Of course, that depends on how you quantify “action.” Barnacles have a fairly short mating season—compared to our non-stop mating season—but they cram a whole lotta nooky into the time they’ve got. Common barnacles (Semibalanus balanoides) get busy—very, very busy—in early November*. Scientists have observed as many as six males copulating with a single female simultaneously, each male inserting his penis up to 10 times within a 100 minute interval. Tetraclita japonica, a common acorn barnacle that mates from late June to early August, is even randier. Researchers counted 11 males copulating with a single female and a total of 582 penis insertions during the female’s eight and a half hour mating bonanza.
Hold on. Before we proceed with our discussion of barnacle sex, there’s something you should know about these crustaceans. Males aren’t always males and females aren’t always females. In other words, barnacles are simultaneous hermaphrodites—always ready to be either male or female. In observing T. japonica, scientists watched one barnacle mate as a female and then, three hours later, as a male, while another mated first as a male and then, a little more than three hours later, as a female. It just so happens that these two barnacles mated with each other twice, once when the first barnacle was a female and again after they changed sexes.
Now back to barnacle sex and the big question: How does it work? Most sessile organisms (critters that spend their lives glued to something else) just shoot their goodies into the water and hope for the best. But barnacles get it on—and even choose their mates—thanks to the magic of the barnacle penis. Actually, it’s not magical at all. It’s just really, really long and covered in chemosensory bristles. The bristles allow functionally male barnacles to “sniff out” attractive barnacles that are acting as females, while the go-go-gadget penis allows barnacles to mate with individuals that aren’t right next to them.
Barnacle penises can be up to ten times the animal’s body length, making barnacles the most well endowed animal in the world (ratio-wise, obviously). Impressive, yes, but such a long schlong is also a liability. They can’t just wave that thing around in the pounding surf without doing some damage. So they don’t. Barnacles on wave-battered shorelines have shorter, fatter (and therefore sturdier) penises than their counterparts in calm intertidal areas. And, barnacles moved from a tranquil shoreline to a wavy one—or vice versa—will adapt their penis shape accordingly.
How do we know so much about the sex lives of plankton-eating critters that glue themselves to rocks, boats and whales? Scientists watch them, they measure them, they move barnacles from one site to another…and when they want to figure out if relaxed penis length is a valid indicator of maximum penis length they use what we believe should be a world-famous methodology known as “artificial inflation of barnacle penises.” Here’s how it works:
…The soma was then cut between the first and second pair of thoracic legs, inserted onto the tapered end of a seawater-filled plastic capillary tube (1.09 mm in outside diameter, 0.38 mm in inside diameter and approx. 25 mm long) and carefully glued in place (Krazy Glue, Elmer’s Products, Columbus, OH) while keeping the penis tissue moist. The capillary tube was then inserted onto the end of a hypodermic needle (0.5 mm in outside diameter and 0.2 mm in inside diameter) and fitted onto a 10 ml plastic syringe filled with seawater. The penis and remaining feeding legs were positioned in seawater under a dissecting microscope. The penis was oriented perpendicular to the field of view, and pressure was applied to the syringe to slowly inflate the penis until (i) the glue failed, (ii) the soma tissue or cuticle ruptured, or (iii) the penis inflated fully. Full inflation was recorded when additional pressure on the syringe failed to extend the penis further and all annulations of the penis cuticle had disappeared. At this point, the penis was photographed again. This process was repeated for approximately 20 individuals per site until we had achieved full penis extension for three individuals from each population. (Neufeld and Palmer 2008)
That’s right. They inflated the penis until (1) the Krazy Glue failed**, (2) the barnacle’s penis exploded or (3) the artificial inflation procedure actually worked.
*Once the mating season ends, the barnacles molt and discard their penises. They grow back—slowly at first, and then rapidly in September and October so the barnacles are ready for action in November.
**and perhaps when the glue failed, the penis shot across the room like a deflating balloon. (This effect isn’t documented in the paper. That’s just how we picture it.)
HOCH, J. (2008). Variation in penis morphology and mating ability in the acorn barnacle, Semibalanus balanoides Journal of Experimental Marine Biology and Ecology, 359 (2), 126-130 DOI: 10.1016/j.jembe.2008.03.002
Hoch, J. (2009). ADAPTIVE PLASTICITY OF THE PENIS IN A SIMULTANEOUS HERMAPHRODITE Evolution, 63 (8), 1946-1953 DOI: 10.1111/j.1558-5646.2009.00668.x
Murata, A., Imafuku, M., & Abe, N. (2001). Copulation by the barnacle Tetraclita japonica under natural conditions Journal of Zoology, 253 (2), 275-280 DOI: 10.1017/S0952836901000243
Neufeld, C., & Palmer, A. (2008). Precisely proportioned: intertidal barnacles alter penis form to suit coastal wave action Proceedings of the Royal Society B: Biological Sciences, 275 (1638), 1081-1087 DOI: 10.1098/rspb.2007.1760