Population genomic structure of the gelatinous zooplankton species Mnemiopsis leidyi in its nonindigenous range in the North Sea

Nonindigenous species pose a major threat for coastal and estuarine ecosystems. Risk management requires genetic information to establish appropriate management units and infer introduction and dispersal routes. We investigated one of the most successful marine invaders, the ctenophore Mnemiopsis leidyi, and used genotyping-by-sequencing (GBS) to explore the spatial population structure in its nonindigenous range in the North Sea. We analyzed 140 specimens collected in different environments, including coastal and estuarine areas, and ports along the coast. Single nucleotide polymorphisms (SNPs) were called in approximately 40 k GBS loci. Population structure based on the neutral SNP panel was significant (F-ST .02; p < .01), and a distinct genetic cluster was identified in a port along the Belgian coast (Ostend port; pairwise F-ST .02-.04; p < .01). Remarkably, no population structure was detected between geographically distant regions in the North Sea (the Southern part of the North Sea vs. the Kattegat/Skagerrak region), which indicates substantial gene flow at this geographical scale and recent population expansion of nonindigenous M. leidyi. Additionally, seven specimens collected at one location in the indigenous range (Chesapeake Bay, USA) were highly differentiated from the North Sea populations (pairwise F-ST .36-.39; p < .01). This study demonstrates the utility of GBS to investigate fine-scale population structure of gelatinous zooplankton species and shows high population connectivity among nonindigenous populations of this recently introduced species in the North Sea. OPEN RESEARCH BADGES This article has earned an Open Data Badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at: The DNA sequences generated for this study are deposited in the NCBI sequence read archive under SRA accession numbers -, and will be made publically available upon publication of this manuscript

In situ mortality experiments with juvenile sea bass (Dicentrarchus labrax) in relation to impulsive sound levels caused by pile driving of windmill foundations

Impact assessments of offshore wind farm installations and operations on the marine fauna are performed in many countries. Yet, only limited quantitative data on the physiological impact of impulsive sounds on (juvenile) fishes during pile driving of offshore wind farm foundations are available. Our current knowledge on fish injury and mortality due to pile driving is mainly based on laboratory experiments, in which high-intensity pile driving sounds are generated inside acoustic chambers. To validate these lab results, an in situ field experiment was carried out on board of a pile driving vessel. Juvenile European sea bass (Dicentrarchus labrax) of 68 and 115 days post hatching were exposed to pile-driving sounds as close as 45 m from the actual pile driving activity. Fish were exposed to strikes with a sound exposure level between 181 and 188 dB re 1 mu Pa-2.s. The number of strikes ranged from 1739 to 3067, resulting in a cumulative sound exposure level between 215 and 222 dB re 1 mu Pa-2.s. Control treatments consisted of fish not exposed to pile driving sounds. No differences in immediate mortality were found between exposed and control fish groups. Also no differences were noted in the delayed mortality up to 14 days after exposure between both groups. Our in situ experiments largely confirm the mortality results of the lab experiments found in other studies

Understanding the impact of sand extraction on benthic ecosystem functioning: a combination of functional indices and biological trait analysis

Marine aggregates have been intensively extracted in the North-East Atlantic over the past decades. This study aimed to assess the effect of sand extraction on benthic ecosystem functioning using a combination of biological traits and functional indices (the bioturbation (BPc) and irrigation potential (IPc) and secondary production (SPc) of the macrobenthic community). Data on macrobenthos, sediment properties and extraction intensity were collected over a time period of ten years (2010 – 2019) for three coarse sediment extraction areas in the Belgian Part of the North Sea, each with a different extraction regime. Sediment parameters such as the medium sand fraction (250 – 500 µm) and median grain size showed a significant effect on all functional indices. Whilst sand extraction variables only significantly affected secondary production estimates. The secondary production of the macrobenthic community decreased following a high yearly extraction intensity, whereas a high cumulative (10-year period) extraction intensity resulted in a slightly increased secondary production. Species-specific responses revealed that these high cumulative extraction volumes increased the abundance of opportunistic species, which could have contributed to the higher SPc values observed in cumulative disturbed areas. Response traits such as tube-living and sessile individuals with a pelagic egg development were positively influenced by a long-term disturbance, an indication of a more disturbance-tolerant community. A short-term disturbance rather seemed to favor a macrobenthic community characterized by a higher burrowing capability. In terms of effect traits, both short- and long-term extraction clearly favored deposit feeders, which can structure organic matter distribution and thus indirectly influence nutrient and oxygen fluxes as well. Future in situ measurements in sand extraction areas could help to unravel and strengthen our understanding of the ecosystem processes linked to these trait-based observations

A comprehensive study to assess the impact of impulsive sound on juvenile sea bass

Given the increasing amount of anthropogenically induced underwater sound into the marine environment, a better understanding of the impact of impulsive underwater sound on marine life is needed. This study tackles the impact of impulsive sound, related to pile-driving activities for offshore wind energy development, on the mortality, stress and behaviour of post-larval and juvenile European sea bass Dicentrarchus labrax. A 'worst-case scenario' field experiment was carried out on board of a piling vessel, exposing 68 and 115 days old fish (<2 g wet weight) to the sound generated during 1.5 hours of pile-driving. The number of strikes ranged from 1740 to 3070, with a single strike sound exposure level between 181 and 188 dB re 1 μPa².s, resulting in cumulative sound exposure levels ranging from 215 to 222 dB re 1 μPa².s. Immediate and long-term survival of the exposed fish was high and comparable to the control groups. However, juvenile fish responded to the impulsive underwater sound by a 50% reduction in their oxygen consumption rates, an indicator of secondary stress response. Primary stress responses, measured through cortisol levels are still to be analysed. We didn't find any effect on the condition and fitness of the exposed fish on the long term. Lab experiments performed with a SIG Sparker and a larvaebrator, respectively producing mid-high and lower frequencies, were inadequate to distinguish the determining sound metric or to pursue the exact origin of the stress response. Further away from the sound source, behavioural and masking effects can be expected. A lab experiment was carried out to study the behaviour of juvenile sea bass before, during and after one hour of impulsive sound exposure. In the aquaria, single strike sound levels reached 162 dB re 1 μPa².s, leading to a cumulative sound exposure level of 196 dB re 1 μPa².s after 2400 strikes. We observed that normal behaviour was disturbed, with an increase in startle responses and stationary behaviour at the beginning of the sound exposure experiment. Also, fish dived to the bottom of the aquaria, which is a typical anxiety-related response. However, no spatial preference was observed and normal behaviour was re-established shortly after the sound exposure ceased. These results indicate that impulsive sound close to the sound source creates sound pressure levels that are below the lethal threshold for fish, but above the stress threshold, at least for sea bass <2 g. Furthermore, lower sound levels at a distance from the sound source (in this case pile-driving) can disturb fish behaviour. Under optimal lab conditions, we did not see effects beyond the sound exposure period, but it remains unknown whether the reduced fitness of juvenile fish after exposure is limited in the real world as well