoidea natalis. In plateau rainforest on Christmas Island, Indian Ocean, the land snail community was surveyed at 28 sites representing four forest states that differed in the density of red crabs, the abundance of yellow crazy ants and management history. One-way ANOVAs and multivariate analyses were used to determine differences in land snail species abundance and composition between forest states. Sample-based rarefaction was used to determine differences in species richness. The removal of the red land crab by supercolonies of yellow crazy ants was associated with a significant increase in the abundance of both invasive (14 species) and native (four species) land snails. Compositional differences in the land snail community were driven most strongly by the significantly greater abundance of a few common species in forest states devoid of red crabs. In forest where the crab population had recovered following management for ants, the land snail assemblage did not differ from intact, uninvaded forest. The land snail community was dominated by exotic species that can coexist alongside red crabs in rainforest uninvaded by exotic ants and scale insects. However, the ant–scale mutualism significantly increased land snail abundance and altered their composition indirectly though the alteration of the recipient community. We suggest these constitute ‘population-release’ secondary invasion in which the impacts of previously successful invaders facilitate a significant increase in abundance of other exotic species already established at low density within the community. Understanding facilitative interactions between invaders and indirect consequences of impacts will provide invaluable insights for conservation in heavily invaded ecosystems.

PRESERVATION POTENTIAL OF GECARCINID LAND CRABS (DECAPODA, BRACHYURA, GECARCINIDAE) FROM SAN SALVADOR

ABSTRACT Land crabs are found in marine and terrestrial sediments and sedimentary rocks from the Pleistocene and Holocene throughout the Caribbean. Field surveys of two widespread and abundant species, Gecarcinus lateralis and Gecarcinus ruricola, were conducted three times a week for 4 weeks along the northern coast of San Salvador Island, the Bahamas, at three localities that varied in vegetation cover, available sediment, and wind and wave energy. A total of 1400 identifiable remains were found and scored for four taphonomic characters: fragmentation, edge modification, surface alteration, and color loss. The majority of specimens exhibited various levels of disarticulation, from complete limbs to only isolated podomeres. Overall, claws are overrepresented, and the distribution of recovered remains differed significantly (p < 0.0001) from that expected based on the anatomy of a single crab (eight legs, two claws, and one carapace per individual). Remains found in direct sunlight and open spaces consist...

Mechanistic target of rapamycin (mTOR) signaling genes in decapod crustaceans: Cloning and tissue expression of mTOR, Akt, Rheb, and p70 S6 kinase in the green crab, Carcinus maenas, and blackback land crab, Gecarcinus lateralis

Mechanistic target of rapamycin (mTOR) controls global translation of mRNA into protein by phosphorylating p70 S6 kinase (S6K) and eIF4E-binding protein-1. Akt and Rheb, a GTP-binding protein, regulate mTOR protein kinase activity. Molting in crustaceans is regulated by ecdysteroids synthesized by a pair of molting glands, or Y-organs (YOs), located in the cephalothorax. During premolt, the YOs hypertrophy and increase production of ecdysteroids. Rapamycin (1μM) inhibited ecdysteroid secretion in Carcinus maenas and Gecarcinus lateralis YOs in vitro, indicating that ecdysteroidogenesis requires mTOR-dependent protein synthesis. The effects of molting on the expression of four key mTOR signaling genes (mTOR, Akt, Rheb, and S6K) in the YO was investigated. Partial cDNAs encoding green crab (C. maenas) mTOR (4031bp), Akt (855bp), and S6K (918bp) were obtained from expressed sequence tags. Identity/similarity of the deduced amino acid sequence of the C. maenas cDNAs to human orthologs were 72%/81% for Cm-mTOR, 58%/73% for Cm-Akt, and 77%/88% for Cm-S6K. mTOR, Akt, S6K, and elongation factor 2 (EF2) in C. maenas and blackback land crab (G. lateralis) were expressed in all tissues examined. The two species differed in the effects of molting on gene expression in the YO. In G. lateralis, Gl-mTOR, Gl-Akt, and Gl-EF2 mRNA levels were increased during premolt. By contrast, molting had no effect on the expression of Cm-mTOR, Cm-Akt, Cm-S6K, Cm-Rheb, and Cm-EF2. These data suggest that YO activation during premolt involves up regulation of mTOR signaling genes in G. lateralis, but is not required in C. maenas.

Names and publication dates of the Brachyura in F.É. Guérin (Guérin-Méneville) (Crustacea: Decapoda).

The names and dates of the publications of Brachyura of Guérin (Guérin-Méneville from 1836) are reviewed, and previously unidentified or overlooked names are identified. Several identical new names used in multiple publications by Guérin (also under the name Guérin-Méneville), and others that appeared in the same year necessitated the accurate determination of publication dates to establish priority. The authorships of three names should be credited to Guérin (1832): Gecarcinus lateralis (Gecarcinidae), Halimus aries (Majidae), and Libinia spinosa (Epialtidae), the first previously attributed to Fréminville (1835), the last two to H. Milne Edwards (1834). The overlooked genus- and species-group names Cyclocarcinus pinnotheroides Guérin-Méneville, 1838, are determined to be senior subjective synonyms of the genus- and species-group names Hapalonotus reticulatus (De Man, 1879) (Pilumnidae). Applying Article 23.9.1 of the International Code of Zoological Nomenclature, precedence is reversed between the genus-group names and the correct name is Hapalonotus pinnotheroides (Guérin-Méneville, 1838). Current and widespread use of the genus-group name Thalamita Latreille, 1829 (31 March) (Portunidae) is threatened by the overlooked Thalamites Guérin, 1829 (21 March), and the precedence of the names is also reversed to maintain usage of the former. The genus-group name Eurypodius Guérin (Inachidae) is shown to have been established in 1828, not 1825. Included is a complete bibliography of the publications in which Guérin (also under Guérin-Méneville) established new names for Brachyura, with their accurate publication dates.

Heterochromatin and genome size inDrosophila

Heterochromatin and genome size in<i>Drosophila</i>

Seed fate in the myrmecochorous Neotropical plant Turnera ulmifolia L., from plant to germination

Myrmecochory (seed dispersal by ants) differs from other dispersal systems in a series of advantages offered by the ants to the plants. Here, seed fate, from fruit to germination, of the myrmecochorous Neotropical plant Turnera ulmifolia L. is described. Seed movement from the fruit to their germination was studied, using different measurements and experiments. The results show that a T. ulmifolia individual produces ca. 5000 seeds per year. The main pre-seed-fall predators are the larvae of the Microlepidopteran Crocidosema plebejana Zeller, which consumed 1% of the seeds on the plant. The red-land crab Gecarcinus lateralis (Freminville) consumed 19% of the seeds beneath the plant and was the main post-seed-fall predator. Seed removal by ants was recorded on and beneath the plant, and ants removed 49% of the total seed production. Considering the seed removal events, the ant Forelius analis contributed with 64% of the total number of events. F. analis took seeds to its nest and discarded 23% of the seeds collected. Germination of seeds collected by F. analis was two to four times higher than that of seeds with and without elaiosome, respectively. The relatively low seed predation was probably related to ant defense, associated with the presence of extrafloral nectaries in this plant and with seed removal on the plant. Our results suggest that F. analis is a quantitatively efficient but qualitatively inefficient seed disperser of T. ulmifolia.

Rheb, an activator of target of rapamycin, in the blackback land crab,Gecarcinus lateralis: cloning and effects of molting and unweighting on expression in skeletal muscle

Molt-induced claw muscle atrophy in decapod crustaceans facilitates exuviation and is coordinated by ecdysteroid hormones. There is a 4-fold reduction in mass accompanied by remodeling of the contractile apparatus, which is associated with an 11-fold increase in myofibrillar protein synthesis by the end of the premolt period. Loss of a walking limb or claw causes a loss of mass in the associated thoracic musculature; this unweighting atrophy occurs in intermolt and is ecdysteroid independent. Myostatin (Mstn) is a negative regulator of muscle growth in mammals; it suppresses protein synthesis, in part, by inhibiting the insulin/metazoan target of rapamycin (mTOR) signaling pathway. Signaling via mTOR activates translation by phosphorylating ribosomal S6 kinase (s6k) and 4E-binding protein 1. Rheb (Ras homolog enriched in brain), a GTP-binding protein, is a key activator of mTOR and is inhibited by Rheb-GTPase-activating protein (GAP). Akt protein kinase inactivates Rheb-GAP, thus slowing Rheb-GTPase activity and maintaining mTOR in the active state. We hypothesized that the large increase in global protein synthesis in claw muscle was due to regulation of mTOR activity by ecdysteroids, caused either directly or indirectly via Mstn. In the blackback land crab, Gecarcinus lateralis, a Mstn-like gene (Gl-Mstn) is downregulated as much as 17-fold in claw muscle during premolt and upregulated 3-fold in unweighted thoracic muscle during intermolt. Gl-Mstn expression in claw muscle is negatively correlated with hemolymph ecdysteroid level. Full-length cDNAs encoding Rheb orthologs from three crustacean species (G. lateralis, Carcinus maenas and Homarus americanus), as well as partial cDNAs encoding Akt (Gl-Akt), mTOR (Gl-mTOR) and s6k (Gl-s6k) from G. lateralis, were cloned. The effects of molting on insulin/mTOR signaling components were quantified in claw closer, weighted thoracic and unweighted thoracic muscles using quantitative polymerase chain reaction. Gl-Rheb mRNA levels increased 3.4-fold and 3.9-fold during premolt in claw muscles from animals induced to molt by eyestalk ablation (ESA) and multiple leg autotomy (MLA), respectively, and mRNA levels were positively correlated with hemolymph ecdysteroids. There was little or no effect of molting on Gl-Rheb expression in weighted thoracic muscle and no correlation of Gl-Rheb mRNA with ecdysteroid titer. There were significant changes in Gl-Akt, Gl-mTOR and Gl-s6k expression with molt stage. These changes were transient and were not correlated with hemolymph ecdysteroids. The two muscles differed in terms of the relationship between Gl-Rheb and Gl-Mstn expression. In thoracic muscle, Gl-Rheb mRNA was positively correlated with Gl-Mstn mRNA in both ESA and MLA animals. By contrast, Gl-Rheb mRNA in claw muscle was negatively correlated with Gl-Mstn mRNA in ESA animals, and no correlation was observed in MLA animals. Unweighting increased Gl-Rheb expression in thoracic muscle at all molt stages; the greatest difference (2.2-fold) was observed in intermolt animals. There was also a 1.3-fold increase in Gl-s6k mRNA level in unweighted thoracic muscle. These data indicate that the mTOR pathway is upregulated in atrophic muscles. Gl-Rheb, in particular, appears to play a role in the molt-induced increase in protein synthesis in the claw muscle.

Rheb REGULATES CRAB MUSCLE ATROPHY

Moulting their shells and misplacing limbs, crabs continually depend on their versatile musculature to reshape their bodies. According to Donald Mykles and colleagues from Colorado State University and the University of Wisconsin – Stevens Point, USA, the massive claw muscle of blackback land crabs withers by as much as 78% before they can slip their shells off. However, Mykles and his colleagues made an interesting discovery in 2010. Instead of shutting down protein synthesis to allow the muscle to wither, the crabs ramp it up 11-fold by shutting down myostatin expression – which usually keeps protein synthesis in check. Knowing that myostatin regulates protein expression in mammals via the mTOR (metazoan target of rapamycin) signalling cascade, which controls mRNA translation, Mykles decided to investigate the mTOR pathway in blackback land crabs to find out whether it also regulates muscle rebuilding in crabs during moulting (p. 590).Stimulating crabs to moult, the team collected muscles from the animals and searched for the mRNA of four key mTOR signalling proteins – Rheb, mTOR, Akt and s6k. The crabs expressed all four genes, confirming that the mTOR pathway is active in moulting crab’s atrophied muscle.Next, the team measured the expression levels of each gene in crabs that had been induced to moult using methods that raise the animals’ moulting hormone (ecdysteroid, which regulates myostatin levels) over different time scales. They found that Rheb levels increased in the moulting crabs’ claws muscles and in the atrophied thoracic muscles of crabs that had lost a limb. Mykles says ‘Rheb is a key activator of mTOR. Its up-regulation by moulting indicates that Rheb plays a role in the stimulation of mTOR-mediated protein synthesis by ecdysteroids’.However, when the team investigated the mechanism by which Rheb is upregulated in the two muscles, there were differences. They found a strong correlation between the levels of Rheb in the crabs’ claws and the ecdysteroid levels, suggesting that ecdysteroid does regulate protein synthesis via mTOR in the claw muscle. Yet, when they compared the ecdysteroid levels and Rheb levels in the thoracic muscles – that wither in response to limb loss – the team found no correlation. Ecdysteroid hormone does not control muscle atrophy in response to limb loss.Analysing the expression patterns of Rheb and myostatin in atrophied claw and thoracic muscles, Mykles and colleagues also saw different expression relationships between the proteins in the two tissues. ‘If blackback land crab myostatin plays a role in both types of atrophy, any unifying mechanism must reconcile the differences between myostatin and Rheb expression in the two muscles’, the team says.Having shown that Rheb is a key component of the signalling pathway that regulates muscle atrophy in crabs, the team says that it is keen to build, ‘a mechanistic understanding of the interactions between ecdysteroids and the signalling pathways that control protein metabolism in crustacean skeletal muscle’.

Cloning of a nitric oxide synthase from green shore crab, Carcinus maenas: A comparative study of the effects of eyestalk ablation on expression in the molting glands (Y-organs) of C. maenas, and blackback land crab, Gecarcinus lateralis

Molting in decapod crustaceans is regulated by ecdysteroids produced by a pair of Y-organs (YOs) located in the cephalothorax. YO ecdysteroidogenesis is suppressed by molt-inhibiting hormone (MIH), a neuropeptide produced in the X-organ of the eyestalk (ES) ganglia. MIH signaling may involve nitric oxide synthase (NOS) and NO-sensitive guanylyl cyclase (GC-I). A full-length cDNA encoding Carcinus maenas NOS (Cm-NOS; 3836 base pairs) of 1164 amino acid residues (estimated mass 131,833 Da) was cloned with 88% identity to Gecarcinus lateralis NOS (Gl-NOS). End-point reverse transcription-polymerase chain reaction (RT-PCR) showed that Cm-NOS was expressed at varying levels in the YO, testis, ovary, hepatopancreas, midgut, hindgut, heart, thoracic ganglion, and skeletal muscle and was not detected in the gill. Immunofluorescence microscopy showed localization of NOS and cGMP in the steroidogenic cells and the surrounding connective tissue layer of the C. maenas YO. ES ablation (ESA) induced molting in G. lateralis; hemolymph ecdysteroid titers increased during premolt and reached a peak of about 400 pg/μL at 20 days and 24 days post-ESA. By contrast, ESA did not induce molting in C. maenas; hemolymph ecdysteroid titers increased about 2-fold (53 to 121 pg/μL) by 3 days post-ESA and remained at that level at 7 days post-ESA. Real time PCR was used to quantify the effects of ESA on the expression of NOS in C. maenas and G. lateralis YOs. ESA caused 32-fold and 5-fold increases in Gl-NOS and Cm-NOS transcripts by 24 days and 7 days post-ESA, respectively, which were correlated with hemolymph ecdysteroid levels. In addition, GC-I catalytic subunit ( Gl-GC-Iβ) mRNA level increased 7.4-fold by 24 days post-ESA, but there was no significant effect of ESA on membrane GC ( Gl-GC-II) mRNA level. These data indicate that the YO up-regulates NO signaling components in response to withdrawal of ES neuropeptides.

Molecular cloning and characterization of the nitric oxide synthase gene from kuruma shrimp, Marsupenaeus japonicus

Nitric oxide (NO) signaling is involved in many physiological processes in vertebrates and invertebrates. In crustaceans, nitric oxide synthase (NOS) plays a significant role in the regulation of the nervous system and in innate immunity. Here, we describe the entire cDNA sequence (4616 bp) of the kuruma shrimp Marsupenaeus japonicus NOS (Mj NOS) generated using the reverse transcriptase-polymerase chain reaction (RT-PCR) and 5′- and 3′- rapid amplification PCRs of cDNA ends from brain and gill mRNAs. The open reading frame of Mj NOS encoded a protein of 1187 amino acids with an estimated mass of 134 kDa, and had an 82.3% sequence homology with the NOS gene of the land crab Gecarcinus lateralis. Highly conserved amino acid sequences in heme and tetrahydrobiopterin were observed in the oxygenase domain. FMN, FAD and NADPH were found in the reductase domain. Mj NOS mRNA was constitutively expressed in the brain, gill, intestine, thoracic ganglion and testis of the kuruma shrimp. When Vibrio penaeicida was injected into the kuruma shrimp, Mj NOS was expressed in the brain, gill, heart, lymphoid organ, intestine and thoracic ganglion. Mj NOS expression in the gill reached its peak 12 h and decreased to its normal level 24 h after V. penaeicida injection.

Molt cycle regulation of protein synthesis in skeletal muscle of the blackback land crab, Gecarcinus lateralis, and the differential expression of a myostatin-like factor during atrophy induced by molting or unweighting

In decapod crustaceans, claw muscle undergoes atrophy in response to elevated ecdysteroids while thoracic muscle undergoes atrophy in response to unweighting. The signaling pathways that regulate muscle atrophy in crustaceans are largely unknown. Myostatin is a negative regulator of muscle growth in mammals, and a myostatin-like cDNA is preferentially expressed in muscle of the land crab, Gecarcinus lateralis (Gl-Mstn). Contrary to prediction, levels of Gl-Mstn mRNA decreased dramatically in both the claw closer and weighted thoracic muscles when molting was induced by either eyestalk ablation (ESA) or multiple limb autotomy (MLA). However, the effect of molt induction was greater in the claw muscle. By late premolt, Gl-Mstn mRNA in the claw muscle decreased 81% and 94% in ESA and MLA animals, respectively, and was negatively correlated with ecdysteroids. Gl-Mstn mRNA in thoracic muscle decreased 68% and 82% in ESA and MLA animals, respectively, but was only weakly correlated with ecdysteroid. Claw and thoracic muscles also differed to varying extents in the expression of ecdysteroid receptor (Gl-EcR and Gl-RXR), elongation factor-2 (Gl-EF-2), and calpain T (Gl-CalpT) in response to molt induction, but levels of the four transcripts were not correlated with ecdysteroid. The downregulation of Gl-Mstn expression in premolt claw muscle coincided with 11- and 13-fold increases in protein synthesis in the myofibrillar and soluble protein fractions, respectively. Furthermore, the rate of the increase in the synthesis of soluble proteins was greater than that of myofibrillar proteins during early premolt (1.4:1, soluble:myofibrillar), but the two were equivalent during late premolt. By contrast, Gl-Mstn mRNA increased 3-fold and Gl-CalpT mRNA decreased 40% in unweighted thoracic muscle; there was little or no effect on Gl-EF-2, Gl-EcR, and Gl-RXR mRNA levels. These data indicate that Gl-Mstn expression is negatively regulated by both ecdysteroids and load-bearing contractile activity. The downregulation of Gl-Mstn in claw muscle may induce the elevated protein turnover associated with remodeling of the contractile apparatus during molt-induced atrophy. The upregulation of Gl-Mstn in unweighted thoracic muscle suggests that this factor is also involved in disuse atrophy when hemolymph ecdysteroid levels are low.

Supercolony mosaics: two different invasions by the yellow crazy ant, Anoplolepis gracilipes, on Christmas Island, Indian Ocean

Invasive species are one of the main reasons for the ongoing global loss of biodiversity. Anoplolepis gracilipes is an invasive ant that has recently received significant attention due to its negative effect on the native fauna and flora of Christmas Island, Indian Ocean. This species has contributed to a drastic change in the structure of the Christmas Island rainforest through its negative impact on the island’s endemic red land crab, the dominant consumer on the islands forest floor. In this study, we investigate the population structure of A. gracilipes on Christmas Island in order to determine whether multiple introductions occurred on the island and how they correspond to known infestations. We genotyped 578 individuals collected from 50 nests across the Island. We identify two distinct subgroups in the population that represent two different supercolonies. These supercolonies are interspersed across the island, however both nuclear (microsatellites) and mitochondrial markers strongly suggest that there is no gene flow between the two colonies. Significant heterozygote excess within the entire sampling area, with all but one worker examined being heterozygous for all seven microsatellite loci, suggests an unusual reproductive system in these ants. Our results are consistent with recent sociogenetic findings in a population of A. gracilipes in Northern Borneo.

DNA evidence of whale sharks (Rhincodon typus) feeding on red crab (Gecarcoidea natalis) larvae at Christmas Island, Australia

Whale sharks (Rhincodon typus) are thought to aggregate in nearshore waters around Christmas Island (105°37′E, 10o29′S) to consume the marine larvae of the endemic red land crab (Gecarcoidea natalis). However, there have been no direct observations of sharks feeding on crab larvae. Whale shark faeces were analysed using genetic testing to confirm the presence of crab larvae in their diet. Primers were designed for amplifying two Gecarcoidea natalis mitochondrial small-subunit (mtSSU) rDNA regions. Gel electrophoresis of polymerase chain reaction (PCR) products amplified from whale shark faecal DNA produced bands of the expected size for G. natalis templates. Specificity of both primer sets for G. natalis mtSSU rDNA was expected to be high from comparisons with mtSSU rDNA regions from closely related crabs and we confirmed their specificity empirically. The amplification of fragments from faecal DNA of the same size as those produced from G. natalis DNA indicates that the whale shark had been feeding on G. natalis and that enough of the crab DNA survived digestion to be detected by these PCRs. Our study provides further evidence that aggregations of whale sharks in coastal waters occur in response to ephemeral but predictable increases in planktonic prey.

Expression of recombinant eyestalk crustacean hyperglycemic hormone from the tropical land crab, Gecarcinus lateralis, that inhibits Y-organ ecdysteroidogenesis in vitro

Crustacean hyperglycemic hormone (CHH) is a pleiotropic neuropeptide that regulates carbohydrate and lipid metabolism, molting, reproduction, and osmoregulation in decapod crustaceans. CHH elevates glucose levels in the hemolymph by stimulating glycogenolysis in target tissues. It also inhibits ecdysteroidogenesis in the molting gland, or Y-organ (YO), possibly as a response to environmental stress. CHH acts via binding to a membrane receptor guanylyl cyclase, which is expressed in most tissues, including the YO. Large amounts of biologically active neuropeptide are required to investigate the mechanism of CHH signaling in the YO. Consequently, the eyestalk ganglia CHH (EG-CHH) isoform was cloned into a yeast (Pichia pastoris) expression vector to express recombinant mature peptide (rEG-CHH) with or without a C-terminal c-Myc/polyhistidine tag. Yeast cultures with untagged or tagged rEG-CHH inhibited ecdysteroidogenesis in YOs from European green crab (Carcinus maenas) 36% (P < 0.002) and 51% (P < 0.006), respectively. Purified tagged EG-CHH inhibited YO ecdysteroidogenesis 32% (P < 0.002), but lacked hyperglycemic activity in vivo. This is the first report of recombinant EG-CHH inhibiting YO ecdysteroidogenesis. The data suggest that the tagged recombinant peptide can be used to elucidate the CHH signaling pathway in the crustacean molting gland.

Expression of alternatively spliced transcripts for a myostatin-like protein in the blackback land crab, Gecarcinus lateralis

Three complete cDNAs for the first myostatin-like gene identified in a crustacean species were cloned from the land crab, Gecarcinus lateralis. Sequence analysis demonstrates a high degree of conservation with myostatin orthologs from vertebrates. The furin cleavage site is identical to that of human myostatin, and all nine cysteines critical to the structure/function of mature myostatin peptides are conserved. Message levels for transcripts encoding the complete crustacean preproprotein were highest in skeletal muscle and heart. Lower levels of expression were observed in nervous tissue, gill, gonad, and hepatopancreas. This expansive distribution is similar to that observed for teleost myostatin, vertebrate GDF-11, and amphioxus GDF8/11, and indicates a potentially broad functional repertoire for the land crab ortholog. In addition to one cDNA encoding a complete preproprotein, two cDNAs encoding C-terminal truncated proteins lacking a mature peptide domain were identified. Expression of these truncated splice variants was restricted to skeletal muscle and heart. Myostatin is a potent negative regulator of muscle mass in mammals, and strong expression of this TGF-β factor in skeletal muscle during intermolt indicates that a myostatin-like gene product could regulate muscle mass in crustaceans when growth is physically restricted by a calcified exoskeleton.

Recruitment dynamics in a rainforest seedling community: context-independent impact of a keystone consumer

The influence of keystone consumers on community structure is frequently context-dependent; the same species plays a central organising role in some situations, but not others. On Christmas Island, in the Indian Ocean, a single species of omnivorous land crab, Gecarcoidea natalis, dominates the forest floor across intact rainforest. We hypothesised that this consumer plays a key role in regulating seedling recruitment and in controlling litter dynamics on the island, independent of the type of vegetation in which it occurred. To test this hypothesis, we conducted crab exclusion experiments in two forest types on the island and followed the dynamics of seedling recruitment and litter processing for six years. To determine if these effects were likely to be general across the island, we compared land crab densities and seedling abundance and diversity at ten sites across island rainforest. Surveys across island rainforest showed that seedlings of species susceptible to predation by land crabs are consistently rare. Abundance and diversity of these species were negatively correlated to red crab abundance. Although red land crabs may be important determinants of seedling recruitment to the overstorey, differences in overstorey and seedling composition at the sites suggests that recruitment of vulnerable trees still occurs at a temporal scale exceeding that of this study. These "windows" of recruitment may be related to infrequent events that reduce the effects of land crabs. Our results suggest that unlike the context dependence of most keystone consumers in continental systems, a single consumer, the red land crab, consistently controls the dynamics of seedling recruitment across this island rainforest.

Effects of elevated ecdysteroid on tissue expression of three guanylyl cyclases in the tropical land crab Gecarcinus lateralis: possible roles of neuropeptide signaling in the molting gland

Two eyestalk (ES) neuropeptides, molt-inhibiting hormone (MIH) and crustacean hyperglycemic hormone (CHH), increase intracellular cGMP levels in target tissues. Both MIH and CHH inhibit ecdysteroid secretion by the molting gland or Y-organ (YO), but apparently through different guanylyl cyclase (GC)-dependent pathways. MIH signaling may be mediated by nitric oxide synthase (NOS) and NO-sensitive GC. CHH binds to a membrane receptor GC. As molting affects neuropeptide signaling, the effects of ecdysteroid on the expression of the land crab Gecarcinus lateralis beta subunit of a NO-sensitive GC (Gl-GC-Ibeta), a membrane receptor GC (Gl-GC-II) and a NO-insensitive soluble GC (Gl-GC-III) were determined. Gl-GC-Ibeta isoforms differing in the absence or presence of an N-terminal 32-amino acid sequence and Gl-GC-III were expressed at higher mRNA levels in ES ganglia, gill, hepatopancreas, ovary and testis, and at lower levels in YO, heart and skeletal muscle. Three Gl-GC-II isoforms, which vary in the length of insertions (+18, +9 and +0 amino acids) within the N-terminal ligand-binding domain, differed in tissue distribution. Gl-GC-II(+18) was expressed highly in striated muscle (skeletal and cardiac muscles); Gl-GC-II(+9) was expressed in all tissues examined (ES ganglia, YO, gill, hepatopancreas, striated muscles and gonads); and Gl-GC-II(+0) was expressed in most tissues and was the dominant isoform in ES and thoracic ganglia. ES ablation, which increased hemolymph ecdysteroid, increased Gl-GC-II(+18) mRNA level in claw muscle. Using real-time RT-PCR, ES ablation increased Gl-GC-Ibeta, Gl-GC-III and ecdysone receptor mRNA levels in the YOs approximately ten-, approximately four- and approximately twofold, respectively, whereas Gl-GC-II mRNA level was unchanged. A single injection of 20-hydroxyecdysone into intact animals transiently lowered Gl-GC-Ibeta in hepatopancreas, testis and skeletal muscle, and certain Gl-GC-II isoforms in some of the tissues. These data suggest that YO and other tissues can modulate responses to neuropeptides by altering GC expression.

Guanylyl cyclases in the tropical land crab, Gecarcinus lateralis: Cloning of soluble (NO-sensitive and -insensitive) and membrane receptor forms

cDNAs encoding three guanylyl cyclases (GCs), which catalyze the production cGMP from GTP, were cloned from the blackback land crab, Gecarcinus lateralis: the β subunit of a NO-sensitive soluble GC (Gl-GC-Iβ; 2380 bp; 603 amino acids; 68,435 Da), a membrane receptor GC (Gl-GC-II; 4609 bp; 1349 amino acids; 150,999 Da), and a NO-insensitive soluble GC (Gl-GC-III; 1416 bp; 285 amino acids; 32,487 Da). All three have a highly-conserved catalytic domain located in the C-terminal region and are therefore likely to be enzymatically active. Gl-GCIβ contains heme/NO-binding (HNOB) and HNOB-associated domains characteristic of the catalytic subunit of NO-sensitive GCs. Gl-GC-II encodes a single-pass membrane protein containing ligand-binding (LB), transmembrane (TM), kinase homology (KH), and dimerization domains. Gl-GC-III is similar to Gl-GC-II but lacks the LB, TM, and most of the KH domains. Isoforms of Gl-GC-Iβ and Gl-GC-II appear to be generated by alternative splicing. Two Gl-GC-Iβ isoforms differed in the absence (Δ32N) or presence (Δ0N) of a 32-amino acid sequence at the N-terminus. The truncated form may not bind a heme group, but still would be able to dimerize with the alpha subunit to form a NO-insensitive enzyme. Three Gl-GC-II isoforms varied in length within the N-terminal ligand-binding domain (+ 18, + 9, and + 0 amino acid insertions). As GC-II is the putative receptor for crustacean hyperglycemic hormone (CHH), these data suggest that the isoforms differ in binding to CHH and related neuropeptides.

Crustacean hyperglycemic hormone from the tropical land crab, Gecarcinus lateralis: Cloning, isoforms, and tissue expression

Crustacean hyperglycemic hormone (CHH) regulates carbohydrate metabolism, molting, and ion and water transport. cDNAs encoding four CHH isoforms (designated EG-CHH-A, -B, -C, and -D) were cloned from eyestalk ganglia (EG) from land crab, Gecarcinus lateralis. The isoforms differed in the 3′ region of the open reading frame and/or the length of the 3′ untranslated region. All encoded essentially identical preprohormones containing a 28-amino acid (aa) signal peptide, a 42-aa precursor related peptide and a 72-aa mature CHH. All deduced aa sequences had the six cysteines, two arginines, one aspartate, one phenylalanine, and one arginine originally identified as characteristic of this neuropeptide family. There was a single aa difference between the EG-CHH-D mature hormone and the other three isoforms. The EG-CHH isoforms were expressed in EG, hindgut, and thoracic ganglion. A fifth CHH isoform, designated pericardial organ (PO)-CHH, was similar to the PO-CHH isoform described in green crab, Carcinus maenas. It was expressed in hindgut and testis, but not in eyestalk ganglia; its expression in PO was not determined. The deduced aa sequence of the PO-CHH was identical to that of the EG-CHH isoforms through aa #40 of the mature peptide. The divergent aa sequence between positions #41 and #73 was encoded by an insertion of a 111-bp sequence absent in EG-CHH cDNAs. The data suggest that EG-CHH and PO-CHH isoforms are generated by alternative splicing of at least two CHH genes.

The Complete Larval Development of Johngarthia Planatus (Brachyura: Grapsoidea: Gecarcinidae) Described from Laboratory Reared Material, with Notes on the Affinity of Gecarcinus and Johngarthia

Abstract The land crab Johngarthia planatus is an eastern tropical Pacific species with a known distribution from Mexico to Colombia. Complete larval development of the species is fully described and illustrated from laboratory-reared material. The five zoeal stages and the megalopa are compared with those of Gecarcinus lateralis lateralis from the Atlantic coast. Main differences for zoeal stages are observed in the carapace and telson lateral spines while minor differences are observed in the mouthparts setation. For the megalopa stage a difference in size of the third maxilliped exopod supports the separation of Johngarthia from Gecarcinus. A brief comparison of all known zoea I and megalopa stages of species of the Gecarcinidae is made, taking into account recent new descriptions.