Carcinus Maenas Research Articles

High-resolution chromosome-level genome of Scylla paramamosain provides molecular insights into adaptive evolution in crabs

BackgroundEvolutionary adaptation drives organismal adjustments to environmental pressures, exemplified in the diverse morphological and ecological adaptations seen in Decapoda crustaceans, particularly brachyuran crabs. Crabs thrive in diverse ecosystems, from coral reefs to hydrothermal vents and terrestrial habitats. Despite their ecological importance, the genetic mechanisms underpinning their developmental processes, reproductive strategies, and nutrient acquisition remain poorly understood.ResultsHere, we report a comprehensive genomic analysis of the green mud crab Scylla paramamosain using ultralong sequencing technologies, achieving a high-quality chromosome-level assembly. The refined 1.21 Gb genome, with an impressive contig N50 of 11.45 Mb, offers a valuable genomic resource. The genome exhibits 33,662 protein-coding genes, enriched in various pathways related to development and environmental adaptation. Gene family analysis shows expansion in development-related pathways and contraction in metabolic pathways, indicating niche adaptations. Notably, investigation into Hox gene regulation sheds light on their role in pleopod development, with the Abd-A gene identified as a linchpin. Post-transcriptional regulation involving novel-miR1317 negatively regulates Abd-A levels. Furthermore, the potential role of fru gene in ovarian development and the identification of novel-miR35 as a regulator of Spfru2 add complexity to gene regulatory networks. Comparative functional analysis across Decapoda species reveals neo-functionalization of the elovl6 gene in the synthesis of long-chain polyunsaturated fatty acids (LC-PUFA), suggesting its importance in environmental adaptation.ConclusionsOur findings shed light on various aspects of crab biology, including genome sequencing, assembly, and annotation, as well as gene family expansion, contraction, and regulatory mechanisms governing crucial developmental processes such as metamorphosis, reproductive strategies, and fatty acid metabolism.

Oyster grower perspectives on green crab (Carcinus maenas) interactions: using applied social science to inform research and engagement

Research that informs the New Hampshire Aquaculture industry, which has grown significantly over the past few years, and helps the overall associated watershed community to keep Great Bay Estuary healthy is vital. Specific study of green crabs, Carcinus maenas, and their potential effects on oysters, Crassostrea virginica, is important to gain a better understanding of the impact invasive species may have on these ecologically and economically important bivalves. The aim of this study was to document and understand New Hampshire (NH) oyster growers’ perspectives on the issue of green crab predation of oysters. Through a survey, this study documented a preliminary understanding of farmer perspectives and research needs regarding the population of green crabs in NH waters. More specifically, this survey provided a social science perspective and information on areas of potential research - abundance, distribution, and diet of green crabs - that could be beneficial to the oyster industry. This study fills knowledge gaps, helps improve future research (including how to better engage with end users), and facilitates creating and implementing solutions that benefit the NH oyster aquaculture industry.

Physiological Responses of the Green Shore Crab, Carcinus maenas, During Acute and Chronic Low Temperature Exposure.

The green shore crab (Carcinus maenas) is native to Western Europe but has spread around the globe and is described as one of the top 100 worst invasive species. On the east coast of North America, their northern-most limit is the island of Newfoundland, Canada, where they can experience water temperatures as low as -1 °C. We investigated the physiological responses of C. maenas to a temperature reduction regime as well as to long-term acclimation to temperatures representative of winter (2 °C) and summer (12 °C) in Newfoundland. Heart rate, oxygen consumption and estimated energy expenditure declined steadily with decreasing temperature, but a marked change was observed between 6 and 4 °C, with lowest levels recorded in 2 °C. After long-term acclimation to 2 °C there was a sustained reduction in physiological parameters. Even though these physiological parameters were very low in 2 °C, the crabs still exhibited intermittent activity. This supports the presence of a dormancy, rather than true torpor/hibernation below 5 °C, in which crabs will continue to actively move and feed, albeit much more slowly. The population in Newfoundland contains haplotypes from both the invasive northern and southern lineages, and they appear to retain a similar low temperature response compared with most other populations of green crab from both their native and expanded range.

Putative Nociceptive Responses in a Decapod Crustacean: The Shore Crab (Carcinus maenas)

Nociceptors are receptors that detect injurious stimuli and are necessary to convey such information from the periphery to the central nervous system. While nociception has been extensively studied in various taxa, there is relatively little electrophysiological evidence for the existence of nociceptors in decapod crustaceans. This study investigated putative nociceptive responses in the shore crabs, specifically their response to mechanical and noxious chemical stimuli. Extracellular multi-unit electrophysiological recordings were conducted from the anterior ganglion and the circumesophageal connective ganglia to assess nociceptive responses. Soft tissues at the joints of the chelae, antennae, and walking legs were stimulated using acetic acid (noxious stimulus) and von Frey hairs (mechanical stimulus), while nearby ganglion activity was recorded. The results indicate the existence of nociceptors in the tested areas, with mechanical stimuli eliciting shorter, more intense neural activity compared with acetic acid. Although acetic acid triggered responses in all areas, the antennae and antennules did not respond to mechanical stimuli. Though we acknowledge the challenges of conducting in vivo electrophysiological recordings, future research should focus on further characterizing nociceptor activity because the results suggest the presence of nociceptors.

Epibionts of mediterranean green crab, Carcinus aestuarii Nardo, 1847 in Çanakkale Strait: Based on seasonal, sexual and color variation

Epibionts of mediterranean green crab, Carcinus aestuarii Nardo, 1847 in Çanakkale Strait: Based on seasonal, sexual and color variation

Exposure to Lead (Pb) influences the outcomes of male-male competition during precopulatory intrasexual selection

Male-male competition is a component of Darwin’s theory of precopulatory intrasexual selection, where males compete for access to resources important for reproduction and successful males pass on traits that endow greater competitive ability to their offspring, thereby exaggerating the trait(s) over evolutionary time. Metals, such as lead (Pb) often occur in urban mangrove forests, where shore crabs reside, and being neurotoxic may cause sub-lethal effects on behaviour. In this study, we asked whether exposure to Pb influences the outcomes of male-male competition in the Semaphore crab, Heloecius cordiformis, thereby altering outcomes of intrasexual selection. Crabs were exposed to an environmentally relevant, and a behaviourally relevant, concentration of lead (i.e. Pb, 10 µg/L and 100 µg/L respectively) under laboratory conditions for 96 h and subsequently allowed to compete for burrow ownership in experimental arenas. Exposed crabs (100 µg/L) were less successful than control crabs in chela size-matched paired competitive interactions, spent less time in burrows and predominantly lost burrow ownership. Further, exposure to Pb contamination (both 10 µg/L and 100 µg/L) negated the chela size advantage in size-asymmetric competitive interactions. This contaminant-induced loss of size-related competitive advantage may result in relaxation in selection pressure on body size and smaller male carapace widths and especially male chela lengths in contaminated locations. Assessment of a range of locations in the wild found a consistent pattern of metal-associated size declines in more contaminated locations for males, but not for females.

Interaction between anthropogenic stressors affects antipredator defense in an intertidal crustacean.

The marine environment is increasingly subject to changes driven by anthropogenic stressors which may alter species' key behaviors and impact phenotypic plasticity. Such stressors rarely occur in isolation, yet our understanding of how simultaneous stresses affect marine organisms is limited. Here, we study the combined impacts of a major global stressor, temperature increase, and a local stressor, anthropogenic noise, upon key defensive traits of the shore crab, Carcinus maenas. We tested the color change and behavioral responses of crabs in relatively colder and warmer water, and in the presence of natural ambient or ship noise. Using image analysis and a model of predator vision, we demonstrate that crabs change color, and improve camouflage, fastest in warmer water in the absence of anthropogenic noise. When anthropogenic noise was present, it adversely impacted crab color change and camouflage, to the extent that the accelerated change due to temperature was negated. In addition, anthropogenic noise affected C. maenas' behavior, reducing the likelihood and increasing the latency of antipredator response to stimuli. This reveals an interaction between the 2 stressors, with the combination of temperature and noise eliciting different biological responses compared with the effects of each stressor in isolation. Our study demonstrates how such interactions between anthropogenic stressors may impact marine life.

Maxent modelling reveals suitable habitat remains unchanged for Carcinus maenas in Australia

ABSTRACT This study assesses the habitat suitability of the invasive green crab, Carcinus maenas, in Australia using MaxEnt modelling under current and future climate scenarios. The species has significant ecological impacts, displacing native species and altering habitats. Using 90 occurrence points from the Global Biodiversity Information Facility, the MaxEnt model shows high performance (Area Under the Curve value of 0.99), indicating reliable predictions. Currently, the green crab's habitat suitability along the Australian coast ranges from high to medium in Tasmania, Victoria, New South Wales and South Australia and low in Western Australia and South Queensland. Future projections suggest a decrease in suitable habitats, confined to Tasmania, Victoria, Adelaide and New South Wales. These findings can guide the development of long-term management and conservation plans for C. maenas, with an emphasis on areas with consistent habitat suitability. Additionally, further field and laboratory experiments are recommended to understand the species’ tolerance to environmental changes. Recognising the ecological impact of C. maenas is crucial for informed decision-making to preserve Australia's ecosystems and ensure the sustainable coexistence of native species amidst invasive threats.

Stuck in the mangrove mud: The risk of trace element exposure to shore crabs in restored urban mangroves

The restoration of mangroves in urban environments can increase the risk of contaminant exposure and subsequent health effects to resident biota, yet this risk is rarely considered in mangrove restoration programs. Here we assessed the influence of sediment chemistry on contaminant bioaccumulation in shore crabs from restored and natural mangroves in urban environments compared to a reference site. The concentrations of some trace elements were several-fold higher in the sediment and crab tissues of the urban restored site compared to the natural reference site (Cd = 6×, Co = 7×, Cr = 4×, Mn = 30×, and Ni = 18× greater in sediments, while Cd = 4×, Co = 2×, Cr = 2×, Mn = 6×, and Ni = 3× greater in crab tissues). NMR-based metabolomics on crabs revealed higher abundances of proline and glutamate at urban sites, which may be indicative of physiological stress from trace element contamination. Choice experiments were used to test habitat selectivity by crabs from each population, and showed that crabs avoided sediments from the contaminated urban sites. Our results suggest that restoring mangroves in contaminated environments could create ecological sinks, where animals take residence in the new habitat but are exposed to sediment-based contaminants, with potential implications for organism and population health.

Habitat-mediated direct and indirect interactions in a marine sedimentary system from Atlantic Canada

Trophic cascades focusing on direct (consumptive) effects have been well studied in marine ecosystems. However, less attention has been given to cascades involving indirect interactions embedded in distinct habitats. We focused on the interactions between a non-indigenous predator (the green crab Carcinus maenas), a consumer (the mud crab Panopeus herbstii), and a prey species (juvenile eastern oysters Crassostrea virginica). These interactions were studied in 3 small yet distinct habitats of increasing complexity: bare sediments, patches of blue mussels Mytilus edulis (BM), and a unique habitat consisting of giant Irish moss Chondrus crispus and mussels combined (IMBM). In the field, green crab predation rates on mud crabs were estimated in each of the abovementioned habitats using tethering experiments. The results showed that green crab foraging was most effective in bare sediments and least effective in IMBM, i.e. the least and most complex habitats, respectively. Trials conducted in the laboratory with mud crabs foraging over oysters showed a similar outcome: oyster mortality rates declined with increased habitat complexity. However, when trials were conducted in the presence of a green crab, this pattern reversed, and oyster mortality was lowest in bare sediments. Mud crab behavior was consistent with these results: in the presence of a green crab, mud crabs were less active and spent more time sheltering, whereas in its absence, the opposite pattern was observed, especially in bare sediments. These behaviorally driven indirect interactions are dependent on the presence of a non-indigenous predator and mediated by the type of habitat.

Neuromodulator-induced temperature robustness in a motor pattern: a comparative study between two decapod crustaceans.

While temperature fluctuations pose significant challenges to the nervous system, many vital neuronal systems in poikilothermic animals function over a broad temperature range. Using the gastric mill pattern generator in the Jonah crab, we previously demonstrated that temperature-induced increases in leak conductance disrupt neuronal function and that neuropeptide modulation provides thermal protection. Here, we show that neuropeptide modulation also increases temperature robustness in Dungeness and green crabs. As in Jonah crabs, higher temperatures increased leak conductance in both species' pattern-generating lateral gastric neuron and terminated rhythmic gastric mill activity. Likewise, increasing descending modulatory projection neuron activity or neuropeptide transmitter application rescued rhythms at elevated temperatures. However, decreasing input resistance using dynamic clamp only restored the rhythm in half of the experiments. Thus, neuropeptide modulation increased temperature robustness in both species, demonstrating that neuropeptide-mediated temperature compensation is not limited to one species, although the underlying cellular compensation mechanisms may be distinct.

Multiple endpoints analysis of the effects of diesel oil on a commercial species, Carcinus maenas

Fuel spills in marine environments pose significant threats to aquatic ecosystems, evidencing the intricate relationship between fuel utilization and its impact on benthic species of commercial value for human consumption. This interconnectedness of human, animal and environmental welfare falls within the One Health framework. The aim of the present study was to evaluate the toxicological effects of diesel oil on the green crab Carcinus maenas, and make a parallelism between tested concentrations and petrogenic hydrocarbon levels in natural environments. Mortality, locomotion and feeding behavior, molting, somatic growth, morphological malformations, stress biomarkers, and nutritional variables were analyzed in three different bioassays. In Bioassay 1, prepuberal females were exposed to diesel oil water accommodated fraction (WAF) to determine the median lethal concentration (LC50) at different periods. In Bioassay 2, prepuberal females were exposed to 168 h LC50 and LC25 of diesel oil WAF for 7 days, and were subsequently exposed to clean water. In Bioassay 3, prepuberal females were exposed to 168 h LC12 and LC6 of diesel oil WAF for 30 days. Petrogenic hydrocarbon levels in the field were quantified at a port and a nature reserve, with concentrations of aromatic hydrocarbons being 1.92 μg/g in the former and below 0.01 μg/g in the latter. In Bioassay 1, the 168 h LC50 was estimated to be 1.04 % of diesel oil. The results obtained in Bioassays 2 (LC50 and LC25) and Bioassays 3 (LC12 and LC6) suggest that environmental exposure to petrogenic hydrocarbons produces high mortality or interferes with the molting process of crabs, leading to reduced growth and developmental abnormalities. Such malformations were observed in chelipeds, pereiopods, gills chambers and eye peduncles, and affected feeding and locomotion behaviors. Overall, this could impact on population size and health, and consequently alter the ecological role and commercial exploitation of economically important species like C. maenas.

Predation scars provide a new method to distinguish native and invasive crab predation on mollusc prey.

Crab species are increasingly important socioeconomic resources that are threatened by human exploitation, climate change, and invasive species, such as European green crabs (Carcinus maenas). However, the continued health of their populations is often uncertain given the limited long-term population data, necessitating alternate approaches to ensure their continued viability. Furthermore, C. maenas are one of the most highly invasive and destructive marine species globally, posing a threat to local ecosystems and species, including socioeconomically important crabs and their mollusc prey. Improved understanding of C. maenas invasions and their impacts on local crab and mollusc resources is therefore vitally important. Here, we present a new method for identifying species-level presence and relative abundances of important crab species, including invasive C. maenas, from the scars they leave on their prey. We conducted controlled manipulative feeding experiments in which individuals of Dungeness crabs (Metacarcinus magister), red rock crabs (Cancer productus), and C. maenas, were allowed to attack snails (Tegula funebralis) and produce sublethal shell damage. Resulting shell damage was photographed and landmarked for geometric morphometric analyses to determine any differences in the shape of shell damage between crab species. There were statistically significant differences between the shape of shell damage created by all three crab species (p < .0001). Shell damage formed a gradient from narrow/deep (C. productus) to shallow/wide (C. maenas) with M. magister as an intermediate form. Our method provides a novel, cost-effective tool for long-term species-specific reconstructions of crab populations and assessing the broader ecological impacts of C. maenas invasions that can inform management and mitigation for these three important crab species.

Size at the onset of morphological sexual maturity, sexual size dimorphism and relative growth of Petrolisthes armatus (Decapoda, Porcellanidae) from Northeastern Brazil

ABSTRACT Our study investigated the size at the onset of morphological sexual maturity, sexual size dimorphism, and relative growth in the green porcelain crab Petrolisthes armatus from a population in Northeastern Brazil. Specimens were collected monthly between February 2011 and January 2012 from the Baía do Pontal, Ilhéus, Bahia. We analysed 576 specimens, with females reaching the size at the onset of morphological sexual maturity at a carapace width of 4.7 mm and males at a carapace width of 5.83 mm. Sexual size dimorphism was confirmed, with females exhibiting a broader pleon and males a wider carapace and larger chelipeds. Positive allometry was observed in cheliped growth, with males exhibiting variation among juveniles and adults. Females showed ontogenetic distinctions in pleon width and cheliped dimensions. Our findings reveal that females and males of P. armatus exhibit different energetic investments in the development of secondary sexual characteristics, especially in cheliped growth for males and pleon enlargement for females. These results from a population of P. armatus in Northeastern Brazil contribute to a deeper understanding of reproductive strategies in females and males of the species.

Vertical arrays of artificial rockpools on a seawall provide refugia across tidal levels for intertidal species in the UK

Eco-engineering of coastal infrastructure aims to address the insufficient intertidal habitat provided by coastal development and flood defence. There are numerous ways to enhance coastal infrastructure with habitat features, but a common method involves retrofitting artificial rockpools. Often these are ‘bolt-on’ units that are fixed to existing coastal infrastructure but there is a paucity of literature on how to optimise their arrangement for biodiversity.In this study, 24 artificial rockpools were installed at three levels between High Water Neaps and Mean Tide Level on a vertical concrete seawall on the south coast of the UK. The species abundance of the rockpools and adjacent seawall were surveyed at low tide for 2 years following rockpool installation and compared. Over the course of the study, sediment had begun to accumulate in some of the rockpools. At the 2-year mark, the sediment was removed and assessed for macrofauna. Algal biomass of the seawall and rockpools was estimated using previously obtained dry weight values for the dominant algae taxa.After 2 years, it was determined that artificial rockpools successfully increase species richness of seawalls, particularly at higher tidal levels where water-retaining refugia are crucial for many species. The rockpools hosted 37 sessile taxa and 9 sessile taxa were recorded on the seawall. Rockpools increased the vertical elevation for brown canopy-forming seaweeds by providing better attachment surfaces. Although the retained sediment only hosted 3 infaunal species, it was observed to provide shelter for shore crabs during surveys. As sea levels and ocean and air temperatures continue to rise, vertical eco-engineering arrangements will play a crucial role in allowing species to migrate up the tidal zone, negating habitat loss and localised extinction.

Thermal performance of native and invasive crab species: investigating the invasion potential of Hemigrapsus takanoi in southern European Carcinus maenas’ habitats

Invasive species’ plasticity and latitudinal spread have become an increasing matter of concern in rapidly changing climate. The brush-clawed shore crab (Hemigrapsus takanoi), native along northwestern Pacific shores, has established dense populations in northern European coasts. Detrimental effects in natura have been reported, such as displacement of native species and competition for shelter, rasing concerns regarding shellfish early life stages and food webs-posing a threat to wild stocks’ recruitment and aquaculture. The species has not yet been observed in southern European countries (e.g., Portugal), where niche habitats such as harbours, mudflats, and lagoons reach higher temperatures than the invaded northern range. The aims of this study were to determine H. takanoi potential to acclimate to a southern thermal range, assess thermal performance, and identify potential competitive advantages over the native shore crab Carcinus maenas. This was accomplished through individual exposure to one of three treatments, resembling a southern European thermal range (17 °C, 21 °C, or 25 °C), for 30 days. H. takanoi thrived in temperatures as high as 25 °C, a realistic thermal scenario in Óbidos Lagoon (the biggest Portuguese lagoon)—while the native shore crab C. maenas underperformed at that treatment, showing lower survival, growth, and feed intake than H. takanoi. These results suggest that H. takanoi may be able to colonise southern and warmer European locations and have higher plasticity than the native species already inhabiting areas close to its upper thermal limits. Under global warming scenarios, inferences of further latitudinal spread and increased invaded range may also be taken to the invaded northern European habitats.

External Radiation Dose to the Shore Crab of Chinnavilai, a High Background Natural Radiation Area

The aim of the study was to determine the activity concentration and radiation dose of radionuclides 226Ra, 228Ra &amp; 40K in shore crab Hemigrapsus Sanguineus, in Chinnavilai, Kayakumari district, Tamil Nadu, India. Gamma spectrometry is used for determine activity concentration and radiation dose of radionuclides (226Ra, 228Ra &amp; 40K.). The total dose values due to both internal and external contributions by all three studied radionuclides fell between 26.51 and 53.19 µGy/day. These values are far below the dose limits (10 mGy/day) prescribed for aquatic animals by the US Department of Energy (2002) and hence the animals are safe, radiologically as far as these radionuclides are concerned. The minimum internal dose for 228Ra in shore crab 0.30 and the external dose 16.42. The total dose for 228Ra in shore crab 16.72. The maximum internal dose for 228Ra in shore crab 0.64 and the external dose 30.03. The total dose for 228Ra in shore crab 30.67.

Holistic assessment of dimethoate toxicity in Carcinus aestuarii's muscle tissues.

Dimethoate (DMT) is one of the most harmful and commonly used organophosphate pesticides in agricultural lands to control different groups of parasitic insects. However, this pesticide is considered a dangerous pollutant for aquatic organisms following its infiltration in coastal ecosystems through leaching. Yet, our investigation aimed to gain new insights into the toxicity mechanism of DMT in the muscles of the green crab Carcinus aestuarii, regarding oxidative stress, neurotransmission impairment, histological aspects, and changes in lipid composition, assessed for the first time on the green crab's muscle. Specimens of C. aestuarii were exposed to 50, 100, and 200µg DMTL-1 for 24h. Compared to the negative control group, the higher the DMT concentration, the lower the saturated fatty acids (SFA), and the higher the monounsaturated fatty acids (MUFA). The significant increase in polyunsaturated fatty acid n-6 (PUFA n-6) was related to the high release, mainly, of linoleic acid (LA, C18: 2n6) and arachidonic acid (ARA, C20: 4n6) levels. Biochemical biomarkers showed that DMT exposure promoted oxidative stress, highlighted by increased levels of hydrogen peroxide (H2O2), malondialdehyde (MDA), advanced oxidation protein product levels (AOPP), and protein carbonyl (PCO). Furthermore, the antioxidant defense system was activated, as demonstrated by the significant changes in the enzymatic activity of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and reduced glutathione (GSH) levels associated with an adaptation process of C. aestuarii to cope with the DMT exposure. This pesticide significantly impairs the neurotransmission process, as evidenced by the inhibition of acetylcholinesterase (AChE) activity. Finally, several histopathological changes were revealed in DMT-treated crabs, including vacuolation, and muscle bundle loss.This research offered new insights into the toxic mechanism of DMT, pointing to the usefulness of fatty acid (FA) composition as a sensitive biomarker in littoral crabs.

Variable Effects of Experimental Sea-level Rise Conditions and Invasive Species on California Cordgrass

Sea-level rise (SLR) will produce unprecedented changes in tidal marsh systems that already cope with daily tidal fluctuation, disturbances from storms, and salinity changes from droughts and runoff events. Additionally, negative impacts from non-native invasive species may alter marsh plants’ ability to respond to SLR stressors like increased inundation and salinity. Increasingly, tidal marsh communities must tolerate both changes in the physical environment from SLR and increased risk of invasion by non-native species. To assess the response of a threatened tidal marsh cordgrass (Spartina foliosa) to both stressors, we implemented a field experiment in San Francisco Bay, CA, USA, exposing cordgrass to a treatment that extended tidal inundation projected with SLR using a recently developed in situ method. At one of two field sites, we also enclosed the cordgrass with or without the invasive European green crab, Carcinus maenas. We found that cordgrass responded negatively to longer inundation, although these effects varied by site and year. In higher inundation treatments, cordgrass survival increased with increasing surface elevation of the plot. Cordgrass survival was lower in the presence of invasive green crabs relative to controls. We did not find interacting effects of responses to increased inundation and invasive species presence, which highlights the need to consider how latent or sequential effects of multiple stressors may affect ecosystems. This study demonstrates significant biological responses to invasive species presence and increased inundation. Evaluating relative effects and timing of multiple stressors, especially those induced by climate change and invasive species, will help us to manage threatened ecological communities in a changing world.

A new genus for the small shore crab previously known as Brachynotus spinosus (H. Milne Edwards, 1853) (Crustacea: Brachyura: Varunidae), with two new species from southern Australia and Samoa

The Australian southern temperate shore crab known as “Brachynotus spinosus” is found to be an endemic new species. The true B. spinosus (H. Milne Edwards, 1852), described from the Solomon Islands, is considered to be known only from its type locality in tropical Melanesia. A record identified as this species from Samoa in western Polynesia in 1891, is also here described as a new species. The temperate and two tropical sister-species are closely similar morphologically, and are placed into a new genus, Parvonotus. Parvonotus gen. nov. is separated from species of Brachynotus De Haan, 1833, and Hemigrapsus Dana, 1852, based on carapace and male pleon characters. Parvonotus spinosus comb. nov., P. samoensis sp. nov. and P. meridionalis sp. nov. can be separated on differences in carapace shape, leg proportions, and in the case of the latter two species, male pleon shape.