Karine Olu

Porifera and Cnidaria are key phyla of marine ecosystem functioning. Slow-growing and long-lived, they show limited resilience to human disturbances and are therefore commonly used as indicator taxa of Vulnerable Marine Ecosystems. Their diversity and distribution largely rely on seafloor images, but image-based identification (ID) of sponges and cnidarians beyond class/order ranks is hindered by morphological plasticity and convergence, and the need of microscopic diagnostic characters or molecular markers that are not accessible on images. Higher-rank taxonomy also poorly reflects their morphological and functional diversity. To address this issue, we applied an image-based classification relying on multiple macro-morphological traits to define statistically morpho-functional groups (MFGs) based on these traits, then used as response variables in ecological analyses, instead of taxa. We applied this framework to seamounts and island slopes in the Mozambique Channel, a poorly sampled region where taxonomic limitations are acute. We aimed to evaluate the relevance of a sponge/cnidarian morpho-functional approach to characterise their diversity and distribution at multiscales, responses to environmental gradients and structuring roles on the megabenthic community. We identified 37 sponge and 34 cnidarian MFGs with varying distributions among sites. MFGs richness and beta diversity varied among seamounts, and between seamounts and island slopes. Hydrology explained 26% of the sponge MFG spatial structure at the channel scale, while topography and substrate played a main role at a smaller scale in the northern part of the channel (17-23%). Some sponge/cnidarian MFGs co-structured with other megabenthic taxa (e.g., sea urchins, comatulids, ophiuroids). This morpho-functional approach, especially effective for sponges, allowed quantifying morphological variation of habitat-forming taxa using qualitative traits, identifying environmental drivers shaping the spatial distribution of these traits, and characterising potential biogenic habitats they provide for other megabenthic organisms. The framework proposes metrics to detect and quantify the heterogeneity and spatial variability of potentially vulnerable biogenic habitats in imagery-based studies.

This study provides an analysis of vesicomyid bivalve–symbiont community distribution across cold seep and hydrothermal vent areas in the Guaymas Basin (Gulf of California, Mexico). Using a combination of morphological and molecular approaches including fluorescent in situ hybridization (FISH), and electronic microscopy observations, vesicomyid clam species and their associated symbionts were characterized and results were analyzed in light of geochemical conditions and other on‐site observations. A greater diversity of vesicomyids was found at cold seep areas, where three different species were present (Phreagena soyoae [syn. kilmeri], Archivesica gigas, and Calyptogena pacifica). In contrast, A. gigas was the only species sampled across the hydrothermal vent area. The same haplotype of A. gigas was found in both hydrothermal vent and cold seep areas, highlighting possible contemporary exchanges among neighboring vents and seeps. In either ecosystem, molecular characterization of the symbionts confirmed the specificity between symbionts and hosts and supported the hypothesis of a predominantly vertical transmission. In addition, patterns of clams could reflect potential niche preferences for each species. The occurrence of numerous traces of vesicomyid movements on sediments in the sites colonized by A. gigas seemed to indicate that this species might have a better ability to move. Furthermore, variation in gill sulfur content could reveal a higher plasticity and sulfur storage capacity in A. gigas. Thus, the distribution of vesicomyid species across the chemosynthetic areas of the Guaymas Basin could be explained by differences in biological traits of the vesicomyid species that would allow A. gigas to more easily exploit transient and punctual sources of available sulfide than P. soyoae.