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21 pages, 4 figures, 1 table, supporting information https://doi.org/10.1002/ece3.9740.-- Data Availability Statement: Raw RNA-seq data and mt-mutS sequences have been uploaded to NCBI's SRA (BioProject ID: PRJNA847883) and GenBank databases (Accessions numbers: ON804207-ON804214), respectively. Final sequence alignments and microsatellite data are available on DRYAD (https://doi.org/10.5061/dryad.r7sqv9sfm), The accurate delimitation of species boundaries in nonbilaterian marine taxa is notoriously difficult, with consequences for many studies in ecology and evolution. Anthozoans are a diverse group of key structural organisms worldwide, but the lack of reliable morphological characters and informative genetic markers hampers our ability to understand species diversification. We investigated population differentiation and species limits in Atlantic (Iberian Peninsula) and Mediterranean lineages of the octocoral genus Paramuricea previously identified as P. clavata. We used a diverse set of molecular markers (microsatellites, RNA-seq derived single-copy orthologues [SCO] and mt-mutS [mitochondrial barcode]) at 49 locations. Clear segregation of Atlantic and Mediterranean lineages was found with all markers. Species-tree estimations based on SCO strongly supported these two clades as distinct, recently diverged sister species with incomplete lineage sorting, P. cf. grayi and P. clavata, respectively. Furthermore, a second putative (or ongoing) speciation event was detected in the Atlantic between two P. cf. grayi color morphotypes (yellow and purple) using SCO and supported by microsatellites. While segregating P. cf. grayi lineages showed considerable geographic structure, dominating circalittoral communities in southern (yellow) and western (purple) Portugal, their occurrence in sympatry at some localities suggests a degree of reproductive isolation. Overall, our results show that previous molecular and morphological studies have underestimated species diversity in Paramuricea occurring in the Iberian Peninsula, which has important implications for conservation planning. Finally, our findings validate the usefulness of phylotranscriptomics for resolving evolutionary relationships in octocorals, This work was funded by program BIOMARES, a Pew Marine Fellowship, and FCT – Foundation for Science and Technology UIDB/04326/2020, UIDP/04326/2020, LA/P/0101/2020, PTDC/BIA-CBI/6515/2020; EU-BiodivERsA BiodivRestore-253 (FCT: DivRestore/0013/2020); EU Horizon 2020 research and innovation program (MERCES: Grant no. 689518); as well as the Inaqua Conservation Fund (Oceanário de Lisboa and National Geographic Channel) and National Geographic Society/Waitt Grant no. W153-11. This article is also a product of the French network on Marine Connectivity (GDR MarCo). M.C. was supported by postdoctoral fellowships of projects HABMAR (Grant no. MAR-01.04.02-FEAMP-0018) co-financed by the European Maritime and Fisheries Fund of the Operational Program MAR 2020 for Portugal (Portugal 2020), and BiodivAMP (Grant no. FA_06_2017_045) financed by the Directorate-General for Sea Policy of the Ministry of Economy and Sea for Portugal under the Operational Program Fundo Azul. JBL was funded by assistant researcher contract framework of the RD Unit—UID/Multi/04423/2019 – Interdisciplinary Centre of Marine and Environmental Research—financed by the European Regional Development Fund (ERDF) through COMPETE2020—Operational Program for Competitiveness and Internationalization (POCI) and national funds through FCT/MCTES (PIDDAC). JB was funded by a FCT PhD scholarship SFRH/BD/72501/2010, With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)

A new high-resolution relative sea-level (RSL) reconstruction is presented for the past 1500 years based on four bio-constructions formed by alga Lithophyllum byssoides (algal rims). Two algal structures have been studied on the southern (Premantura site) and two on the eastern Istrian coast (Uboka and Brsec sites) in the Northeastern Adriatic. The data from the algal rims (47 radiocarbon data points) enabled the distinction of four major phases of RSL change which corresponds to periods of climate change. RSL between AD 400 and 800 during the Dark Ages Cold Period (DACP), was almost stable. After AD 800, during the Medieval Climate Anomaly (MCA) the RSL increased up to ∼0.8 mm/yr. The following Little Ice Age period, (LIA) interval I (AD 1400 till 1600) is again characterised by RSL stability (RSL slowed down) which allows the rims at the southern coast to reach the width of ∼40–80 cm at their uppermost part and up to 20 cm for those along the eastern coast. Between AD ∼1600 and 1750, during the colder LIA II interval, algal rims do not form, as LIA II is assumed to be a period of RSL fall. Algal rims reveal that from the second part of the 19th century the RSL rose by 13–15 cm at the Premantura location and around 10 cm at the Brsec and Uboka areas, providing rates between 1 and 0.7 mm/yr respectively for the Current Warm Period (CWP). The sea-level trends were quantitatively defined using an Errors-In-Variables Integrated Gaussian Process (EIV-IGP) model, with full consideration of the available uncertainty. Following correction for the total land-level change (assumed to be around −0.4 mm/yr), four successive trends in sea-level change were confirmed. Sea-level dropped during the DACP at a mean rate of −0.4 mm/yr and increased to 0.5 mm/y as a consequence of Medieval warmth. Thereafter it was relatively stable during LIA I, fell up to −0.1 mm/yr during LIA II interval and has been slowly rising again during the CWP. Moreover, L. byssoides δ18O records show that these periods of sea-level changes are consistent with changes in temperature and thus with periods of rapid climate change.

We present a new sea-level reconstruction for the past 1500 years based on biological indicators from the Central Adriatic islands. Biogenic littoral rims built by the coralline rhodophyte Lithophyllum byssoides were found on the particularly exposed sites on the rocky coasts of the islands of Vis, Ravnik and Bisevo in Croatia. The presence of thick and well-developed Lithophyllum rims, considered to be precise (± 10 cm) sea-level indicators, points directly to the rising sea-level environment. Biogenic rims were mapped, measured and sampled for 14C dating. The obtained results point to four phases of sea-level changes. The sea-level was near stable from around 550 till 770 cal AD, in the Dark Ages Cold Period (DACP), then during the Medieval Climate Anomaly (MCA) (770 till 1330 cal AD) the sea-level increased at a rate of 0.71 mm/yr. During the Little Ice Age (LIA) (1330 till 1640 cal AD) it was near stable again. Later, the sea-level started to rise at a much higher rate particularly during the Current Warm Period (CWP). These data were compared with local predictions derived from a glacio-hydro-isostatic models associated with the Last Glacial cycle. If the isostatic–eustatic component is separated, this area seems to have almost stable tectonic conditions during the past 1500 years. Our results show that the large algal rims most likely grew during near-stable sea-level conditions that occurred during two relatively colder periods in the past 1500 years. They also reveal that well-developed (up to 1.8 m wide) upper levels of algal rims were formed during ~ 300 years of stabilisation throughout the LIA.

The sponges Oopsacas minuta (Hexactinellida) and Asbestopluma hypogea (Demospongiae, Cladorhizidae) belong to groups that are typical of deep water, but both species have already been recorded from the shallow water Trois Pepes cave at La Ciotat (Marseille area). We here report on subsequent findings of A. hypogea in two other littoral caves: one at Garmenjak Island in the Middle Adriatic (Croatia), the other at Jarre Island (Marseille, France). The maximum temperatures experienced in situ by these populations are significantly higher than those from the deep Mediterranean Sea, but they are also up to 6°C higher than those reported for the Trois Pepes cave population, up to 23.1 °C in July 2002 for Garmenjak population and 22.8 °C in August 2004 for Jarre population (exposures from a few hours to a few days). The average summer temperature inside both caves remains several degrees lower than the outside temperature at the same depth. Long time survival of A. hypogea populations at the relatively high summer/autumn temperatures in both Jarre cave (4 years monitoring) and Garmenjak cave (5 years monitoring) indicates that the species can successfully colonise such shallow habitats in spite of its presumed deep-water origin. We also report of additional cave records of O. minuta from the Middle Adriatic (Croatia): at Hvar Island in the marine part of the Živa Voda anchialine cave, and from the marine caves at Iski Mrtovnjak, Lastovo and Fraskeric Islands. The wide distribution of this species in the deep Mediterranean is substantiated by a record on Santa Lucia Bank north of Corsica (303 m) and photographs taken by the submersible Cyana in the Central Tyrrhenian Sea (2912 m, 2845 m) and the Ionian Sea (2447 m). Possible colonisation patterns of littoral caves by both sponges are discussed.

The Croatian coastal karst abounds in submerged caves that host a variety of environmental conditions depending on the geomorphology, depth and submarine groundwater discharge. One example is the Y-Cave, a shallow, mostly submerged, horizontal cave on Dugi Otok Island, on the eastern Adriatic coast. This study was aimed at examining the temporal and spatial variability of the marine cave environment, including temperature, salinity, light intensity, cave morphology and hydrodynamism, along with the dissolutional effect caused by the mixing of sea and freshwater. The general distribution of organisms in the Y-Cave was positively correlated to the light gradient and reduced water circulation, thus the highest species diversity and abundance were recorded in the front part of the cave. The phylum Porifera was the most dominant group, and the poriferan species diversity in the cave ranks among the ten highest in the Mediterranean. The middle part of the cave, although completely dark, hosts an abundant population of the gastropod Homalopoma sanguineum and clusters of the gregarious brachiopod Novocrania anomala, whose presence could be connected to tidal hydrodynamics. The absence/scarcity of sessile marine organisms and pronounced corrosion marks at shallow depths inside the cave suggest a freshwater impact in the upper layers of the water column. A year long experiment with carbonate tablets revealed three different, independent ongoing processes affected by the position in the cave: bioaccumulation, dissolution and mechanical erosion. The results of long-term temperature readings also revealed water column stratification within the cave, which was not disturbed by either tidal or wave action. The shallow, partly submerged and relatively small Y-Cave is characterised by a suite of complex environmental conditions, which, together with the resulting distribution of organisms, are unique to this cave.

12 páginas, 5 figuras, 3 tablas., Gorgonians are important structuring species of the Mediterranean hard-bottom communities that are threatened by disturbances such as increasing seawater temperature, mucilaginous events and destructive fishing, among others. In this study we assessed for the first time the population structure and conservation status of one of the most common gorgonians in the Eastern Adriatic Sea, the red gorgonian Paramuricea clavata. During late spring 2009, nine populations dwelling between 30 and 50 m depth were examined by SCUBA diving along 200 km of the Croatian coastline. The density ranged between 7 and 20 coloniesm 2 . The mean and maximum colony heights were 31.2 22.7 cm ( SD) and 138 cm, respectively. Two main patterns of P. clavata size frequency distributions were observed: the first one with a higher proportion of juveniles (~30%) observed mostly in the northernmost populations, and the second one with a higher proportion of larger colonies (>25% of colonies >40 cm in height). Regarding the disturbance impact level, the proportion of healthy colonies (with 60%) and the mean extent of injury (i.e. denuded axis or epibiosis) was 9.7 4% ( SD), indicating low impacts. Contrasting population size structures with high recruitment in mature populations provides new insights into the demographic structure of the Mediterranean gorgonian forests dwelling in their upper bathymetric range (, Part of the funding was provided by the Croatian Ministry of Culture through the grant allocated to Marine Explorers Society –“20,000 leagues” in 2009 and by the Croatian Ministry of Science and Education (project 119- 0362975-1226).

In late summer and early autumn of 1989 benthic mortality in the upper part of Krka estuary extended from the Skradinski buk waterfalls to approximately 5 km downstream. Dead benthic organisms were found on the bottom at depths of up to 4.5 m. During August, a freshwater phytoplankton bloom was recorded in Visovac lake (which supplies the Skradinski buk waterfalls). The green tide in the lake was composed of up to 11.9 million cells. 1--1, predominantly of Synedra acus and Dinobtyon sertularia. At that time the maximal total inflow of phytoplankton cells through the waterfalls was approx. 0.41 S-I (38000 1. d-1). Cells of these two species died after reaching the salt wedge and the cummulative effect of their decomposition apparently caused an oxygen demand sufficient enough to trigger the benthic mortality. Due to the water mass dynamics, the spreading of mortality downstream was slow (from August to October, approx. 5 km). Certain benthic fishes (like Gobius sp.) were observed on the bottom of the affected area in mid-October 1989.

The Adriatic Sea can be considered as a large, mostly shallow embayment of the Mediterranean Sea. The latitudinal SE-NW extension of the Adriatic results in considerable climate differences along its coast. In this work we compiled the listof sponges from classes Demospongiae and Homoscleromorpha recorded in the Adriatic up to now: we performed a thor-ough literature check and we added the results of our decade long research with a special emphasis on sponge fauna inmarine caves. All the records were assigned to the North, Middle or South Adriatic according to their geographic location,and the analysis showed a very high similarity among the sponge fauna of the 3 areas. The likeness between the N. andthe M. Adriatic is over 83 %, between the M. and the S. Adriatic over 76 % and between the N. and the S. Adriatic over66 %. Altogether 283 species of sponges from these two classes (our field data and literature survey) have been recordedin the whole Adriatic up to now, which is a considerably higher number than in previous reviews. Among the 125 speciesthat we found so far in our research along the Croatian coast, 77 were found in marine caves. We are still discovering spe-cies not previously found in the Adriatic Sea (especially from cryptic habitats) and here we report 15 new records for the Adriatic Sea, 9 of which were noted only in caves.

10 pages, 6 figures, Ongoing sea temperature increase threatens reefs worldwide. In this study we experimentally examined for the first time the response of the early life stages of a long-lived structural gorgonian, the Mediterranean species Paramuricea clavata (Risso 1826), to the highest summer temperature recorded so far in the study area (25°C; the Medes Islands, Northwestern Mediterranean). This temperature also simulates the end-of-century predicted warming (+3°C) over the temperature maxima recorded during the species’ reproductive period. The results showed a severe negative impact of constant thermal stress on the viability of P. clavata embryos and larvae, resulting in reduced survivorship, completely abnormal embryonic development and impaired metamorphosis. The deteriorating effect was rapid for embryos (after 7 h), whereas the impact on larvae became evident after 10 d of stress. Larval modifications started 7 d earlier under elevated temperature conditions, but the appearance of spherical, aberrant shapes prevented an assessment of whether metamorphosis was initiated but abnormal, or deterred altogether. The apparent higher sensitivity of embryos suggests that thermal stress during embryonic development may be the most critical factor for the viability of P. clavata larvae. In the context of an ongoing warming trend, the reduced viability of early life stages would jeopardize the persistence of P. clavata, which relies on successful recruitment for replenishment of its populations. Further research into the effects of thermal stress on the reproduction of structural invertebrate species and the viability of their early life stages will improve our understanding of the long-term consequences of environmental global change in marine benthic communities, We thank J. M. Fortuño for assistance with scanning electronic microscopy, the Medes Islands MPA Staff for their continuous support, and O. Ross and M. Buklijas Caleta for help in image editing. We further thank the 3 anonymous reviewers for their constructive comments. S.K. acknowledges Research Fellowship 2009-11 from the Spanish Agency for International Development Cooperation (AECID). This study was funded by the Spanish Ministry of Science and Innovation (Biorock project ref. CTM2009−08045), Juan de la Cierva and Ramon y Cajal contracts to C.L. and Beatriu de Pinós contract to N.T. This is a contribution from the Medrecover research group, funded by the Generalitat de Catalunya (Catalan government)

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.

Recent ROV (Remotely Operated Vehicle) exploration and bottom sampling in the southern Adriatic Sea (Apulian and Montenegrin margins) resulted in the discovery of cnidarian-rich deep-sea habitats in the depth range of ca. 400-700 m. In particular, ROV inspection of Montenegrin canyons reveals the existence of megabenthic communities dominated by a variety of cnidarians, including scleractinians (Madrepora oculata, Lophelia pertusa, Dendrophyllia cornigera), antipatharians (Leiopathes glaberrima) and gorgonians (Callogorgia verticillata) as major habitat forming taxa, often in association with sponges and, subordinately, serpulids. All such cnidarians are new records for the southeastern side of the Adriatic Sea. Our investigation indicates that an almost continuous belt of patchy cold water coral sites occurs along the entire southwestern margin (Apulian), basically connecting the Adriatic populations with those inhabiting the Ionian margin (Santa Maria di Leuca coral province).