Frequent explorers of the North Coast’s rocky intertidal zone have likely encountered more conspicuous creatures like urchins, anemones, hermit crabs, and sea stars. But there’s another exceptional tidepool dweller often overlooked, or sometimes mistaken for orange-flecked snot: the nudibranch.
More commonly known as the sea slug, the term nudibranch actually describes a diverse order of shell-less mollusks comprised of 3,000+ unique species. “Nudi-branch” translates to naked gills, a reference to the exposed bronchial structures protruding from nudibranch backsides. These critters generally grow between 0.25 - 12 inches in length, and inhabit tidepools, coral reefs, and other coastal habitats around the world.
Nudibranches use their sensitive “antennae,” called rhinophores, to seek out prey such as algae, sponges, corals, anemones, and sometimes other nudibranches. Additionally, select species possess the remarkable ability to transfer photosynthetic algae into their own tissues after consumption. Thereafter, the slugs feed off sugars produced through photosynthesis, much like a plant.
Most nudibranch species live no longer than about one year. However, the creature doubles its chances of procreation via a “simultaneous hermaphroditic” anatomy. Individual slugs bear both male and female reproductive parts, allowing them to mate with any other adult slug.
Not all nudibranch species resemble speckled mucus, and, in fact, the family comes in a dazzling array of diverse and vibrantly beautiful forms. One particularly pink member of the nudibranch clan has made a surprise appearance on the North Coast in recent times, astonishing scientists with its newfound range and abundance.
The Hopkins’ rose nudibranch (Okenia rosacea) is common on the southern coast, but very seldom seen in northern California. Yet, dense populations of this flowery species have been discovered in tidepools from San Luis Obispo to Humboldt County. The last time Hopkin’s rose was seen this far north in such great numbers was in 1992 and 1998 during especially strong El Nino events.
Hopkins’ rose nudibranch sightings on the North Coast, in addition to other unusual marine happenings like “melting” sea stars, jellyfish blooms, and sea turtles off San Francisco, are interpreted by many scientists to be indicators of a strong approaching El Niño event, as well as larger shifts in ocean climate and marine ecosystems.
Whether or not they’re here to stay, for local beachgoers, the pretty sea slugs are a striking reminder of nature’s wondrous diversity. Enjoy their pink presence and remember to tread lightly at local tidepools.