2000-06 Post-larval Fish

By Ned and Anna DeLoach with Paul Humann

Over the years I have heard curious tales of fish falling from the sky. Possibly sucked from lakes by swirling tempests, the reluctant skyriders later tumble back to earth far from home. Although I have never seen fish drop to earth, my wife, Anna, and I have been underwater at night when settling post-larval fish cascaded down around us like autumn leaves.

The perilous odyssey of the little immigrants begins weeks earlier, possibly hundreds of miles away, when thousands of gametes are abandoned in the currents by spawning parents. Within 24 hours, the buoyant fertilized eggs develop into yolk-sac larvae with long tails and distended bellies packed with a temporary store of fat. These high-energy eating machines must find a food source within days or starve. Fortunately, tropical seas transport patches of concentrated planktonic marine life, primarily single-celled plants and microscopic crustaceans known as copepods—the most abundant group of animals on earth. The maturing larval fish, which increase in body mass by a third each day, dine on anything that fits inside their mouths. Simultaneously, creatures with larger jaws prey upon the youngsters.

Evolution hasn’t forsaken the little fish during these dangerous times. As the yolk sacs disappear, the larvae develop elaborate morphological adaptations that delight the imagination. Spikes, spines and spinelets, streaming fins bristling with barbs, and bulbous heads deter predators, while silver or transparent bodies aid in their death-defying game of open water hide-and-seek.

In the Caribbean, from spring through summer, on the dark nights of the new moons, mature fish larvae, ready to begin life on the bottom, surf toward the shallows on incoming tides. We have been underwater at night as patches pass over the reef. One moment the water is clear and in the next instant the beams from our dive lights attract a blizzard of squirming creatures, and odd little fish begin to rain down. Surgeonfish the size of quarters, with bodies so clear you can count their ribs, bump about the bottom, while transparent eel-like fish burrow into the sand, and tiny silver-burnished butterflyfish pause as if collecting their thoughts before vanishing into the reef. Ten minutes later, the patch moves on, leaving behind a new generation to renew the reef.