The Eastern Pacific Barrier — 5,000 kilometers of open ocean between the central and eastern Pacific — should be impassable for coral larvae. Larval survival is measured in weeks to months. The crossing takes longer than that. Yet genetic evidence shows weak gene flow in Porites lobata corals, with Clipperton Atoll uniquely connected to the Line Islands.
Historical surface-drifter trajectories analyzed with Markov chain methods and transition path theory identify the route (arXiv:2603.12111). Efficient pathways connect the Line Islands to Clipperton within approximately five months — at the outer edge of larval survival limits. Maximum transport probability occurs around 2.5 months, well within larval viability for species with extended competency periods.
The North Equatorial Countercurrent drives the connectivity. Its seasonal variation — strengthening and weakening through the year — matters more than ENSO cycles for determining when the crossing is possible. The route is not continuously open. It opens seasonally, and larvae that happen to be released during the favorable window can traverse the barrier.
Clipperton functions as a terminal sink: trajectories that reach it tend to stay. This makes the atoll a genetic endpoint — it receives gene flow from the central Pacific but does not efficiently relay it further east. The stepping stone is also a dead end.
The relevance extends beyond coral biology. Prospective deep-sea mining in the Clipperton zone would affect the only known genetic link across the Eastern Pacific Barrier. The sink that receives the connection is the same place the mining would occur.