On The Trails: Barnacles

An acorn barnacle (on the left of the photo) opens up to wave its legs and feed. (Photo by Bob Armstrong)

By Mary F. Willson

On a recent stroll down Sandy Beach, I stopped to look at the barnacles encrusting the tall pillars left by the mining industry. I marvel at the way the plates of the operculum make a tight closure, keeping the little critter inside from drying out.

Although some barnacles live in deep water, we are, of course, most familiar with the intertidal ones that have to deal with varying water levels. And most of those we see are so-called acorn barnacles, apparently getting that name because somebody imagined that they looked like acorns. Although they may not look to us like crustaceans (or acorns), they are in fact crustaceans, related to crabs and shrimps.  

Larval barnacles are free-living. The first stage, called a nauplius, feeds on particles suspended in the water. The second stage, called a cypris, does not feed. It swims with its legs, looking for a place to settle. When it does, it attaches to the substrate with its head: at the bases of the first antennae there are glands that secrete special glue that sticks the larva head-down.

A raven gnaws barnacles from a rock. (Photo by Bob Armstrong)

Then it metamorphoses into the adult phase, secreting its shell, which consists of plates of calcium carbonate that surround the side of the animal and an operculum of four plates to close off the opening at the top. Adults are sessile, stuck to the substrate for the rest of their lives. They are sometimes subjected to destructive buffeting by surf-borne timber that can wipe out whole colonies. 

The adult barnacle feeds by waving it long legs, filtering small bits from the water. Clearly, that does not work when the barnacle is not submerged, so when the tide is out, feeding is impossible. Since the barnacles stuck on the upper parts of those industrial pillars are out of the water more than those below, of necessity they must feed somewhat less and presumably grow more slowly. Unless they somehow live longer or compensate in other ways, they typically will be smaller than those below.

Most barnacles are hermaphroditic, with both male and female sex organs. But being sessile, how do they mate? Conveniently, the male intromittent organ is extraordinarily long (relative to body size) and so it can reach from one barnacle to another, if they settled close together. If cross-fertilization is impossible, some barnacles can self-fertilize.

Lots of animals feed on barnacles. The pelagic larval stages are consumed by various fish, mussels, and others that filter particles from the water — including adult barnacles. The adults are gnawed off the substrate by bears, otters, gulls, crows, and ravens; oystercatchers and sandpipers snack on them too. We find evidence of this dietary choice by the smaller predators in small piles of undigestible, regurgitated bits of shell or, in the case of bears, in defecated scat.

A barnacle fly inserts her eggs into a barnacle. (Photo by Bob Armstrong)

One type of fly specializes on barnacles, laying eggs on the operculum (somehow not drowning when the tide rises). When the eggs hatch, the larvae crawl under the plates and eat the barnacle, often moving from one barnacle to another, wedging their way between the plates. The larvae pupate in the empty shells of the dead barnacles.

Barnacles are subject to another, perfidious danger. The oceans are being acidified by absorption of the increasing amount of carbon dioxide in the atmosphere.  Acidity "dissolves" the calcium carbonate that barnacle shells are made of, weakening the shells (and the shells of mollusks, the skeletons of corals). Repairing the damage puts a demand on the animals’ metabolism, detracting from other activities such as reproduction. As a result, the abundance and distribution of these critters change. And that’s just a beginning: there are many other effects of acidification too.

There are thought to be over 1,400 species of barnacles, mostly the acorn type. There are two other kinds of barnacles. A gooseneck barnacle has a stalk that often attaches to driftwood as well as rocks; the stalk allows it to bend with water flow. The number of gooseneck barnacle species seems to be uncertain.

In addition, there is a strange bunch of barnacles called Rhizocephala; there may be around 300 species. They are shellless parasites inside other crustacea, in some cases quite specific in their choices of host and even modifying host morphology. In contrast to most other barnacles, in these the sexes are separate. Females enter a crab, shrimp, or another barnacle and take over the interior, effectively castrating the host. Males have to find the right host, one with a female in place. They enter the reproductive sac of a nicely settled female, which protrudes from the host, and promptly degenerate into little more than a sperm-producing organ. That’s a rather different life history than that of the other barnacles — I have to wonder how it evolved.

• Mary F. Willson is a retired professor of ecology. "On The Trails" appears periodically in the Juneau Independent.

A barnacle fly lands on an acorn barnacle and may insert her eggs there. (Photo by Bob Armstrong)