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Gelatinous zooplankton (GZ) such as medusae, ctenophores, siphonophores, pyrosomes and salps are important components of oceanic pelagic communities and small calycophoran siphonophores (CS) are typically abundant at shallow depths. The Sargasso Sea spawning area of the Atlantic catadromous freshwater eels has a regular pattern of shallow autumn to spring temperature fronts. There is limited information about the southern Sargasso Sea GZ fauna, and it is not known which species are distributed across these frontal zones. Plankton samples from a survey of larval European eel (Anguilla anguilla) abundance in March and April 2017 using an Isaacs-Kidd Midwater Trawl (0–300 m, 35 stations, three transects) were used to examine the distribution and abundance of net-captured CS and other GZ species in relation to oceanographic characteristics. More than 2200 specimens of 15 taxa were sub-sampled, with five CS (Abylopsis tetragona, A. eschscholtzii, Chelophyes appendiculata, Eudoxoides spiralis and E. mitra) dominating catches at every station. GZ were most abundant around the 22 and 24 °C isotherms, and higher abundances of CS in the north were correlated with lower water temperature. The widespread presence of CS across the European eel spawning area is consistent with a recent study detecting their DNA sequences in the gut contents of young eel larvae collected in the Sargasso Sea, suggesting CS material was either eaten directly or as part of ingested marine snow particles. The present study shows that both types of organisms occupy the southern Sargasso Sea during the European eel spawning season.

Here, we report on a comprehensive collection of mainly early life cephalopods that were sampled during a research cruise to the Sargasso Sea with the RV Maria S. Merian in April 2015. A total of 2466 cephalopods were caught which belonged to at least 27 species (17 families). The most abundant family was represented by the hooked squids (Onychoteuthidae) with a share of 19.18% of the total cephalopod catch. The subtropical convergence zone (STCZ) was found approximately around 27 °N. The STCZ was characterised by a sharp near-surface temperature gradient and divided the Sargasso Sea into a northern and a southern area. This division was also reflected in the cephalopod community composition. For example, the cranchiid Leachia lemur prevailed in the northern part, and the incirrate octopod, Japetella diaphana, was mainly distributed in the southern part of the study area. Principal component analysis (PCA) and redundancy analysis (RDA) analyses detected a significant correlation between species occurrence and sea surface temperature (SST) as well as a longitudinal gradient with distinct clusters along the five transects from west to east. Ordination analysis (MDS) showed significant differences in the cephalopod assemblages between day and night with midwater forms (Enoploteuthidae, Pyroteuthidae) dominating the night catches, probably due to their upward migration into the top 200 m during the night.

The role of intracontinental migration patterns of European eel (Anguilla anguilla) receives more and more recognition in both ecological studies of the European eel and possible management measures, but small-scale patterns proved to be challenging to study. We experimentally investigated the suitability of fatty acid trophic markers to elucidate the utilization of feeding habitats. Eight groups of juvenile European eels were fed on eight different diets in a freshwater recirculation system at 20°C for 56 days. Three groups were fed on freshwater diets (Rutilus rutilus, Chironomidae larvae, and Gammarus pulex) and four groups were reared on diets of a marine origin (Clupea harengus, Crangon crangon, Mysis spec., and Euphausia superba) and one on commercial pellets used in eel aquaculture. Fatty acid composition (FAC) of diets differed significantly with habitat. FAC of eel muscle tissue seemed to be rather insensitive to fatty acids supplied with diet, but the general pattern of lower n3:n6 and EPA:ARA ratios in freshwater prey organisms could be traced in the respective eels. Multivariate statistics of the fatty acid composition of the eels resulted in two distinct groups representing freshwater and marine treatments. Results further indicate the capability of selectively restraining certain fatty acids in eel, as e.g. the n3:n6 ratio in all treatments was <4, regardless of dietary n3:n6. In future studies on wild eel, these measures can be used to elucidate the utilization of feeding habitats of individual European eel.

In recent years, thicklip grey mullet Chelon labrosus has shown increasing expansion of its native habitats in the north-eastern Atlantic into northerly adjacent areas including the North Sea and the brackish Baltic Sea. Despite the regular annual and seasonal occurrence of C. labrosus in the western Baltic during the warm months, nothing is known of the origin or whereabouts of the mullet during the cold season. As different possible migration scenarios can be considered, we performed otolith microchemistry analyses on specimens from the western Baltic Sea to identify the origin of this nonindigenous species. Comparison with North Sea samples revealed common habitat preferences and underlined the highly euryhaline nature of C. labrosus in different recently occupied habitats. Occasional fluctuations of Sr/Ca ratio along the growth axis suggest periodical migration between waters of different salinities but did not reveal distinct migration pathways.

The present study experimentally tested the influence of water temperature on the inclusion of 15 elements into juvenile European eel (Anguilla anguilla) otoliths in freshwater. It should be investigated (1) if temperature effects on otolith Sr/Ca might impair the interpretation of migration studies and (2) if the elemental composition of otoliths can be used to reconstruct experienced temperature histories of eels. Therefore, eels were kept under full experimental conditions at three different water temperatures (14 °C, 19 °C and 24 °C) for 105 days. Thereafter, laser ablation inductively coupled mass spectrometry (LA-ICPMS) was conducted on the outer edge of their otoliths. Our analyses revealed significant temperature effects on otolith Na/Ca, Sr/Ca, Mg/Ca, Mn/Ca, Ba/Ca, Zr/Ca and Y/Ca ratios. Variations of Sr/Ca caused by temperature were far below those used to detect eel movements between waters of different salinities and will therefore not affect the interpretation of migration studies. Elemental fingerprints of Sr/Ca, Mg/Ca, Mn/Ca and Ba/Ca ratios resulted in clearly separated groups according to temperature treatments, indicating that changes in water temperature might lead to characteristic changes in otolith element composition. However, the successful application of elemental fingerprints to reconstruct moderate changes of water temperature seems doubtful because the influence of somatic growth on otolith microchemistry still remains unclear, and temperature-induced variations could be overlaid by changes of water element concentrations during growth periods. Nevertheless, our results contribute to the completion of knowledge about factors influencing element incorporation and help to explain variations in element composition of fish otoliths.

Otolith microchemistry is widely used as a tool to track individual migration pathways of diadromous fish under the assumption that the elemental composition of fish otoliths is directly influenced by the physicochemical properties of the surrounding water. Nevertheless, several endogenous factors are reported to affect element incorporation into fish otoliths and might lead to misinterpretations of migration studies. This study experimentally examined the influence of eight different diets on the microchemical composition of European eel (Anguilla anguilla) otoliths using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Seven natural prey types and one artificial diet were fed during 8 weeks in freshwater circuits. Results show for the first time that food has no significant influence on the incorporation of Na, Sr, Ba, Mg, Mn, Cu and Y into European eel otoliths. This indicates that the incorporation of elements usually chosen for migration studies is not affected by diet and that individual feeding behaviour of A. anguilla will not lead to any misinterpretation of migration pathways.

Species of the genus Hysterothylacium are among the most common marine nematode fish parasites in the northern Atlantic. Due to recent findings of cryptic speciation in other parasitic ascaridoid nematodes, a similar pattern of sibling species was hypothesized also for Hysterothylacium aduncum. By investigating a 886- to 890-bp-long genomic DNA fragment including ITS-1, 5.8S rDNA and ITS-2 of 40 specimens of H. aduncum of sprat (Sprattus sprattus) of four different biogeographical regions (North Sea, English Channel, Bay of Biscay, Adriatic Sea), we could not detect significant genetic variability and therefore cryptic speciation. Nevertheless, while ITS-1 and 5.8S rDNA sequences were identical for all analysed specimens, ITS-2 sequences showed a population-specific pattern with the differentiation of an English Channel/Bay of Biscay group from a North Sea/Mediterranean Sea group.

The family Labridae is a large assemblage of marine fish composed of about 580 species in 82 genera distributed in tropical and temperate marine waters around the world. Several subgroups, currently classified as tribes, have been identified in this large family, yet only a few phylogenetic analyses have been performed on labrid clades. We confirm monophyly of the labrid tribe Labrini and propose a phylogeny of the 23 species of the genera Acantholabrus, Centrolabras, Ctenolabrus, Labrus, Lappanella, Symphodus, Tautoga, and Tautogolabrus occurring in the eastern and western Atlantic and the Mediterranean. We analyzed a 577-bp segment of the mitochondrial 16S rDNA and a 506-bp segment of the mitochondrial control region in 22 species, for a total of up to 1069 bp per species. We used both parsimony and likelihood approaches under a variety of assumptions and models to generate phylogenetic hypotheses. The main features of the molecular phylogeny for the Labrini turned out to be the same for the two algorithms applied. The tree structure is similar to a previous, unpublished morphological phylogeny for a subset of labrine species. Estimated divergence times of the Labrini based on fossils and a molecular clock range from about 15 mya for the deepest splits to less than 1 mya for younger clades. Biogeographic patterns of the Symphodus species group and the genus Labrus are dominated by speciation events driven by the closing and opening of the Mediterranean Sea and periodic glaciation events during the past 1 million years. The Labrini are the only clade in the entire Labridae that exhibit nest-building and broodcare behavior. We use the phylogeny to show that similar broodcare behavior has evolved twice in the labrine fish and discuss scenarios for the evolution of broodcare from the diandric protogynous hermaphroditism found in ancestral labrines and many other wrasses.

Seabreams are among the most valuable fish, not only for small-scale and semiindustrial fisheries but also for aquaculture throughout the Mediterranean. Nevertheless, their phylogenetic relationships are not at all clear. The current taxonomy is based solely on trophic morphology and rests on the assumption that each trophic type evolved only once from a less specialized ancestral condition. We analyzed a 486-bp segment of the mitochondrial 16S rDNA of all 24 seabream species described for the northeastern Atlantic and the Mediterranean to elucidate their generic and subfamily-level relationships. Three major mitochondrial lineages, each comprising species of different feeding strategy and dentition, were found that do not agree with the present taxonomic assignments. Most of the investigated genera were resolved paraphyletically, indicating that the structure and arrangement of oral teeth must have repeatedly evolved from a less specialized ancestral condition. Further, the genus Sparus was resolved as distantly related to the genus Pagrus, in that it was assigned to a different major mitochondrial lineage. Oblada melanura was consistently placed within the Diplodus radiation as sister group to Diplodus puntazzo. Our phylogenetic hypothesis thus suggests multiple independent origins of similar trophic specializations within the Sparidae and indicates that the currently recognized three or four subfamilies need to be redefined.