Herbivorous surgeonfishes are an ecologically successful group of reef fish that rely on marine algae as their principal food source. Here, we elucidated the significance of giant enteric symbionts colonizing these fishes regarding their roles in the digestive processes of hosts feeding predominantly on polysiphonous red algae and brown Turbinaria algae, which contain different polysaccharide constituents. Using metagenomics, single-cell genomics, and metatranscriptomic analyses, we provide evidence of metabolic diversification of enteric microbiota involved in the degradation of algal biomass in these fishes. The enteric microbiota is also phylogenetically and functionally simple relative to the complex lignocellulose-degrading microbiota of terrestrial herbivores. Over 90% of the enzymes for deconstructing algal polysaccharides emanate from members of a single bacterial lineage, "Candidatus Epulopiscium" and related giant bacteria. These symbionts lack cellulases but encode a distinctive and lineage-specific array of mostly intracellular carbohydrases concurrent with the unique and tractable dietary resources of their hosts. Importantly, enzymes initiating the breakdown of the abundant and complex algal polysaccharides also originate from these symbionts. These are also highly transcribed and peak according to the diel lifestyle of their host, further supporting their importance and host-symbiont cospeciation. Because of their distinctive genomic blueprint, we propose the classification of these giant bacteria into three candidate genera. Collectively, our findings show that the acquisition of metabolically distinct "Epulopiscium" symbionts in hosts feeding on compositionally varied algal diets is a key niche-partitioning driver in the nutritional ecology of herbivorous surgeonfishes.piscine herbivores | marine algae | carbohydrases | giant enteric symbionts | Epulopiscium M arine herbivorous fishes are an ecologically dominant force, structuring tropical reef ecosystem functions and relationships through their grazing activities (1-3). However, they are one of the least understood guilds of herbivores regarding the mechanisms by which they digest and assimilate algal biomass, especially in contrast to plant material assimilation by terrestrial vertebrate herbivores and insects (1, 4). In terrestrial herbivores, the community of microbes inhabiting an organism's gut-the gut microbiota-is considered to play a significant role in the organism's ability to digest plant matter. It is thought to be a major factor leading to the evolution of vertebrate and insect-based herbivory (1-3, 5) and constitutes a potentially critical component of diet-driven speciation in mammals (1,4,6). Contrary to the immense literature on the nutritional ecology and plant biomass assimilation by mammals, ruminants, and arthropods (4,7,8), mechanistic insights into the digestive physiology of marine herbivores, especially the roles of gastrointestinal microbes of piscine herbivores in gut digestive processes, remain largely uncharacteriz...

Scleractinian corals ascribed to the family Agariciidae represent an important component of Red Sea coral reef fauna, though little genetic data are currently available for this group, and existing information shows polyphyly in the examined mesophotic taxa from the Pacific Ocean. In this work, we provide a first genetic survey of Agariciidae from the Saudi Arabian Red Sea, based on a collection of shallow-water material (<30 m) from the Gulf of Aqaba to the Farasan Islands. Two molecular markers were sequenced to infer morphospecies monophyly and relationships, the intergenic region between COI and 16S rRNA from mitochondrial DNA and the ribosomal ITS1 region from nuclear DNA. A total of 20 morphospecies were identified based on classical macromorphological characters. Six, namely Gardineroseris planulata, Pavona maldivensis, Pavona clavus, Pavona decussata, Leptoseris fragilis, and Leptoseris yabei, were resolved with both DNA loci. The molecular boundaries among the remaining 14 species remain unclear. Our results further confirm that the morphology-based taxonomy of most agariciid species is in disagreement with genetics. In order to disentangle the systematics of these taxa, the inclusion of more sampling locations, additional variable loci, and a micromophological approach are likely needed. Our genetic data represent a first step towards the comparison of biodiversity and connectivity between the Red Sea and the rest of the Indo-Pacific.

Fire corals of the hydrocoral genus Millepora provide an important ecological role as framework 18 builders of coral reefs in the Indo-Pacific and the Atlantic. Recent works have demonstrated the incongruence between 19 molecular data and the traditional taxonomy of Millepora spp. based on overall skeleton growth form and pores. In an 20 attempt to establish a reliable and standardized approach for defining species boundaries in Millepora, we focused on 21 those from the Red Sea. In this region, three species are currently recognized the fan-shaped branching M. dichotoma, 22 the blade-like M. platyphylla, and the massive/encrusting M. exaesa. A total of 412 colonies were collected from six 23 localities. Two mitochondrial marker genes (COI and 16S rDNA) were sequenced to obtain phylogeny reconstructions 24 and haplotype networks. Eight morphological traits of pores and the nematocysts of both polyp and eumedusoid stages 25 were measured to determine if significant morphological differences occur among the three species. Both markers 26 clearly resolved M. dichotoma, M. platyphylla, and M. exaesa as distinct, monophyletic lineages in the Red Sea. 27 Nevertheless, they also revealed deep genetic breaks with Southwestern Indian Ocean populations of the three species. 28 Manuscript Click here to access/download;Manuscript;CORE Arrigoni et al Millepora.docx Click here to view linked References In the Red Sea, the three species were further distinguished based on their pore and nematocyst features. A 29 discriminant analyses revealed dactylopore density, number of dactylopores per gastropore, dactylopore distance, and 30 gastropore diameter as the most informative discriminative characters. The heteronemes, the large and small stenoteles 31 of polyps, and the distribution of mastigophores of eumedusoids also showed significantly interspecific differences. 32 An integrated morpho-molecular approach proved to be decisive in defining species boundaries of Millepora 33 supported by a combination of pore and nematocyst characters which may be phylogenetically informative. colonial organisms building persistent calcareous skeletons and as such playing an important ecological role as 38 framework builders of coral reefs (Lewis 2006). These hydrocorals are among the most relevant reef builders in 39 shallow-water tropical seas, second only to scleractinians (Lewis 1989; Edmunds 1999; Smith et al. 2014). Millepora 40 occurs in tropical and subtropical coral reefs of both the Atlantic and the Indo-Pacific and its depth distribution is 41 restricted from less than 1 m to about 50 m deep because of the obligate symbiosis with zooxanthellae of the genus 42

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