Sven R. Laming

Aspects adaptatifs de la biologie du développement et des symbioses chimiosynthétiques chez les petites moules Bathymodiolinae de l'Océan profond: Plusieurs habitats de l'Océan profond sont caractérisés par la présence de composés chimiques réduits. Les communautés benthiques prospèrent dans ces "habitats réducteurs", en raison de liens trophiques avec des bactéries chimiosynthétiques, qui tirent l'énergie de donneurs (sulfures, hydrocarbures) et accepteurs (O2) d'électrons. Les moules bathymodiolines sont un taxon clé dans ces habitats. À l'âge adulte, presque toutes possèdent des bactéries symbiotiques, mais les données sont rares concernant leur cycle de vie. Cette recherche explore la biologie de la reproduction, le développement et la nutrition dans le cycle de vie des espèces Idas modiolaeformis et "I." simpsoni. Les analyses anatomiques, histologiques et moléculaires à divers stades du développement sont associées à des observations in vivo. Une comparaison est proposée entre ces espèces. Chez les deux, les post-larves n'ont pas de symbiotes, suggérant des larves strictement hétérotrophes. L'infection par les symbiotes est environnementale, extracellulaire et initialement non-spécifique, se restreignant aux surfaces latéro-abfrontales des filaments branchiaux chez les adultes. La maturation est rapide, de même que la transition de l'hétérotrophie à la mixotrophie chimiosymbiotique: les symbioses, l'alimentation par filtration, et la rétention d'un système digestif complet coïncident. Les différences entre espèces et habitats sont discutées dans le contexte de leur évidente réussite évolutive à s'adapter aux habitats réduits et éphémères de l'Océan profond.

Deep-sea mining in the Pacific Clarion-Clipperton Fracture Zone (CCZ), a low-energy sedimentary habitat with polymetallic nodules, is expected to have considerable and long-lasting environmental impact. The CCZ hosts extraordinarily high species diversity across representatives from all Domains of Life. Data on species biology and ecology remain scarce, however. The current study describes the reproductive biology of Ophiosphalma glabrum (Ophiosphalmidae) and Ophiacantha cosmica (Ophiacanthidae), two ophiuroids frequently found in the CCZ. Specimens collected in Spring 2015 and 2019 in four contract areas were examined morphologically and histologically. Size-class frequencies (disc diameter and oocytes feret diameters), sex ratios, gametogenic status, putative reproductive mode, and a simple proxy for fecundity are presented. Habitat use differs in each. While O. glabrum is epibenthic, occurring as single individuals, O. cosmica often forms size-stratified groups living on stalked sponges, suggesting gregarious settlement or retention of offspring (though no brooding individuals were found). Further molecular analyses are needed to establish whether O. cosmica groups are familial. In O. glabrum, for which sample sizes were larger, sex ratios approximated a 1:1 ratio with no size-structuring. In both species, individuals were at various stages of gametogenic maturity, but no ripe females were identified. Based on this, O. glabrum is most probably gonochoric. Reproductive mode remains inconclusive for O. cosmica. Both species are presumptively lecithotrophic, with vitellogenic-oocyte feret diameters exceeding 250 μm. Oocyte feret diameters at times exceeded 400 μm in O. glabrum, indicating substantial yolk reserves. Estimates of instantaneous fecundity (vitellogenic specimens of O. glabrum only) were confounded by interindividual variability in gonad characteristics. The well-furnished lecithotrophic larvae of O. glabrum would be capable of dispersing even under food-impoverished conditions. The current study examines ophiuroid reproductive biology over multiple localities in the CCZ concurrently for the first time, at sites characterised by differing productivity regimes. The reproductive biology of each species is thus discussed with reference to past evolutionary (habitat stability), contemporary (food supply), and future environmental drivers (potential impacts of deep-sea mining).

Deep-sea mining in the Pacific Clarion-Clipperton Fracture Zone (CCZ), a low-energy sedimentary habitat with polymetallic nodules, is expected to have considerable and long-lasting environmental impact. The CCZ hosts extraordinarily high species diversity across representatives from all Domains of Life. Data on species biology and ecology remain scarce, however. The current study describes the reproductive biology of Ophiosphalma glabrum (Lütken & Mortensen, 1899) (Ophiosphalmidae) and Ophiacantha cosmica (Lyman, 1878) (Ophiacanthidae), two ophiuroids frequently found in the CCZ. Specimens collected in Spring 2015 and 2019 in four contract areas were examined morphologically and histologically. Size-class frequencies (disc diameter and oocytes feret diameters), sex ratios, gametogenic status, putative reproductive mode and a simple proxy for fecundity are presented. Habitat use differs in each. While Ophiosphalma glabrum is epibenthic, occurring as single individuals, Ophiacantha cosmica often form size-stratified groups living on stalked sponges, suggesting gregarious settlement or retention of offspring (though no brooding individuals were found). Further molecular analyses are needed to establish whether O. cosmica groups are familial. In Ophiosphalma glabrum , for which sample sizes were larger, sex ratios approximated a 1:1 ratio with no size-structuring. In both species, individuals were at various stages of gametogenic maturity but no ripe females were identified. Based on this, O. glabrum is most probably gonochoric. Reproductive mode remains inconclusive for Ophiacantha cosmica . Both species are presumptively lecithotrophic, with vitellogenic-oocyte feret diameters exceeding 250 µm. Oocyte feret diameters at times exceeded 400 µm in Ophiosphalma glabrum , indicating substantial yolk reserves. Estimates of instantaneous fecundity (vitellogenic specimens of O. glabrum only) were confounded by interindividual variability in gonad characteristics. The well-furnished lecithotrophic larvae of O. glabrum would be capable of dispersing even under food-impoverished conditions. The current study examines ophiuroid reproductive biology over multiple localities in the CCZ concurrently for the first time, at sites characterised by differing productivity regimes. The reproductive biology of each species is thus discussed with reference to past evolutionary (habitat stability), contemporary (food supply) and future environmental drivers (potential impacts of deep-sea mining).

Recent advances in technology have enabled an unprecedented development of underwater research, extending from near shore to the deepest regions of the globe. However, monitoring of biodiversity is not fully implemented in political agendas and biological observations in the deep ocean have been even more limited in space and time. The Foresight Workshop on Advances in Ocean Biological Observations: a sustained system for deep-ocean meroplankton was convened to to foster advances in the knowledge on deep-ocean invertebrate larval distributions and improve our understanding of fundamental deep-ocean ecological processes such as connectivity and resilience of benthic communities to natural and human-induced disturbance. This Meroplankton Observations Workshop had two specific goals: 1) review the state-of-the-art instrumentation available for meroplankton observations; 2) develop a strategy to implement technological innovations for in-situ meroplankton observation. Presentations and discussions are summarised in this report covering: i) key challenges and priorities for advancing the knowledge of deep-sea larval diversity and distribution: ii) recent developments in technology and future needs for plankton observation, iii) data integration and oceanographic modelling; iv) synergies and added value of a sustained observation system for meroplankton; v) steps for developing a sustained observation system for deep-ocean meroplankton and plans to maximise collaborative opportunities.

The chemosymbiotic gastropod Alviniconcha (Provannidae), first described in 1988, is one of the most emblematic hydrothermal-vent taxa described from the Central Indian Ridge and the Southwest (SW) Pacific. Symbiotic bacteria found in the gill of Alviniconcha are thought to be their principal source of nutrition. In the SW Pacific, species distributions for A. kojimai, A. boucheti – and to a lesser extent A. strummeri – overlap. While Alviniconcha species do not appear to truly co-exist in these highly energetic but spatially limited habitats, certain species regularly co-occur within a single vent field and in rare instances, the same edifice. Past research suggests that SW-Pacific Alviniconcha species might aggregate around fluids with distinct geothermal profiles. These small-scale distribution patterns have been attributed to differences in their symbiont assemblages or host physiologies. However, little is known about the anatomy of most Alviniconcha species, beyond that detailed for the type species Alviniconcha hessleri, whose geographic range does not overlap with other congeners. In fact, species within this genus are currently described as cryptic, despite the absence of any comparative morphological studies to assess this. To test whether the genus is genuinely cryptic and identify any functional differences in host anatomy that might also mediate habitat partitioning in SW Pacific species, the current study examined the morphoanatomy of A. kojimai, A. boucheti and A. strummeri from the Fatu Kapa vent field, an area of hydrothermal activity recently discovered north of the Lau Basin near the Wallis and Futuna Islands and the only known example where all three species occur within adjacent vent fields. A combination of detailed dissections, histology and X-ray computed tomography demonstrate that A. kojimai, A. strummeri and A. boucheti are readily identifiable based on shell morphology and ornamentation alone, and therefore not truly cryptic. These traits provide a rapid and reliable means for species identification. However, aside from some subtle differences in radular morphology, these species of Alviniconcha exhibit conserved anatomical features, providing no evidence that functional host anatomy is implicated in habitat partitioning. This provides support for the current belief that host-species distributions are probably governed by symbiont-mediated physiological factors.

Hydrothermal vents are places where seawater exits cracks in the sea floor, having been super-heated and enriched with metals and minerals deep in the underlying bedrock. They are an example of an ecosystem based on chemosynthesis, where life is sustained by energy from chemicals rather than energy from sunlight. The discovery of an abundance of life around deep-sea hydrothermal vents emitting hot and toxic fluids demonstrated that animals and other organisms could thrive in the dark, cold and high-pressure deep oceans. Mussels are among the most studied animals found near hydrothermal vents. Scientists discovered that mussels rely on a close, living relationship—a “symbiosis”—with bacteria for their nutrition. In this symbiosis, bacteria use chemicals from the hydrothermal fluid and seawater to produce organic compounds, while the mussels provide the bacteria with essential compounds and protection. The mussel life cycle is uniquely adapted to finding and colonizing their unusual habitat and then finding suitable symbiotic bacteria, almost immediately. Despite its remoteness, the deep sea is already under threat. Although there is still much work to be done, research into mussels and other animals that have evolved similar symbioses has revealed not only their beauty, but also their fragility.

Mussels within the subfamily Bathymodiolinae, in particular the larger Bathymodiolus species (sensu lato) thriving at cold seeps and hydrothermal vents, are among the most iconic fauna to colonize deep-sea reducing habitats globally. Fuelled by energy derived from chemosynthetic symbioses, their contribution to ecosystem productivity is conspicuous, with many bathymodioline species forming dense, extensive aggregates. Chemosymbiotic mussels play crucial roles as ecosystem engineers, both through the formation of spatially heterogeneous biogenic reefs and in redistributing reduced-fluid emissions. The notable absence of Bathymodiolinae outside of reducing ecosystems affirms their dependency on these ephemeral habitats, placing spatiotemporal constraints on dispersal to, and colonization of nascent, chemosynthetically active substrata. Thus, although symbioses may explain why these mussels are so productive in deep-sea reducing habitats, species' survival over successive generations depends largely upon the adaptive characteristics of their lifecycle as a whole. Despite accumulating data on the biology and ecology of adults however, details remain fragmented regarding earlier developmental junctures during their development. This paper therefore brings together results from research undertaken over recent years on this topic, providing a synthesis of various lifecycle aspects of bathymodiolins from the earliest stages of development, gametogenesis, through to sexual maturity, including the intrinsic, emerging role of symbionts. The review provides a comprehensive overview of our current understanding and identifies areas where further study into these keystone organisms is warranted. The benefits of applying an integrated, lifecycle approach when evaluating the potential impacts of global change and anthropogenic activities upon deep-sea fauna and their habitats are then discussed.

The Mediterranean Sea and adjoining East Atlantic Ocean host a diverse array of small-sized mussels that predominantly live on sunken, decomposing organic remains. At least two of these, Idas modiolaeformis and Idas simpsoni, are known to engage in gill-associated symbioses; however, the composition, diversity and variability of these symbioses with changing habitat and location is poorly defined. The current study presents bacterial symbiont assemblage data, derived from 454 pyrosequencing carried out on replicate specimens of these two host species, collected across seven sample sites found in three oceanographic regions in the Mediterranean and East Atlantic. The presence of several bacterial OTUs in both the Mediterranean Sea and eastern Atlantic suggests that similar symbiont candidates occur on both sides of the Strait of Gibraltar. The results reveal markedly different symbiotic modes in the two species. Idas modiolaeformis displays high symbiont diversity and flexibility, with strong variation in symbiont composition from the East Mediterranean to the East Atlantic. Idas simpsoni displays low symbiont diversity but high symbiont fidelity, with a single dominant OTU occurring in all specimens analysed. These differences are argued to be a function of the host species, where subtle differences in host evolution, life-history and behaviour could partially explain the observed patterns. The variability in symbiont compositions, particularly in Idas modiolaeformis, is thought to be a function of the nature, context and location of the habitat from which symbiont candidates are sourced.