Biophysical modeling and population genetics of the vent mussel Bathymodiolus along the Mid-Atlantic Ridge (MAR) indicate that known vent fields are too far apart for direct larval dispersal, so connectivity requires intermediate, still-undiscovered chemosynthetic habitats ('phantom' stepping stones), which may be either extant-but-undetected or now extinct. A decade of follow-up work has advanced pieces of the question — genome-wide markers for MAR gastropods, ghost-population demographic modeling on Indian Ocean vents, and evidence of larval vertical migration in Pacific limpets — but the central test remains open: no study has detected intermediate habitats in the ridge gaps, demonstrated that MAR genetic patterns are a legacy of extinct vents, ground-truthed abyssal circulation, or measured MAR Bathymodiolus larval behavior and mortality. The task is to reach that determination, weighing the coupled model, marker, larval-biology, and cross-taxon evidence on which MPA-spacing recommendations depend.

Background

Deep-sea hydrothermal vents are patchy, ephemeral ecosystems on the seafloor where chemosynthetic microbes — bacteria that draw energy from oxidizing reduced compounds in vent fluids rather than from light — sustain dense animal communities in the dark ocean. Mussels of the genus Bathymodiolus are foundation species (organisms that create habitat for others) at many vents: they form beds that structure entire communities. Because adult mussels are sessile, exchange among vent fields happens through a planktonic larval stage whose duration (the pelagic larval duration, PLD — up to about one year in bathymodiolins) sets an upper bound on passive drift. Vent populations are therefore textbook metapopulations — local populations prone to extinction and recolonization — and their resilience depends on connectivity, i.e. the successful exchange of recruiting larvae among fields.

Breusing et al. (2016, Current Biology 26:2257–2267) combined two indirect methods to study MAR mussel connectivity: (i) biophysical modeling — releasing ~950,000 numerically tracked particles per vent site in the velocity fields of the high-resolution ocean general circulation model (OGCM) VIKING20 — and (ii) population genetics with 90 SNPs (single nucleotide polymorphisms), nine microsatellites, and mitochondrial DNA, analyzed for structure (Bayesian clustering, pairwise FSTF_{ST} differentiation indices — a standard measure of genetic differentiation between populations) and recent migration rates. The model, run for larvae drifting passively for 6 to 12 months from the Azores vent fields (Menez Gwen, Lucky Strike, Rainbow) southward, showed that median effective dispersal distances never exceeded 110–150 km and that arrival at the next known vent field was essentially impossible, even though independent estimates place known MAR hydrothermal systems roughly every 30–125 km (northern MAR) to 112–230 km (southern MAR), with plume-sensing surveys suggesting even closer spacing of undiscovered fields (Baker et al. 2016). Both method legs agreed that migration among presently known fields within a single generation is unlikely. The authors concluded that connectivity requires intermediate, presently unknown chemosynthetic habitats — 'phantom' stepping stones — which could either be extant but overlooked (small, diffusive vents are hard to detect; mussels were later found at the off-axis Lost City field and at a diffuse site only 5 km from the well-studied Menez Gwen field) or be hydrothermal systems that are now extinct. The inference, however, rested on unvalidated assumptions on both legs: the OGCM deep flows lacked field ground-truthing (e.g. by deep drifter releases) and its grid covered only part of the MAR; the genetic markers were designed to work across species and so traded away within-species resolution; larval vertical migration, swimming, and planktonic mortality were excluded from the simulations although larval behavior can change modeled deep-sea dispersal by an order of magnitude (Gary et al. 2020); and the stepping-stone inference itself had not been tested across taxa with different PLDs.

The stakes are managerial as well as conceptual: polymetallic seafloor massive sulfide (SMS) deposits at MAR vents are targets for deep-sea mining under International Seabed Authority exploration licenses, and reserve-design principles translate connectivity estimates into required spacing of marine protected areas (MPAs) — Breusing et al. suggested conservation areas spaced less than roughly 21–26 km apart for bathymodiolin mussels, while subsequent regional network design (Dunn et al. 2018) and three-dimensional management frameworks (Gollner et al. 2026) must otherwise assume dispersal scales that remain unverified.

Problem Statement

Determine how the known hydrothermal-vent ecosystems of the Mid-Atlantic Ridge (MAR) are actually connected, by decisively testing the 'phantom' stepping-stone hypothesis of Breusing et al. (2016): do intermediate chemosynthetic habitats (seafloor communities fueled by chemical energy rather than light) exist today between the known vent fields (extant but undiscovered — detectable by targeted exploration and mapping), are the genetic and biophysical signatures of connectivity a legacy of hydrothermal systems that are now extinct, or is no intermediate habitat required, because larval behavior and mortality allow direct dispersal among the known fields?

References

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