barnacle sex gets even stranger


“I hate a barnacle, as no man ever did before.”

-Charles Darwin, in an 1852 letter to William Darwin Fox

If you’ve ever been to the beach, chances are you’ve seen a barnacle. Probably hundreds of them, in fact, just sitting out at low tide, peppering the environs with grayish-white encrustations. They’re everywhere, studding the rocks and driftwood and even the shells of other creatures. Perhaps, while clambering across the beach, you’ve cut your hand or foot on a barnacle’s calcareous housing, or watched in a tide pool as aggregations of them rhythmically beat the water for food.  They are quite literally a fixture of the seashore—being sessile invertebrates, they are glued to the spot—and their steadfastness gives rise to some interesting adaptions.

Barnacles are crustaceans, distant kin to crabs, shrimp and lobsters. Being arthropods, they have bodies split into segments—head, thorax, abdomen—but none of this is visible on an adult, intact barnacle. Most of its anatomy is held within a ring of calcium-carbonate plates, roughly conical, that protect the animal from predators, the pounding surf, and the desiccating nature of the sun. This is the part of a barnacle you’re most likely to see: its outer aegis, homologous to the shells of crabs and other crustaceans. Inside this cone is a tiny creature perched, rather improbably, on its head. Its back is to the bottom of the cone, and its limbs are pointed upward. What corresponds to the forehead of the barnacle is cemented to the substrate. Its six limbs, called cirri, are long and feathered and are used to sieve plankton from the water, bringing it down to the mouth to be eaten. As an infraclass, the 1,220 species of barnacles worldwide are known as Cirripedia, Latin for “curl-footed”.


The Pacific Northwest is home to twenty-nine of those species, including what is arguably the weirdest-looking of them all: the gooseneck barnacle (Pollicepes polymerus), a grotesque creature that really, um, sticks its neck out there in its zest for life. If you’re thinking, “Hey, that thing really does look like a goose’s head, hanging off a rock,” then you might forgive the Welsh monk Giraldus Cambrensis’s error in judgment, made in 1188 in his natural history treatise Topographia Hiberniae, when he deduced that the barnacles were, in fact, baby geese. Over in Wales, there is another species of gooseneck barnacle, Lepas anserifera, which attaches to driftwood and other flotsam. Cambrensis saw a physical similarity between these barnacles and a local goose species, Branta leucopsis—a bit of a stretch, this was—and claimed that, since the goose had never been seen to nest in Europe, and since the barnacles were always found on branches as from a tree, the two were simply different forms of the same plant-like organism. (Keep in mind that this was long before bird migration had been figured out.) For a while people believed him, took his assertion that the geese were “neither flesh, nor born of flesh” to heart, and even went so far to deem B. leucopsis fit to eat on religious holidays when the consumption of meat was forbidden.


The goose tree, where geese grow

Back to our goosenecks in the Pacific Northwest, letting it all hang out in the intertidal churn. The “neck” is more accurately known as a peduncle, and it acts as a holdfast, anchoring the creature to rocks, logs, boat hulls, etc.  It also contains the animal’s sexual organs. (Things get a little juicy here—ready yourself.)  Almost all barnacles are hermaphroditic: individuals generally possess all the hardware needed for sexual reproduction, and could theoretically fertilize themselves in a pinch. Remember that a barnacle is rooted in place, limited in its ability to shop around for a suitable partner. In the sessile dating life of barnacles, where everybody’s similarly equipped and stuck at home, mating essentially consists of flailing your penis around until you find a receptive mantle, then depositing some sperm in the hope that it takes. This roughly circular area of penetrative potential is known, in scientific parlance, as the “penis range”. It is often remarked that the barnacle has Animalia’s longest penis relative to body size, and the rumors are true: most species boast members two to three times their body length, and one species, Anthrobalanus of Chile, packs a penis eight times longer. (In 1853 Charles Darwin, expressing his admiration of the Anthrobalanus phallus, wrote to a friend, “The probosciformed penis is wonderfully developed…when fully extended it must equal between eight and nine times the entire length of the animal!”) P. polymerus apparently got the short shrift, its meager penis only slightly longer than its 18-millimeter body.

Nevertheless, the gooseneck barnacle manages to make do with what it was given. A recent Canadian study published in the journal Proceedings of the Royal Society B found evidence that P. polymerus reproduces by so-called “spermcast”, a mode of gamete dispersal commonplace in marine species but heretofore unknown to crustaceans. Similar to “spray and pray” dispersal, spermcasting involves little more than mass releases of millions of sex cells to the current, in the off chance that a successful union—sperm meets egg inside barnacle—will result. Using paternal testing, the Canadian scientists found that fertilized egg masses of selected P. polymerus contained genetic markers from individuals outside of their penis range—that is, sperm from other, more distant barnacles had somehow drifted in. It is the first documentation of such a phenomenon occurring in crustaceans, and it calls into question all that scientists had previously assumed about barnacle reproduction.

Scientists like Darwin, for instance. After returning from his Beagle voyage in October 1836, Darwin spent more than eight years researching and classifying extant and extinct barnacle species from around the world, eventually publishing the four-volume monograph Cirripedia in two parts, in 1851 and 1854. He was troubled by the implications of his theory of evolution, iconoclastic as it was, and to ease his mind and cogitate on less incendiary things, he studied barnacles. Basically, Darwin’s procrastinating resulted in his creating one of the most seminal works in barnacle biology to this day. Would that I could be half as productive whilst putting off my responsibilities.

Next time you’re at the beach, give a closer look to those barnacles underfoot. And if you find yourself out on the Pacific Coast, where the waves and currents wash powerfully, spilling relentless over the rocky shore, keep an eye out for colonies of P. polymerus, that goose-necked eccentric of Crustacea.


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