The Ocean’s Most Abundant Fish Evolved in Extreme Heat — and That May Be Good News
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By Daniel Hentz, whoi.edu.
The most numerous fish in the ocean isn’t one most boaters or anglers will ever see. Bristlemouths — tiny, deep-dwelling fish of the genus Cyclothone — number in the quadrillions and play a major role in the ocean’s biological carbon pump, helping move CO₂ from the surface into the deep. A new WHOI-led study has pushed their origins back nearly 40 million years earlier than previously believed, to one of the hottest periods in Earth’s history — a finding that carries surprisingly hopeful implications for how ocean ecosystems may handle a warming climate.
As Daniel Hentz writes for WHOI’s Oceanus magazine:
Where myctophids (lanternfish) represent the most abundant kind of fish in the ocean by weight, or biomass, bristlemouths win out by sheer population size — think trees compared with blades of grass. Data from trawl net surveys estimate roughly a quadrillion of these fish live in the ocean today (yes, with a quad!).
Like lanternfish, bristlemouths play an outsized role in the ocean’s biological carbon pump — the marine food web responsible for moving planet-warming carbon dioxide from the sea surface into the deep, thus regulating Earth’s temperature.
The breakthrough came from an unlikely source: a single fossilized tooth with distinctive spiral grooves, discovered under a microscope by WHOI paleoceanographer Elizabeth Sibert. After comparing the specimen against teeth from over 40 deep-sea fish species across three natural history collections, her team confirmed the spiral shape belongs exclusively to bristlemouths — and that the fossil dates to roughly 55.6 million years ago, during the early Eocene, when average global temperatures were at least 11°F warmer than today and the seafloor sat at a sweltering 60°F.
The fact that bristlemouths not only survived but thrived through that extreme warmth — and multiple ice ages since — suggests a level of resilience in deep-ocean ecosystems that could matter as today’s oceans continue to warm.
