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Summary: Terpios hoshinota is an aggressive, space‐competing sponge that kills various stony corals. Outbreaks of this species have led to intense damage to coral reefs in many locations. Here, the first large‐scale 16S rRNA gene survey across three oceans revealed that bacteria related to the taxa Prochloron, Endozoicomonas, SAR116, Ruegeria, and unclassified Proteobacteria were prevalent in T. hoshinota. A Prochloron‐related bacterium was the most dominant and prevalent cyanobacterium in T. hoshinota. The complete genome of this uncultivated cyanobacterium and pigment analysis demonstrated that it has phycobiliproteins and lacks chlorophyll b, which is inconsistent with the definition of Prochloron. Furthermore, the cyanobacterium was phylogenetically distinct from Prochloron, strongly suggesting that it should be a sister taxon to Prochloron. Therefore, we proposed this symbiotic cyanobacterium as a novel species under the new genus Candidatus Paraprochloron terpiosi. Comparative genomic analyses revealed that 'Paraprochloron' and Prochloron exhibit distinct genomic features and DNA replication machinery. We also characterized the metabolic potentials of 'Paraprochloron terpiosi' in carbon and nitrogen cycling and propose a model for interactions between it and T. hoshinota. This study builds a foundation for the study of the T. hoshinota microbiome and paves the way for better understanding of ecosystems involving this coral‐killing sponge. [ABSTRACT FROM AUTHOR]

Planktonic larvae of sessile metazoans select substrates for settlement based on various factors. Phallusia philippinensis larvae (Ascidiacea: Phlebobranchia: Ascidiidae) showed a negative preference for nano-scale nipple arrays (dense arrays of papillae-like nanostructures approximately 100 nm in height). To clarify whether ascidian larvae discriminate between nano-structure sizes for substrate selection, three different sizes of periodic nano-folds were fabricated using two-beam interference exposure, and substrate selection assays were performed on the three types of nano-folds and flat surfaces made of the same material. The substrate selection assay with 500-2000 freshly hatched larvae was carried out in nine replicates. The ascidian larvae showed a positive preference for flat surfaces and a negative preference for substrates with a height of 120 nm and pitch of 600 nm. Manly's selection indices differed with the size of the periodic nano-folds, supporting the hypothesis that larvae directly or indirectly discriminate between nano-scale differences upon settlement. The present study is the first to show that differences in nanostructure size affect substrate selection during larval settlement of sessile animals. The evolutionary adaptive reasons for larvae to discriminate between nano-scale structures and select substrates for settlement are potentially important to effectively manage ascidian biofouling using non-toxic methods.

Soft tunic syndrome is an infectious disease caused by the flagellate Azumiobodo hoyamushi, which severely damages the aquaculture of the edible ascidian Halocynthia roretzi. Tunic is a cellulosic extracellular matrix entirely covering the body in ascidians and other tunicates, and its dense cuticle layer covers the tunic surface as a physical barrier against microorganisms. When the tunic of intact H. roretzi individuals was cut into strips, electron-dense fibers (DFs) appeared on the cut surface of the tunic matrix and aggregated to regenerate a new cuticular layer in seawater within a few days. DF formation was partially or completely inhibited in individuals with soft tunic syndrome, and DF formation was also inhibited by the presence of some proteases, indicating the involvement of proteolysis in the process of tunic softening as well as cuticle regeneration. Using pure cultures of the causative flagellate A. hoyamushi, the expression of protease genes and secretion of some proteases were confirmed by RNA-seq analysis and a 4-methylcoumaryl-7-amide substrate assay. Some of these proteases may degrade proteins in the tunic matrix. These findings suggest that the proteases of A. hoyamushi is the key to understanding the mechanisms of cuticular regeneration inhibition and tunic softening.

Sea slugs are always covered in a mucus layer that has various functions including chemical defense that often involves aposematism and mimicry. Therefore, it is necessary for sea slugs to exhibit their body colors and patterns exactly, and the optical properties of mucus should support this requirement. We examined body mucus from heterobranch sea slugs collected in the Okinawan coral reefs. The refractive indices of mucus from 32 species ranged from 1.3371 to 1.3854 and were similar or slightly greater than the refractive index of seawater (ca. 1.34), indicating that light reflectance on the mucus layer is generally small. Moreover, dissolution of mucus into seawater would form a gradient of refractive indices and enhance the reduction of reflectance. We also obtained relative absorption spectra of the mucus from 32 species. In the range of visible light, absorption spectra of mucus suggest that the mucus layer is almost transparent and is not likely to interfere with the body colors. The presence of absorption peaks and/or shoulders in the UV (ultraviolet) range (280-400 nm) indicates that the mucus layer potentially serves as a sunscreen that absorbs UV radiation in 23 species, whereas prominent UV absorption was not found in the other 9 species. In a kleptoplasty sacoglossan Plakobranchus ocellatus , the refractive indices and presence or absence of UV-absorption showed that the optical properties of the mucus varied to some extent but did not show seasonal fluctuation. The UV-absorption in the mucus may also protect kleptoplasts in kleptoplasty sacoglossans. The present results support the importance of mucus as a functional optical layer for the shell-less life of sea slugs.