Giant ostracod

About the Giant Ostracod

What’s round, orange, and can see in the darkness of the ocean twilight zone? It’s the giant ostracod, of course! Although their name makes them sound huge, the largest of these species (G. agassizii and G. australis) only measure about an inch long. Size is relative, however: that’s more than 30 times the size of an average ostracod.

These tiny crustaceans go through life suspended in a semi-translucent, bubble-like carapace. Through a narrow slit in the bubble, they extend and retract feathery antennae, which are useful for everything from sensing their environment to swimming and snapping up food. Unlike many of their twilight zone neighbors, giant ostracods rely on their superior vision to track glowing prey and hide from predators. With eyes like parabolic mirrors, ostracods are thought to be capable of perceiving more light than any other animal—quite an advantage for a species that lives in perpetually dark conditions, 600-2,300 meters (2,000–7,500 feet) below the ocean surface.

To travel up and down in the water column, giant ostracods adjust the sulphate content in their hemolymph, which is like blood for invertebrates. Scientists estimate that they’re 95% water, a trait shared by jellyfish. But with their large eyes and hearts, these tiny giants are some of the most active hunters in the ocean twilight zone.