Seeing like a Siphonophore: vertical migration above and below

Three views of a single Rhizophysa. Credit: Larry Madin, Woods Hole Oceanographic Institution

Almost midnight, in our scuba gear on the deck of a research ship somewhere off the coast of Papua New Guinea. We take one last long look up to the awe-inspiring beauty of the Milky Way above. Inhaling the first breath of our oxygen-nitrogen mixture, our eyes train down to that other unfathomable and mysterious depth below, beneath the “thin blue line” of earth’s atmosphere. We step off the deck, splash down, and slip beneath the surface of the warm, dark equatorial water.

Credit: https://reeftrip.com/scuba-diving/night-scuba-diving/

Weightless as astronauts, we’re suddenly transfixed by another natural luminous wonder, as captivating as the starry expanse above. A massive, seemingly endless vertical procession of green and blue bioluminescence stretches out as wide and deep as our eyes can plumb. Diving closer, we become immersed in an entire galaxy of glowing siphonophores, slowly revealing their detail and variety (at last count, 175 species, the longest over 40 meters). Colonies of polyps, medusae, gas-filled pneumatophores, each playing a role. Some stinging, others digesting, reproducing, younger zooids providing thrust propulsion, elders orienting the movement.

Credit: Larry Madin, Woods Hole Oceanographic Institution

A mammoth, nocturnal, vertical marathon is underway. And it will happen again tomorrow night, and the next, just as it has since prehistory, dwarfing all other earthly migrations in longevity and scale.

We are hypnotised, gradually, subconsciously losing a sense of ourselves, becoming one with the colony, seeing like a siphonophore. Meanwhile, just above the surface, another all-too-human species begins their own slow upward migration, driven by rising sea levels to higher ground within a progressively, worryingly thin blue line…

(to be continued)

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