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Krill is a vital food source for many marine animals but also strongly impacted by climate change. Genetic adaptation could support populations, but remains uncharacterized. We assembled the 19 Gb Northern krill genome and compared genome-scale variation among 74 specimens from the colder Atlantic Ocean and warmer Mediterranean Sea. The genome is dominated by methylated transposable elements and contains many duplicated genes implied in molting and vision. Analysis of 760 million SNPs indicates extensive homogenizing gene-flow among populations. Nevertheless, we detect extreme divergence across hundreds of genes, governing ecophysiological functions like photoreception, circadian regulation, reproduction and thermal tolerance. Such standing variation may be essential for resilience in zooplankton, necessitating insight into adaptive variation to forecast their roles in future marine ecosystems and support ocean conservation.One-Sentence SummaryGenome-scans of Northern krill link genes for photoreception, reproduction and thermal tolerance to ecological adaptation.

Cumaceans are small peracarid crustaceans that can be remarkably diverse and important benthic organisms. Despite their ubiquitous presence in soft sediments, no well-resolved phylogeny currently exists, which impedes ecological and evolutionary studies of the group. We present a phylogeny based on Bayesian inference of six markers (18S, 28S, 12S, 16S, CytB and COI), which recovers monophyly of the order, a deep split between telson and pleotelson bearing groups, and monophyly of four of the seven included families, including monophyletic Pseudocumatidae, Lampropidae, Bodotriidae and Nannastacidae. The only species representing the family Gynodiastylidae in our dataset was positioned among members of Diastylidae in the phylogenetic analyses. However, this result is based on a single partial COI sequence; thus, we consider it doubtful, and the family Diastylidae are otherwise recovered as a monophyletic family. The family Leuconidae is split into two well-supported clades, a clade containing Antarctic members of the genus Leucon and a separate clade containing non-Antarctic members of the genera Leucon and Eudorella. The phylogeny is a great stride forwards, as it supports most families as monophyletic, making generic level phylogenies a plausible endeavour in the future. publishedVersion

The endoparasitic crustacean Sacculina carcini (Cirripedia: Rhizocephala) has a much simpler morphology than conventional filter-feeding barnacles, reflecting its parasitic lifestyle. To investigate the molecular basis of its refined developmental program, we produced a draft genome sequence for comparison with the genomes of nonparasitic barnacles and characterized the transcriptomes of internal and external tissues. The comparison of clusters of orthologous genes revealed the depletion of multiple gene families but also several unanticipated expansions compared to non-parasitic crustaceans. Transcriptomic analyses comparing interna and externa tissues revealed an unexpected variation of gene expression between rootlets sampled around host midgut and thoracic ganglia. Genes associated with lipid uptake were strongly expressed by the internal tissues. We identified candidate genes probably involved in host manipulation (suppression of ecdysis and gonad development) including those encoding crustacean neurohormones and the juvenile hormone binding protein. The evolution of Rhizocephala therefore appears to have involved a rapid turnover of genes (losses and expansions) as well as the fine tuning of gene expression.

Acorn barnacle adults experience environmental heterogeneity at various spatial scales of their circumboreal habitat, raising the question of how adaptation to high environmental variability is maintained in the face of strong juvenile dispersal and mortality. Here we show that 4% of genes in the barnacle genome experience balancing selection across the entire range of the species. Many of these genes harbor mutations maintained across 2 million years of evolution between the Pacific and Atlantic oceans. These genes are involved in ion regulation, pain reception, and heat tolerance, functions which are essential in highly variable ecosystems. The data also reveal complex population structure within and between basins, driven by the trans-Arctic interchange and the last glaciation. Divergence between Atlantic and Pacific populations is high, foreshadowing the onset of allopatric speciation, and suggesting that balancing selection is strong enough to maintain functional variation for millions of years in the face of complex demography.

Molecular and morphological methods are used to describe Lernaeodiscus rybakovi, a new rhizocephalan species parasitizing the porcellanid crab Pachycheles stevensii Stimpson, 1858, collected in Russian waters of the Sea of Japan. Molecular analysis of three species, including the new one, confirms the monophyly of the genus Lernaeodicus Muller, 1862 and its recent transfer to the family Peltogastridae. The main morphological features of the new species are also common with characters of the other species of the genus Lernaeodiscus Muller, 1862. Externa of L. rybakovi differs from the well studied Lernaeodiscus porcellanae from the E. Pacific by molecular markers, color and the absence of pronounced marginal lobes. Retinacula on the internal cuticle of L. rybakovi are found in the genus Lernaeodiscus for the first time. Some of the female hosts with adult externae were unusual in also carrying their own eggs on the pleopods, a rare situation among rhizocephalans. The complete larval development in the genus Lernaeodiscus is described here for the first time and includes five naupliar and one cypris instar. The main morphological features of Lernaeodiscus nauplii (the presence of flotation collar, morphology of the frontolateral horns and furcal spines, and the arrangement of dorsal setae on the shield head) are common with those of other known peltogastrid larvae, but also resemble nauplii of Peltogasterella in the presence of naupliar eyes and thin structure of the flotation collar. The sizes of male and female cyprids of L. rybakovi overlap slightly. In summer months, larval sex ratio is male-biased. We briefly review important larval characters in the Rhizocephala.

The association between anemonefish and anemone is a classical example of mutualism in coral reefs. Although mutualism is probably the key innovation that triggered the adaptive radiation o...

The Nordic Seas have one of the highest water-mass diversities in the world, yet large knowledge gaps exist in biodiversity structure and biogeographical distribution patterns of the deep macrobenthic fauna. This study focuses on the marine bottom-dwelling peracarid crustacean taxon Cumacea from northern waters, using a combined approach of morphological and molecular techniques to present one of the first insights into genetic variability of this taxon. In total, 947 specimens were assigned to 77 morphologically differing species, representing all seven known families from the North Atlantic. A total of 131 specimens were studied genetically (16S rRNA) and divided into 53 putative species by species delimitation methods (GMYC and ABGD). In most cases, morphological and molecular-genetic delimitation was fully congruent, highlighting the overall success and high quality of both approaches. Differences were due to eight instances resulting in either ecologically driven morphological diversification of species or morphologically cryptic species, uncovering hidden diversity. An interspecific genetic distance of at least 8% was observed with a clear barcoding gap for molecular delimitation of cumacean species. Combining these findings with data from public databases and specimens collected during different international expeditions revealed a change in the composition of taxa from a Northern Atlantic-boreal to an Arctic community. The Greenland-Iceland-Scotland-Ridge (GIS-Ridge) acts as a geographical barrier and/or predominate water masses correspond well with cumacean taxa dominance. A closer investigation on species level revealed occurrences across multiple ecoregions or patchy distributions within defined ecoregions. publishedVersion

Molecular and morphological methods are used to describe the rhizocephalan Lernaeodiscus kasyanovi sp. nov. from Russian waters of the Sea of Japan, the second of two Lernaeodiscus species parasitizing the porcellanid crab Pachycheles stevensii Stimpson, 1858. Lernaeodiscus kasyanovi sp. nov. differs from the other species found on this host, Lernaeodiscus rybakovi Korn et al., 2020a, by molecular markers, by a smaller size, a lighter color, by the shape of receptacles, by the presence of marginal lobes in mature specimens, and by the retinacula type on the internal cuticle. Lernaeodiscus kasyanovi sp. nov. is the sister taxon to Lernaeodiscus porcellanae in the monophyletic genus Lernaeodiscus. Lernaeodiscus kasyanovi is rather rare compared to L. rybakovi, their prevalence on the porcellanid crab P. stevensii does not exceed 2%. It is the second example of two rhizocephalan species infesting the same brachyuran species in Peter the Great Bay.

The relationships of crustaceans and hexapods (Pancrustacea) have been much discussed and partially elucidated following the emergence of phylogenomic data sets. However, major uncertainties still remain regarding the position of iconic taxa such as Branchiopoda, Copepoda, Remipedia, and Cephalocarida, and the sister group relationship of hexapods. We assembled the most taxon-rich phylogenomic pancrustacean data set to date and analyzed it using a variety of methodological approaches. We prioritised low levels of missing data and found that some clades were consistently recovered independently of the analytical approach used. These include, for example, Oligostraca and Altocrustacea. Substantial support was also found for Allotriocarida, with Remipedia as the sister of Hexapoda (i.e., Labiocarida), and Branchiopoda as the sister of Labiocarida, a clade that we name Athalassocarida (=”nonmarine shrimps”). Within Allotriocarida, Cephalocarida was found as the sister of Athalassocarida. Finally, moderate support was found for Hexanauplia (Copepoda as sister to Thecostraca) in alliance with Malacostraca. Mapping key crustacean tagmosis patterns and developmental characters across the revised phylogeny suggests that the ancestral pancrustacean was relatively short-bodied, with extreme body elongation and anamorphic development emerging later in pancrustacean evolution., J.L.-F. was supported by a Marie Skłodowska-Curie Fellowship (655814) and J.O. was supported by a grant from the Danish Agency for Science, Technology and Innovation (0601-12345B). F.P. was supported by the grant CHALLENGEN (CTM2013-48163) from the Spanish Government and a post-doctoral contract funded by the Beatriu de Pinós Programme of the Generalitat de Catalunya (2014-BPB-00038). J.F.F. was supported by a NERC GW4+ PhD studentship and J.V. and D.P. were supported by a NERC BETR grant (NE/P013678/1). M.G. is supported by a European Marie Curie (ITN – IGNITE) PhD studentship

We present a comprehensive revision and synthesis of the higher-level classification of the barnacles (Crustacea: Thecostraca) to the genus level and including both extant and fossils forms. We provide estimates of the number of species in each group. Our classification scheme has been updated based on insights from recent phylogenetic studies and attempts to adjust the higher-level classifications to represent evolutionary lineages better, while documenting the evolutionary diversity of the barnacles. Except where specifically noted, recognized taxa down to family are argued to be monophyletic from molecular analysis and/or morphological data. Our resulting classification divides the Thecostraca into the subclasses Facetotecta, Ascothoracida and Cirripedia. The whole class now contains 14 orders, 65 families and 367 genera. We estimate that barnacles consist of 2116 species. The taxonomy is accompanied by a discussion of major morphological events in barnacle evolution and justifications for the various rearrangements we propose.

Rhizocephalan cirripedes are a very unique group of parasites infecting decapod crustaceans, but apart from a few well-studied species, little is known on their ecology and impact on hosts. Here we report on the resultsof a 14-month study of infestations of the rhizocephalanParasacculina leptodiae in the rocky shore crab Leptodius exaratus along the shores of Kuwait in the Persian (Arabian) Gulf. Monthly samples along an intertidal gradient revealed a slightly higher prevalence of P. leptodiae in female (18%) compared to male crabs (11%) and marked differences in prevalence among the sampling sites. Crabs from more sheltered locations in Kuwait Bay showed lower prevalence of P. leptodiae compared to crabs from more exposed sites. Seasonal patterns were largely absent, but prevalence in female crabs showed some monthly variation depending on the site. Rhizocephalan prevalence was generally highest in both crab sexes at the lower shores. This possibly resulted from lower exposure of crabs to infective stages in the higher intertidal and movements of infected crabs to lower parts of the shore. Prevalence of ovigerous females significantly declined with increasing local parasite prevalence. This suggests that the well-known castrating effects of rhizocephalans on individual hosts can also affect local crab reproduction at the population level which has notbeen shown before. Our results indicate that the rhizocephalanP. leptodiae is a common parasite of the rocky shore crab L. exaratus along the shores of Kuwait, with potential effects on the crab’s population dynamics which warrants further study.

The barnacle Austrominius modestus, native of New Zealand and Australia was introduced to the South of England around 1943 and has since spread to most coastal waters in Western Europe, including the southern North Sea. Apart from an ephemeral incursion into the Danish Wadden Sea in 1978, since 2010 it has established permanent populations capable of reproduction along the North Sea coast of the Jutland peninsula, the Limfjord, the north-western Kattegat, and the Skagerrak. It has probably invaded Danish waters by pelagic larvae originating in the German Wadden Sea. The species has since spread to other Danish localities, likely following the prevailing currents, but other means, as for instance transport by vessels, are possible. The barnacle inhabits stones, stone-reefs, mollusk shells, and live shore crabs in shallow waters. Based on hydrographical data from its native and recently invaded areas, we predict its future distribution to extend to most coasts of continental Europe except for brackish waters (< 20 PSU), and the Arctic seas. The northernmost distribution limit may include the Lofoten Islands of Norway publishedVersion

The northern acorn barnacle (Semibalanus balanoides) is a robust system to study the genetic basis of adaptations to highly heterogeneous environments. Adult barnacles may be exposed to highly dissimilar levels of thermal stress depending on where they settle in the intertidal (i.e., closer to the upper or lower tidal boundary). For instance, barnacles near the upper tidal limit experience episodic summer temperatures above recorded heat coma levels. This differential stress at the microhabitat level is also dependent on the aspect of sun exposure. In the present study, we used pool-seq approaches to conduct a genome wide screen for loci responding to intertidal zonation across the North Atlantic basin (Maine, Rhode Island, and Norway). Our analysis discovered 382 genomic regions containing SNPs which are consistently zonated (i.e., SNPs whose frequencies vary depending on their position in the rocky intertidal) across all surveyed habitats. Notably, most zonated SNPs are young and private to the North Atlantic. These regions show high levels of genetic differentiation across ecologically extreme microhabitats concomitant with elevated levels of genetic variation and Tajima's D, suggesting the action of non-neutral processes. Overall, these findings support the hypothesis that spatially heterogeneous selection is a general and repeatable feature for this species, and that natural selection can maintain functional genetic variation in heterogeneous environments. publishedVersion

Background By comparing spatial geographical structures of host populations with that of their symbionts light can be shed on their biological interactions, and the degree of congruence between host and symbiont phylogeographies should reflect their life histories and especially dispersal mechanisms. Methods Here, we analyzed the genetic diversity and structure of a host, the blue swimming crab, Portunus pelagicus, and its symbiotic pedunculate barnacle Octolasmis angulata from six location sites representing three geographic regions (north, central and south) along the Vietnam coastline. High levels of congruence in their phylogeographic patterns were expected as they both undergo planktonic larval stages. Results Based on the COI mtDNA markers, O. angulata populations showed higher genetic diversity in comparison with their host P. pelagicus (number of haplotype/individuals, haplotype and nucleotide diversity are 119/192, 0.991 ± 0.002 and 0.02; and 89/160, 0.913 ± 0.02 and 0.015, respectively). Pairwise Fst and AMOVA analyses showed a more pronounced population structure in the symbiotic barnacle than in its crab host. The DAPC analyses identified three genetic clusters. However, both haplotype networks and scatter plots supported connectivity of the host and the symbiotic barnacle throughout their distribution range, except for low subdivision of southern population. Isolation by distance were detected only for the symbiont O. angulata (R2 = 0.332, P = 0.05), while dbMEM supported spatial structure of both partners, but only at MEM-1 (Obs. 0.2686, P < 0.01 and Obs. 0.2096, P < 0.01, respectively).

The Sognefjord is a geologically unique, long, and deep glacial valley, stretching more than 200 km inland with a maximum depth of 1300 m. Surprisingly little is known about the seafloor environment and bottom communities of this, in a global perspective, spectacularly long and deep fjord. Megafaunal species richness, seabed substrates, and biotopes of the fjord were studied in 2000, 2001, 2014, and 2015 using underwater video as part of a joint project between the University of Bergen and the Institute of Marine Research with funds from the Norwegian Biodiversity Information Centre. Clear gradients in species richness and composition were found related to distance into the fjord, depth, and landscape features (fjord sides, basin plain, shallower side fjords). Detrended Correspondence Analyses of results from detailed video annotations indicated the presence of six biotopes with characteristic species composition and environment. Earlier fjord studies have shown that deep-sea species often occur shallower in fjord basins than in adjacent offshore areas. Furthermore, the isolation of fjord basins behind one or more sills can lead to mass occurrence of species that by chance have been introduced and been able to establish dense populations. The limited contact between fjord basins in relation to the open ocean and between side fjords makes the extremely large and branched Sognefjord particularly interesting for studies of the effects of connectivity on bottom communities. Similarities to communities outside the fjord are discussed.

Infestation of Sacculina beauforti on an aquaculture species, mud crab Scylla olivacea is alarming due to its high prevalence and the extreme morphological changes in hosts. To further understand its pathological effect on growth and reproduction of S. olivacea, gonadal and hepatopancreatic histological changes of infected individuals were compared with healthy individuals. Also, the histological characteristics of S. beauforti's mature externa was described. Hepatopancreases of infected individuals were loosely packed and rootlets were observed in the intertubular spaces. Although hepatopancreatic tubule count was significantly lower, tubule diameters were unaffected. Gonads, however, were severely affected. No germ cells were found in the infected testes (except for remnants of spermatozoa), indicating the arrest of spermatogenesis. Rootlets were also present in testes of infected individuals. Ovarian tissues of infected females were severely damaged with no rigid cell structures. Gonadosomatic index (GSI) of infected males and females were lower, but the hepatosomatic index (HSI) were higher than their healthy counterparts. No significant differences in GSI and HSI values were observed among infected males and females. Multiple regression analysis revealed that carapace width, GSI and HSI are statistically important for the prediction of infection status. Given the high prevalence of S. beauforti infection and its disruptive effects on the morphology and internal physiology of S. olivacea, this study, albeit fundamental and incomprehensive, highlights to farmers and researchers the emergence of a castrating parasite and the urgency for the development of preventive measures and treatments for this disease in an economically important aquaculture species.

Rhizocephalans are abundant members of marine ecosystems and are important regulators of crustacean host populations. Morphological and ecological variation makes them an attractive system for evolutionary studies of advanced parasitism. Such studies have been impeded by a largely formalistic taxonomy, because rhizocephalan morphology offers no characters for a robust phylogenetic analysis. We use DNA sequence data to estimate a new phylogeny for 43 species and use this to develop a revised taxonomy for all Rhizocephala. Our taxonomy accepts 13 new or redefined monophyletic families. The traditional subdivision into the suborders Kentrogonida and Akentrogonida is abandoned, because both are polyphyletic. The three ‘classical’ kentrogonid families are also polyphyletic, including the species-rich Sacculinidae, which is split into a redefined and a new family. Most species of large families remain to be studied based on molecular evidence and are therefore still assigned to their current genus and family by default. We caution against undue generalizations from studies on model species until a more stable species-level taxonomy is also available, which requires more extensive genus- and species-level sampling with molecular tools. We briefly discuss the most promising future studies that will be facilitated by this new phylogeny-based taxonomy.

In this study we describe a unique fossil comprising 13 intact specimens of a pedunculate cirripede attached to a sponge (Codites serpentinus). The fossil comes from the Upper Jurassic lithographic limestones of southern Germany. Based on the shape and distinctive sculpture of the plates, a new genus and species Litholepas klausreschi gen. et sp. nov. (Eolepadidae: Neolepadinae) is introduced. Although lateral plates are indistinct and the rostrum is unknown, plate disposition and strong external sculpture indicate a position within Neolepadinae tough the exact relationships are still uncertain. Representatives of L.klausreschi gen. et sp. nov. are considered to have lived either in a parasitic or commensal relationship partially buried within the sponge.

The blue swimming crab (Portunus pelagicus Linnaeus, 1758) is one of the commercially exploited crab fishery resources in Vietnam. This is the first study to provide a broad survey of genetic diversity, population structure and migration patterns of P. pelagicus along the Vietnamese coastline. The crab samples were collected from northern, central and southern Vietnam. Here, we used a panel of single nucleotide polymorphisms (SNPs) generated from restriction site-associated DNA sequencing (RADseq). After removing 32 outlier loci, 306 putatively neutral SNPs from 96 individuals were used to assess fine-scale population structure of blue swimming crab. The mean observed heterozygosity (Ho) and expected heterozygosity (He) per locus was 0.196 and 0.223, respectively. Pairwise Fst and hierarchical AMOVA supported significant differentiation of central and northern from southern populations (P

Barnacles and their allies (Thecostraca) are a biologically diverse, monophyletic crustacean group, which includes both intensely studied taxa, such as the acorn and stalked barnacles, as well as cryptic taxa, for example, Facetotecta. Recent efforts have clarified phylogenetic relationships in many different parts of the barnacle tree, but the outcomes of these phylogenetic studies have not yet been combined into a single hypothesis for all barnacles. In the present study, we applied a new “synthesis” tree approach to estimate the first working Barnacle Tree of Life. Using this approach, we integrated phylogenetic hypotheses from 27 studies, which did not necessarily include the same taxa or used the same characters, with hierarchical taxonomic information for all recognized species. This first synthesis tree contains 2,070 barnacle species and subspecies, including 239 barnacle species with phylogenetic information and 198 undescribed or unidentified species. The tree had 442 bifurcating nodes, indicating that 79.3% of all nodes are still unresolved. We found that the acorn and stalked barnacles, the Thoracica, and the parasitic Rhizocephala have the largest amount of published phylogenetic information. About half of the thecostracan families for which phylogenetic information was available were polyphyletic. We queried publicly available geographic occurrence databases for the group, gaining a sense of geographic gaps and hotspots in our phylogenetic knowledge. Phylogenetic information is especially lacking for deep sea and Arctic taxa, but even coastal species are not fully incorporated into phylogenetic studies.

We use Rhizocephala to illustrate the problems inherent in estimating the phylogeny of parasitic crustaceans. The adult rhizocephalan parasite has such a reduced morphology that little else than the presence of a moulted cuticle relegates them to Arthropoda. Therefore, until the advent of molecular phylogenetic analysis relegation of the Rhizocephala to taxon relied exclusively on larval characters. The reduced adult morphology also entailed that very few characters were available for intrinsic rhizocephalan systematics and virtually none that could be compared with any outgroup. Thus, rhizocephalan taxonomy relied only on analysis within the group and, with few exceptions, was not based on any phylogenetic principles. The advent of DNA methods in phylogeny confirmed, with high confidence, rhizocephalans as cirripedes and nested them within the taxon as the sister group to Thoracica. This result was supported by SEM studies of the cypris larvae. Additional molecular phylogenetic studies yielded detailed insight into rhizocephalan relationships, culminating in the phylogeny presented here, which includes species from all but one family. Contrary to traditional hypotheses, the new phylogeny shows that infection of the host using a kentrogon stage represents the plesiomorphic condition, while rhizocephalans (Akentrogonida) without this stage are advanced. In this analysis, both Kentrogonida and the species-rich Sacculinidae are paraphyletic. In addition, recent hypotheses on family-level relationships in Akentrogonida, based on larval structure and the sexual system, were largely confirmed. This shows that when accurately analysed in a strict homology regime, morphological characters can be powerful partners to molecular data in elucidating rhizocephalan phylogeny.

Crustaceans play essential roles as food sources for global coastal communities, and their fisheries and aquaculture support the economic growth of many coastal countries. Thus, the understanding of diseases in crustaceans, especially those with high economic values, are important for their cure and prevention. Among other parasites that parasitise crustaceans, parasitic barnacles (Rhizocephala) are considered the most intriguing, both in terms of their unique life cycle and the detrimental changes they induced upon their hosts such as host sterilisation, moult inhibition (anecdysis), growth reduction, feminisation of male individuals, and behaviour alteration. In addition to discussing in detail the morphological, physiological and behavioural changes caused by rhizocephalan infestations, this review also shows that rhizocephalans are found in most economically important crustacean species around the world. Based on their reported adverse effects on their hosts, their potential implications on various aspects of crustacean aquaculture including broodstock selection, culture and fattening, soft-shell crab production and breeding programs are discussed. Although treatment or cure for rhizocephalan infestation has yet to be found, several recommendations are provided to enable accurate identification and handling of infected hosts and exposed aquaculture facilities in order to minimize losses and prevent parasite outbreaks.

Anelasma squalicola is a barnacle found attached to deep-water lantern sharks of the family Etmopteridae and is the only known cirriped on fish hosts. While A. squalicola is equipped with mouth and thoracic appendages (cirri), which are used for suspension feeding in conventional barnacles, its attachment device (peduncle) appears to have evolved into a feeding device, embedded into the tissue of its host. Here we demonstrate, through comparisons of the feeding apparatuses between A. squalicola and conventional suspension-feeding barnacles, that mouthparts and cirri of A. squalicola are highly reduced, and incapable of suspension-feeding activities. We show that in conventional suspension-feeding barnacles strong symmetries exist within these vital trophic structures. In A. squalicola strong asymmetries are widespread, indicating that those structures have been uncoupled from natural selection. The digestive tract is consistently empty, suggesting that feeding via cirri does not occur in A. squalicola. In addition, comparisons of stable isotope ratios (δ13C and δ15N) between A. squalicola, its shark host, and a conventional suspending feeding barnacle indicate that A. squalicola is taking nutrition directly from its host shark and not from the surrounding water. Our results strongly indicate that this barnacle has abandoned suspension feeding and now solely relies on obtaining nutrition from its host by a de novo evolved feeding mechanism. publishedVersion

Rhizocephalan barnacles have been reported to parasitize a wide range of king crab species (Lithodidae). So far all these parasites have been assigned to a single species, Briarosaccus callosus Boschma, 1930, which is assumed to have a global distribution. Here we investigate Briarosaccus specimens from three different king crab hosts from the fjord systems of Southeastern Alaska: Lithodes aequispinus Benedict, 1895, Paralithodes camtschaticus (Tilesius, 1815), and Paralithodes platypus (Brandt, 1850). Using molecular markers and by morphological comparison we show that Briarosaccus specimens from these three commercial exploited king crabs are in fact morphologically distinct from B. callosus, and further represent two separate species which we describe. The two new species, Briarosaccus auratum n. sp. and B. regalis n. sp., are cryptic by morphological means and were identified as distinct species by the use of genetic markers (COI and 16S). They occur sympatrically, yet no overlap in king crab hosts occurs, with B. auratum n. sp. only found on L. aequispinus, and B. regalis n. sp. as parasite of the two Paralithodes hosts.

The ecologically important shore crab Carcinus maenas is commonly infected in its native range by the rhizocephalan Sacculina carcini. However, several aspects of this host-parasite interaction are poorly understood. Here, we analyse data from approximately 60000 Danish crabs to unravel factors governing infection patterns in time and space, and according to host sex and size. Female crabs were more frequently infected (12.6%) than males (7.9%). Sites with high salinity supported the highest infection prevalence. Infection prevalence peaked in summer (10 to 15%) and winter (20 to 35%) due in part to emergence of virginal externae in summer (main outbreak) and autumn (minor outbreak) preceded by peaks in crabs with lost externa (scars). Younger externae and scars dominated among males, whereas adult externae were most frequent among females. Infection prevalence increased with size in females but decreased in males, and modified (feminized) males showed lower scar frequency than unmodified ones. Modified males occurred frequently among the smaller size classes, whereas unmodified males dominated the larger size classes. Externa size was positively related to host size in both genders (same linear relationship). Molecular analyses suggested that hosts below 16 mm in carapace width do not become infected. Dissections of infected hosts revealed marked reduction of ovaries, whereas testes were unaffected by sacculinization. Our study demonstrates great spatio-temporal variation in infection prevalence mainly related to the parasite's life history. S. carcini appears capable of infecting all host sizes except the smallest. Owing to incomplete feminization of males, infections are rapidly lost from the larger and highly profitable male hosts.

Summary The barnacle Anelasma squalicola is a marine epibiont found on members of the species-rich, deep-sea lantern shark family Etmopteridae ( Figure 1 A) but is unlike any other epibiotic thoracian barnacles [1] . While many barnacle species are associated with various marine animals including turtles and whales, with the exception of Anelasma these all retain a filter-feeding lifestyle and have a commensal relationship with their host; despite often being deeply embedded in the dermis, no other species has been reported as feeding on its host. Although Anelasma is fully equipped with cirri (thoracic appendages), these are no longer used for filter feeding [1] . Instead, Anelasma embeds a stalk with root-like structures into the flesh of the shark ( Figure S1 C in Supplemental Information, published with this article online) that it uses to parasitize its host. Here, we show that specimens of Anelasma sampled from all over the world show very little genetic differentiation, suggesting that this innovation coincided with a rapid worldwide expansion.

All available biological, ultrastructural, and molecular data support the conclusion that the cirripedian suborders, Rhizocephala and Thoracica, are closely related. Williamson’s speculations (Williamson, D. I., 2014. The origin of barnacles (Thecostraca, Cirripedia). Crustaceana,87: 755-765) that the so-called hybridogenesis can explain how Rhizocephala evolved, rest on a selective choice of other authors’ data and an ignorance of solid facts derived from decades of comparative anatomical and molecular studies.