One of the most enchanting and graceful gelatinous mid-water blobs has to be the sea butterfly, Corolla spectabilis. They are planktonic members of the wide world of sea slugs, and yes, they are opisthobranchs. Sea butterflies belong to a family of animals that are descendants of gastropods and portions of the molluscan foot have been modified to form swimming wings. Corolla is generally a passive drifter, however, at any disturbance it vigorously flaps its oval wingplate, propelling itself past surprised divers with remarkable speed. This transparent slug has a swimming rate of eighteen inches per second, so a fleeting glimpse is often all one gets.
Planktonic survival depends on maintaining buoyancy, and expending as little energy as possible to offset sinking. Consequently, the fragile sea butterfly has no shell. Its internal organs are supported by a gelatinous pseudoconch, which is neutrally buoyant. Perhaps tubercles serve to increase surface area, which would further reduce the sinking rate.
The darkly pigmented spots around the periphery of the wing margin are light sensing organs. A strobe flash can send the animal bobbing and weaving.
A fascinating and almost unimaginable feature of the sea butterfly is the unique way in which it captures food. It starts with a row of mucous glands embedded in the wing margin. In combination with an internal pallial gland, these glands produce sticky mucous which is then formed by the proboscis into a delicate feeding web, which can be many times larger than the animal.
The web traps planktonic particles ranging from bacteria to single cell algae to copepods and other crustaceans. As ciliary pathways convey the web to the mouth, the grooves of the proboscis consolidate the whole enchilada into a string. Sorting occurs, some food and mucus are rejected, and the rest is consumed.
Corolla spectabilis actually swims backwards, pushing its pseudoconch forward, with the mouth and feeding web bringing up the rear. The proboscis always stays in contact with the web. If disturbed, the animal may shed the web and swim away. Unfortunately, a strobe flash sometimes qualifies as a disturbance!
The mother of all net builders, this Corolla spectabilis was dragging a huge mucus web, laden with tiny animals that are beyond the resolution of my lens. It contains dozens of radially symmetrical spheres.