Sea slug a vision without rose-colored glasses

The base of the Hopkins rose comprises a fairly wide, flattened body that also includes a foot and head. Since all these parts are fused, guesswork is needed to determine where one body portion transitions to the next. Atop the base lies a tangle of tall, smooth, soft projections called papillae. The papillae aren't just for fun. They are at the heart of the rose's evolution from snail (slug with shell) to slug (a snail with no shell). Over time, as the protective shell was lost, it was replaced by chemical weapons stored in the papillae missiles, ready to fire when necessary.

In addition to the thicket of papillae, two stiff, hornlike projections (rhinophores) are anchored at the base above the head region. The rhinophores are equivalent to a nose, permitting the slug to sniff out food, mates and predators. Though challenging to separate out the rhinophore trees from the papillae forest, at least the horns' ringlike segments contrast with the smooth papillae.

Peeking out from the opposite end are feathery tufts laid out in a close-knit, circular format. These are the gills, and while they do not resemble the surrounding papillae, they too are hard to detect. Not only are the gills about one third as tall as the papillae, they are attired in matching pink. In regards to rosiness, color intensity varies among Hopkins roses. And an individual may buck the monochromatic theme by embellishing the pink papillae with white accents.

Following mating, the rose's spawn continues the color scheme by depositing a flattened coil consisting of tiny fertilized pale-pink eggs held together by a mucus sheath. To create the swirl, the egg string is laid down starting from the outside and curling inward. Other related slugs lay similar coils but do so by beginning from the inside and working outward. That's a good idea because there will always be room no matter the coil's length. The rose's method, however, may prove problematic if the center is reached and more egg string exists than there is substrate. It begs the question of why the rose would choose a strategy that required perfect planning.

The diet of a Hopkins rose is limited to the rose-pink bryozoan Integripelta bilabiata. Bryozoans are primitive minute-sized invertebrates that grow in colonies on various surfaces and frequently contain biologically active chemicals. The rosy hue of I. bilabiata originates from its self-manufactured pigment identified as hopkinsiaxanthin. This pigment has yet to be found elsewhere in nature, be it plant or animal, that is except for the Hopkins rose.

The difference is that the bryozoan synthesizes the pigment, while the sea slug ingests it along with the bryozoans toxic chemicals, color and meaty, nutritional parts. The Hopkins rose somehow delegates the pigment to the skin cells, the chemicals to the papillae, and the actual food to the digestive system. Amazingly, the defensive chemicals are selectively localized and stored until being detonated.

For a very good reason, the arsenal is sequestered solely in the papillae and not spread evenly throughout the body. Should a potential predator see the slug and think it an easy meal, the long, waving papillae will be tasted because they are noticed first. After that unpleasant dose of reality, the predator moves on. The bite taken and rejected won't be fatal to the Hopkins rose because the papillae are not vital organs.

Just to clarify, the slug's defensive adaptation dissuades most predators but not necessarily all. Fish are known to prey on the distasteful dorid but probably only when nothing else is available. I can't imagine a circumstance in which a meal of Hopkins rose is worth it. Aside from its vile taste and toxicity, it's not fast food because the fish must first act to fire off all the chemical defense missiles before consuming the animal. After such painful and laborious maneuvering, if I were the fish, I would have lost my appetite.

— 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.