Carapace Shape Research Articles

Shape variation in the carapace of Chinese mitten crabs (Eriocheir sinensis H. Milne Edwards, 1853) in Yangcheng Lake during the year-long culture period

To evaluate whether ontogenetic development over the grow-out culture period can cause geographical plasticity of carapace shape in a certain region, geometric morphometric analysis was used, in this study, to determine the ontogenetic changes in the carapace morphology of Chinese mitten crabs (Eriocheir sinensis) originating from Yangcheng Lake, China, during a year of growth from coin-sized juveniles to market-sized adults. The morphological differences in the carapace throughout the year of culture were characterized using a 35-landmark point system. We used principal component analysis and linear discriminant analysis to determine morphological variation using thin-plate spline analysis and mesh deformation. During the growth process, the changes in the carapace were mainly concentrated in the third and fourth anterolateral teeth and on the M-shaped pattern. During the growth of male and female crabs throughout the year, the shape of the carapace changed considerably over the first six months. Afterwards, the shape of the carapace began to stabilize and could not be differentiated through discriminant analysis. This study is the first to use geometric morphometrics to analyze the ontogenetic changes in the carapace shape of E. sinensis crabs native to Yangcheng Lake. The results demonstrate that it takes a long time for the carapace morphology of native crabs to stabilize.

Confirmed feasibility of a satellite tracker attachment method on small juvenile hawksbill turtles Eretmochelys imbricata

Satellite trackers can be used for studying sea turtle movement, illuminating their migrations and behaviours. However, many studies have focused on adult turtles, with uneven species representation, despite the importance of understanding movement and habitat use of turtles at all life-stages. Furthermore, few publications detail successful satellite tracker attachment methods, particularly for juveniles. Smaller-sized juvenile sea turtles often have an irregularly shaped carapace and are fast growing, rendering the attachment of rigid trackers in a safe and durable manner challenging. Juvenile hawksbill turtles’ specific carapace shape and imbricated scute arrangement further complicate satellite tracker attachment compared to juveniles of other turtle species. This study’s objective was to confirm the feasibility of an attachment method that would allow small-sized juvenile hawksbill turtles (~267-345 mm curved carapace length) to continue growing, without tracker loss or damage to underlying scutes. Replica trackers were made of resin (simulating Wildlife Computer Spot-387 trackers), and attached with epoxy, silicone and neoprene, using a technique modified from those used on neonate loggerheads and Kemp’s ridleys. Throughout the study (3.5 mo), replica trackers remained attached, the turtles grew up to 114% heavier and 25% longer, and all turtles appeared clinically healthy and active. Furthermore, all scutes were undamaged after tracker removal. As a critically endangered species, the paucity of data on hawksbill turtles, and specifically juveniles, can hinder evidence-based management decision-making. The improved ability to satellite track juvenile hawksbills can, therefore, help better our understanding of their ecology and inform management and conservation practices for this species.

A New Species of Amazon Freshwater Toad-Headed Turtle in the Genus Mesoclemmys (Testudines: Pleurodira: Chelidae) from Brazil

We describe a new extant species of freshwater turtle from Brazil: Mesoclemmys sp. nov., a medium-sized toad-headed turtle (mean carapace length = 189.5 ± 25.8 mm), which to date has only been found in the Brazilian Amazon Basin, western Pará State, Central Amazon, and the lower Amazonas River Basin. It is easily distinguished from its sympatric congeners by differences in body size, head size, carapace shape, and coloration. The head is triangular and completely black, with large red eyes positioned anteriorly and a pair of long, light yellow barbels positioned anteriorly close the mandible. The carapace is oval and colored dark reddish brown. The plastron is black in the central region and burnt-yellow along the edges and ventral aspects of the marginal scutes and bridge, with dark seams. The plastral formula is 3-5-7-4-1-6-2. The new species inhabits temporary rainwater ponds under closed-canopy rainforest and has a known geographic range of 2183 km2. Sequences of fragment mitochondrial DNA (cytochrome c oxidase subunit I and 16S) of the new species were determined and compared with the available GenBank sequences. Our phylogenetic results indicate that the specimens represent a divergent taxon within Mesoclemmys and a robust analysis of morphological data corroborates the existence of a new distinct species of Mesoclemmys, described herein.

Does the transport of larvae throughout the south Atlantic support the genetic and morphometric diversity of the Sally Lightfoot Crabs Grapsus grapsus (Linnaeus, 1758) and Grapsus adscensionis (Osbeck, 1765) (Decapoda: Grapsidae) among the oceanic islands?

Grapsus grapsus and Grapsus adscensionis are supralittoral crabs that are known to inhabit oceanic islands and depend on surface currents to recruit in the rocky shores. The ornamentation of the cephalothorax is very distinct among species, but morphological differences are controversial, and integrative studies with different approaches are needed. This study investigated the genetic variation among the populations of G. grapsus from Saint Peter and Saint Paul Archipelago (SP), Fernando de Noronha (FN), Rocas Atoll (RA) and Trindade Island (TR) in the western Atlantic, and G. adscensionis from the islands of Ascension (AI) and Saint Helena (SH) in the mid-Atlantic. Morphology was assessed by geometric morphometric analyses of the carapace and chelae, and numerical analyses of tubercles in the frontal plate of SP, FN, RA, TR and AI populations. In addition, dispersal of the larvae in the Atlantic Ocean was simulated performing a Lagrangian analysis using HYCOM reanalysis dataset as the ocean surface velocity field. The data obtained for the mitochondrial D-loop gene confirmed the distinctness of the two putative species and demonstrated the connectivity between the populations of G. grapsus from the three equatorial islands. The TR population presented unique haplotypes, as well as AI and SH. The geometric morphometric analyses showed differentiation between the carapace shapes for G. grapsus and G. adscensionis, however, the chelae shape does not allow to distinguish between the species or the population. The morphometric and molecular results were consistent with the pattern of particles dispersion in the Atlantic ocean. The larvae of SP, FN and RA mix after two months of drift, while the larvae of TR, AI, SH circulate only around the respective islands. The results reinforce the validation of the two species and the isolation of populations of G. grapsus in TR and of G. adscensionis in AI and SH. The populations of these islands might be maintained by self-recruitment, through larval behavior associated with the local current system, and therefore should be the target of conservation measures.

Body shape variations help to diminish taxonomy uncertainty in juvenile swimming crab Callinectes Stimpson, 1860

The American continent harbors thirteen species of the genus Callinectes, eight of which are distributed in Atlantic South America. This genus has three identification keys for adult individuals only, proposed by Williams and Melo. The first one uses sexual characters to distinguish the species, which prevents the identification of juveniles due to the presence of a sealed abdomen. The other two are similar, both use the form and arrangement of the carapace teeth and have never been systematically tested on juveniles. Therefore, no formal identification keys have been proved to be useful for identify juvenile Callinectes based on any characters. Consequently, there is a lack of baseline studies in taxonomic variation regarding the differences between individuals of this genus. In this study, we investigated the suitability of identification keys, developed for adult individuals of the genus Callinectes, for juvenile individuals. Therefore, we evaluated the degree of dissimilarity of the body shape using the 2D landmark geometric morphometric technique. We compared the results of intra- and interspecific body shape with the key descriptions proposed by Williams and Melo for adults. This study included 537 individuals (juveniles and adults) and is the first effort to propose diagnosis for juvenile Callinectes, demonstrating that the carapace shape is a variable structure with potential to identify juveniles of these species. The results of mean body shape showed the possibility of extending the identification keys developed for adults to earlier ontogenetic classes (juveniles) when analyzed in conjunction with more distinguishable morphological characters tested in this study.

The megalopal stage of the hydrothermal vent crab Austinograea rodriguezensis Tsuchida amp; Hashimoto, 2002 (Decapoda: Bythograeidae): a morphological description based on CLSM images.

The Bythograeidae is unique amongst brachyuran crab taxa as it is the only family where all 6 genera and 16 species are endemic to hydrothermal vents. During the research conducted by German Federal Institute for Geosciences and Natural Resources to identify inactive polymetallic sulphide deposits along Central and Southeast Indian Ridges, the INDEX project collected from hydrothermal vent fields 6 Bythograeidae megalopae. Entire specimens and dissected appendages were stained, mounted on slides and examined using Light Microscopy and Confocal Laser Scanning Microscopy. Additional molecular analysis using mtCOI confirmed the identification of the specimens as Austinograea rodriguezensis Tsuchida Hashimoto, 2002. The megalopal stage of A. rodriguezensis shows similarities and distinctions between the characters of two other bythograeid megalopae, Bythograea thermydron Williams, 1980, and B. microps Saint Laurent, 1984. Unlike other brachyuran megalopae, B. thermydron and A. rodriguezensis lack long serrate setae and stout serrate spines on the dactylus of the fifth pereiopod. In both species no coxal spines on the pereiopods were observed. The elliptical carapace of B. thermydron is broader than long vs longitudinally rectangular in A. rodriguezensis. This carapace shape resembles B. microps more than B. thermydron, however, the dorsal surface of B. microps carapace is densely covered in short setae vs only covered with short setae on the anterior 1/4 and the posterior 1/6 length of carapace in A. rodriguezensis. Furthermore, the amount of plumose natatory setae on the pleopods in B. microps is in total larger and more variable, than in A. rodriguezensis. Bythograeid megalopae seem quite generalized and miss specific features that reveal them distinctively as endemic vent crab. A distinction from other species is possible by observing the unique combinations of certain characters. This is the first megalopal stage description of Austinograea and the fourth within the Bythograeidae.

A giant nektobenthic radiodont from the Burgess Shale and the significance of hurdiid carapace diversity.

Radiodonts, stem-group euarthropods that evolved during the Cambrian explosion, were among the largest and most diversified lower palaeozoic predators. These animals were widespread geographically, occupying a variety of ecological niches, from benthic foragers to nektonic suspension feeders and apex predators. Here, we describe the largest Cambrian hurdiid radiodont known so far, Titanokorys gainesi, gen. et sp. nov., from the Burgess Shale (Marble Canyon, Kootenay National Park, British Columbia). Estimated to reach half a metre in length, this new species bears a very large ovoid-shaped central carapace with distinct short posterolateral processes and an anterior spine. Geometric morphometric analyses highlight the high diversity of carapace shapes in hurdiids and show that Titanokorys bridges a morphological gap between forms with long and short carapaces. Carapace shape, however, is prone to homoplasy and shows no consistent relationship with trophic ecology, as demonstrated by new data, including a reappraisal of the poorly known Pahvantia. Despite distinct carapaces, Titanokorys shares similar rake-like appendages for sediment-sifting with Cambroraster, a smaller but much more abundant sympatric hurdiid from the Burgess Shale. The co-occurrence of these two species on the same bedding planes highlights potential competition for benthic resources and the high diversity of large predators sustained by Cambrian communities.

Vertically migrating Isoxys and the early Cambrian biological pump.

The biological pump is crucial for transporting nutrients fixed by surface-dwelling primary producers to demersal animal communities. Indeed, the establishment of an efficient biological pump was likely a key factor enabling the diversification of animals over 500 Myr ago during the Cambrian explosion. The modern biological pump operates through two main vectors: the passive sinking of aggregates of organic matter, and the active vertical migration of animals. The coevolution of eukaryotes and sinking aggregates is well understood for the Proterozoic and Cambrian; however, little attention has been paid to the establishment of the vertical migration of animals. Here we investigate the morphological variation and hydrodynamic performance of the Cambrian euarthropod Isoxys. We combine elliptical Fourier analysis of carapace shape with computational fluid dynamics simulations to demonstrate that Isoxys species likely occupied a variety of niches in Cambrian oceans, including vertical migrants, providing the first quantitative evidence that some Cambrian animals were adapted for vertical movement in the water column. Vertical migration was one of several early Cambrian metazoan innovations that led to the biological pump taking on a modern-style architecture over 500 Myr ago.

How to become a crab: Phenotypic constraints on a recurring body plan.

A fundamental question in biology is whether phenotypes can be predicted by ecological or genomic rules. At least five cases of convergent evolution of the crab-like body plan (with a wide and flattened shape, and a bent abdomen) are known in decapod crustaceans, and have, for over 140 years, been known as "carcinization." The repeated loss of this body plan has been identified as "decarcinization." In reviewing the field, we offer phylogenetic strategies to include poorly known groups, and direct evidence from fossils, that will resolve the history of crab evolution and the degree of phenotypic variation within crabs. Proposed ecological advantages of the crab body are summarized into a hypothesis of phenotypic integration suggesting correlated evolution of the carapace shape and abdomen. Our premise provides fertile ground for future studies of the genomic and developmental basis, and the predictability, of the crab-like body form.

Crab carapace shape as a biomarker of salt marsh metals pollution

Coastal environmental pollution is a global problem that has been growing for decades. For this reason, different approaches have been sought to address and detect its environmental implications. Some organisms are considered bioindicators or biomonitors of contamination, which provide information about environmental quality. Previous studies used the crab Neoelice granulata (Brachyura, Varunidae) as bioindicator of the presence of metals by the analysis of soft tissues to evaluate physiological and molecular markers. However, the contaminant levels accumulated in these soft tissues have shown to be affected by seasonal variations, suggesting that environmental assessments based on soft tissue samples may be unreliable. Within this framework, we aimed to describe the crab body (carapace) variations related to a known soil metal gradient in a Patagonian salt marsh and to evaluate the use of the body shape as an alternative biomarker for monitoring the quality of salt marsh systems. We studied the carapace shape variations using geometric morphometrics (GM) based on a 2D structure with object symmetry. We observed symmetric and asymmetric components of carapace shape variation. While the latter was not found associated with a gradient of contamination by metals, the symmetric component responded to environmental changes; therefore, it could be considered a stress biomarker related to metal contamination. Consequently, we recommended using GM analysis because it is inexpensive, faster and non-seasonal and could be used on living organisms, avoiding destroying individuals to measure the environmental stress.

DNA barcodes combined with geometric morphometry challenge species hypothesis in palaemonid shrimp

Although the Mediterranean Region is known as a hotspot for biodiversity and endemism its freshwater fauna is still greatly unexplored, and even the emblematic taxa such as decapods require in-depth integrative investigation. In our research we used integrative approach composed of various geometric morphometric and molecular methods to challenge the taxonomic status of two freshwater shrimps representing Palaemonidae: Palaemon antennarius and Palaemon minos. Basing on 352 COI sequences, three Molecular Operational Taxonomic Units (MOTUs) were defined. Two of them belonged to P. antennarius: first inhabiting Apennine Peninsula and Sicily, the second one from the Balkan Peninsula. The third MOTU corresponded to Palaemon minos from Crete. The Balkan MOTU of P. antennarius was closer to P. minos in terms of genetics, than to the other conspecific MOTU. The carapace shape variation, studied on 180 individuals, was mainly explained by the geographic distribution. Balkan and Cretan groups were clearly distinguished, while other samples were distributed along the shape gradient from Sicily and southern Apennine Peninsula to the Balkans. The results of our study showed that, either the MOTU assigned to the Apennine Peninsula and Sicily constitutes a separate species or, alternatively, P. minos should be synonymised with P. antennarius.

Tritodynamia bengalensis n. sp., a new species of brachyuran crab from West Bengal state, India (Decapoda: Brachyura: Macrophthalmidae).

Tritodynamia bengalensis n. sp. is described on the basis of a male specimen dredged from the shallow coastal waters of West Bengal state, India. The new species is morphologically similar to T. bidentata Yang Tang, 2005 and T. serratipes Anker Ng, 2014, but can be distinguished by the carapace shape, dentition of the pollex, relative length of the dactylus of the third maxilliped, proportions of the propodus of the third pereopod, and structure of the male left gonopod. This is the first record of the genus Tritodynamia from India.

Influence of nest temperature on morphology of Leatherback Turtle (Dermochelys coriacea) hatchlings incubated in hatcheries in Oaxaca, Mexico

The influence of nest incubation temperatures on carapace shape and morphological traits of Leatherback Turtle (Dermochelys coriacea (Vandelli, 1761)) hatchlings incubated in two hatcheries in Oaxaca, Mexico, was evaluated. This study was carried out from October 2016 through May 2017. On each beach, there are community groups consisting of volunteers not affiliated with universities who protect and relocate the nests to increase hatching success. In each translocated nest, a data logger was placed in the centre of the egg mass. Hatchlings were collected as they emerged from each nest. The carapaces of the hatchlings were photographed and subjected to geometric morphometric analysis; also, hatchlings were weighed and their bodies measured. The mean temperature of 12 nests in each hatchery was recorded, with no significant differences between hatcheries. The principal component analysis revealed an overlapping of the carapace shape under different temperature durations. Temperature had a significant influence on hatchling morphology. Higher mean incubation temperatures produced hatchlings with low mass, smaller appendages, narrower carapace widths, and shorter flipper lengths. Lower mean incubation temperatures produced hatchlings with greater mass, wider appendage widths, wider carapace widths, and longer flipper lengths. Results indicate that the Leatherback hatchlings incubated in hatcheries demonstrate morphology that varies in relation to nest incubation temperature in a similar way to hatchlings produced in natural environments.

A Different Perspective on Sex Dimorphism in the Adult Hermann's Tortoise: Geometric Morphometry.

Eastern Hermann's tortoise (Testudo hermanni boettgeri) is a subspecies of Hermann's tortoise (Testudo hermanni) found in Albania. Gender determination is one of the crucial elements in determining the population dynamics in all species. Female and male adult tortoises look different, but these differences can be difficult to distinguish in captivity or when their sex indicators are deformed or injured. Therefore, the aim of this study was to use indirect and non-invasive methods such as geometric morphometric analysis to determine the sex of adult tortoises. For the geometric morphometry, 17 female and 23 male Hermann's tortoises were collected and photographed from the hills and mountains around the Tirana district of Albania between August and October 2019. Sexes were discriminated based on geometric morphometry, and plastron shape was a better indicator than carapace shape. In addition, abdominal, femoral and anal scutes on the plastron and the ratio of femoral to pectoral suture lengths were important for the sex distinction. Females had a larger plastron than males; this may have been supported by fecundity selection, because a large plastron suggests more volume in which to store eggs. The femoral and anal scutes were larger in male tortoises, and serve as a stronger base during mating. This study was conducted for adults only, and future studies are needed to determine if these indicators also apply to hatchlings and juveniles.

Morphological and Genetic Variation Among Populations of the Fiddler Crab Minuca burgersi (Holthuis, 1967) (Crustacea: Brachyura: Ocypodidae) from Shores of the Caribbean Basin and Western South Atlantic Ocean.

For this study, in addition to museum vouchers, 1437 specimens of Minuca burgersi (Holthuis, 1967) were collected from crab colonies at 105 locations in the western Atlantic Ocean to examine diversity in a species with a large geographic range. Both allometric and geometric morphometry were coupled with the molecular analysis of DNA to give a broader perspective of intraspecific variability in this species. A total of 1153 specimens from the Caribbean Sea and the Atlantic coast of South America demonstrated that M. burgersi from both regions are very similar in their pattern of growth. The average carapace width (CW) for Caribbean is larger than the average for South American males and females. However, size distribution based on CW is unimodal in Caribbean and bimodal in South American populations. The carapace length-width ratio is about 0.68 in females and 0.66 in males. South American males express asymmetric elongation of the cheliped in smaller CW intervals than Caribbean males. In a sample of 259 females, carapace shape is distinct between South American and Caribbean populations. Caribbean populations have less swelling in the branchial regions than South American populations. The swelling correlates primarily with geographic region and to a lesser degree with substrate and salinity. Molecular data from the 16S rDNA and cytochrome c oxidase subunit I (COI) reveal three clades within Minuca burgersi. Two clades are distributed in the Caribbean and the third in eastern South America. The timing of divergence between Caribbean and South American clades is coincident with an increased rate of water and sediment outflow from the Amazon as inferred from the geologic record. Current patterns and associated gene flow within the Caribbean were subsequently influenced by the closing of the Isthmus of Panama. We speculate that various populations may employ different larval dispersion mechanisms resulting in genetic heterogeneity. Consequently, there is considerable biological divergence among populations of M. burgersi in the Caribbean and South America.

Preliminary Study of Temperature Effects on Size and Shape in the Modern Spinicaudatan Eulimnadia texana (Crustacea: Branchiopoda).

Studies of temperature effects on morphology in Spinicaudata have focused on length, with no data on shape. To fill this gap, size and shape variability in response to temperature fluctuations was investigated by rearing the modern spinicaudatan Eulimnadia texana. Two days after hydration, juvenile individuals were separated into four different temperature treatments: 20°C, 23°C, 26°C, and 29°C. Hermaphrodite size and shape were analysed by looking at linear combinations of size variables and using Fourier shape analysis; methods that are also used to describe fossil size and shape for better comparison. Size differences were considerable, with reduced growth at low and high temperatures and accelerated growth at the optimum temperature of 26°C, revealing that the reaction of size to increasing temperature is non-linear. The height of the dorsal margin, which is associated with space for egg production in Eulimnadia texana, accounts for a high amount of size variability in this species and, presumably, in most of the Limnadiidae. Hermaphrodite shapes reared under temperatures of 20°C and 29°C are statistically distinct, while intermediate temperatures yield intermediate shapes. The rate of shape change along temperature is comparatively low between 23°C and 26°C and accelerated at lower and higher temperatures. With increasing temperature, the highest point of the dorsal margin is shifted towards the anterior of the carapace, while it assumes a median position at 20°C. Our result that temperature has strong effects on carapace size and shape implies considerable ecophenotypic variability in Spinicaudata.

An integrative approach challenges species hypotheses and provides hints for evolutionary history of two Mediterranean freshwater palaemonid shrimps (Decapoda: Caridea)

The Mediterranean Region is a biodiversity/endemism hotspot whose freshwater fauna remains largely unexplored. Our integrative study challenges the taxonomic status of two freshwater palaemonid shrimps, Palaemon antennarius and Palaemon minos. Three molecular operational taxonomic units (MOTUs) were defined based on 352 cytochrome oxidase subunit I (COI) sequences and 88 haplotypes. Two belonged to P. antennarius: one inhabiting the Apennine Peninsula and Sicily, and the other from the Balkan Peninsula. Palaemon minos was the third MOTU, found on Crete. The Balkan MOTU of P. antennarius was genetically closer to P. minos than to the other conspecific MOTU. Data from a nuclear marker (Histone 3) is congruent with such a pattern. The carapace shape variation (based on 180 individuals) was mainly explained by the geographical distribution. Balkan and Cretan groups were clearly recovered, while other samples clustered along a shape gradient from Sicily, through the Apennine Peninsula to the Balkans. Our results show that, for taxonomic consistency, the MOTU inhabiting the Balkan Peninsula should be either described as a new separate species or synonymised with P. minos. The third possible option would be treating all the populations as part of P. antennarius. Geometric morphometrics supports the first option, phylogenetic reconstructions point to the second one, yet the low genetic divergence favours the third one, illustrating that even emblematic taxa such as shrimps require an in-depth integrative approach.

Habitat, Sexual and Allometric Influences on Morphological Traits of Intertidal Crabs

Intertidal crabs display distinct morphological traits that allow differential interactions with biotic and abiotic features of the intertidal landscape, but are also influenced by allometry and sexual selection. This study aimed to explore the influence of sexual, allometric and habitat factors on morphological variation in the intertidal mangrove crab assemblage. A standardized photographic protocol was developed using readily available, low-cost technology to capture the morphology of carapaces and claws as sets of Cartesian landmarks. Digitization errors were 1 to 2 orders of magnitude smaller than the variation among individuals. In Tubuca seismella and Tubuca signata (superfamily Ocypodoidea), species that have traditionally been studied for sexual dimorphism, standardized major claw sizes were 2.8 and 3.7, respectively, times larger for males than females. Sexual dimorphism in claw size was also observed for Metopograpsus frontalis and Parasesarma longicristatum (superfamily Grapsoidea), with the largest claw in males being 15% and 33%, respectively, larger than in females. In contrast to size, claw shape did not relate to sex, except for T. seismella. Carapace shape, although variable among individuals and displaying asymmetry, was unrelated to sex. Claw and carapace shapes displayed high correlations, with values around 0.78. Carapace shapes grouped into taxonomic families and linked to habitat preferences, while claw shapes varied along a taxonomic gradient. These results complement studies on crab morphology that focus on specific factors or species, and stress the importance of multiple, interacting factors including sexual, allometric and habitat influences as drivers of morphological trait variability.

Camouflage by the masking crab,Notomithrax ursus(Herbst, 1788) (Decapoda: Brachyura: Majidae): is it a decorator or a dressmaker?

AbstractMany spider crabs attach various kinds of living material to camouflage themselves and thereby escape detection by predators. This strategy uses a suite of co-evolved characters including a pyriform carapace shape, long agile chelipeds, hooked setae and a stereotyped behaviour pattern which is used to gather and plant pieces of other organisms amongst hooked setae on their body. Notomithrax ursus (Herbst, 1788) is a New Zealand majid crab that lives amongst seaweeds and mainly masks itself with the same. In doing so, it uses all the skills that we expect of a dressmaker. Denuded of its mask, the crab chooses algae it can manipulate, measures and cuts each piece, passes them to its mouth, and then plants them on its body in a roughly regular order, such that the severed ends are inserted into the hooked setae with only intact end visible. Longer pieces are planted on the body and smaller pieces on its walking legs to make a profile resembling a clump of seaweed. Distribution of the seaweed follows the distribution of the hooked setae on the crab. The number of pieces attached is independent of crab size and, as carapace width increases, the crabs compensate by cutting out longer pieces of seaweed. Cheliped allometry explains scaling of the mask. Total length of seaweed used can range 0.6–2.6 m (mean 1.36 m). Increasing mask load is accompanied by increases in the number and size of hooked setae thereby strengthening attachment. The behaviour of N. ursus should be called “dressmaking” not “decorating.” It uses a stereotyped behaviour pattern to follow a design, dictated by the distribution of hooked setae, and produce a customized garment concealing it from predators. A hypothesis on the evolution of masking, from passive to active camouflage is proposed, based on the original ideas of C.W.S. Aurivillius.

Shape matters: relevance of carapace for brachyuran crab invaders

The increasing trend of biological invasions is one of the most concerning threats for ecosystems functioning. The identification of optimal characters determining the invasive potential of non-native species has always been a challenge in conservation studies. Morphological features can be used as a good proxy to address the invasive success in fish species, assuming that anatomical differences in comparison to native species can provide to newcomers ecological opportunities increasing their probability to become successful. Considering this, the present study constitutes the first applicative attempt of a model based on geometric morphometrics to demonstrate the importance of the carapace shape in the invasive ability in marine decapod crustaceans. The study was performed on the native brachyuran community of Alfacs Bay, in the Ebro Delta, the largest estuarine zone along the north-western Mediterranean, in which two recently established non-native crab species coexist: Dyspanopeus sayi and Callinectes sapidus. Results suggested that invaders with extreme carapace traits located peripherally in the community morphospace, such as C. sapidus, usually possess ecological advantages contributing to understand their success. Conversely, intermediate morphologies within the morphospace, such as D. sayi, imply ecological overlapping with native species reducing their community relevance. Besides, we also assessed the effect on the community morphospace of a hypothetical future invasive event from another crab species (Portunus segnis) with high probabilities to also colonize the community. Our outcomes confirm that the morphometric approach could be an alternative tool for assessing the potential ability of invasive crab species. However, further studies at different spatial and temporal scales, including additional traits and quantitative data from invasions, would be necessary to confirm the efficacy and usefulness of the methodology.