Maja Brachydactyla Research Articles

Long-term changes in the French fisheries for crustaceans in the Eastern English Channel: 1901 to 2020

Abstract Small-scale crustacean fisheries in the English Channel have existed for centuries but have never been formally assessed; hence, concerns about their ecological and socio-economic sustainability could be raised. This study explores trends in this fishery’s landings from 1901 to 2021, based on historical and recent landings data in two of the main administrative fishing divisions of the area, Cherbourg and Boulogne-sur-Mer quartiers. Crustacean fisheries were reported for both divisions from the beginning to the end of the time period considered, demonstrating that a dedicated crustacean fishery existed in the area since at least the 20th century. The species composition of the crustacean’s catch (e.g., edible crab, Cancer pagurus Linnaeus, 1758; European lobster, Homarus gammarus (Linnaeus, 1758); European spider crab, Maja brachydactyla Balss, 1922) was almost impossible to track before the 2000s. Important fluctuations were observed in catch volume reports along time that might be related to two phenomena: technological advances that would have improved the fishery performances and the decline of some previously targeted species that might have led to a transfer in fishing effort.

Biogenic reef creation and biodiversity enhancement by an offshore longline mussel farm

As the mussel farming industry moves offshore, there is a need to fill evidence gaps on its ecological impacts. Over eight years, this research presents the results of a long-term monitoring study of the UK’s first large scale offshore mussel farm following a BACI (Before-After Control-Impact) approach to evaluate the temporal and spatial effects of the farm on benthic habitat and epibenthic species assemblages. Situated in historical fishing ground, the farm is located on a degraded seabed ecosystem dominated by soft sediment. After seven years of farming, damaged seabed increased its complexity, with biogenic reef being created (30 % increase). The structure is attracting a significantly increasing diversity and abundance of sessile, sedentary and mobile species to the area compared to control sites. The abundance and diversity of commercial species increased compared to far control sites and certain commercial species were exclusively recorded beneath the mussel headlines: Cancer pagurus, Maja squinado and Necora puber. Results are discussed through a three mechanisms framework: Species recruitment, settlement, and subsidy. Results from this study are crucial to support policymakers develope a sustainable offshore industry as part of the blue economy that can both produce sustainable protein and contribute to marine conservation objectives as ‘other effective area-based conservation measures’ (OECM). This study reports how low-trophic farming can support nature restoration and recovery.

Spillover and competitive exclusion in the crustacean community following the implementation of a marine reserve

Abstract Flamanville marine protected area (MPA) located in Normandy, France, was created in 2000 to sustain the local crustacean fishery. In 1985, an annual survey targeting crustacean populations (e.g. European lobster Homarus gammarus, edible crab Cancer pagurus, and European spider crab Maja brachydactyla) that include the future MPA started. The MPA implementation effects were investigated in this study. The sampling design allowed the integration of spatial and temporal covariances to estimate the MPA effects. With respect to the initial objective, the MPA significantly improves the most economically valuable species, H. gammarus, abundance. Lobster catch per unit effort increased by 597% in the MPA, whereas outside it only increased by 156%. The MPA creation also led to an increase in lobster size inside the protected area. Furthermore, a few years after the MPA implementation, lobster catches showed a significant non-linear decline with distance from the centre of the reserve, suggesting a spillover effect. However, the edible crab catches were halved following the MPA implementation suggesting an opposite effect. Spider crab abundance seemed unaffected. Finally, the MPA implementation had no effect on edible crab and spider crab sizes. These differential responses appear to result from these species’ variable movement and competitive abilities.

Microparasite screening across four species of decapod crustaceans (Decapoda: Brachyura, Astacidea) in Ireland

ABSTRACT Decapod crustaceans and their associated fisheries are an important resource globally, and demand for seafood continues to grow alongside a growing human population. With a combined 2023 value of €36 million in Ireland, three species of brachyuran crabs, the velvet crab (Necora puberLinnaeus, 1767), edible crab (Cancer pagurusLinnaeus, 1758), spider crab (Maja squinado Herbst, 1788)), and the European lobster (Homarus gammarusLinnaeus, 1758) are decapods of commercial importance. These species are also important components of their respective ecosystems. Parasite infection can affect the health of crustaceans and may affect population productivity. Microparasites, including paramyxids (Hematodinium spp.) and microsporidians can infect decapod crustaceans and have a range of effects on their hosts. We assessed three microparasites in four crustacean species. Paramarteilia canceri was found in all three crab species, and we provide the first genetic confirmation in spider crabs. A microsporidian associated with cotton crab disease, Ameson sp., was identified in N. puber, along with a second species of microsporidian which will require further classification. None of the three parasite groups were identified in Homarus gammarus. Species of Hematodinium were not identified in any of the species studies. Our work highlights the need for parasite surveys to uncover parasite diversity and prevalence in crustaceans to help elucidate host-parasite interactions, and the potential impacts of parasites.

Gut content metabarcoding of six crustaceans provides detailed diet description and insights into their roles as predators and scavengers

While many studies have analyzed the diet of marine consumers using traditional morphology-based methods, the integral diet of many species of functionally important groups such as benthic invertebrates remains poorly resolved. This hinders our ability to precisely establish trophic links among species, which is necessary to understand food webs and ecosystem dynamics. Here, we applied DNA metabarcoding to obtain a high resolution picture of the animal fraction of the diet of six species of crustaceans in the Bay of Biscay; the edible crab Cancer pagurus, the velvet crab Necora puber, the circular crab Atelecyclus undecimdentatus, the spider crab Maja brachydactyla, the shrimp Crangon crangon, and the hermit crab Pagurus bernhardus. We used two pairs of primers targeting the 18S rDNA (generalist) and 12S rDNA (fish-specific) regions to identify prey that makes up the animal fraction of the diet. Based on occurrence data, 54 prey taxa belonging to 5 different phylum mostly represented by teleost fishes and malacostracans were identified. Through a high-level of prey detection, results indicate a scavenging strategy across all consumers mostly based on the consumption of preys likely originating from fishing discards. This study also reveals patterns of niche overlaps among consumers differentiating C. crangon from others and suggesting high overlap in animal resource use between N. puber and M. brachydactyla as well as C. pagurus. High resolution diet description revealed the complexity of trophic interactions on a local scale.

Influence of Storage Time on the DNA Integrity and Viability of Spermatozoa of the Spider Crab Maja brachydactyla.

Natural populations of the spider crab Maja brachydactyla constitute a fishery resource of great economic importance in many countries. As in the rest of eubrachyurans, the females of this species have ventral-type seminal receptacles where they store sperm from copulations. Sperm can be stored in these structures for months and even years before egg fertilisation, with the consequent degradation of the sperm cells during the time. In this work, we analyse the viability and the possible genetic damage in sperm accumulated in the seminal receptacles of M. brachydactyla females as a function of the storage time (from 0 to 14 months) using the comet assay technique. On one hand, we developed an algorithm for comet image analysis that improves the comet segmentation compared with the free software Open comet v1.3.1 (97% vs. 76% of detection). In addition, our software allows the manual modification of the contours wrongly delimited via the automatic tool. On the other hand, our data show a sharp decline in sperm viability and DNA integrity in the first four months of storage, which could lead to a decrease in the fecundity rate and/or viability of the embryos or larvae from the second and third clutches of the annual cycle if the repair capacity in these gametic cells is low.

Bycatch in northeast Atlantic lobster and crab pot fisheries (Irish Sea, Celtic Sea and Bristol Channel)

Bycatch in valuable NE Atlantic baited ‘pot’ fisheries for lobster Homarus gammarus and edible crab Cancer pagurus has not been well documented, potentially limiting evidence-based management. Using onboard observers supplemented by fishers’ ecological knowledge (FEK) we characterised bycatch of fish and larger invertebrates in 10,741 pot hauls around the coast of Wales, UK, over 4 years in all seasons. A total of 1529 fish from 30 species, and around 15 species of invertebrate, were recorded. Bycatch abundance varied seasonally and spatially and was dominated by eight common and widespread taxa comprising six ‘core’ fish (two catsharks: bullhuss (Scyliorhinus stellaris) and smallspotted catshark (S. canicula); ballan wrasse Labrus bergylta, sea scorpions (Myoxocephalus scorpius &Taurulus bubalis combined), three-bearded rockling Gaidropsarus vulgaris, conger eel Conger conger, together 86% of fish abundance), velvet swimming crab Necora puber and spider crab Maja brachydactyla. Commercially important fish species were only caught in low numbers (3.0% of all fish), with cod Gadus morhua the most frequent (1.8% of all fish). Only two species of conservation interest were recorded: the large catshark Scyliorhinus stellaris which is locally abundant (assessed by the IUCN Red List of Threatened Species as ‘Near Threatened’ and ‘Vulnerable to extinction’ at European and global scales respectively), and a single legally retained spiny lobster Palinurus elephas. Retention or live release status was recorded for nearly all (96%) of the core fish individuals and varied widely with taxa: sea scorpions (never retained), Scyliorhinus stellaris (13% retained), smallspotted catshark, three-bearded rockling, conger eel (44–59% retained), with ballan wrasse having the highest retention (90% of 242 individuals; average of 5.5 individuals retained per trip when present). Observed high spider crab abundance combined with FEK of previous absence or scarcity suggest that this species has increased in northwards range and abundance in recent decades, possibly because of warming seas. No incidents of in-pot mammal or bird entrapment were recorded. The retention of fish for bait is not currently acknowledged in consumer advice or landings data. Possible effects of bait retention on populations of data-poor species which may be vulnerable to overexploitation (e.g., long-lived ballan wrasse) and coastal ecosystems are unknown and warrant further research.

Quantifying use of kelp forest habitat by commercially important crustaceans in the United Kingdom

AbstractKelp forests are regarded as important nursery and foraging habitats for commercially important species of finfish and shellfish despite an absence of fishery-independent data in many regions. Here, we conducted targeted surveys at 12 subtidal reefs, distributed across 9° of latitude in the UK, using three complementary techniques (Underwater Visual Census (UVC), Baited Remote Underwater Video (BRUV) and deployment of prawn pots) to quantify the abundance of crustaceans within kelp forests. Commercially important species were recorded at all sites; Cancer pagurus (brown/edible crab) and Necora puber (velvet swimming crab) were the most abundant and commonly observed, although Maja brachydactyla (spider crab), Homarus gammarus (European lobster) and Palaemon serratus (common prawn) were also recorded. The abundance of some species exhibited pronounced regional variability, with higher abundances of C. pagurus within northern regions and, conversely, higher abundances of M. brachydactyla and P. serratus within southern regions. Each sampling technique yielded similar spatial patterns for the most abundant species but had varying sensitivity to some species. Most individuals observed were juvenile or sub-adults, suggesting kelp forests serve as important nursery grounds for commercially and ecologically important crustaceans. Further monitoring efforts, conducted across greater spatiotemporal scales and in different habitat types, are needed to provide a robust baseline against which to detect changes and to inform management and conservation actions.

The socio-economic impact of Marine Protected Areas in Jersey: A fishers’ perspective

Marine Protected Areas (MPAs) are advocated as tools to support sustainable fisheries and biodiversity by excluding the most destructive fishing practices. Some MPAs protect the whole-site of habitat mosaics from bottom-towed fishing but most only restrict damaging activities from specific evidenced conservation features. Social and economic factors influence the success of an MPA but the impact of spatial management changes to local fisheries is rarely captured in post MPA designation monitoring. In Jersey, Channel Islands, two offshore MPAs were designated in 2017 that are managed following the whole-site approach. This study provides an insight into the overall economic importance of key fishery species (whelk (Buccinum undatum), brown crab (Cancer pagurus), scallop (Pecten maximus), lobster (Homarus gammarus), and spider crab (Maja brachydactyla) to local fishing fleets and the impact of the MPAs on local, small-scale fishers in Jersey. Total landings (kg and £) in Jersey pre and post MPA designation were calculated from logbook and primary sales data and a structured interview was developed and tested as a research tool to document the impact (positive and negative) of the MPA designation on local fishers. Specific questions were designed to elicit both quantitative and qualitative data relating to the participants (n = 21) fishing activity; support for the MPAs; income and job satisfaction; subjective well-being and current sales strategies. The results demonstrate that one mobile gear vessel respondent was displaced from traditional fishing grounds as a result of the MPA designation. Fishers also reported an increase in static fishing inside the MPAs, post designation. Generally, there are high levels of support for the MPA from static fleet respondents. In the first year since designation there have been no reported changes to catch (kg, £). Mobile gear fishers reported a lower subjective wellbeing and satisfaction with their job and income than static gear fishers, despite greater profits. The major concern reported by the mobile fleet that contributed to their stress levels, was not attributed to the MPA designations but related to conflict with French static net fisheries operating outside of the MPAs. Ongoing support for Jersey’s fishers and securing a sustainable and fulfilling livelihood will require further spatial management of fisheries outside of the MPAs with a possibility of setting gear or effort limits on static fishereis within the MPAs.

Removal of bottom-towed fishing from whole-site Marine Protected Areas promotes mobile species biodiversity

Marine habitats are being altered by anthropogenic pressures, influencing the diversity and distribution of species. Marine Protected Areas (MPAs) are increasingly used as spatial management tools to mitigate these human impacts on marine systems, but levels of protection vary. MPAs that exclude bottom-towed fishing activities from all habitats within their boundaries while still allowing static gear fishing, taking a “whole-site approach”, have shown benefits in terms of increasing biodiversity and biomass on reef habitat. In this study we use Baited Remote Underwater Videos (BRUVs) to quantify differences in mobile species assemblage composition, diversity and abundance on mixed sediment habitats inside and outside three MPAs adopting this whole-site approach within Jersey's territorial waters. Greater numbers of taxa were recorded within all three MPAs compared to nearby unprotected areas, with an average of 4.9 ± 1.8 taxa observed per deployment inside the MPAs compared to 3.9 ± 1.7 taxa outside the MPAs. No significant difference was observed in the overall abundance. The proportion of presence of two indicator species (Labridae Spp. and juvenile bream Spondyliosoma cantharus) was greater within the MPAs than the nearby unprotected areas. IUCN threatened and vulnerable shark species were observed on more BRUVs inside the MPAs, while IUCN threatened and vulnerable ray species were present on fewer BRUVs in the MPAs, but both occurred in too few numbers to statistically assess. Not all species responded positively to the MPAs, with scavenging species such as spider crab (Maja brachydactyla) recorded in lower numbers in two out of the three MPAs compared to the nearby unprotected areas. BRUV surveys that have been used to survey MPAs that exclude bottom-towed fishing in Europe are sparse and have primarily focussed on reef habitat. This study provides the first account of how this whole-site approach for mixed sediment habitats in three MPAs affects mobile species. These results evidence the merit of an ecosystem-based approach to MPA management for species of both conservation and commercial importance.

First Record of the Epizoic Octolasmis angulata (Cirripedia) on Maja squinado (Herbst, 1788) (Majoidea, Crustacea) from Çanakkale, Türkiye

Some morphometric and biological characteristics of a European spider crab, Maja squinado (Herbst, 1788) caught off Aksaz region, (Biga, Çanakkale, Türkiye) were investigated. In addition, an epizoic species infesting this M. squinado specimen has been identified. The crab specimen was caught by a commercial fisherman using trammel nets. Carapace length, carapace width and weight of the crab were 15.9 cm, 14.5 cm, and 1109 g respectively. The specimen was an ovigerous female with a calculated fecundity of 3.48x104 eggs. Eggs were in stage 4th of development and covered the whole carapace region. Additionaly, it was observed that the specimen was infested with the epixoic barnacle Octolasmis angulata (Aurivillius, 1894). A total of 46 O. angulata individuals were observed; 32 from the gill lamellae, and 14 from the carapace. This study reports the existence of the epizoic O. angulata on Maja squinado for the first time in Türkiye.

Morphological description of the midgut tract and midgut–hindgut junction in the larvae of the spider crab Maja brachydactyla Balss, 1922 (Malacostraca: Decapoda)

The midgut tract of decapods is a digestive organ involved in the synthesis of peritrophic membrane, food transport, absorption of nutrients, and osmoregulation. The midgut tract has been described in detail in adult decapods, but little information is available regarding the morphology and ultrastructure of the midgut tract in larval stages. The present study describes the midgut tract and the midgut–hindgut junction of the larvae of the common spider crab Maja brachydactyla Balss, 1922 using techniques that included dissection, light microscopy, and electron microscopy. The study is mainly focused on the stages of zoea I and megalopa. The results obtained in this study show that the larval midgut tract is a short and simple tube positioned anteriorly, between the stomach and the hindgut tract. During larval development, the maximum length of the midgut tract increases significantly, but no differences were found on either the maximum diameter or the morphological traits of the organ. The midgut tract is active at least ca. 12 h after hatching, as suggested by the presence of the peritrophic membrane in the lumen, the presence of abundant electro-dense vesicles in the cell apex, and the release of the vesicle content on the organ lumen. The midgut–hindgut junction forms an abrupt transition between the midgut tract and the hindgut tract in which epithelial cells with mixed features of midgut and hindgut do not occur.

Value of coastal habitats to commercial fisheries in Jersey, English Channel, and the role of marine protected areas

AbstractFisheries are in decline worldwide, and Marine Protected Areas (MPAs) are being advocated as tools that can not only protect and restore biodiversity but also improve fisheries sustainability and protect fisher livelihoods. To understand the role of Marine Protected Areas (MPAs) in underpinning commercial fisheries, this study demonstrates the economic value of Jersey's benthic substrates (habitats) for five predominant species fished by Jersey and French vessels: Homarus gammarus, Cancer pagurus, Maja brachydactyla, Pecten maximus and Buccinum undatum. Value was apportioned between habitats that support these commercial species across their essential life history stages, and the proportional economic value that was protected from bottom‐towed fishing within MPAs was analysed. Multiple habitats across Jersey's territorial waters contributed a total economic value of £14,664,729, with £4,127,999 protected within MPAs. Overall, subtidal sediment was the most valuable habitat to both Jersey (£2.12 million) and French (£2.47 million) fisheries but was also the least protected habitat from bottom‐towed fishing (2.73%). Our findings support an ecosystem‐based approach to fisheries management and emphasise the importance of considering species life histories, and their habitat requirements, in management plans.

Morphological and histological description of the midgut caeca in true crabs (Malacostraca: Decapoda: Brachyura): origin, development and potential role

BackgroundThe decapods are a major group of crustaceans that includes shrimps, prawns, crayfishes, lobsters, and crabs. Several studies focused on the study of the digestive system of the decapods, constituted by the oesophagus, stomach, midgut tract, midgut gland, and hindgut. Nevertheless, in the midgut tract there are associated a set of organs called “midgut caeca”, which are among the most controversial and less studied digestive organs of this group. This work used the common spider crab Maja brachydactyla Balss, 1922 as a model to resolve the origin, development, and potential role of the midgut caeca. Such organs were studied in the larvae (zoea I, zoea II, megalopa), first juveniles, and adult phases, being employed traditional and modern techniques: dissection, micro-computed tomography (Micro-CT), and light and electron microscopical analyses (TEM and SEM).ResultsThe common spider crab has a pair of anterior midgut caeca and a single posterior caecum that originate from the endoderm germ layer: they develop from the midgut tract, and their epithelium is composed by secretory cells while lacking a cuticle lining. The midgut caeca are small buds in the newly hatched larvae, enlarge linearly during the larval development, and then continue growing until became elongated and coiled blind-tubules in adults. The adult midgut caeca are internally folded to increase their inner surface. The electron microscopy observations showed that the midgut caeca are highly active organs with important macroapocrine and microapocrine secretory activity. Our results suggest that the role of the caeca might be related to the digestive enzyme secretion. The secretory activity should increase as the animal grows in size.ConclusionThe present study resolves the embryonic origin of the midgut caeca (endoderm derived organs), development (general lengthening starting from small buds), and role (active secretory organs). The secretory activity of the midgut caeca should be incorporated in the current models of the digestive physiology in different decapod taxa.

Diurnal and nocturnal scavenger communities differ at two shallow-water depths in an Irish marine lough

We report on still/video photographic studies in 2018/9 of diurnal and nocturnal benthic scavenging on carrion in shallow water (2 m and 12 m depths) within Lough Hyne (a biodiverse Irish sea lough) using natural light during replicated 6 h daytime deployments and dim red light (630 nm) during 6 h deployments at night. In 2018 at 2 m depth in the North Basin, bait was present throughout day-time deployments, showing little evidence of scavenging; scavenger species' richness and abundance were low. At night, baits disappeared or were skeletonised; they were scavenged by a significantly more abundant and species-rich scavenger guild. At 12 m depth in the same basin, baits were removed or skeletonised during day and night. This confirmed our hypothesis that scavengers avoid well-lit very shallow water during the day, probably to avoid predation. Multivariate analysis of 2018/19 still photo data (12 m depth) revealed pronounced and statistically significant separate clustering of day, night and basin abundance/species data, although overall species' richness values did not differ significantly. These analyses together reflected differences in species’ compositions associated with contrasting sediments, flow regimes and night/day. Nephrops norvegicus, Pagurus bernhardus and Crangon crangon were only found in the low flow rate North Basin; Homarus gammarus, Trigla sp., Callionymus lyra, Agonus, Tritia reticulata and Marthasterias glacialis were only found in the high flow rate South Basin. Most scavenger species foraged in both day and night (though in different proportions). However, prawns (Palaemon serratus) only appeared at night. Small mobile scavenging fish, small crabs and prawns arrived at baits within seconds to minutes, as did locally-abundant slow movers (e.g. anemones (Anthopleura ballii), whelks (T. reticulata)). These times are shorter than recorded in earlier studies in deeper water. Larger crustaceans (e.g. Cancer pagurus, H. gammarus) took tens of minutes to hours before arrival; similar times have previously been recorded in deeper water. Large crustacean scavengers (C. pagurus, Maja squinado, H. gammarus) took substantial quantities of bait during long feeding bouts, but catsharks (Scyliorhinus canicula) often took similar quantities in a few seconds. All of the dominant scavenger species are broad-spectrum omnivores; only one obligate scavenger species (Tritia reticulata) was present (only in South Basin) and consumed negligible quantities of bait.

Differences in prevalence of multiple paternity in the spiny spider crabMaja brachydactylain two localities that differ in female fecundity, fishing intensity, and management measures

AbstractMany marine species are under high fishing pressure, which can cause alterations in their mating systems and the structure of their populations. The spiny spider crab Maja brachydactyla is a commercial decapod on the east Atlantic coast. In Galicia, the most exploited region in Spain, the landing of ovigerous females is prohibited, favouring exploitation focused on males. The removal of males, especially the largest ones, may lead to sperm limitation and a reduction in the average size over generations. In these cases, polyandry can work as a mechanism to protect females against sperm limitation and to increase genetic diversity and effective population size. This study analyses the multiple paternity in M. brachydactyla in two localities that differ in female fecundity, fishing intensity, and management measures. Our results showed multiple paternity in this species for the first time, with a moderate bias between sires. However, the frequency of multiple paternity was almost seven times higher in the intensely exploited Golfo Ártabro (NW Spain; 69%) than in Carna (W Ireland; 10%), where exploitation levels and female fecundity are lower.

Description of the larval and adult hindgut tract of the common spider crab Maja brachydactyla Balss, 1922 (Brachyura, Decapoda, Malacostraca).

Arthropods are the most diversified animals on Earth. The morphology of the digestive system has been widely studied in insects; however, crustaceans have received comparatively little attention. This study describes the hindgut tract of the common spider crab Maja brachydactyla Balss, 1922, in larvae and adults using dissection, light and electron microscopical analyses. The hindgut tract maintains a similar general shape in larvae and adults. Major differences among stages are found in the morphology of epithelial cells and microspines, the thickness of the cuticle and connective-like tissue, and the presence of rosette glands (only in adults). Here, we provide the description of the sub-cellular structure of the folds, epithelium (conformed by tendon cells), musculature, and microspines of the hindgut of larvae and adults of M. brachydactyla. The morphological features of the hindgut of M. brachydactyla are compared with those of other arthropods (Insecta, Myriapoda and Arachnida). Our results suggest that the morphology of the hindgut is associated mainly with transport of faeces. In adults, the hindgut may also exert an osmoregulatory function, as described in other arthropods. At difference from holometabolous insets, the hindgut of M. brachydactyla (Decapoda) does not undergo a true metamorphic change during development, but major changes observed between larval and adult stages might respond to the different body size between life stages.

High survivability of discarded invertebrates from bottom trawling fisheries

The impacts of bottom trawling in coastal ecosystems are significant, not only for targeted species but also for non-commercial ones. However, the critical gaps in scientific knowledge associated with these impacts on untargeted but functionally important species remain to be filled. This is notably the case for a great proportion of discarded invertebrates, for which only a handful of studies have investigated their survival. In this study, 600 individuals from six different benthic invertebrate species were collected in commercial conditions in the Bay of Biscay for short-term survival experiments. Overall, the observed survival after 100–130 h and the predicted survival via mixture models were very high (>93%) for Asterias rubens, Aphrodita aculeata, Buccinum undatum and Pagurus sp. Survival of Maja brachydactyla was lower though still high (>80% overall) and Atelecyclus undecimdentatus was more vulnerable to trawling and handling, with ∼50% of survival. Both showed 100% mortality when presenting carapace cracks and survival of M. brachydactyla was lower when missing appendages. No biotic nor abiotic parameters were correlated to survival, except injury class for M. brachydactyla and A. undecimdentatus. This study shows an overall high survival but highlights the fact that otter trawl fisheries may differentially affect discarded benthic invertebrates, therefore bringing light for future studies on longer-term impacts on benthic communities and coastal ecosystems. Furthermore, we recommend incorporating survival rates of discarded invertebrates in ecosystem-level modelling studies and encourage the use of detailed information and biomass reports of benthic invertebrates for a better management of natural populations and fisheries resources.

High incidence of heteroplasmy in the mtDNA of a natural population of the spider crab Maja brachydactyla.

Mitochondria are mostly inherited by maternal via, that is, only mitochondria from eggs are retained in the embryos. However, this general assumption of uniparentally transmitted, homoplasmic and non-recombining mitochondrial genomes is becoming more and more controversial. The presence of different sequences of mtDNA within a cell or individual, known as heteroplasmy, is increasingly reported in several taxon of animals, such as molluscs, arthropods and vertebrates. In this work, a considerable frequency of heteroplasmy were detected in the COI and 16S genes of the spider crab Maja brachydactyla, possibly associated to hybridisation with the congeneric species Maja squinado. This finding is a fact to keep in mind before addressing molecular analyses based on mitochondrial markers, since the assumption of maternal inheritance could lead to erroneous results. As M. brachydactyla is a commercial species, heteroplasmy is an important aspect to take into account for the fisheries management of this resource, since effective population size could be overestimated.

Can crustacean cuticle preserve a record of chronological age? Investigating the utility of decapod crustacean eyestalks for age determination of two European spider crab species

Until recently it was not thought possible to directly and accurately age crustacean species as any calcified structure that could preserve a record of chronological age is lost at moulting. Recent research has suggested that it may be possible to directly infer age from the number of endocuticular bands present within the eyestalks of snow crab (Chionoecetes opilio) and Antarctic krill (Euphausia superba). Using captive-reared Mediterranean spider crabs (Maja squinado), and wild-caught north-east Atlantic spider crabs (Maja brachydactyla), the present study aimed to determine if endocuticular bands of potentially annual periodicity are present within the eyestalks of these brachyurans. Although clear and readable bands were present within the endocuticle of their eyestalks, no bands of potentially annual periodicity were identified. The number of endocuticular bands did not correlate with the known-age of the M. squinado individuals; but did correlate positively with carapace length for both M. squinado and M. brachydactyla, suggesting they form as a result of growth and the size of individuals. It is hypothesised that these endocuticular bands, and potentially those identified by other studies purporting them to be annual growth bands, are in fact lamellae that appear due to the layers of calcite crystal aggregates interspersed with chitin and protein fibres that form the mineral of the cuticle in decapod crustaceans. Further research is needed to determine the processes that control and result in the deposition of crustacean cuticle, particularly during the post-moult period.