In the barely macro world of the sea, there are no better examples that “good things come in small packages” than the sea slug Corambe and the bryozoan Membranipora. Not one but two local Corambe species (C. pacifica and C. steinbergae) exist, and though they are scarcely big enough to see with the naked eye, I never tire of searching for and finding these diamonds in the kelp rough. Most sea slugs (called nudibranchs) lack a shell for protection and so have evolved more sophisticated defense mechanisms. A Corambe achieves near-perfect invisibility by mimicking the same colors, patterns or even outline of the bryozoa on which it settles and preys upon.
The bryozoa Membranipora is a colonial animal where many tiny individuals live together. In this case, hundreds of thousands of bryozoan individuals, called zooids, each sized no larger than a pinhead, are easily visible as a group. Disk-shaped, white colonies radiate outward to form “nets” made up of rectangular zooids. The colonies that carpet the fronds and bind to the air bladders of the giant kelp Macrocystis pyrifera are typically at least an inch in diameter. Bryozoan colonies grow between late spring and early autumn, where colonies may clash or cooperate with each other over kelp real estate. Small colonies converse with each other using electrical signals; large colonies send out runners from which new zooids bud to lodge attacks on colonies that get too close.
The two Corambe species look similar, having an oval, flattened, disk-like shape, but C. pacifica is distinguished by a notch at one end of the body. Their see-through bodies are adorned with orange-brown patches and opaque-white spots. Two sensory organs (rhinophores) that sprout like horns are found toward the front of the body. This look then mimics both the amber kelp and the white bryozoan mats. Because the slugs grow to no more than a half-inch long, they are nearly impossible to find while under water, so I prefer to inspect kelp washed onto the beach. Nearby I can usually find Corambe’s opaque spawn ” C. pacifica deposits eggs in a coil, while C. steinbergae lays half-moons.
Because the Corambe species are beholden to their Membranipora food source, predator and prey have coordinated life cycles. The slugs and bryozoa begin life as larvae in the plankton soup. This devil-may-care dispersal method greatly affects where and how many bryozoan larval zooids settle in the kelp. Corambe larvae have an internal trigger mechanism that allows them to sniff out, then settle on, a bryozoan colony. It’s a delicate balance, because if no bryozoan colonies are around, Corambe larvae won’t establish themselves. To complicate matters, the two Corambe species often struggle for food and frond off the same host bryozoa.
That the Corambe species compete with each over the same prey, and Membranipora colonies haggle with each other over space, doesn’t preclude the wrangling between slug and bryozoan. Zooids fend off slug squatters by growing large, chitinous spines, which research has shown take under a day and a half from when a Corambe starts munching. Apparently, the slug inadvertently releases chemicals into the water that spark the zooids to erect a shield. The side effect of growing spines is slower bryozoan growth due to less available energy for colony expansion compared to nonspined colonies. This excellent example of natural selection demonstrates that while producing spines limits growth, it still trumps the disadvantages of slower growth. The result is a fitter animal for improved survival over the long haul.
On a final note, Corambe isn’t known to crawl around much since its best defense is camouflage. Some nudibranch species demonstrate a “swimming” (so-called flexion) motion best described as a convulsive or spastic movement. I was snorkel-swimming off La Jolla Caves when I noticed a Corambe a foot below the water’s surface performing the jerky forward-backward movement. I grabbed a nearby piece of bryozoan-encrusted Macrocystis and placed it under the animal. It alighted and instantly “disappeared” onto the colonial carpet.
Nudibranch expert Bill Rudman responded to my e-mail query about this behavior: “Since few people find this highly cryptic animal in the first place, I am sure you are likely the first to see one ‘swimming.’ This is the first report of this behavior I have heard of.” Though I am confident of what I saw, I’ll need to validate my observation with another sighting. Fortunately, I’m good at sweating the small stuff.
” 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? E-mail [email protected].