California has such a rich variety of bright, colorful nudibranchs, and as a consequence, many divers have a pretty good grasp of the major groupings of these sea creatures. The basic “seat-of-the-[wetsuit]-pants” generalization is that there are two forms of nudibranchs (or “suborders” in more scientific jargon)—the dorids, which are the sluggy-looking sea slugs, flattened from top to bottom, and the aeolids, the more showy species with all the frilly protrusions down their backs. (There are, in actuality, a few other suborders that aren’t so stereotype-able, but also contain fewer species and are, thus, understandably often ignored.) So by using these basic criteria for the two major suborders, diver can often have a fairly good idea of what sort of nudibranch they’re looking at.
Except—wouldn’t you know it?—there are always those non-conformist that don’t fit generalizations.
A most notable and spectacular California non-conformist nudibranch is the Hopkin’s Rose Nudibranch (Hopkinsia roseacea). It has the flattened, slug-like body and the gill-ring on its rear portion that should make it obvious that it is a dorid. But those features are overshadowed by the numerous long frilly growths all over its back, causing it, in effect, to do a good imitation of an aoelid.
Strictly speaking the frilly projections on aeolids are called “cerata” and those of Hopkin’s Rose are “papillae”—not that most divers particularly care about some semantics. And while the small, one-inch long nudibranch probably doesn’t really care either, there is some significance in the difference. The cerata of aeolids service as their gills (and also contain part of their digestive system). But Hopkin’s Rose, being an actual dorid, has its gill-ring for its respiration, even though this is often hidden from diver’s views.
Hopkins’ Rose does fit well into the generation that nudibranchs—whether aeolid or dorid—are bright colored and beautiful. In fact, “little hoppy” excels in this quality, being a brilliant pink. This is no ordinary pink either, mind you, but a special type of carotesid colorant called “hopkinsiaxanthid.” As you might have already guessed, hopkinsiaxanthid isn’t to be found in a lot of other places.
But one important place where it is to be found is in the pink encrusting bryozoa or moss animal Eurystomella bilabiata —and not just coincidentally so. This bryozoa is the favorite food of the Hopkin’s Rose; in fact, the nudibranch seems to eat nothing else at all! The shell-less snail uses its rasping tongue to make a hole into the hard exterior of the moss animal and then, just sucks up the soft parts inside. Yum!
Yes, the Hopkin’s Rose nudibranch is really “in the pink”—and the pink into it —and all the days of its life. The spiral ribbons of eggs it lays are likewise a similar pink color.
Many aeolids that are so appealing to divers are also appealing to predators, like the voracious sea slug, Navanax inermis. But not so with the pseudo-aeolid Hopkin’s Rose. It evidently secretes some sort of non-acidic substance that keeps Navanax and other potential enemies at bay.
This paradoxical non-conformist dorid is found from Oregon to Baja California, but is more common in the northern portion of its range. Like many other nudibranchs, its appearance tends to be seasonal. Thus, often, it seems to be nowhere around, then, all of a sudden, the intertidal rocks seem to blossom with roses—Hopkin’s Roses—and divers that are “in the drink” will find the whole undersea garden around them “in the pink.”