Marine hard-bottom communities are undergoing severe change under the influence of multiple drivers, notably climate change, extraction of natural resources, pollution and eutrophication, habitat degradation, and invasive species. Monitoring marine biodiversity in such habitats is, however, challenging as it typically involves expensive, non-standardized, and often destructive sampling methods that limit its scalability. Differences in monitoring approaches furthermore hinders inter-comparison among monitoring programs. Here, we announce a Marine Biodiversity Observation Network (MBON) consisting of Autonomous Reef Monitoring Structures (ARMS) with the aim to assess the status and changes in benthic fauna with genomic-based methods, notably DNA metabarcoding, in combination with image-based identifications. This article presents the results of a 30-month pilot phase in which we established an operational and geographically expansive ARMS-MBON. The network currently consists of 20 observatories distributed across European coastal waters and the polar regions, in which 134 ARMS have been deployed to date. Sampling takes place annually, either as short-term deployments during the summer or as long-term deployments starting in spring. The pilot phase was used to establish a common set of standards for field sampling, genetic analysis, data management, and legal compliance, which are presented here. We also tested the potential of ARMS for combining genetic and image-based identification methods in comparative studies of benthic diversity, as well as for detecting non-indigenous species. Results show that ARMS are suitable for monitoring hard-bottom environments as they provide genetic data that can be continuously enriched, re-analyzed, and integrated with conventional data to document benthic community composition and detect non-indigenous species. Finally, we provide guidelines to expand the network and present a sustainability plan as part of the European Marine Biological Resource Centre (www.embrc.eu).

The genetic diversity of the soft-shell clam Mya arenaria from seven locations in Europe (two stations in the southern Baltic Sea (the Gulf of Gdansk) and two in the North Sea (Veerse Meer and Oosterschelde), and three additional stations in the Denmark Straits and Bay of Biscay) was determined using starch gel electrophoresis of allozymes. The results showed a low level of genetic variability and a lack of genetic di¡erentation among the populations studied. Basic polymorphism characteristics calculated for populations from the North Sea estuaries and the Gulf of Gdansk were: He 0.094^0.145, Ho 0.092^0.130, percentage of polymorphic loci 33 (0.95 criterion), mean number of alleles per locus 2.0^2.7. The mean value of F ST was 0.0133 and not signi¢cant. It is concluded that in spite of a low level of genetic polymorphism the soft-shell clam is a successful colonizer. The genetic homogeneity among the populations re£ects rapid population extension, alleles neutrality and a high gene £ow.

The effect of eutrophication on benthos is usually reported as negative. In the case of the Gulf of Gdańsk, eutrophication has increased the availability of food resources to filter feeders such as mussels, affecting their physiology and vertical distribution. Comparative studies of the mussel Mytilus trossulus from two depth zones (10 m and 40 m) over a seasonal cycle revealed ecophysiologic differences between deep-and shallow-water animals. For the same shell length, the shallow-water mussels were heavier (dry weight ϭ 0.004L 2.297 ) and showed a higher weight index (dry weight/volume), averaging 3.8 Ϯ 1.9 mg cm Ϫ3 , than the deep-water mussels (dry weight ϭ 0.0002L 1.726 and 2.5 Ϯ 1.1 mg cm Ϫ3 ), primarily because of a nearly twofold greater carbohydrate store. In the shallow zone, females contained more carbohydrates (on average 5.8% dry weight) than males (3.8% dry weight) because females conserve energy for reproduction. Differences in physiologic variables, and subsequently physiologic performance of the mussels, were related to different nutritional conditions in the ambient water. The deep habitat had lower Chl a, averaging 1.1 mg L Ϫ1, total particulate matter (TPM) 4.3 mg L Ϫ1 and particulate organic matter (POM) 1.1% of TPM as compared with the shallow habitat, with average Chl a 2.5 mg L Ϫ1 , TPM 4.9 mg L Ϫ1 , and POM 1.5%. The eutrophication of the Gulf of Gdańsk has led to an increase in the food availability in the water column, which allowed efficient colonization of the deep zone by the mussels; however, the dominance of males over females (ϳ3 : 1) suggests that the food sustains only spawning-related metabolic demands and is not sufficient for energy conservation in this zone. Starch gel electrophoresis of eight enzyme loci showed no statistical differences in the allele and genotype frequencies between the shallow-and deep-water mussels; thus, the differences in ecophysiologic traits between depths are due to acclimatization.

Contemporary genetic constitution of marine species carries signatures of Pliocene-Pleistocene glacial cycles. Molecular studies of polar organisms show that isolation in refugia during glaciation often results in loss of genetic diversity. However, recent studies of marine organisms from the Southern Ocean have highlighted their remarkably high level of infraspecific genetic differentiation and the presence of cryptic species. Thus, demographic responses to climate change vary substantially with geography and life history. To elucidate the relative role of glacial period in driving the evolution of Antarctic and Arctic fauna we examined the genetic diversity and historical demography of the pelagic marine gastropods Limacina antarctica from Drake Passage in the Antarctic and Limacina helicina from Spitsbergen fjords in the Arctic. Diversity was assessed by comparing nucleotide sequences from part of the mitochondrial gene encoding the cytochrome c oxidase subunit I (COI). Sequences from 60 individuals of L. antarctica collected at 7 stations along Drake Passage were compared with those of 67 individuals of L. helicina from the fjords Hornsund and Isfjorden. We identified 47 different haplotypes for L. antarctica and 25 for L. helicina. No spatial genetic structure was found in either species, indicating that studied populations in each species belong to a single evolutionary unit. Demographic analyses of haplotype networks and significant negative Tajima's D and Fu's F S indices suggest recent rapid population expansion in both species. However, L. antarctica populations displayed a higher level of haplotype and nucleotide diversity than L. helicina populations, which suggests that the impact of glaciations was less prominent in L. antarctica.KEY WORDS: Polar regions · Glaciations · Limacina antarctica · Limacina helicina · Cytochrome c oxidase subunit I (COI) · Genetic diversity · Median-joining network · Bottleneck effect Resale or republication not permitted without written consent of the publisher

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