Biological invasion is one of the main components of global changes in aquatic ecosystems. Unraveling how establishment in novel environments affects key biological features of animals is a key step towards understanding invasion. Gut microbiome of herbivorous animals is important for host health but has been scarcely assessed in invasive species. Here, we characterized the gut microbiome of two invasive marine herbivorous fishes (Siganus rivulatus and Siganus luridus) in their native (Red Sea) and invaded (Mediterranean Sea) ranges. The taxonomic and phylogenetic diversity of the microbiome increased as the fishes move away from the native range and its structure became increasingly different from the native microbiome. These shifts resulted in homogenization of the microbiome in the invaded range, within and between the two species. The shift in microbial diversity was associated with changes in its functions related with the metabolism of short-chain fatty acids. Altogether, our results suggest that the environmental conditions encountered by Siganidae during their expansion in Mediterranean ecosystems strongly modifies the composition of their gut microbiome along with its putative functions. Further studies should pursue to identify the precise determinants of these modifications (e.g. changes in host diet or behavior, genetic differentiation) and whether they participate in the ecological success of these species.

This Collective Article presents information on 20 taxa belonging to five (5) Phyla: Cnidaria (2), Mollusca (8), Arthropoda (4), Echinodermata (1) and Chordata (5) recorded from the Alboran Sea to the Levantine Sea. These new records were found in seven (7) different ecoregions as follows: Alboran Sea: new record of the rare football octopus Ocythoe tuberculata in the harbour of Algeciras (Spain); Western Mediterranean Sea: first record of the isopod Arcturinella deltensis in the Ligurian Sea, and the third in the whole Mediterranean Sea; third Mediterranean site for the Muricidae Coralliophila ahuiri along the Italian coasts (Alghero); first and westernmost record of the goby Corcyrogobius liechtensteini from the Iberian Peninsula; new record of the sea slug Glaucus atlanticus from the coast of the Iberian Peninsula; first and easternmost record of Holothuria (Vaneyothuria) lentiginosa lentiginosa along the Italian coasts; Tunisian Plateau/Gulf of Sidra: seventh record of the oceanic squid Thysanoteuthis rhombus in Tunisia, and first for the Gulf of Gabes. Ionian Sea: second occurrence of the Atlantic tripletail Lobotes surinamensis in the Ionian Sea and first record from the Strait of Messina area; new record of the rare sea elephant Pterotrachea coronata from the Strait of Messina; Adriatic Sea: first record of a facies of football ascidian Diazona violacea in the South Adriatic Sea (Tremiti Island, Italy); two records of the sevengill shark Heptranchias perlo in Croatian waters after more than 70 years of absence of documented records in this area; first known occurrence of the nudibranch Jalonus hyalinus in Slovenian waters and also the first known occurrence of this species in the Adriatic Sea; first record of the nudibranch Okenia elegans in Slovenian waters; Aegean Sea: First record of the parasitic isopod Nerocila milesensis along Greek coasts (Crete island) as well as the second record worldwide; additional record of the brachyuran Paragalene longicrura collected from the Saronikos Gulf (Greece); first record of the siphonophore Rhizophysa filiformis in Greek waters as well as the second record of this species in the eastern Mediterranean basin; new record of the rare and protected angelshark Squatina aculeata along Turkish coasts; Levantine Sea: first record of the marine amphipod Caprella andreae in the Levantine Mediterranean shore of Israel, based on both morphological and molecular data; first occurrence of the cephalopod Tremoctopus violaceus along Turkish coasts, which confirms its presence in the north-eastern coasts of Turkey; record of a bloom of the thermophilic jellyfish Pelagia noctiluca in the north Levantine Sea.

In the marine environment, fish contribute to key ecological processes such as controlling food‐webs through top‐down impacts, especially on algae. To date, the assessment of fish grazing activity has mostly been performed using short‐term (<1 h) censuses by divers or remote cameras which do not allow estimating the variability of grazing rate within and between days. However, understanding the temporal variation of fish activity and hence contribution of species to ecosystem functioning is of particular interest in the context of biological invasion. Here, using long‐duration remote underwater cameras, we recorded fish abundance and grazing events over three consecutive days in October 2019 in a shallow Mediterranean ecosystem from northern Crete. This novel approach allowed us to assess temporal variation of abundance and grazing activity of the two native (Sarpa salpa and Sparisoma cretense) and the two non‐indigenous fish species (Siganus rivulatus and Siganus luridus). Non‐indigenous Siganus rivulatus was the most common species in the studied coastal habitat, followed by the two native species while the non‐indigenous Siganus luridus was scarce. Overall, the non‐indigenous S. rivulatus and the native S. salpa are responsible for more than 90% of the recorded grazing activity with similar bite rates between the two species. More than 70% of the grazing activity arose in grazing pulses in the afternoon, supporting the diel feeding hypothesis according to which feeding is greater in the afternoon when nutritive quality of macrophytes is the highest. In addition, some of the highest peaks in grazing activity were driven by a few individuals. Hence, surveys of only abundance could not provide accurate estimates of herbivory. Last, Siganus rivulatus presence did not significantly affect grazing activity of the native Sarpa salpa. Our results demonstrate that long‐duration remote underwater videos are a useful tool to accurately assess the contribution of fishes to ecosystem functioning.

Molecular methods such as DNA/eDNA metabarcoding have emerged as useful tools to document the biodiversity of complex communities over large spatio‐temporal scales. We established an international Marine Biodiversity Observation Network (ARMS‐MBON) combining standardised sampling using autonomous reef monitoring structures (ARMS) with metabarcoding for genetic monitoring of marine hard‐bottom benthic communities. Here, we present the data of our first sampling campaign comprising 56 ARMS units deployed in 2018–2019 and retrieved in 2018–2020 across 15 observatories along the coasts of Europe and adjacent regions. We describe the open‐access data set (image, genetic and metadata) and explore the genetic data to show its potential for marine biodiversity monitoring and ecological research. Our analysis shows that ARMS recovered more than 60 eukaryotic phyla capturing diversity of up to ~5500 amplicon sequence variants and ~1800 operational taxonomic units, and up to ~250 and ~50 species per observatory using the cytochrome c oxidase subunit I (COI) and 18S rRNA marker genes, respectively. Further, ARMS detected threatened, vulnerable and non‐indigenous species often targeted in biological monitoring. We show that while deployment duration does not drive diversity estimates, sampling effort and sequencing depth across observatories do. We recommend that ARMS should be deployed for at least 3–6 months during the main growth season to use resources as efficiently as possible and that post‐sequencing curation is applied to enable statistical comparison of spatio‐temporal entities. We suggest that ARMS should be used in biological monitoring programs and long‐term ecological research and encourage the adoption of our ARMS‐MBON protocols.

Diadema setosum is an echinoid of Indo-Pacific origin that invaded the Mediterranean Sea in 2006. It is an ecosystem engineer with an important ecological function in its native range, but it can have a detrimental effect on Mediterranean reefs. Recently in 2022 a mass mortality event (MME) affecting this species was recorded in the east Aegean Sea in the eastern part of the Mediterranean basin. We are reporting herein a westward progression of the MME in 2023 affecting established populations in various locations around the island of Crete.