Aim Invasive species are of increasing global concern. Nevertheless, the mechanisms driving further distribution after the initial establishment of non‐native species remain largely unresolved, especially in marine systems. Ocean currents can be a major driver governing range occupancy, but this has not been accounted for in most invasion ecology studies so far. We investigate how well initial establishment areas are interconnected to later occupancy regions to test for the potential role of ocean currents driving secondary spread dynamics in order to infer invasion corridors and the source–sink dynamics of a non‐native holoplanktonic biological probe species on a continental scale. Location Western Eurasia. Time period 1980s–2016. Major taxa studied ‘Comb jelly’ Mnemiopsis leidyi. Methods Based on 12,400 geo‐referenced occurrence data, we reconstruct the invasion history of M. leidyi in western Eurasia. We model ocean currents and calculate their stability to match the temporal and spatial spread dynamics with large‐scale connectivity patterns via ocean currents. Additionally, genetic markers are used to test the predicted connectivity between subpopulations. Results Ocean currents can explain secondary spread dynamics, matching observed range expansions and the timing of first occurrence of our holoplanktonic non‐native biological probe species, leading to invasion corridors in western Eurasia. In northern Europe, regional extinctions after cold winters were followed by rapid recolonizations at a speed of up to 2,000 km per season. Source areas hosting year‐round populations in highly interconnected regions can re‐seed genotypes over large distances after local extinctions. Main conclusions Although the release of ballast water from container ships may contribute to the dispersal of non‐native species, our results highlight the importance of ocean currents driving secondary spread dynamics. Highly interconnected areas hosting invasive species are crucial for secondary spread dynamics on a continental scale. Invasion risk assessments should consider large‐scale connectivity patterns and the potential source regions of non‐native marine species.

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Prey availability to seabirds has a profound influence on individual decisions about allocating somatic and reproductive investment. These decisions can be expressed in foraging behavior and prey utilization and have consequences for establishing relationships between changes in the fish populations and responses in seabird breeding performance. We report here results of an unusual opportunity to investigate the relationships between fish abundance and at-sea foraging behavior, prey utilization and food provisioning of partners and chicks of Common Terns Sterna hirundo breeding in the German Wadden Sea. High quality prey was carried out of the foraging area disproportionately often, while almost all low quality prey items were ingested by the foraging adult bird itself. Proportions of prey being used for provisioning were more similar to prey being carried out of the foraging area than to prey caught. The preferential utilization of high quality food for provisioning suggests that large proportions of low quality food being delivered to the colony may indicate a shortage of high quality food and, consequently, poor prospects of good breeding performance. Moreover, seabirds feeding whole, undigested prey items may indicate a higher abundance of high quality fish in the sea, due to selecting high quality prey for provisioning. This may result in overestimating the abundance of high quality prey fish when calculated from colony-based diet studies of single-loading seabird species such as terns alone.Zusammenfassung Die Beuteverfügbarkeit für Seevögel hat eine zentrale Bedeutung für die individuelle Entscheidung, ob die verfügbare Energie in das eigene Ü berleben oder die Reproduktion investiert wird. Diese Entscheidungen werden u.a. in Jagdverhalten und Beutenutzung manifest, was sich wiederum auf die funktionelle Beziehung zwischen den Beutepopulationen, dem Brutverlauf und der Kükenaufzucht der Seevögel auswirken kann. Basierend auf direkten Beobachtungen beschreibt der vorliegende Artikel die Beziehung zwischen der Fischabundanz und dem Jagdverhalten auf See, der Beutenutzung sowie der Balz-und Kükenfütterung bei Flussseeschwalben Sterna hirundo im deutschen Wattenmeer. Qualitativ hochwertige Beute wurde überdurchschnittlich oft aus dem Jagdgebiet in die Kolonien getragen, während nahezu alle Beuteorganismen von geringer Qualität vom jagenden Altvogel selbst konsumiert wurden. Die relativen Anteile verschiedener Beutetiere an der Balz-und Kükennahrung stimmten besser mit der aus dem Jagdgebiet abtransportierten Beute überein als mit der Beute, die insgesamt Communicated by T.

This study was undertaken to further clarify whether the brown shrimp, Crangon crangon (Linnaeus 1758), is a gonochorist, a facultative or an obligate hermaphrodite. Juvenile shrimps were sampled from intertidal habitats along the German Wadden Sea coast with a push net and from a power plant water inlet to quantify the share of primary females. Length-based sex ratios were determined for about 27,000 individuals using external characteristics. Observed sex ratios were mainly female-biased, and also large males occurred regularly in the catch. This indicates that sex at hatch is not male as would be characteristic for an obligate protandric hermaphrodite and that not all male shrimps change sex. A cohort-based computer simulation, including sex-specific growth rates, mortality and seasonally varying recruitment, generated sex ratios comparable to the field. The observed decline in the proportion of males with increasing size can be explained solely by faster growth of females without involving hermaphroditism. Based on temperature-dependent growth and moult rates as well as length-specific numbers of eggs per female, the potential egg production of primary and secondary females was modelled, yielding contributions of secondary females of < 1%. Sex change in C. crangon has previously been observed and may be interpreted as an evolutionary relict of this species having evolved in a habitat characterized by lower population densities, lower predation levels and increased longevity compared to today's living conditions in North Sea coastal waters

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Whereas the anthropogenic impact on marine biodiversity is undebated, the quantification and prediction of this change are not trivial. Simple traditional measures of biodiversity (e.g. richness, diversity indices) do not capture the magnitude and direction of changes in species or functional composition. In this paper, we apply recently developed methods for measuring biodiversity turnover to time-series data of four broad taxonomic groups from two coastal regions: the southern North Sea (Germany) and the South African coast. Both areas share geomorphological features and ecosystem types, allowing for a critical assessment of the most informative metrics of biodiversity change across organism groups. We found little evidence for directional trends in univariate metrics of diversity for either the effective number of taxa or the amount of richness change. However, turnover in composition was high (on average nearly 30% of identities when addressing presence or absence of species) and even higher when taking the relative dominance of species into account. This turnover accumulated over time at similar rates across regions and organism groups. We conclude that biodiversity metrics responsive to turnover provide a more accurate reflection of community change relative to conventional metrics (absolute richness or relative abundance) and are spatially broadly applicable. This article is part of the theme issue ‘Integrative research perspectives on marine conservation’.

Bimodal depth distribution patterns observed for sprat Sprattus sprattus larvae in previous field studies conducted in the deep basins of the Baltic Sea have led researchers to hypothesise that larval sprat condition was depth-dependent. We examined this hypothesis by measuring morphological, biochemical and otolith-based proxies for nutritional condition in sprat larvae collected in discrete 5 m depth intervals from the surface to the bottom in the central Bornholm Basin. Similar to earlier studies, larval sprat were most abundant in 2 depth strata (0 to 10 and 65 to 75 m). Their nutritional condition in surface and deep waters was not uniformly expressed by the different indices. For example, sprat larvae from 0 to 10 m could not be distinguished from conspecifics caught at 65 to 75 m by a long-latency condition proxy (otolith-based growth rates). Similarly, a mediumlatency proxy (RNA:DNA) did not suggest differences in condition between the depths. However, short-latency proxies (protein:standard length and DNA:dry weight) supported the depth-dependent condition hypothesis. The lack of correspondence and pitfalls associated with the use and interpretation of multiple condition indices (e.g. the influences of temperature and body size) are discussed and recommendations to strengthen these various metrics are provided.