Photograph by Minette Layne
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Gooseneck barnacles
Photograph by Minette Layne

Poorly-Endowed Barnacles Overthrow 150-Year-Old Belief

In absolute terms, the blue whale has the largest penis of any animal—a huge mobile appendage that can reach 10 feet in length. But the blue whale itself is enormous. If you take body size into account, the animal kingdom’s champion penis belongs to a much smaller creature, and one that often lives on the faces of whales. It’s the barnacle.

Barnacles are found wherever hard surfaces meet seawater, including boats, moorings and whale heads. They look like little rocks, but they’re actually crustaceans—close relatives of crabs and shrimp. While their relatives walk about, barnacles affix themselves to a surface, and filter food from the water with protruding paddling legs. This stationary life poses a problem when it comes to mating, especially since barnacles apparently have to fertilise each other internally.

They do so with a huge penis, which blindly reaches across into neighbouring shells and deposits sperm inside. This giant organ can stretch up to eight times a barnacle’s own body length, making it proportionately the biggest penis in the animal world. Since most barnacles are hermaphrodites, every individual can fertilise and be fertilised by all of its neighbours. And if there’s no one else within reach, the barnacles apparently fertilise themselves.

This view of barnacle sex has been a stalwart of textbooks ever since a barnacle-obsessed Charles Darwin devoted eight difficult years of his life to these strange creatures, and published an epic four-volume monograph on their biology. Here he is, waxing wonderstruck about their penises:

“The males are attached at a considerable distance from the orifice of the sack of the female, into which the spermatozoa have to be conveyed; and to effect this, the probosciformed penis is wonderfully developed, so that in Cryptophialus, when fully extended, it must equal between eight and nine times the entire length of the animal!”

But barnacles still hold surprises. Graduate student Marjan Barazandeh from the University of Alberta has found clear evidence that the gooseneck barnacle Pollicipes polymerus does something that barnacles are really not meant to do—it spermcasts. That is, individuals can fertilise each other by ejaculating directly into the surrounding water and sieving out each other’s sperm. “These observations overturn over a century of beliefs about what barnacles can, or cannot, do,” she writes.

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Left: Gooseneck barnacle extruding its feeding legs and penis. Right: Gooseneck barnacle leaking sperm from its shell. From Barazandeh et al, Biology Letters.

Barazandeh, together with fellow student Chris Neufeld and team leader Richard Palmer, collected almost 600 gooseneck barnacles from Canada’s west coast, and confirmed that their penises are shorter and less stretchy than those of their more famously endowed kin. They only extend to two thirds of the animal’s body.

[An interlude: How, you might ask, does one measure the penis of a barnacle? I will tell you. To measure the relaxed penis, Neufeld just pulled it out and assessed it under a microscope. To measure one in all its fully extended glory, he needed the following contraption: a system of pulleys, which controls an open bottle, which leads to a rubber tube, which is connected to a hypodermic needle, which feeds into a capillary tube, which is glued to the base of a severed barnacle penis. All of these elements are full of seawater. By using the pulleys to raise and lower the bottle, he could control the pressure in the needle and carefully pump a specific amount of water into the penis. The team describes it as a “gravity-fed pressure system for inflation”. It’s as if Rube Goldberg built a fluffing device.]

The team found that many of these goosenecks were carrying developing embryos, despite sitting well outside the penis range of any immediate neighbour. Scientists first found isolated but fertilised barnacles back in 1960, but they always assumed that these individuals had fertilised themselves. Spermcasting runs so against the textbook wisdom about barnacles that no one considered it as an explanation.

For the gooseneck barnacle, that assumption is especially bizarre since no one has ever seen these animals fertilise each other. Equally, scientists have failed to see solo goosenecks fertilise themselves in a lab. “DNA markers were an obvious way to test these alternative hypotheses,” says Palmer.

Baranzandeh collected embryos from 37 barnacles and checked their DNA, she found that almost all of them carried genes from a second parent. They couldn’t possibly have arisen through self-fertilisation. Nor could these genes have come from a neighbouring barnacle that then died, since barnacles take longer to decay than eggs take to hatch. Spermcasting is the only remaining alternative.

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Gooseneck barnacles at Friday Harbor, by Daniel Foucachon

We don’t know how it happens, how often it happens, or whether other barnacles can do the same thing (although the team is checking). We do know that the goosenecks can capture sperm from the water even if there’s a penis within reach, since a quarter of the individuals with an adjacent partner were carrying embryos that had been fertilised by a distant one.

And since Barazandeh saw goosenecks leaking sperm from their shells at low tide, it’s possible that these ejaculates wash away to be captured by barnacles downshore. As she writes, “Quite contrary to all prior expectations about mating in barnacles, P. polymerus appear able to obtain sperm from the water in the field and do so even when an adjacent partner is available,”

Reference: Barazandeh, Davis, Neufeld, Coltman & Palmer. 2013. Something Darwin did not know about barnacles: spermcast mating in a common stalked species. Biology Letters.

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