From the depths of the oceans to the shallow estuaries and wetlands of our coasts, organisms of the marine environment are teeming with unique adaptations to cope with a multitude of varying environmental conditions. With millions of years and a vast volume of water to call their home, they have become quite adept at developing specialized and unique techniques for survival and – given increasing human mediated transport – biological invasions. A growing world human population and a global economy drives the transportation of goods across the oceans and with them invasive species via ballast water and attached to ship hulls. In any given 24-hour period, there are about 10,000 species being transported across different biogeographic regions. If any of them manage to take hold and establish a range in an exotic habitat, the implications for local ecosystems can be costly. This review on marine invasions highlights trends among successful non-indigenous species (NIS), from vectors of transport to ecological and physiological plasticity. Apart from summarizing patterns of successful invasions, it discusses the implications of how successfully established NIS impact the local environment, economy and human health. Finally, it looks to the future and discusses what questions need to be addressed and what models can tell us about what the outlook on future marine invasions is.

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How Do They Do It? – Understanding the Success of Marine Invasive Species

In a warming ocean, temperature variability imposes intensified peak stress, but offers periods of stress release. While field observations on organismic responses to heatwaves are emerging, experimental evidence is rare and almost lacking for shorter-scale environmental variability. For two major invertebrate predators, we simulated sinusoidal temperature variability (±3 °C) around todays’ warm summer temperatures and around a future warming scenario (+4 °C) over two months, based on high-resolution 15-year temperature data that allowed implementation of realistic seasonal temperature shifts peaking midpoint. Warming decreased sea stars’ (Asterias rubens) energy uptake (Mytilus edulis consumption) and overall growth. Variability around the warming scenario imposed additional stress onto Asterias leading to an earlier collapse in feeding under sinusoidal fluctuations. High-peak temperatures prevented feeding, which was not compensated during phases of stress release (low-temperature peaks). In contrast, increased temperatures increased feeding on Mytilus but not growth rates of the recent invader Hemigrapsus takanoi, irrespective of the scale at which temperature variability was imposed. This study highlights species-specific impacts of warming and identifies temperature variability at the scale of days to weeks/months as important driver of thermal responses. When species’ thermal limits are exceeded, temperature variability represents an additional source of stress as seen from future warming scenarios.

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Warming and temperature variability determine the performance of two invertebrate predators

Traditionally, the internal morphology of crustacean larvae has been studied using destructive techniques such as dissection and microscopy The present study combines advances in micro-computed tomography (micro-CT) and histology to study the internal morphology of decapod larvae, using the common spider crab (Maja brachydactyla Balss, 1922) as a model and resolving the individual limitations of these techniques The synergy of micro-CT and histology allows the organs to be easily identified, revealing simultaneously the gross morphology (shape, size, and location) and histological organization (tissue arrangement and cell identification) Micro-CT shows mainly the exoskeleton, musculature, digestive and nervous systems, and secondarily the circulatory and respiratory systems, while histology distinguishes several cell types and confirms the organ identity Micro-CT resolves a discrepancy in the literature regarding the nervous system of crab larvae The major changes occur in the metamorphosis to the megalopa stage, specifically the formation of the gastric mill, the shortening of the abdominal nerve cord, the curving of the abdomen beneath the cephalothorax, and the development of functional pereiopods, pleopods, and lamellate gills The combination of micro-CT and histology provides better results than either one alone

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Micro-computed tomography and histology to explore internal morphology in decapod larvae

Life-history traits and interactions with native species play an important role for the successful establishment of non-native species in new habitats. We investigated the recent successful invasion of the Pacific crabs Hemigrapsus takanoi and H. sanguineus to the southeastern North Sea coast with respect to their recruitment patterns, as well as interactions of juvenile with sub-adult individuals among the Pacific crabs and with native shore crabs Carcinus maenas. A field survey of juvenile native and introduced crab abundances (carapace width 1.4–10 mm) was conducted in the northern Wadden Sea, spanning 24 months from 2014 to 2016. The survey revealed different seasonal recruitment patterns of native C. maenas and both introduced Hemigrapsus species. Native shore crabs showed a single recruitment peak from June to July, while Hemigrapsus spp. mainly recruited from August to early September, but recruits occurred in low densities throughout the winter until the end of the following spring season. Field experiments on the effects of larger crabs on the recruitment intensity showed that recruitment of H. takanoi was enhanced by the presence of larger congeners, but remained unaffected by larger C. maenas. Recruitment of juvenile C. maenas, by contrast, was reduced by the presence of larger Hemigrapsus spp. Additional laboratory experiments revealed high rates of cannibalism on newly recruited C. maenas by subadult conspecifics as well as strong predation by larger Hemigrapsus spp. In contrast, newly recruited Hemigrapsus spp. had a much lower risk of being preyed on by subadult conspecifics and native shore crabs. Our results suggest that the timing of recruitment in combination with low intraspecific competition and reduced predation pressure by native shore crabs are crucial for the rapid and ongoing establishment of Hemigrapsus spp. in the Wadden Sea.

Recruitment patterns, low cannibalism and reduced interspecific predation contribute to high invasion success of two Pacific crabs in northwestern Europe

A new family Leptograpsodidae n. fam. is erected to include an unusual genus of grapsoid crabs, LeptograpsodesMontgomery, 1931, represented by a burrowing species from South Australia L. octodentatus (H. Milne Edwards, 1837), with setal pouches on the coxae of pereiopods 2 to 4 that are evidence of a high degree of terrestrial adaptation. With nearly 600 extant species, the superfamily Grapsoidea MacLeay, 1838 encompasses a diverse range of morphologies and ecologies. Genetic data question its monophyly, whereas the inter- and intrarelationships of included taxa have not been recently examined through a morphological review using a reliable complex of characters (primarily, proepistome, thoracic sternum, male pleon, male genital region, male gonopore, and penis). The reevalution of the morphological characters of L. octodentatus in order to reappraise its proper taxonomical status has afforded us the opportunity to review most of grapsoids and, in particular, the Gecarcinidae H. Milne Edwards, 1837 (see Appendix), Grapsidae MacLeay, 1838, Varunidae H. Milne Edwards, 1853 (especially Cyclograpsus H. Milne Edwards, 1837), and Xenograpsidae N. K. Ng, Davie, Schubart & Ng, 2007. The discovery that Leptograpsodes shares some characters with Gecarcinidae, especially a stridulatory apparatus similar to that of Discoplax A. Milne-Edwards, 1867, a feature not previously recorded in either genus, has revealed the need to re-examine the taxonomy of Gecarcinidae. This resulted in an unexpected outcome. Discoplax must be restricted to its three troglobitic species: D. longipes A. Milne-Edwards, 1867 (type species), D. gracilipesNg & Guinot, 2001, and D. michalisNg & Shih, 2015. Cardisoma Latreille, 1828 sensu stricto only accommodates C. guanhumi Latreille, in Latreille, Le Peletier, Serville & Guerin, 1828 (type species), C. armatum Herklots, 1861, C. carnifex (Herbst, 1796), and C. crassum Smith, 1870. A new genus, Tuerkayana n. gen. is hereby established to include two species previously assigned to Cardisoma: T. rotundum (Quoy & Gaimard, 1824) n. comb. (type species), T. hirtipes (Dana, 1851) n. comb., plus two others previously regarded as Discoplax: T. celeste (Ng & Davie, 2012) n. comb. and T. magnum (Ng & Shih, 2014) n. comb. Compared to Varunidae, Leptograpsodes bears a superficial resemblance to Cyclograpsus H. Milne Edwards, 1837, a paraphyletic genus that must be restricted to its type species C. punctatus H. Milne Edwards, 1837 and only its closest congeners. The validity of the subfamilial taxon Heliceinae Sakai, Turkay & Yang, 2006 is recognised. The putative sister-group relationship of Leptograpsodes and Xenograpsus supported by molecular analyses of Schubart (2011) and Ip et al. (2015) is not confirmed by morphology, even if the two families share some traits. The synapomorphies of Leptograpsodes justify its separation from all grapsoid lineages as currently conceived. The use of previously overlooked traits, notably related to the thoracic sternum, proves to represent an optimal support for the brachyuran systematics and phylogeny, and presently to redefine the grapsoid taxonomical categories. The complete fusion of thoracic sternites 3 and 4 without any visible suture, even laterally, is proposed as a synapomorphy of Grapsoidea; this character state could be extented to other Thoracotremata in the future. Species of three genera Leptograpsodes, Discoplax and Epigrapsus are reported to exhibit stridulating structures (suborbital pars stridens and plectrum on inner margin of P1 merus). Some nomenclature issues, notably the authorship of the family-group name Gecarcinidae that is here credited to H. Milne Edwards, 1837, are addressed in the Appendix.

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Review of grapsoid families for the establishment of a new family for Leptograpsodes Montgomery, 1931, and a new genus of Gecarcinidae H. Milne Edwards, 1837 (Crustacea, Decapoda, Brachyura, Grapsoidea MacLeay, 1838)

Twenty years after its initial introduction to Europe, the Asian brush clawed crab Hemigrapsus takanoi is now a successful invader of the European Atlantic and North Sea coasts from northern Spain to southern Denmark. In July 2014, this species was recorded in the Baltic Sea for the first time, as eight individuals were found in the inner Kiel Fjord, Schleswig-Holstein, Germany. To date, a total number of 19 individuals have been collected. Findings of an ovigerous female and of juvenile crabs among the collected individuals indicate that H. takanoi most likely already reproduces successfully in the Baltic Sea.

First record of the Asian crab Hemigrapsus takanoi Asakura & Watanabe, 2005 (Decapoda, Brachyura, Varunidae) in the Baltic Sea

Ever more complex: a new type of organization of reproductive organs in female Dorippe sinica Chen, 1980 (Decapoda: Brachyura: Dorippidae)

What's in this crab? MRI providing high-resolution three-dimensional insights into recent finds and historical collections of Brachyura.

Brachyuran crabs have a distinct outer morphology with a highly reduced pleon, which is folded underneath a usually broad cephalothorax. The pleon is secured in this position with a specialised holding mechanism, which consists of a sternal knob, or press-button, and, in most eubrachyurans, a corresponding pit or socket on the pleon. The effectiveness of this mechanism has not been described so far. In this study a method to investigate the force needed to move the pleon from its folded position was measured in male and female crabs under different experimental conditions is presented. In male specimens a median pull-off force of 167.20 mN was measured in air. The measured force decreased slightly underwater. When the press-buttons were removed artificially, no pleon attachment was feasible underwater. The variation observed in the pull-off forces measured is most likely a result of different levels of wear or damage of the holding structures. In mature females, which lack these structures, no pleon attachment was possible underwater. In air, residual moisture and the morphology of the pleon resulted in attachment via capillary interactions. The method presented in this study can be used in further investigations to evaluate the effectiveness of pleon-holding structures in various brachyuran crabs.

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Biomechanics of pleon attachment in the European shore crab Carcinus maenas (Linnaeus, 1758) (Brachyura: Portunoidea: Carcinidae)

Eubrachyurans not only exhibit a crab-like body shape, consisting of a mostly wide cephalothorax and a thin, shield-like pleon folded underneath it, but reportedly hold the pleon in this po...

Stephanie Köhnk

Papers

First record of the Asian crab Hemigrapsus takanoi Asakura & Watanabe, 2005 (Decapoda, Brachyura, Varunidae) in the Baltic Sea

Ever more complex: a new type of organization of reproductive organs in female Dorippe sinica Chen, 1980 (Decapoda: Brachyura: Dorippidae)

What's in this crab? MRI providing high-resolution three-dimensional insights into recent finds and historical collections of Brachyura.

Biomechanics of pleon attachment in the European shore crab Carcinus maenas (Linnaeus, 1758) (Brachyura: Portunoidea: Carcinidae)

The morphological and functional variability of pleon-holding mechanisms in selected Eubrachyura (Crustacea: Decapoda)