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Medusafish seeks protection amid serpent’s potent coils

I’m not finished talking about jellies. Those in the know already read my recent column (“When jellies collide and barnacles hitch a ride,” Aug. 12). Now I’m following up with another web-of-life connection, this time linking jellies with fish. Jellies are highly effective survivalists, what with a mostly transparent appearance to foil near- and farsighted predators, and stinging cells that do double duty by capturing a meal and thwarting many an overambitious predator. It’s no surprise then that other marine life species seek shelter under a jelly’s cushy umbrella.

For example, during the juvenile phase, medusafish search out jellies to commence a symbiotic relationship. Though not known for sure, the fish may be oblivious to a jelly’s prickly parts by excreting an extra layer of mucus, which would suppress the jelly’s stinging missiles. In this cohabitation, the guest may benefit more than the host because medusafish not only reap protection but make a meal off the jelly’s leftovers or even munch on the jelly itself. Talk about biting the hand that feeds you. As for the jelly’s welfare, medusafish remove parasites and attract careless prey.

This summer, there has been no shortage of jelly sightings. Just the other day, I discovered a young medusafish (Icichthys lockingtoni) buried in an egg-yolk jelly (Phacellophora camtschatica) floating off La Jolla Shores. The blunt snout, fanlike fins, beady eyes and pasty coloring resembled no local yokel found in these parts. Dwarfed by prominent fins positioned atop and underneath the body are two small pectoral fins attached below the gills. These little fins look like an afterthought, but they aren’t. Juvenile medusafish have no swim bladder and very low body fat content, so medusafish have slightly negative buoyancy. The pectoral fins act to supply lift and thrust. One fin flaps in a semi-rotary manner, then the other pectoral fin takes over. Alternating fin flapping makes for efficient braking and turning in a small space. And the paired pectorals allow medusafish to swim, hover and otherwise maneuver at various attitudes, even upside down. Not many fish species feed in any orientation but it makes sense for medusafish, particularly when snacking on the host. Here, medusafish use their mouth to grasp some jelly tissue, then roll and twist their body to tear off the chunk.

When young medusafish grow to about 7 inches long, they vacate the boarding house and head to deeper water, all the while continuing to grow, which can be to about 18 inches long. Researchers in submersibles diving to depths of up to about 3,000 feet have videotaped adult medusafish swimming in a slow, snakelike motion, which contrasts with a juvenile’s movement. In another departure from juveniles, adults may increase body fat percentage and decrease muscle and bone mass to achieve neutral buoyancy. Now that they are neither negatively buoyant nor confined by the boundaries and whims of a jelly, adult pectoral fins are superfluous. Consequently, these appendages shrink as the adult ages, demonstrating that, when there’s a good enough reason, physical and behavioral changes are possible even later in life.

— Judith Lea Garfield, naturalist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. Send comments to