A significant proportion of the food in the deepest ocean falls from discarded giant larvacean houses.
Up until a few years ago the food chain in the darkest depths of the ocean floor was a puzzle. The volume of nutrients sinking down from above wasn’t enough to support the amount of life found in the abyss, at least as measured by sediment traps. These are essentially big underwater funnels, pointing towards the surface, that let researchers capture and measure stuff floating down from above. The numbers from these traps just didn’t add up; they were missing something.
Enter larvaceans. These little ocean-dwelling creatures look a little like tadpoles, with a bulging head and a long tail. They’re found around the world, and it turns out they are responsible for a significant proportion of that missing food. Not the larvaceans themselves, though… the creatures of the seabed feed on their falling houses.
“House” here is a technical term for a kind of translucent shell, similar to the exterior of sea urchins (itself known as a “test” for some reason). The larvaceans construct their houses out of mucous and use them to feed. The houses are enormous traps for tiny plankton; the larvaceans beat their tails to pump water through the house and filter the food.
The larvacean house is fragile but also enormous. Giant larvaceans such as the beautifully named Bathochordaeus charon get about 6cm long, but their houses are up to a metre in diameter. Nevertheless, these houses are temporary; when the filters get clogged (for example, by seaborne plastic particles) the larvacean simply detaches from the house and builds a new one. The house itself sinks down into the depths and becomes that missing link in the deep ocean food chain.
[Thanks to Beneath the Blue]