This is not a cnidarian, though it is often mistaken for one. These beautifully transparent, planktonic animals are actually members of the order opistobranch and have much more in common with nudibranchs than hydrozoans. They are technically sea slugs and anyone who has the gift of spotting one will likely never forget how resilient they are given their diminutive size. I have watched these little angels (some as small as 5 mm) flap their “wings” for over half an hour straight, making their way up and down the water column, swirling in currents and caught in beds of leaf kelp.
During their larval stage these sea slugs have shells which they “lose” as they morph into adulthood. What would usually be considered the foot of the sea slug or nudibranch becomes its “wing-like” appendage. . . .more later. These ares fascinating creatures!