Mating system and evidence of multiple paternity in the Antarctic brooding sea urchin Abatus agassizii

Investigating population dynamics from parentage analysis in the highly endangered fan mussel Pinna nobilis

Understanding dispersal patterns is a major focus for conservation biology as it influences local survival and resilience in case of local disturbance, particularly for sessile species. Dispersal can be assessed through parentage analyses by estimating family structure and self-recruitment. This study documents the family structure of a pelagic spawner, Pinna nobilis, which is facing a major crisis that threatens its survival as most of its populations have been decimated by a parasite, Haplosporidium pinnae. In this context, we focused on a single population (Peyrefite, Banyuls-sur-mer, France) where 640 individuals were sampled in 2011, 2015, and 2018 and genotyped for 22 microsatellite markers. Genetic diversity was high and homogeneous among years, with mean allele numbers ranging between 13.6 and 14.8 and observed heterozygosities (H o) between 0.7121 and 0.7331. Low, but significant, genetic differentiations were found between 2011-2015 and 2015-2018. A parentage analysis described 11 clusters, including one prevailing, and revealed that 46.9% of individuals were involved in half-sib relationships, even between years, suggesting that source populations were recurrent year after year. There were few individuals resampled between years (30 in 2015 and 14 in 2018), indicating a rapid turnover. Considering the large number of half-sib relationships but the low number of relations per individual, we conclude that P. nobilis exhibit homogeneous reproductive success. Self-recruitment was not detected, making this population highly vulnerable as replenishment only relies on connectivity from neighboring populations. In the context of the pandemic caused by H. pinnae, these results will have to be considered when choosing a location to reintroduce individuals in potential future rescue plans.

Characterization of the Gut Microbiota of the Antarctic Heart Urchin (Spatangoida) Abatus agassizii

Abatus agassizii is an irregular sea urchin species that inhabits shallow waters of South Georgia and South Shetlands Islands. As a deposit-feeder, A. agassizii nutrition relies on the ingestion of the surrounding sediment in which it lives barely burrowed. Despite the low complexity of its feeding habit, it harbors a long and twice-looped digestive tract suggesting that it may host a complex bacterial community. Here, we characterized the gut microbiota of specimens from two A. agassizii populations at the south of the King George Island in the West Antarctic Peninsula. Using a metabarcoding approach targeting the 16S rRNA gene, we characterized the Abatus microbiota composition and putative functional capacity, evaluating its differentiation among the gut content and the gut tissue in comparison with the external sediment. Additionally, we aimed to define a core gut microbiota between A. agassizii populations to identify potential keystone bacterial taxa. Our results show that the diversity and the composition of the microbiota, at both genetic and predicted functional levels, were mostly driven by the sample type, and to a lesser extent by the population location. Specific bacterial taxa, belonging mostly to Planctomycetacia and Spirochaetia, were differently enriched in the gut content and the gut tissue, respectively. Predictive functional profiles revealed higher abundance of specific pathways, as the sulfur cycle in the gut content and the amino acid metabolism, in the gut tissue. Further, the definition of a core microbiota allowed to obtain evidence of specific localization of bacterial taxa and the identification of potential keystone taxa assigned to the Desulfobacula and Spirochaeta genera as potentially host selected. The ecological relevance of these keystone taxa in the host metabolism is discussed.