Gooseneck barnacles and Barnacle Geese

One of the joys of biology lies in appreciating how strange and varied the world is. When humanity starts to feel claustrophobic, you can imagine the life of an albatross, aloft over the ocean for most of her life, searching out schools of delicious fish by their scent; or a cuttlefish, flashing colored signals at his companions as he shoots through the currents, flexible tentacles waving. When the world feels narrow and limiting, you can remember that clownfish change sexes depending on their place in the dominance hierarchy, with males becoming female when they advance to the position of top dog.

Yet—amazingly—biology used to be even wilder. Before satellite tracking and genetic analysis, before “biology” was a recognized science at all, natural philosophers looked at a perplexing natural world and invented some truly outside-the-box explanations for what they saw.

Some of these are fairly well known: for example, the idea that there is a “homunculus”—a tiny human—inside the head of each human sperm cell.


As drawn by N. Hartsoeker in 1695. Public Domain,

To be fair, we now know that sperm (and eggs, etc.) contain the genetic blueprint for building a human, which isn’t all that far off from containing a tiny human.

But my favorite old science myth involves—of course!—birds.

Medieval European natural philosophers were puzzled by Barnacle Geese, which disappeared during summer, but reappeared every winter. Even stranger, these geese were always adults: they were never seen hatching from eggs or as young goslings.


The mysterious Barnacle Goose. Photo by Stefan Berndtsson*

Therefore, they must be reproducing in some strange new way—but how? In 1187, Giraldus Cambrensis explained:

[Barnacle Geese] are produced from fir timber tossed along the sea, and are at first like gum. Afterwards they hang down by their beaks as if they were a seaweed attached to the timber, and are surrounded by shells in order to grow more freely. Having thus in process of time been clothed with a strong coat of feathers, they either fall into the water or fly freely away into the air… I have frequently seen, with my own eyes, more than a thousand of these small bodies of birds, hanging down on the sea-shore from one piece of timber, enclosed in their shells, and already formed.

– Giraldus Cambrensis “Topographica Hiberniae” (1187), quoted in Edward Heron-Allen, Barnacles in Nature and in Myth, 1928, reprinted in 2003, p. 10. ISBN 0-7661-5755-5.

In case you’re having trouble picturing it, he is describing this:


Gooseneck barnacles

The geese are holding on by their dark bills, while their bodies grow inside the protection of those white shells.


See? There’s even a few feathers visible coming out of the bottom of this one! (Photo by Teddy Llovet*)


Um… no. (Photo by Caleb Putnam*)

This is an inspired answer to the puzzle of Barnacle Goose reproduction. They don’t reproduce somewhere far away, or hide their babies: they develop inside barnacles out at sea. The idea seems to be related to the fact that the geese were seen in the same areas as the barnacles, and perhaps to the similar balck-and-white color scheme of the geese and barnacles.

But surely someone cut open a gooseneck barnacle and observed the lack of tiny goose inside? The cirri of the barnacle—the little appendages used to filter-feed in the water—may look feathery from afar, but they don’t remotely resemble an entire goose.


To review: barnacles. (Photo by Dawn Endico*)


Geese. (Photo by Tomi Tapio K*)

There may have been motivation not to look too critically at this explanation. If Barnacle Geese were not birds, but instead were “born of trees” (because the barnacles grow on timber), then it was okay to eat them during fast days and Lent. Another popular myth also connected geese to barnacles to trees, albeit in an even less plausible scenario:

Whether the barnacles grow on trees or on driftwood, the important message is: the Barnacle Geese are, religiously speaking, vegetables. Or maybe fish.

Unfortunately, several religious leaders made a point of denying this. Pope Innocent III explicitly declared Barnacle Geese to be birds in 1215, as did Rabbeinu Tam in the 12th century. So much for that loophole.

As for the mystery of Barnacle Goose reproduction: they breed in the Arctic, which is probably why continental Europeans never saw them do it. They lay their eggs high on inaccessible cliffs, then get their goslings to jump down from these terrifying heights and walk to wetlands where they feed and grow.


Barnacle Geese with goslings. If you look closely you will see that they are not inside gooseneck barnacles. Photo by Stefan Berndtsson*


Barnacle Geese with an older gosling. Still not in a barnacle. Photo by Roine Johansson*

But isn’t it a lovely thought: all those tiny geese, safe in their shells, biting hard onto the driftwood as they bob about in the ocean? Spreading their wings at last and bursting fully-formed from the water?

It does make you wonder why the various ducks that nest in trees were never accused of being “born of trees.” They seem like much more plausible candidates.


Australian Wood Ducks guarding their nest in this tree cavity. Photo by Rhys Moult*


Newly-hatched ducklings in a tree cavity. Photo by Steve Brace*

Maybe they didn’t taste as good as the Barnacle Geese.

*Photos obtained from Flickr and used via Creative Commons. Many thanks to these photographers for using Creative Commons!


3 thoughts on “Gooseneck barnacles and Barnacle Geese

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