Excerpts from Jim Conrad's
Naturalist Newsletter

from the February 18, 2006 Newsletter issued from near Telchac Pueblo in northwestern Yucatán state, , México

During low tides when I'm wandering around Hotel Reef I see lots of barnacles. They're stuck to any rock that's flooded at high tide, to older mangrove roots and pneumatophores -- even to plastic bottles and other trash that's been floating for some time. During my weekly walks I cross a shallow stream by stepping on cinderblocks someone has placed there. The blocks are submerged only briefly at the very peak of high tides, but even they are completely encrusted with barnacles. This week I picked up a barnacle-covered seashell, brought it home in a jug of saltwater, and you can see it below:

Barnacles on a seashell

That picture shows that a barnacle consists of a low, stony, crater-like structure inside which resides something like a clam composed of two shells standing on end, opening upward through the crater's hole. In the picture, some craters are empty but in others the closed, clam-like shells are clearly visible. The stony crater sides slant over the clam-like shells enough to imprison them. The question then arises, how does a barnacle make its living stuck in such a prison?

When the tide comes in the clam-like shells inside the crater open and the organism's modified legs, called "feeding legs," or cirri, extend outside the shell, and direct food particles such as algae suspended in the water into the organism's mouth. You can see a diagrammatic cross-section of all this at http://www.mesa.edu.au/friends/seashores/barnacles.html.

I placed the barnacle-encrusted seashell into a bowl of saltwater brought from the lagoon and waited for the clam-like shells to open and the animal inside to go to work. For the first couple of days I saw nothing, but then one morning there they were. The upended, clam-like shells were open and the feeding legs were being rapidly flicked into the water and then just as quickly withdrawn into the shells at a rate ranging from one to two flicks per second. The two shells inside the craters stayed cracked open all the time but they opened wider when the legs were flipped outside. From above, each barnacle's shells looked like two thick lips surrounded by a stony beard, with a furry tongue darting in and out. From the side, the legs' movement was just like you might make with your cupped hand if you were trying to coax wispy incense toward your nose. The barnacle-creature was coaxing algae-rich water into its shell-guarded mouth.

Now I understood how a barnacle cemented to a rock fed itself, but not how it attends to the matter of sex. Barnacles are arthropods, along with insects, spiders and lobsters, so I guessed that barnacle procreation was more complex than simply releasing sperm into the water on the theory that a female might be in the vicinity.

On the Web I read that Mother Nature has come up with a neat solution to the problem. She has equipped barnacles with what may be -- proportional to the barnacle's size -- the biggest penis in the animal kingdom. The barnacle simply cracks his clam-like shell a bit, sticks out his penis and probes the neighborhood for receptive female organs. Here anthropomorphism leads only to feelings of inadequacy. Anyway, after fertilization the eggs are brooded until they hatch as microscopic larvae -- as many as 10,000 in a brood. The larvae then float in the sea until they've grown enough to settle down, attach themselves to firm surfaces and start secreting the crater-like structures seen on my seashell.

Lots of barnacle species exist, but they fall into two main groups -- acorn barnacles and goose barnacles. The crater-like ones on my seashell are acorn barnacles. Goose barnacle shells reside atop large, fleshy stalks.