Cryptic habitats and cryptic diversity: unexpected patterns of connectivity and phylogeographical breaks in a Mediterranean endemic marine cave mysid.

The marine cave-dwelling mysid Hemimysis margalefi is distributed over the whole Mediterranean Sea, which contrasts with the poor dispersal capabilities of this brooding species. In addition, underwater marine caves are a highly fragmented habitat which further promotes strong genetic structuring, therefore providing highly informative data on the levels of marine population connectivity across biogeographical regions. This study investigates how habitat and geography have shaped the connectivity network of this poor disperser over the entire Mediterranean Sea through the use of several mitochondrial and nuclear markers. Five deeply divergent lineages were observed among H. margalefi populations resulting from deep phylogeographical breaks, some dating back to the Oligo-Miocene. Whether looking at the intralineage or interlineage levels, H. margalefi populations present a high genetic diversity and population structuring. This study suggests that the five distinct lineages observed in H. margalefi actually correspond to as many separate cryptic taxa. The nominal species, H. margalefi sensu stricto, corresponds to the westernmost lineage here surveyed from the Alboran Sea to southeastern Italy. Typical genetic breaks such as the Almeria-Oran Front or the Siculo-Tunisian Strait do not appear to be influential on the studied loci in H. margalefi sensu stricto. Instead, population structuring appears more complex and subtle than usually found for model species with a pelagic dispersal phase. The remaining four cryptic taxa are all found in the eastern basin, but incomplete lineage sorting is suspected and speciation might still be in process. Present-day population structure of the different H. margalefi cryptic species appears to result from past vicariance events started in the Oligo-Miocene and maintained by present-day coastal topography, water circulation and habitat fragmentation.

Origins and evolutionary flexibility of chemosynthetic symbionts from deep-sea animals.

Bathymodiolin mussels dominate hydrothermal vent and cold seep communities worldwide. Symbiotic associations with chemosynthetic sulfur- and methane-oxidizing bacteria that provide for their nutrition are the key to their ecological and evolutionary success. The current paradigm is that these symbioses evolved from two free-living ancestors, one methane-oxidizing and one sulfur-oxidizing bacterium. In contrast to previous studies, our phylogenetic analyses of the bathymodiolin symbionts show that both the sulfur and the methane oxidizers fall into multiple clades interspersed with free-living bacteria, many of which were discovered recently in metagenomes from marine oxygen minimum zones. We therefore hypothesize that symbioses between bathymodiolin mussels and free-living sulfur- and methane-oxidizing bacteria evolved multiple times in convergent evolution. Furthermore, by 16S rRNA sequencing and fluorescence in situ hybridization, we show that close relatives of the bathymodiolin symbionts occur on hosts belonging to different animal phyla: Raricirrus beryli, a terebellid polychaete from a whale-fall, and a poecilosclerid sponge from a cold seep. The host range within the bathymodiolin symbionts is therefore greater than previously recognized, confirming the remarkable flexibility of these symbiotic associations.

High symbiont diversity in the bone-eating worm Osedax mucofloris from shallow whale-falls in the North Atlantic.

Osedax worms are whale-fall specialists that infiltrate whale bones with their root tissues. These are filled with endosymbiotic bacteria hypothesized to provide their hosts with nutrition by extracting organic compounds from the whale bones. We investigated the diversity and distribution of symbiotic bacteria in Osedax mucofloris from shallow-water whale-falls in the North Atlantic using comparative 16S rRNA sequence analysis and fluorescence in situ hybridization (FISH). We observed a higher diversity of endosymbionts than previously described from other Osedax species. Endosymbiont sequences fell into eight phylogenetically distinct clusters (with 91.4-98.9% similarity between clusters), and considerable microdiversity within clusters (99.5-99.7% similarity) was observed. Statistical tests revealed a highly significant effect of the host individual on endosymbiont diversity and distribution, with 68% of the variability between clusters and 40% of the variability within clusters explained by this effect. FISH analyses showed that most host individuals were dominated by endosymbionts from a single cluster, with endosymbionts from less abundant clusters generally confined to peripheral root tissues. The observed diversity and distribution patterns indicate that the endosymbionts are transmitted horizontally from the environment with repeated infection events occurring as the host root tissues grow into the whale bones.