Hakai Magazine

Coastal science and societies

barnacle flies mating
The fertilized eggs of these amorous flies are laid inside a barnacle’s shell. Once hatched, the larvae will eat their hapless host. Photo by Shane Lighter/Royal British Columbia Museum

Two Flies Walk into a Barnacle

Then they mate and their babies eat the barnacle. Seriously.

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by Amorina Kingdon

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The recent Oscar-winning documentary Free Solo follows rock climber Alex Honnold as he climbs El Capitan, a kilometer-high vertical mountain face, without any ropes or safety gear. The film is compelling for the same reason as space travel or free diving: it’s about surviving and even thriving in a place that by all rights should be fatal.

On a much, much humbler scale, that principle is why barnacle flies fascinate a handful of scientists.

These otherwise ordinary, terrestrial relatives of dung flies live, breed, and die exclusively in barnacles, a lifestyle choice that submerges them in salty water twice a day. This should drown them, yet it doesn’t, and no one knows why. In fact, no one knows much of anything about these coastal insects, but that may be about to change thanks to new data from British Columbia’s central coast.

Joel Gibson and Henry Choong, biologists from the Royal British Columbia Museum in Victoria, were on Calvert Island in 2018 for the Hakai Institute’s bioblitz, during which scientists documented as many species as possible in the surrounding environs. The pair was surveying a section of the coast when they came to a vertical rock formation at the end of a beach. “It looked like a walk-in closet,” Gibson says. The rock walls were coated with barnacles, Choong’s specialty, and buzzing with flies, which captivated Gibson who is an entomologist.

The two combined their expertise to study this bonanza. “When he collects the flies, I collect the barnacles,” Choong laughs. As they did, they noticed patterns. The flies concentrated in barnacles near, but not quite at, the top of the “closet.” There were also more flies on the vertical surfaces than on adjacent sloped surfaces, although both had otherwise similar species and concentrations of barnacles. (Gibson and Choong hypothesize that living higher on the vertical wall limits how long the flies are underwater or washed by waves.)

Their new observations add to a surprisingly small body of knowledge. Only a handful of scientists have studied these intertidal flies, but the little that is known is fascinating. The female fly lays her eggs inside a barnacle, which the larvae then eat to fatten up. (With macabre practicality, a larval fly seems to save the muscle that holds the barnacle closed until the very end so it can still use the barnacle’s hollowed out shell for protection.) Once its barnacle host is consumed, the larva then crawls to another. One larva might gobble three or four barnacles before it’s grown, Gibson says. As an adult, the fly then finds a mate, lays eggs, and starts the cycle again.

Chris Neufeld, the director of education at the Bamfield Marine Sciences Centre, did his PhD on barnacles and saw many, many flies. With so little data on them, they were initially more oddity than anything else. “It’s a pretty weird niche,” he says.

But he thinks barnacle-munching intertidal flies might have more to tell us. Discovering how the flies adapt to the sea could illuminate larger patterns of why animals end up in the niches that they do. For example, Neufeld muses, why aren’t there more insects like the barnacle flies in the sea? They’re numerous on land yet almost none have adapted to the ocean. Neufeld wonders if the ecological niches filled by insects on land are already filled in marine environments. “The sea is already very full of very successful arthropods,” he says. Barnacle flies might provide insight.

Gibson and Choong are working on a paper quantifying their new observations, including flies’ predilection for vertical surfaces and how they move from barnacle patch to barnacle patch. How they ended up in this niche in the first place Gibson isn’t sure, but he looks forward to figuring out how they make this surprising environment work for them. It’s no El Capitan, but it’s still inspiring.