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Vessel biofouling is the putative vector for many marine invasions. Biofouling communities are often dominated by organisms with sessile life history strategies that cannot themselves exit a transport vector, thereby requiring another means of dispersal such as reproduction, which releases mobile propagules. For many sessile organisms, reproduction and associated propagule delivery is tightly coupled with environmental conditions, which frequently vary along vessel transit routes or in ports. However, there are still major gaps in our understanding of specific environmental effects on the reproductive biology of most marine non-indigenous species (NIS), limiting our ability to predict when and where introductions by reproduction will occur. Here, we conducted a manipulative laboratory experiment to explore how variation in temperature and food availability – two conditions that vary among vessel transit routes and ports – affect reproductive output of a common marine biofouling species, the barnacle Amphibalanus improvisus . The experiment had two phases: (1) a simulated “route” portion where temperature and food availability were manipulated independently followed by (2) a simulated “port” portion where temperature and food co-varied. We found that temperature and food availability during both phases affected barnacle fecundity, with reproductive output varying by an order of magnitude between some treatments. Barnacles tended to produce more propagules when temperatures were moderate and food was more plentiful as well as when entering relatively warm ports with greater food availability. Our results show how reproduction by marine biofouling species can be regulated by environmental conditions, which vary predictably with transit route, such that some routes and ports may be especially susceptible to introduction via reproduction. Identifying triggers for reproduction in common marine NIS is therefore key to evaluating the risk of marine invasions.

Understanding the ecological and evolutionary forces that determine the genetic structure and spread of invasive species is a key component of invasion biology. The bay barnacle, Balanus improvisus (= Amphibalanus improvisus), is one of the most successful aquatic invaders worldwide, and is characterised by broad environmental tolerance. Although the species can spread through natural larval dispersal, human-mediated transport through (primarily) shipping has almost certainly contributed to the current global distribution of this species. Despite its worldwide distribution, little is known about the phylogeography of this species. Here, we characterize the population genetic structure and model dispersal dynamics of the barnacle B. improvisus, and describe how human-mediated spreading via shipping as well as natural larval dispersal may have contributed to observed genetic variation. We used both mitochondrial DNA (cytochrome c oxidase subunit I: COI) and nuclear microsatellites to characterize the genetic structure in 14 populations of B. improvisus on a global and regional scale (Baltic Sea). Genetic diversity was high in most populations, and many haplotypes were shared among populations on a global scale, indicating that long-distance dispersal (presumably through shipping and other anthropogenic activities) has played an important role in shaping the population genetic structure of this cosmopolitan species. We could not clearly confirm prior claims that B. improvisus originates from the western margins of the Atlantic coasts; although there were indications that Argentina could be part of a native region. In addition to dispersal via shipping, we show that natural larval dispersal may play an important role for further colonisation following initial introduction.

This study reports for the first time the co‐invasion of two Western Atlantic native species, the parasitic leech Myzobdella lugubris and the commensal barnacle Amphibalanus improvisus on the invasive Atlantic blue crab Callinectes sapidus in southwestern Europe. Two individuals of M. lugubris parasitizing two male crabs were collected from the Guadalquivir River estuary (SW Spain) in August 2024 and September 2025, while A. improvisus was found on the carapace of an ovigerous female in April 2025. This is the first worldwide record of M. lugubris parasitizing C. sapidus in the host's non‐native range and the third European record for the leech, the first two having been documented on fish and turtles. Also, it is the first documentation of both associations with a crustacean host in European waters. Molecular analyses confirmed the identity of both non‐native species, with the M. lugubris COI sequence showing the highest similarity (99.85%) to a non‐native Hawaiian specimen, and 100% similarity for A. improvisus. Given the blood‐feeding nature of M. lugubris and its role as a recognized pathogen vector, the establishment of this parasitic association poses an unquantified but substantial risk for the translocation and emergence of novel infectious diseases in native European fish and crustacean populations. Furthermore, this finding highlights the need for comprehensive health surveillance of commercially exploited C. sapidus populations across the non‐native range. This study documents the first worldwide record of the parasitic leech Myzobdella lugubris and the commensal barnacle Amphibalanus improvisus associated with invasive Atlantic blue crabs (Callinectes sapidus) in southwestern Europe. Molecular analyses confirmed the identity of these Western Atlantic native species, highlighting a complex multi‐level co‐invasion in the Guadalquivir River estuary. Given the role of M. lugubris as a pathogen vector, this finding poses a significant risk for disease emergence in native European aquatic populations and highlights the need for comprehensive health surveillance.

The spread of marine non-indigenous species (NIS) is driven largely by shipping and global trade. Biofouling on vessel hulls is a major source of invasions, but many biofouling organisms are sessile (non-motile) and require reproduction, which often produces mobile, waterborne propagules (e.g., larvae) that can colonize new regions. The ability of biofouling organisms to reproduce, however, can depend strongly on abiotic conditions at the time of reproduction but also potentially prior to any reproductive event. For many organisms, past environments can influence performance later in life via within-generation carryover effects, but how carryover effects influence potential NIS introductions is unclear. We conducted a laboratory experiment to explore within-generation carryover effects of salinity and how they operate across different environments in the barnacle Amphibalanus improvisus, a common biofouling organism. We exposed newly settled barnacles to two different salinities (15 and 28 psu) for three weeks, maintained them in a common salinity for five weeks, then re-exposed them to the two initial salinities in a fully-factorial design prior to measuring their reproductive output. We found that barnacles that switched salinities between the initial and final exposures tended to produce more nauplii larvae (up to 50%) than those that remained in the same salinity conditions, possibly because of a stress response induced by physiological acclimation to early life environments. Because adult biofouling species are likely to be exposed to variable environmental conditions during vessel transits, carryover effects and their impact on propagule output may be important to consider when evaluating potential NIS introductions via biofouling.

The sponge derived 2,5-diketopiperazine metabolite barettin is a potent antifouling compound effective against the settlement and metamorphosis of barnacles. Simplified derivatives of barettin have previously been shown to display similar inhibitory properties. The synthetic derivative benzo[g]dipodazine has been reported to display significantly improved antifouling properties in comparison with the native barettin with inhibitory activities as low a 0.034 µM reported against barnacle cyprid settlement. In the current study we report the antifouling activity of 29 synthetic analogs designed and inspired by the potent antifouling effect seen for benzo[g]dipodazine. The library contains mainly not only dipodazine derivatives but also disubstituted diketopiperazines and compounds incorporating alternative heterocyclic cores such as hydantoin, creatinine, and rhodanine. Several of the prepared compounds inhibit the settlement of Amphibalanus improvisus cyprids at low micromolar concentrations, in parity with the natural barettin. While several highly active compounds were prepared by incorporating the benzo[g]indole as hydrophobic substituent, the remarkable antifouling effect reported for benzo[g]dipodazine was not observed when evaluated in our study.

Background Salinity plays an important role in shaping coastal marine communities. Near-future climate predictions indicate that salinity will decrease in many shallow coastal areas due to increased precipitation; however, few studies have addressed this issue. The ability of ecosystems to cope with future changes will depend on species’ capacities to acclimatise or adapt to new environmental conditions. Here, we investigated the effects of a strong salinity gradient (the Baltic Sea system – Baltic, Kattegat, Skagerrak) on plasticity and adaptations in the euryhaline barnacle Balanus improvisus. We used a common-garden approach, where multiple batches of newly settled barnacles from each of three different geographical areas along the Skagerrak-Baltic salinity gradient were exposed to corresponding native salinities (6, 15 and 30 PSU), and phenotypic traits including mortality, growth, shell strength, condition index and reproductive maturity were recorded. Results We found that B. improvisus was highly euryhaline, but had highest growth and reproductive maturity at intermediate salinities. We also found that low salinity had negative effects on other fitness-related traits including initial growth and shell strength, although mortality was also lowest in low salinity. Overall, differences between populations in most measured traits were weak, indicating little local adaptation to salinity. Nonetheless, we observed some population-specific responses – notably that populations from high salinity grew stronger shells in their native salinity compared to the other populations, possibly indicating adaptation to differences in local predation pressure. Conclusions Our study shows that B. improvisus is an example of a true brackish-water species, and that plastic responses are more likely than evolutionary tracking in coping with future changes in coastal salinity.

Many marine animals produce numerous larvae, few of which survive to adulthood. While larval mortality is generally attributed to environmental causes such as predation and transport to unsuitable habitats, mortality deriving from the process of ontogeny has rarely been investigated. This study examines 2 hypotheses (Acquisition of Robustness Hypothesis and Transitional Timing Hypothesis) that relate ontogenescence (high mortality early in life that declines with age) to the biological process of development. We conducted 2 experiments with larvae of the barnacle Amphibalanus improvisus to test these hypotheses. In Expt 1, where the survival and duration-in-stage of hundreds of individual larvae were tracked under low stress conditions, half of the developing individuals died. Deaths were concentrated in the larval stages immediately adjacent to the 2 major transitions (nauplius-to-cyprid and cyprid-to-juvenile). 89% of deaths occurred in individuals that had delayed their transition to the next stage. In almost every stage, delays were associated with increased risk of death before reaching the next stage. In Expt 2, which examined stage-based tolerance of temperature and salinity stress, the cyprid stage was most susceptible to ecological stressors. Results of both experiments closely follow the predictions of the Transitional Timing Hypothesis, while neither support an Acquisition of Robustness across development. Stages adjacent to major transitions have reduced physiological tolerance to stressors. Simultaneously, these individuals must achieve competence for the next transition or remain in the current stage until death. The resulting suppression of adult recruitment likely plays an important and under-appreciated role in the population ecology of marine animals.

Aim High-latitude regions host many fewer non-native species than temperate ones. The low invasion loads of these colder regions may change with increases in human-mediated propagule supply. We test the hypothesis that colonization by non-native species that have already invaded temperate shorelines would be precluded by environmental conditions if they were introduced to Alaska and other high-latitude regions by shipping or other vectors. Location Pacific coast of North America as well as coastal oceans world-wide. Methods Using 16 habitat descriptors in ecological niche models, we characterized the conditions throughout the native and introduced distributions of four marine species (Amphibalanus improvisus, Carcinus maenas, Littorina saxatilis and Styela clava) that have invaded multiple global regions to test the extent to which suitable conditions for these species exist in Alaska and other high-latitude regions under current and predicted future climate scenarios. Results Models projected environmental match for all four species in many areas beyond their present range limits, suggesting that Alaska and other high-latitude shorelines are currently vulnerable to invasion by non-native species that occur in lower latitudes. Main conclusions Given current and possibly increasing human-mediated species transfers and suitable environmental conditions that exist now and with projected warming, policy and management efforts are urgently needed to minimize invasion opportunities at high latitudes.

Levels of pollution, including contamination by toxic metals, in the Thames estuary reduced over the last four decades of the 20th century. This 2014 study investigates whether the declines in the bioavailabilities of trace metals (Ag, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V, Zn) have continued in the 21st century, using a suite of littoral biomonitors also employed in 2001 – the brown seaweed Fucus vesiculosus, the strandline, talitrid amphipod Orchestia gammarellus and the estuarine barnacle Amphibalanus improvisus. Bioaccumulated concentrations represent relative measures of the total bioavailabilities of each metal to the biomonitor over a previous time period, and can be compared over space and over time. Trace metal bioavailabilities varied along the estuary, and, in general, fell between 2001 and 2014, a reflection of the continuing remediation of the Thames estuary from its severely polluted state in the middle of the 20th century.

Benthic species associated with hard substrate are known to form communities characterized by high abundance and biomass. Although the bottom of the Southern Baltic Sea is dominated by soft sediments, such communities find favorable conditions to thrive on artificial substrates of offshore constructions. The aim of this research was to characterize both short-term and long-term benthic communities associated with artificial hard bottom provided by man-made structures in the Southern Baltic Sea. Species composition as well as the abundance and biomass were examined at various sites. An inactive World War II torpedo testing facility in the Gulf of Gdansk served as a site for sampling long-term communities while short-term communities were sampled using settlement panels and PVC cylinders. Panels were deployed at the torpedo testing facility for 127 days. PVC cylinders were deployed for 141-190 days in the Polish Exclusive Economic Zone. Twenty six macrofaunal taxa, including 12 crustaceans, were identified during the research. and were the most abundant invertebrates at sampled surfaces. Six non-indigenous species were found. For the first time adult individuals of were found in the Polish Marine Areas indicating that it is possible for this non-indigenous species to reproduce in this region.