Lemons of the Sea

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Sea lemons are nudibranchs (Latin nudus, “naked”, Greek brankhia, “gills”), a class of gastropod mollusks commonly referred to as sea slugs. Technically, nudibranchs are further defined as strictly marine, carnivorous, hermaphroditic gastropods that shed their shell after the larval stage, but let’s not get esoteric about it. Nudibranchia comprises more than 3,000 species worldwide, many of them widespread in their localities, and so it is fitting that the family to which sea lemons belong, Dorididae, is named for the Greek mythological nymph Doris, the embodiment of the sea’s bounty. When life gave the ocean lemons, they speciated like mad and gave rise to some of the most colorful creatures known to humankind.

Dorids, true to their nudibranchian name, do indeed have exposed gills with which to extract dissolved oxygen from their aqueous surroundings. (In contrast, members of the nudibranch family Aeolidioidea have no gills, and instead rely on thin extensions of the digestive tract called cerata for gas exchange.) Sea lemons carry their seven gills on their backside, near the tail. At the front end are sensory organs called rhinophores; these horn-like appendages detect chemical traces in the water and keep the sea lemon close to its preferred prey, the yellow breadcrumb sponge.

That old saw “You are what you eat” rings especially true with the nudibranchs. Many species take their coloration from the animals they eat—Archidoris and its breadcrumb fixation are a textbook example. Some resorb the toxins of their prey and become toxic themselves. Remarkably, the aeolids devour sea anemones and jellyfish with impunity, the defensive stinging cells of those hapless blobs passing unharmed through the gut of the nudibranch. These poison-filled cells, called nematocysts, are incorporated into the cerata on the nudibranch’s back—a canny feat of biological hijacking.

Sea lemons live for around a year or so, and grow to four inches in length. The adults have crazy hermaphroditic sex, exchanging sperm packets so that both partners come away fertilized and ready to lay eggs—up to two million from a single individual. These eggs, which are laid in a cream-colored ribbon and glued to a rock, hatch in about twenty days. Of the millions of free-swimming larvae that emerge from a typical brood, less than one percent will survive to adulthood.

The bounty of the sea is staggering, and nowhere is it more eclectically on display than at low tide on a rocky shore along the Oregon coast. Go on, visit during a diurnal negative tide and give everything a close look: peer beneath the rugged basalt and sandstone boulders, poke around the tidepools, get your hands and feet wet. Marvel at the wonderfully tactile, living landscape. Laid bare by the regularly receding waters, it is a world utterly alien to us, its denizens quite literally out of their element but adaptive in the extreme.