Longest bioluminescent animal

The world's longest bioluminescent animal is the giant siphonophore or swimming bell (Praya dubia), an extremely elongate, almost transparent species of marine invertebrate related to the more familiar, jellyfish-resembling Portuguese man o' war. Measuring up to 40-50 m (131-164 ft) long, and found off coasts worldwide, each specimen of this species is actually a super-organism, i.e., an intimately connected colony of tiny individual creatures termed zooids, each with its own specific function. The super-organism attracts prey by way of its intrinsic bioluminescence – emitting a bluish light that lures prey victims into its curtain of stinging tentacle-like structures termed tentillae that swiftly immobilize and kill them.

Some argue that Praya dubia is, in fact, the longest animal in the world altogether, though others suggest colonial animals should not be compared with singular organisms.

In a Praya dubia colony, some of the zooids take the form of individual tentacles, others assume the form of swimming bells, others become feeding organs, and so on. This species exists in deep water, with a core habitat at approximately 75-127 m (246-417 ft), and its colony is held together by the considerable hydrostatic pressure exerted at such depths. Consequently, in the past when a specimen was brought to the surface to examine, it would simply burst, yielding an amorphous jelly-like mass. So for a long time, its true shape and structure remained unknown to science, until living specimens were eventually observed at depth underwater.