The glossy, bright-orange droplet adhering to a giant kelp frond caught the corner of my eye. Moving in close, I determined it was the tiny brooding anemone Epiactis prolifera, which grows to no more than an inch across. The center of the oral (mouth) disk is marked with white lines that radiate outward until they meet a ring of tentacles. The thick pedal disk (area under the tentacles) and stubby column or stalk have similar striations but are fainter and more diffuse. In the same family as corals, the brooding or proliferating anemone is so called because progeny are raised from eggs deposited outside the parent’s body. I find brooding anemones latched onto plant and algal life, like that found in the rocky reefs and kelp bed off La Jolla Cove, and on mats of algae that coat areas of sandy bottom in the La Jolla Shores submarine canyon. They aren’t strangers to tide pools or the beach, where they may be dumped unceremoniously onto the sand as hapless passengers of uprooted kelp strands. Out of water, they draw in their tentacles to stave off moisture loss, but the next incoming tide can’t arrive too soon for these fragile flower animals. Despite the risk of a beach stranding, attaching to something solid and (at least relatively) fixed remains the best defense against currents or wave action. In fact, once a brooding anemone finds a place to put down stakes, it typically spends most of its life there or within close range. Not being a go-getter, it is fitting that the tentacles circling the anemone’s mouth contain stinging bodies (nematocysts), which are used to capture prey floating by, particularly small crustaceans. Once immobilized, the prey is taken into the mouth whole. Indigestible material and waste is excreted through the mouth. Even with its formidable arsenal of nematocysts, some sea slugs are immune to the anemone’s defenses. When a slug devours its prey, the undetonated packets of stinging cells are absorbed and saved as a rainy day defense. Brooding anemones may also succumb to some hungry sea stars and fish. Though vibrantly hued, a brooding anemone may be overlooked because of its diminutive scale, but its proliferating proclivities make it a standout. From a reproductive perspective, a brooding anemone is described as “gynodioecious.” Gynodioecy is a particular breeding system (seen in plants but rarely in animals) in which females coexist with hermaphroditic females (harbor both male and female gonads). Why no males? No one has the complete answer to date, but it can be said that with this arrangement, reproduction can occur between hermaphrodites, within a hermaphrodite or between a hermaphrodite and a female. Whatever the combo, it’s a win-win because after all is said, done and fertilized, every player in the brooding anemone population has the plumbing to birth its own brood. The brooder eggs journey from inside the parent to outside on the pedal disk via an exceptional migration. The eggs are first fertilized in the parent’s digestive cavity, then the anemone’s body expands and contracts to release a mixture of eggs and milky mucus out (where else?) the mouth. Fine hairs (cilia) on the parent’s surface move the spawn down to small pits on the edges of the pedal disk, where the mucus and specialized cells in the parent’s tissue promote attachment. Since the area is exposed to the great underwater outdoors, wave action may limit the number of eggs able to reach their destination. Successfully attached spawn develop and grow by feeding off their yolk. When this starter food is depleted, the progeny don’t leave their nook but employ their now-functioning nematocysts to capture prey like an adult. Only when they are at least 3 months old do they at last crawl off to make their own home. Between the brooding anemone’s reproductive oddities and almost marsupial way of rearing its offspring, it is easily a species worthy of further investigation. How and why did this mating system evolve in this animal, and what advantages does it confer over the more traditional methods? I hope there is interest in studying this species more deeply to reveal the answers. — 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].
