A catastrophic seafloor eruption at 9°50'N on the East Pacific Rise in 2005–2006 devastated the vent communities within the eruption footprint — although one community at the southern margin (V-vent, 9°47′N) survived with no detectable change in species composition — and created a natural clearance experiment. When larval supply and colonization monitoring resumed, the pioneer assemblage included the limpet Ctenopelta porifera, an immigrant inferred to have originated more than 300 km away, and the formerly rare limpet Lepetodrilus tevnianus, demonstrating that larval supply can change drastically once the local communities seeding recruitment are removed. The unresolved core of this finding is threefold: (1) determining whether the colonist assemblage reflects altered larval supply, post-eruption changes in environmental suitability (thermal, chemical, and structural habitat change), or both, including larval flux in the first post-eruption months and species-specific tolerances to post-eruption conditions; (2) determining by molecular genetics whether the post-eruption Lepetodrilus larval cohort represents the pioneer L. tevnianus or remnant predisturbance congeners, which are morphologically indistinguishable as larvae; and (3) quantifying, across the vent fields of the East Pacific Rise ridge system, the frequency and duration of episodic "open" states — windows in which disturbed sites are recolonizable by remote immigrants — and the role of temporally variable currents and stochastic larval supply in enabling rare long-distance exchanges. The long-term trajectory question posed by the same study (regime shift versus transient succession) has since been substantially addressed by an 11-year monitoring series, which documented a prolonged succession returning toward the pre-eruption fauna rather than a lasting regime shift of the pioneer assemblage, although composition later deviated from or bypassed the pre-eruption state and was still changing at the end of the series.

Background

Deep-sea hydrothermal vents are springs of hot, chemically reduced fluid on the seafloor that host animal communities fueled by microbial chemosynthesis rather than photosynthesis. Vent habitats are patchy and short-lived on geological timescales: along fast-spreading ridges such as the East Pacific Rise (EPR), magmatic eruptions periodically bury entire vent fields under lava and kill the resident fauna. Most vent species disperse only as planktonic larvae, so the persistence of their populations depends on larvae traveling tens to hundreds of kilometers through cold, food-poor deep water between isolated vent fields. This makes vent species a textbook metapopulation — a set of local populations connected by dispersal and subject to local extinction and recolonization.

The EPR vent fields near 9°50'N, at roughly 2500 m depth, are the most intensively monitored vent system in the world, with eruption cycles in 1991 and 2005–2006. The communities are structured by foundation species (large tubeworms and mussels that create habitat for smaller animals) and include small gastropod limpets of the genus Lepetodrilus that graze bacterial mats. Larvae of many vent gastropods are morphologically cryptic: larvae of different Lepetodrilus species cannot be separated by microscope, so plankton collections can be identified only to genus unless molecular markers are used.

Before the 2005–2006 eruption, field observations suggested that established vent populations largely retain their own larvae and are self-sustaining. The eruption removed the vent communities at the study sites — one community survived at the southern margin of the eruption footprint (V-vent at 9°47′N, within ~6 km to the south), with no detectable change in species composition — and allowed a direct test. Mullineaux and colleagues, who had been monitoring larval supply in plankton traps and colonization on deployed surfaces before the eruption and resumed shortly afterward, found a striking switch: prominent predisturbance species dropped out, while the post-eruption colonists included Ctenopelta porifera — whose only known populations occur around 13°N, more than 300 km to the south — and Lepetodrilus tevnianus, previously rare. This demonstrated that connectivity is temporally variable: a vent site alternates between a "closed" state, in which local larvae dominate recruitment, and an "open" state after a disturbance, when remote immigrants can establish. The same group showed that mesoscale eddies (rotating currents tens of kilometers wide) and the ridge-segment circulation can transport vent larvae and fluids over long distances, and developed a staged morphological-plus-molecular method (restriction-fragment-length polymorphism digests and an 18S ribosomal-DNA database) capable of identifying five Lepetodrilus species in larval samples from this ridge segment.

Follow-up work has substantially resolved one of the four questions the study raised. An 11-year colonization time series (through 2017) showed that C. porifera disappeared after about 22 months, L. tevnianus declined after about 33 months, and species composition approached the pre-eruption state by roughly 96 months while still changing at 135 months, at times bypassing or deviating from the pre-eruption state — a prolonged succession rather than a lasting regime shift of the pioneer assemblage, and evidence that the pre-eruption "baseline" is not a fixed climax community. In contrast, three other threads remain open. First, the observed recolonization could reflect either a post-eruption "larval vacuum" (collapsed local larval supply opening the site to dispersive immigrants) or altered environmental suitability (changed vent-fluid temperature and chemistry, and replacement of the tubeworm Riftia by Tevnia as the foundation species), or both; the larval traps began roughly six months after the eruption, and pilot samples collected in the eruption region in May–June 2006 revealed very low fluxes but came from collectors sited several hundred meters from vents, where larval abundances are known to be reduced, so flux in the critical first months remains effectively unconstrained; and species-specific physiological tolerances to post-eruption conditions have not been measured. Second, the identity of the post-eruption Lepetodrilus larval cohort was inferred only from colonists — the larvae themselves were never identified to species — and the taxonomy of the local L. elevatus species complex has since been shown to need molecular revision. Third, no study has quantified how often and for how long vent sites across a ridge system are open to remote colonists, or how stochastic, time-varying currents produce rare long-distance exchanges such as the inferred 300-km Ctenopelta colonization, even though this temporal connectivity underpins predictions of vent-ecosystem resilience to natural eruptions and to anthropogenic disturbances such as seafloor massive-sulfide mining.

Problem Statement

Provide a mechanistic, predictive account of post-eruption recolonization and temporal connectivity for the deep-sea hydrothermal vent metapopulation of the East Pacific Rise, centered on the intensively studied vents near 9°50'N, resolving three linked questions that the 2005-2006 eruption study left open. (1) Determine whether the observed post-eruption colonist assemblages developed through altered larval supply, through settlement responses to post-eruption changes in environmental suitability (thermal and chemical habitat alteration, and structural change such as replacement of foundation tubeworm species), or through both: this requires estimating or bounding larval flux in the first months after the disturbance — the hypothesized 'larval vacuum', which the trap series begun roughly six months after the eruption and the caveated May–June 2006 pilot samples collected several hundred meters from vents leave effectively unconstrained — and determining the tolerances of the relevant gastropod species to the post-eruption thermal and chemical regimes at the colonized surfaces. (2) Determine by molecular genetic identification whether the post-eruption Lepetodrilus larval cohort corresponds to the pioneer L. tevnianus, to remnant predisturbance species such as L. elevatus and L. pustulosus, or to a mixture, given that larvae in this genus are morphologically indistinguishable. (3) Quantify the frequency and duration of episodic 'open' states — the post-disturbance windows in which sites are colonizable by larvae from remote populations — across the vent fields of the East Pacific Rise ridge system, and establish the role of temporally variable currents and stochasticity in larval supply in enabling rare long-distance exchanges such as the inferred ~300 km arrival of Ctenopelta porifera; such quantification underpins predictive metapopulation modeling and the assessment of anthropogenic disturbances including seafloor massive-sulfide mining. The community-trajectory component of the original question — whether the post-eruption composition represents a lasting regime shift or a transient successional state — has been substantially addressed by an 11-year monitoring series (a prolonged succession returning toward, and later at times deviating from or bypassing, the pre-eruption fauna rather than a lasting regime shift of the pioneer assemblage) and is part of the background.

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