Everything You Wanted to Know About Barnacles, But Were Afraid to Ask (or, How I Came to be a Marine Scientist)

I'm often asked why I became a marine scientist. I come from upstate New York--farm country. We have lakes, ponds, creeks, rivers. No ocean. While there's not really a single answer, I could give the standard, boiler-plate: Jacques Cousteau, family vacations at the shore, my love of the outdoors, being a swimmer. But the truth is it was barnacles that really captured my interest.

True confessions, I spent much more of my college career socializing (a euphemism for drinking) than studying. There are many lectures, indeed, many semesters that are a bit hazy in my mind, not due to the 30-intervening years, but due to the hangover I had during those classes. But, I remember barnacles from zoology class. This was while I was still at SUNY Binghamton, still a vague "maybe-sort of something in biology" major who hadn't decided what to do with my life.

I was probably half-dozing, sleeping off a hangover, or planning that night's activities, when the professor said something that caught my attention and has stuck with me to this day.

A barnacle's penis can reach up to ten-times its body length.

The barnacle penis is a modified cirrus

Wait. It gets better.

 

The barnacle penis has transverse and longitudinal muscles.

That means it can extend, contract, and move left, right, around in circles, any old way!

I'll admit, that is a really strange thing to focus on, but I did. I paid attention to the rest of that lecture. More surprising, I went home and read the chapter on arthropods, and particularly, crustaceans.

What I learned stayed with me. Not only are barnacles the John Holmes of the invertebrate world...hell, of the WHOLE animal kingdom, but they're pretty darn interesting for little buggers that stay latched on to one place for their whole lives.

Barnacles are hermaphroditic--they're he/shes with both male and female parts. They can self-fertilize, but usually don't. With a penis that's 10-times your body length, you can reach out and touch your nearest (or not so near) neighbor pretty easily, and your neighbor can reciprocate.

After fertilization, the eggs develop inside the adult's shell and, when ready, are released into the water. Ready means the eggs have developed into nauplii larvae (barnacle infancy), a traveling-with-the-currents, feeding, growing stage. The nauplius has a simple eye spot, so can detect light and shadows, antennae to sense the environment, and swims (or more appropriately, steers, since the currents would overpower any swimming they did) using setae (hair-like projections) as oars. The barnacle spends about 6 months as a nauplius, molting to grow, as do all arthropods. At the last molt before adulthood, it will undergo metamorphosis into a cyprid larva (adolescence).

The cyprid stage is really messed up, in a fascinating way. Cyprid larvae don't eat. At all. And barnacles can spend anywhere from days to 6 or 7 weeks in this stage.

The cyprid larvae's job is to find a place to settle down. This isn't a decision to be made lightly. Barnacles are sessile: once they settle down, it's forever.

Cyprid larva

The perfect forever home for a barnacles is on a hard surface; one with some pits and grooves to snuggle into so it can't be easily wiped off. Somewhere with room to grow, but with some neighbors, too (to take advantage of that long, dexterous penis); some of its own kind to associate with...just not too close or crowded.  And food should be readily available. 
 

Food comes from the same place as enemies or predators, from the water.  Live high enough above the water line and the aquatic predators can't reach you, but too high and neither can the food. Then there's a whole other set of terretrial predators that can get you. And the sunlight up high can be pretty harsh, beating down on you during the day when the tide is out. The cyprid larvae has to balance all of these considerations when seeking its permanent home.

Once it's found a suitable location, the cyprid settles down. It has a short period when it can crawl around, use its antennae to make sure it this is where it wants to spend the rest of its life, and the do the deed.

The larva places its head down on the spot it selected and uses an adhesive gland located near its antennae to release a cementing substance. The decision is now final and irrevocable. The cyprid larva is permanently glued to this spot. Now, the barnacle undergoes one last metamorphosis into adulthood.

The newly cemented-in-place barnacle undergoes many changes so it can be more efficient in its sedentary adult life. It secretes calcareous plates around its soft body for protection from predators, desiccation, and wave action. The eyespot that detected light/dark serves no purpose. Maintaining a photoreceptor is a waste of energy, so adult barnacles lose their “eye.”

Adult acorn barnacle

The larval swimming appendages (setae) also become unnecessary. The swim appendages turn into "cirri" or feeding legs.  These "new-" (or modified old-) appendages provide the scientific name given to barnacles‑‑Cirripedia. In Latin, that means hairy feet.  Their hairy feet wave in the water above their shell, catching small organisms. So, really, the barnacle spends its whole adult life doing a handstand and kicking food into its mouth.

As in most arthropods, the appendages, or cirri, become highly specialized. Some of the cirri are used for feeding, some are for grasping, and then there's that one special one that captured my interest: the reproductive cirri, aka, the penis. I've since found that my professor's claim of "10 times it's body length" may have been a bit of an exaggeration. It's more typically "only" seven times the animal's length. Still no slouch in the endowment department (see photo above). 

Once a barnacle’s metamorphosis to adulthood is complete, he/she can extend this impressive appendage in a neighborly‑barnacle sort of way to any old neighbor at all (within about a 7-barnacle-length circumference) and get on with important business:  passing on its genetic material to a new generation. And while this is going on with any given barnacle, chances are that one of its neighbors is doing the same thing back to it, creating a cirri‑connected, hermaphroditic, totally sessile, intertidal mass‑orgy.

Fascinating, isn't it? There are thousands more stories of life in the sea, all equally interesting.

Is it any wonder why I became a marine scientist?