Antipathella subpinnata (Ellis and Solander 1786) is one of the most frequently observed black corals at mesophotic depths (60–200 m) of the Mediterranean Sea, particularly in the northwestern part of the basin, where its populations can reach high densities and create forest-like aggregations, both along the coast and in offshore locations such as seamounts. Similar to other marine underwater forests, black coral gardens host a rich associated fauna and attract numerous species of commercial interest. As such, these corals are targeted by recreational and artisanal fisheries and are vulnerable to human impact due to their arborescent morphology and low growth rates. Genetic connectivity can provide valuable insight into the processes of population maintenance and replenishment following environmental disturbance and is often used as a proxy for population resilience. In our study, a restriction-site associated DNA analysis (2bRAD) was used to evaluate fine-scale population structure of the Mediterranean black coral A. subpinnata, and to understand which populations could serve as a potential source of genetic diversity for adjacent populations. Colonies from two offshore localities (a Ligurian seamount and a Tyrrhenian canyon) and four coastal populations from Liguria and Sicily were sampled and genotyped. Significant genetic differentiation was recorded between coastal and offshore localities. Moreover, offshore localities were genetically distinct from one another, while all coastal populations were characterized by panmixia. This indicates that offshore A. subpinnata gardens are potentially less resilient to human impact (i.e., demersal fishing activities) due to a limited influx of larvae from adjacent habitats. In addition, they are unlikely to supply coral propagules to coastal populations. Overall, this study highlights the vulnerability of Mediterranean A. subpinnata forests, and the importance of enforcing conservation and management measures to achieve Good Environmental Status (GES, EU Marine Strategy Framework Directive) of these valuable marine ecosystems.

coralligenous assemblages are among the most species-rich and vulnerable habitats of the Mediterranean Sea. Nevertheless, data on connectivity patterns on species inhabiting these habitats, crucial to define management and protection priorities, are largely lacking. Moreover, unreliable species-level taxonomy can confound ecological studies and mislead management strategies. In the northwestern Mediterranean two Parazoanthus axinellae morphotypes differing in size, color and preferred substrate are found in sympatry. In this study, we used COI and ITS sequence polymorphism to assess (1) the genetic divergence between the two morphotypes, (2) their connectivity patterns and (3) their phylogenetic position within the Parazoanthidae. Specimens of P. axinellae were sampled in 11 locations along the northwestern Mediterranean; in 6 locations, samples of the two morphotypes were collected in sympatry. Small genetic diversity and structure were found within morphotypes, while marked and consistent differentiation was detected between them. Moreover, the less widespread morphotype appeared to be closer to Pacific species as P. juanfernandezii and P. elongatus. Our findings confirmed the limited knowledge on Parazoanthus species complex, and how this gap can have important implication for the conservation strategies of this widespread and valuable genus in the Mediterranean Sea. Biogenic reefs are made by calcareous encrusting algae and animals that change the geological primary habitat on which they settled through superimposition of their skeletons. They are among the most productive and diverse benthic ecosystems, providing habitat, feeding grounds, recruitment and nursery areas for a variety of invertebrate and vertebrate species 1. In the Mediterranean Sea, the main biogenic reefs are the coralligenous reefs. They are among the most important Mediterranean ecosystems due to the high diversity of species and ecological processes that they support 1,2. These complex habitats are threatened by several human activities (e.g. recreational fishing and trawling, sediment deposition, anchorage, diving), which lead to their fragmentation and loss 3,4. Monitoring spatio-temporal changes in species composition is crucial to quantify human-induced biodiversity loss and habitat fragmentation, but the absence of clear taxonomic identifiers to distinguish between species makes this a challenging task. Many relevant coralligenous taxa (e.g. Porifera, Echinodermata, Cnidaria) lack diagnostic morphological characteristics, and therefore these groups are recognized as taxonomically problematic, in particular at lower taxonomic levels 5. Moreover, in these species, with high morphological plasticity 6 , some characters such as shape and coloration might not be reliable for species delimitation. It happened that morphospecies have turned out to be a single morphologically variable species 7-9 , and, conversely, what was thought to be multiple growth forms of a single species turned out to be a complex of species 10,11 .

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Sponges are the oldest extant Metazoans, with fossils dating back to the Precambrian (Li et al., 2014) and recent work supporting sponges as sister to all other animals (Feuda et al., 2017). They form an abundant and diverse component of benthic communities, with 8,866 formally described species (20,000 estimated species), that occupy both marine and freshwater habitats from the poles to the tropics (Van Soest et al., 2017). Sponges also play integral roles in ecosystem processes such as bioerosion and consolidation, benthic-pelagic coupling through their immense filtering capabilities, as well as nutrient cycling through their complex symbiosis with microorganisms (Bell, 2008; de Goeij et al., 2013; Diaz & Rüetzler, 2001). Sponges undertake this nutrient cycling using exceptionally diverse microbial communities, with over 40 microbial phyla (including candidate phyla) known to associate with sponges (Thomas et al., 2016). Nutrient levels in coastal areas have been increasing due to inputs from both point (e.g., sewage effluent) and nonpoint (e.g., agricultural and urban runoff) sources (Carpenter et al., 1998), and this has contributed to the degradation of reef ecosystems worldwide (D'Angelo & Wiedenmann, 2014; Fabricius et al., 2011). Various aspects of coral physiology (e.g., reproductive success, calcification rates and growth) are negatively impacted by elevated nutrients