It’s a jelly. It’s an egg case. It’s a . . . worm?
The balloon worm (Poeobius meseres) hardly looks like a worm at all. It lives in the midwater—the vast expanse of open water deep below the surface and far above the seafloor. This worm has evolved unique ways of living suspended in the water column instead of crawling along the seafloor.
Most marine polychaete worms—the much more elaborate relatives of earthworms and leeches—have a body that is clearly segmented. Their bodies are divided into many nearly identical, repeated parts. Typically, each of those repeated parts is studded with several stiff bristles. Poeobius, however, has a bag-like body filled with fluid that, together with its thick gelatinous coat, provides buoyancy to help it float in the water column effortlessly. Its smooth body lacks the obvious segmentation and the bristles of its bottom-dwelling kin. But if you look carefully, there is evidence of segmentation: several repeated clusters of nerves spaced along the worm’s belly.
Poeobius is a common and very abundant resident of the midwater in Monterey Bay. It drifts through the water, collecting and eating bits of sinking organic matter in a mucous net. This little worm is actually an important part of cycling nutrients like carbon from the ocean’s surface to its depths.
Maximum size: 36 millimeters (1.5 inches)
Depth: 300–2,500 meters (980–8,200 feet)
Habitat: midwater, in both twilight (mesopelagic) and midnight (bathypelagic) zones
Range: North Pacific Ocean (from Japan to Alaska to the Gulf of California) and South America
Diet: marine snow
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