Nudibranchs

Rocky intertidal habitats can burst with color. In Pacific tidepools, among the most colorful organisms are beautiful sea slugs, or nudibranchs. They are roughly similar in size to their shell-less cousins on land, but they are not at all drab like terrestrial slugs.

Scientifically, nudibranchs are classified in an order (Nudibranchia) within a subclass of mollusks known as the opisthobranchs. There are several other groups of opisthobranchs, including sea hares. The nudibranchs themselves are divided into a few groups. The most common are dorid nudibranchs, which have a tuft of gills on the top of the body midway, or towards the posterior end of the animal. Another group, the aeolid nudibranchs, is showy, having rows of protrusions (cerata) along the top of the body. While many molluscs have hard shells for protection (clams and limpets for example), the sea slugs do without. The nudibranchs are predators, feeding on other invertebrates and sometimes on other nudibranchs. Their bright colors may serve as a warning to potential predators that might be tempted to eat these soft-bodied morsels.

Hermissenda crassicornis, an aeolid nudibranch, from Greyhound Rock, Santa Cruz Co, CA, June 2015.

With such delicate bodies, it is a wonder that sea slugs can survive on wave-pounded coasts. Clinging to seaweeds in calm pools at low tide, within a few short hours, the rising tide will bring crashing waves and turbulence. Other intertidal invertebrates – mussels, limpets, chitons and barnacles for example – cling with great tenacity to intertidal rocks to withstand strong hydrologic forces within their environment. The marine algae have a different strategy: they typically have flexible branches or stipes that allow them to be passively whipped back and forth as waves rush over them. How do the delicate nudibranchs make it? Do they retreat into refuges among the rocks when hydrologic chaos is at its greatest?

Triopha catalinea, a dorid nudibranch, at Greyhound Rock. 

I’ve seen quite a few nudibranchs on intertidal trips this year to the central California coast. For example, during a very brief stop at Greyhound Rock in northern Santa Cruz County in June I found four different species in the low intertidal. The most colorful was a single individual of Triopha catalinae, a few centimeters long. Clinging to a strand of feather boa kelp (Egregia menziesii) right at the water’s surface, the milky white body punctuated with bright orange spots was a striking contrast with the chocolate brown kelp below.

One of the most common nudibranch species this year has been the distinctive Okenia rosacea, or Hopkin’s rose. I found close to a dozen animals in just a half hour of tidepooling at Greyhound Rock. In fact, I’ve seen this species on many of the intertidal trips I took this spring in California: Pigeon Point in San Mateo County, Glass Beach in Mendocino County, and at Sand Dollar and Mill Creek in Big Sur. Essentially a hot pink mess of slug, there is nothing quite like it on the coast and you can’t miss it!

Okenia rosacea (formerly known as Hopkinsia rosacea) from Pigeon Point,
San Mateo County, CA, October 2009.

References

Behrens DW. 1991. Pacific Coast Nudibranchs. Sea Challengers, Monterey, CA.

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Hunting heron

Duxbury Reef is a low-relief intertidal bench at the west end of Bolinas Bay in Marin County in northern California. Tidal seaweeds and invertebrates live in a matrix of cobbles and silty sand which left the water rather murky – more like an estuary than the outer California coast. It seems like the unusual substrate of this site limits algal diversity and results in a fairly unique community. Several parallel ridges of bedrock provide some rare additional relief that hosts more typical mid-intertidal species like rockweeds and mussels.

Birds were actually the highlight of my low tide visit earlier this month to the reef. Unnerved by my presence, a great blue heron hunted in the shallow water with the rising tide. I crouched and observed for some time, witnessing at least two successful hunts of intertidal fish. The bird’s patience was obvious, but they must have remarkable eyesight to see fish which are probably well hidden in the dark tidepools.