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Marine underwater habitats dominated by seagrass Posidonia oceanica play an essential role in fish community assembly, affecting taxonomic and functional diversity, abundance and fish behavior. The value of seagrasses as habitat depends on the spatial arrangement of the seascape elements and the availability of alternative habitats. Little is known about the effect of the seascape context of P. oceanica meadows on fish assemblages in the Mediterranean Sea. To identify P. oceanica meadows' relative importance as a habitat for fishes, fish communities in the Croatian Adriatic Sea were investigated, using SCUBA lure-assisted visual census. The results show a significant effect of different arrangements of P. oceanica meadows' seascape elements and surrounding habitats on fish community structure. Fragmented mosaic meadows with P. oceanica growing directly on and between rocky-algal reefs/boulders had significantly higher fish abundances compared to both types of continuous meadows (bordering rock and bordering sand). Continuous meadows bordering sand harbored the highest number of unique species. Evidence that alternative structured habitats within proximity to seagrass beds may affect the community structure of associated fish assemblages is provided, highlighting the need to consider P. oceanica meadows' seascape context in conservation management and experimental design for fish community structure., Naselja morske cvjetnice Posidonia oceanica igraju važnu ulogu u strukturiranju zajednica riba te utječu na taksonomsku i funkcionalnu raznolikost, brojnost i ponašanje riba. Relativna vrijednost naselja morskih cvjetnica ovisi o prostornoj organizaciji elementa podmorskog okoliša te dostupnosti alternativnih staništa. Međutim, nedovoljno je poznato koji sve čimbenici utječu na strukturu zajednice riba povezanih s ovom morskom cvjetnicom. S ciljem utvrđivanja utjecaja različite prostorne organizacije elementa podmorskog okoliša na zajednice riba, proveden je vizualni cenzus uz pomoć mamca u naseljima cvjetnice P. oceanica u hrvatskom dijelu Jadranskog mora. Utvrđen je značajan utjecaj različite prostorne organizacije elementa podmorskog okoliša na strukturu zajednice riba – u rascjepkanim mozaičnim naseljima u kojima se P. oceanica isprepliće s kamenom podlogom obraslom makro-algama zabilježena je značajno veća brojnost riba u odnosu na kontinuirana cjelovita naselja (uz sediment ili uz kamenitu obalu). U radu su predstavljeni dokazi da dostupnost alternativnih strukturiranih staništa u neposrednoj blizini naselja morske cvjetnice P. oceanica utječe na strukturu zajednice riba koje u njoj žive. Neophodno je uzeti u obzir kontekst prostornog rasporeda elemenata podmorskog okoliša kojima dominira morska cvjetnica P. oceanica prilikom planiranja znanstvenih istraživanja, ali i kod upravljanja priobalnim područjima.

Seascape connectivity studies, informing the level of exchange of individuals between populations, can provide extremely valuable data for marine population biology and conservation strategy definition. Here we used a multidisciplinary approach to investigate the connectivity of the marbled crab (Pachygrapsus marmoratus), a high dispersal species, in the Adriatic and Ionian basins. A combination of genetic analyses (based on 15 microsatellites screened in 314 specimens), Lagrangian simulations (obtained with a biophysical model of larval dispersal) and individual-based forward-time simulations (incorporating species-specific fecundity and a wide range of population sizes) disclosed the realized and potential connectivity among eight different locations, including existing or planned Marine Protected Areas (MPAs). Overall, data indicated a general genetic homogeneity, after removing a single outlier locus potentially under directional selection. Lagrangian simulations showed that direct connections potentially exist between several sites, but most sites did not exchange larvae. Forward-time simulations indicated that a few generations of drift would produce detectable genetic differentiation in case of complete isolation as well as when considering the direct connections predicted by Lagrangian simulations.Overall, our results suggest that the observed genetic homogeneity reflects a high level of realized connectivity among sites, which might result from a regional metapopulation dynamics, rather than from direct exchange among populations of the existing or planned MPAs. Thus, in the Adriatic and Ionian basins, connectivity might be critically dependent on unsampled, unprotected, populations, even in species with very high dispersal potential like the marbled crab. Our study pointed out the pitfalls of using wide-dispersing species with broad habitat availability when assessing genetic connectivity among MPAs or areas deserving protection and prompts for the careful consideration of appropriate dispersing features, habitat suitability, reproductive timing and duration in the selection of informative species.

20 pages, 9 figures, 5 tables, supplementary data https://doi.org/10.12681/mms.25306, The Mediterranean Sea has a long-lasting history of fishery exploitation that, together with other anthropogenic impacts, has led to declines in several marine organisms. In particular, elasmobranch populations have been severely impacted, with drastic decreases in abundance and species diversity. Based on their experience, fishers can provide information on marine species occurrence, abundance and behavioural traits on a long-term scale, therefore contributing to research on the poorly studied biological aspects of elusive or rare elasmobranch species. In this study, for the first time, the use of the Local Ecological Knowledge (LEK) of fishers was applied to study the behavioural traits of sharks, rays and skates in 12 FAO-GFCM geographical sub-areas (GSAs) of the Mediterranean Sea. This study found both new insight and proved the reliability of LEK-based catch seasonality, reflecting seasonal movements, by comparing LEK-based findings and available literature on five elasmobranch taxa (Mustelus spp., Squalus acanthias, Raja spp., Myliobatis aquila and Scyliorhinus stellaris) in the Adriatic Sea and 7 taxa (Mustelus spp., Raja spp., Prionace glauca, Scyliorhinus canicula, Torpedo spp., Pteroplatytrygon violacea and Isurus oxyrinchus) in the remaining Mediterranean GSAs. In addition, LEK provided new insights into a novel comprehensive representation of species aggregations (Mustelus spp., S. acanthias, M. aquila and S. canicula) in the sampled GSAs and supplied the first descriptions of the size, number of individuals and sex composition of the aggregations. When the limits and shortcomings of LEK-based research are considered, this methodology can be a complementary and cost-effective tool used to study elasmobranchs in either a data-poor scenario or a scenario in which a baseline is missing. LEK can also be useful for the evaluation and inclusion of fishers’ perceptions on bottom-up management and to provide important evidence for conservation plans, This work was supported by funding of Ministero dell’Istruzione, dell’Università e della Ricerca (MIUR) to CM, With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)

Posidonia oceanica meadows are thought to provide refuge for prey species, which suggests that fish communities are organized according to the abundance and behavior of predators. We collected published research on fish assemblages associated with P. oceanica meadows in the Mediterranean Sea and performed a meta- analysis to test hypotheses regarding the drivers of community structure. In 14 studies documenting the relative abundance of fish species, 112 taxa within a depth range of 1-40 m were reported at local P. oceanica meadows. Sampling method had the most significant effect on community abundance. Total predator abundance and abundance of resident predators each had a significant effect on the prey fish community. A higher abundance of resident predator species such as Scorpaena notata and Zosterisessor ophiocephalus was associated with a lower abundance of small Symphodus spp., Chromis chromis, and Gobiidae, and overall lower prey species richness. Fish communities in P. oceanica meadows are, therefore, in part organized in response to the presence and abundance of piscivorous species, which reduces the value of seagrass as a refuge habitat for potential prey species. For more robust estimates of the habitat value of P. oceanica, a concerted international, collaborative research effort that applies uniform, non- destructive sampling methods, such as visual census or stereo-videography within and outside meadows, is needed.

Connectivity between populations shapes the genetic structure of species being crucial for an effective management of environmental resources. Genetic approaches can provide indirect measures of connectivity, allowing the identification of genetically differentiated – unconnected – populations. In this study, we applied a 2b-RAD approach based on hundreds of polymorphic loci to provide the first detailed insight into the population genomics of the Mediterranean mussel Mytilus galloprovincialis in part of its native geographical range. We sampled 19 localities within the Mediterranean and Black Seas, and analyzed a total of 478 samples. We detected strong differences between the two seas, whereas no differences were found between samples from the Western and Central Mediterranean and within Western Mediterranean samples. In the Central Mediterranean a significant differentiation emerged comparing Central Adriatic samples with those from South Adriatic and Ionian Seas. Furthermore, an East-to-West genetic structuring was found in the Central Adriatic Sea, which was not present in the Southern Adriatic and Ionian Seas. These results possibly reflect the local oceanography, with a Middle Adriatic gyre unable to prevent genetic differentiation in this species, and a Southern Adriatic gyre that effectively mixes propagules in Southern areas. In the Black Sea, no signal of genetic structure was found, although samples were spaced at similar distances as in the Adriatic-Ionian area. Genetic connectivity patterns of M. galloprovincialis reveal peculiar species-specific features respect to other species with similar larval duration, suggesting caution in using genetic connectivity data of single species in defining conservation units. We recommend of using genetic connectivity data of many species representing a variety of life history traits, and we call for new investigations using high resolution population genomics, particularly in the Black Sea, to understand if areas separated by hundreds of kilometers can be considered genetically connected as mussels’ data suggest. This information will be critical to ensure “a well-connected system of protected areas” according to Aichi Target 11 of the Convention on Biological Diversity.

We report the first record of vertical rhizome growth in the temperate seagrass Zostera marina. In a population of Z. marina occurring on subtidal sand in the Novigrad Sea (Croatia), an area subject to episodic high sediment transport, collected plants of Z. marina showed vertical rhizomes with shorter and narrower inter-nodes (mean length = 3.4 ± 1.5 SD mm, mean width = 1.9 ± 0.3 SD mm) than those recorded for horizontal rhizomes (mean length = 9.0 ± 3.5 SD mm, mean width = 2.8 ± 0.4 SD mm). Out of a sample of 1130 rhizome fragments, 288 (25.5%) were vertical. Repeated moderate burial events may have stimulated the production of vertical rhizomes, and the ability of Z. marina to produce vertical rhizomes may enable it to withstand moderate burial in this highly dynamic environment.

A lure assisted, visual census, belt transect method has been developed and compared with the traditional visual census. Presentation of a lure was predicted to improve visibility of fish that are difficult to detect or identify. These include species hidden within three dimensional structures, buried in or camouflaged with the substratum, and fast swimming. A snorkeler presenting an artificial lure near the seabed along 3-m transects recorded all fish visiting the lure or present near the lure's path. Totals of 1527 lure-assisted transects and 1292 lure-absent transects intersecting a diversity of benthic habitats were carried out at thirty sites spanning the Croatian Adriatic coast in spring 2010. More than half the observed fish visited the lure, and lure presentations increased the probability of census for 76% of the 58 observed taxa. The overall significantly higher fish density at lure-assisted transects than at lure-absent transects was also significant within all common habitat types and for the entire investigated depth range (0.25–9 m). The positive effect of the lure on estimates of density was significant for 33% of taxa, including: ambush predators ( Trachinus draco ), fast moving cruise-chase predators (Sparidae), predators chasing prey from a hidden waiting position ( Coris julis and Serranidae) or from a position of camouflaging against bare sand (Gobiidae). Adding lure presentation to regular visual census is logistically simple and flexible in the detail of its design and application.

Within the scope of a seagrass monitoring program in the Novigrad Sea, Central Croatian Adriatic, we predicted that the annual variability in coverage of seagrasses ( Zostera marina , Zostera noltii , and Cymodocea nodosa ) can be partially explained by the annual variability in sediment translocation. From 23 fixed DGPS-referenced monitoring video transects followed over three years (June 2007–2009), we calculated annual (i) changes in interior bed seagrass coverage, (ii) gain in seagrass at the lower edge of the bed and seagrass bed expansion, and (iii) accumulation of sediment, its depth dependence, and the associated changes in transect slope. We found that in 2007 to 2008, the year with net sediment accumulation, seagrass coverage increased and the bed expanded. In both years seagrass cover within the seagrass bed increased with increasing sediment accumulation, while seagrass bed expansion was highest under intermediate sedimentation rates. Boat-based videographic monitoring can document both natural sediment movement along the depth gradient, and species-specific responses necessary for informed management of submerged aquatic vegetation in the Adriatic Sea.

We performed a series of laboratory tank experiments designed to identify possible mechanisms for the strict seagrass avoidance of juvenile Gobius niger previously observed in their natural habitat, the Novigrad Sea, northern Adriatic. Overall our results suggest that juvenile G . niger strongly prefers seagrass when predator absence is certain, avoids seagrass in the presence of predators, and chooses habitat based on both predator identity and on the degree of certainty of predator presence or absence. In the presence of predators, juvenile G . niger occupies the same habitat as adult size G . niger but avoids the habitat chosen by Zosterisessor ophiocephalus . Juvenile G . niger is most likely to switch habitat in response to addition or removal of Z . ophiocephalus , the predator with the stronger seagrass preference. A significantly lower likelihood of selecting seagrass was exhibited when predator absence was uncertain than when predator absence was certain. Our results suggest that juvenile G . niger identifies predators by species and responds differentially to perceived risk posed by each predator through differential habitat selection, which also depends on the certainty of predator absence. These results are consistent with the hypothesis that the importance of structured refuge habitats, such as seagrass, is relative: it depends on both the prey's perception of the relative predation risk posed by multiple predators in alternative habitats, and on the prey's specific antipredator response strategies. These responses could explain the avoidance of seagrass by G . niger in the Novigrad Sea.

In the Novigrad Sea, Croatia, we tested the hypothesis that lower fish abundance in seagrass is associated with higher predation risk by assaying predator attack behavior with lure trolling over seagrass and neighboring bare substrate, by monitoring predator ingestion of prey in experimental tanks, and by analyzing gut contents of predators collected in nature. Lure trolling indicated that (1) total abundance of fish was higher at bare sediment where small fish ($< 5 \mbox{; ; ; ; ; ; cm}; ; ; ; ; ; $), including juveniles, predominated ; (2) abundance was lowest in seagrass where large fish ($> 15 \mbox{; ; ; ; ; ; cm}; ; ; ; ; ; $) predominated ; (3) large ambush predators, primarily the grass goby and European eel, were almost completely restricted to seagrass ; (4) the predation mode in seagrass was almost entirely ambushing or stalk-attacking, while the predation mode at bare sediment was almost entirely chase-attacking ; (5) ambush predation was far more successful than chase-attack predation ; and (6) overall predation risk was approximately $3 \times$ higher at seagrass. Tank experiments showed that predation success of the grass goby was higher than that of the most common chase-attacker, the black goby, and presence or absence of artificial seagrass, regardless of density, had no significant effect on predation success of either species. Guts of the grass goby contained food items of a wider size range that averaged twice the size of those of the black goby. Our results confirm our prediction that the risk of predation is higher in seagrass meadows than at adjacent bare substrate, and this risk differential is explainable by the presence of larger, more efficient ambush predators restricted to seagrass, and the scarcity of large chase-attack predators in the Novigrad Sea.

Seagrass meadows have been hypothesized to protect mobile macrofauna from preda- tion by providing structural complexity that interferes with predator movement and vision. However, if ambush predators are common in seagrass, they may exclude many prey species, resulting in lower total abundance and diversity in seagrass relative to neighboring bare substrates, and segregation of predators and their prey into different habitats. We tested these abundance and diversity predictions using visual censuses of fish and invertebrates employing georeferenced videography to quantify habitat and pinpoint every observation, and using bait stations and drop nets over 38 km of scuba transects in all 4 seasons and by day and night in an enclosed estuary of the northeast Adriatic (Novi- grad Sea, Croatia). The primary fish predators at our study site were the ambush or stalk-attack predators, grass goby Zosterisessor ophiocephalus and European eel Anguilla anguilla, and we found that these species strongly preferred seagrass habitat. In contrast, most other fish, including small juveniles, avoided seagrass and preferred bare sediment to vegetated habitats. Species strongly pre- ferring seagrass were primarily sessile invertebrates not preyed on by the goby or eel. Both total macrofaunal density and total taxon richness were greatest on bare sediment and lowest in dense seagrass habitats. The 3-dimensional aspect of habitat structure was as important in delineating the communities as the presence or absence of habitat structure. These results are consistent with the hypothesis that ambush predators reduce the value of seagrass habitat for their prey relative to neighboring bare sediment or rock; that structured habitat might carry a higher mortality risk for many prey species, and that neighboring unvegetated habitat, including bare sediment, can be at least as important as seagrass in harboring biodiversity.

Understanding the causes of variation in faunal abundance and diversity across shallow coastal habitats is a fundamental goal of marine ecology. Field methods for inferring a habitat effect on population density and predation risk are informative only if method biases are equal across habitats and species. We hypothesised that observation of fixed lures has a species by bias interaction if sampled species have different modes of predation, and that these biases are overcome by use of moving lures. We tested this hypothesis by observation of fixed and moving lures within seagrass and bare sediment in the Novigrad Sea, Croatian Adriatic. Both methods showed that ambush predators peaked in seagrass, wait–chasers peaked over bare sediment, and move–chasers were abundant in both. Stationary lures underestimated wait–chase and wait–ambush predators relative to moving lures, whereas moving lures did not underestimate the density of predators. These results indicate that stationary lures can underestimate both fish abundance and predation risk in the presence of waiting predators, and that if waiting predators are more abundant in structured habitat, then stationary lures will underestimate the predation risk within such habitats. Use of moving lures may be preferable for comparing habitats differing in structural complexity and frequency of predation modes.