It is the jellyfish that isn’t. Numbers of Leucothea pulchra, a comb jelly, appeared in our nearby waters to give ocean-goers a glimpse into one of nature’s more transparent residents.
While not uncommon, little is known about this group of marine dwellers because they are likely destroyed in plankton nets and other collecting devices before they can be noted if not identified.
I saw some in the shallows off the Marine Room and deeper in the nearby submarine canyon this past fall. Though I haven’t seen this sort-of-a-jelly before, it is distinctive because the flattened, oval body looks to be covered by a gossamer piece of dotted Swiss fabric. The brownish-orange bumps speckling the clear tissue are thought to be sensory chemical or mechanical structures, but this is not known for certain. While Leucothea can grow large, reaching 10 inches long, the ones I saw were no more than 6 inches long.
So why isn’t a comb jelly a jellyfish? At one time, both groups were placed in the same phylum, but upon closer inspection, the greater complexity of the comb jelly’s anatomy and behavior could not be ignored. For example, a jellyfish is made up of only two layers but a comb jelly has three. A jellyfish swims vigorously by contracting its entire body.
Contrarily, a comb jelly has a graceful, complicated way to maneuver through the water that, surprisingly, begins with its mouth. It is here that eight independent swimming plates, or comb rows, are attached, and these extend lengthwise to cover the body. The thousands of “teeth” on the combs are actually cilia, tiny hairs closely spaced and anchored at the comb’s base. The cilia beat in a wave pattern to push the animal forward.
As an aside, at certain angles a comb jelly looks like it is bioluminescent, but this trick to the eye is due to the beating of the cilia, which pick up and refract light.
While the comb jelly made progress past me, I enjoyed a rainbow-colored neon light show that travels in chainlike fashion across the comb rows.
I watched as the combs wafted gently and rhythmically on the outside of the body, looking like a cloak made of wide flaps. The flaps opened and closed like an unbuttoned cloak gently billowing in the breeze. Leucothea is one special comb jelly because it has an added swimming pattern that eclipses other comb jellies: jet propulsion. This it accomplishes by speeding up the beat of its cilia, which then creates a vortex wake. In this way, it has a choice of speeds. The ones I saw held to low-speed cruise control.
Unlike a jellyfish, a comb jelly does not have stinging abilities, which brings us back to those swimming combs. As stated, the combs originate at the mouth. During swimming, as the combs billow out and are drawn back in, Leucothea captures tiny zooplankton (such as larval clams and shrimp) within its folds. Once sucked in, the victims are brought to sticky tentacles around the comb jelly’s mouth, then moved into the mouth.
In all, those big oral combs are multitaskers: They pilot the ship, capture dinner and even serve it up. And although this animal is easy prey for those seeking an easily digestible food fix, such as fish, turtles and some jellyfish, the carnivorous comb jelly is likely an important predator in the upper waters of the open sea. Leucothea surely deserves credit for being more behaviorally elaborate than what I would expect of a gelatin-based life form.
” Judith Lea Garfield, biologist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. www.judith.garfield.org. Questions, comments or suggestions? Email [email protected].
