Seaweed flies

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A seaweed fly in the genus of Fucellia, on a beach in Vineyard Haven on New Year's Day. - Matt Pelikan

The Christmas bird count (see bit.ly/mvbirdcount) held on Dec. 28 was a snoozer for me: Low numbers, no good birds, many expected species missed. But as I contemplated the nonexistent waterfowl on Vineyard Haven Harbor around 2 that afternoon, I found something that made the whole day worthwhile: swarms of little flies, frolicking around a dollop of seaweed.

I’ve seen flies on seaweed on summertime beaches, of course, but the idea of insects actually enjoying winter always intrigues me. So I headed back to the same site on New Year’s Day to learn, and photograph, what I could. Despite the 42° air, I found flies still rollicking around on every respectable clump of weed.

The flies themselves looked pretty generic, distinguished by little except the inhospitable date and habitat. But by combining the photos I managed with what I already knew, it was pretty easy to pin my quarry down to the genus fucellia. Known more commonly as “seaweed flies,” fucellia (there are about 20 known species in North America) is in the root-maggot fly family, closely related to the familiar housefly.

I can’t identify my fly to species just from photos — this is a difficult genus — but they rank among a surprising variety of flies that have adopted coastal wrack line as their only habitat. Worldwide, that’s scores or hundreds of species across a half-dozen families. The name “kelp fly” also gets imprecisely applied to these specialists. Our wintertime fucellia may or may not be the same species found on wrack lines in the summer. And I don’t yet know if other species or genera may be present on our beaches. (I can feel a study of wrack-line flies coming on.)

Why specialize in such a hostile setting, replete with salt, relentless sun, sterile sand, limited shelter, and few dining options other than skanky seaweed? The answer surely includes reliability — there is almost always wrack on a beach — and the absence of much competition (because the conditions are so challenging).

Seaweed flies have simply worked out the knack of exploiting this harsh and limited habitat. One key adaptation is a short lifecycle: Fucellia cranks out a new generation in about 26 days, tolerably fast work by fly standards but, more important, shorter than the lunar cycle (which largely governs tide height). Seaweed flies that lay eggs on wrack right after the highest tides of the month can be reasonably sure their progeny will mature before the next tides high enough to disturb the wrack.

Even if a storm tide washes high onto the beach, all is not lost for an immature seaweed fly. The larvae are impervious to salt, and can tolerate prolonged submersion. And the pupae of seaweed flies are typically waterproof and buoyant. At least a few of any immature seaweed flies that are washed away probably come ashore elsewhere and simply continue their development.

As with most seaweed flies, fucellia larvae feed exclusively on decaying seaweed. There is some evidence that they’re actually feeding not so much on the weed itself as on the bacteria that are breaking it down. But in any case, there are often strong associations between a particular fly and a particular species or genus of seaweed.

Bacterial action in clumps of rotting weed generates heat that helps seaweed flies remain active through the winter, and of course the absence of shade on a beach gives these flies the benefit of whatever the wintertime sun can cough up by way of energy. In summer, when overheating could be a problem, clumps of weed provide an infinite number of shady, moist crannies to hide in.

Adults, which can be appallingly plentiful in summer, feed on anything from deliquescent seaweed to flowers on the nearby dunes to your sandwich. So seaweed flies can be annoying companions. But although plenty of invertebrates will bite you on a beach, a seaweed fly is not one of them. As is pretty typical among the house fly’s relatives, seaweed flies have sponge-like mouth parts that are incapable of piercing skin. And as far as I’ve been able to determine, these flies are not known to carry disease.

Combined with the bacteria they feed on, these flies are a potent force for recycling the nutrients in defunct seaweed. Both the larvae and adults of seaweed flies represent a bountiful food for sandpipers and plovers fueling up on beaches: You could almost say that seaweed flies are machines for converting washed-up weed into energy for breeding and migrating shorebirds.

And astonishingly, these specialized flies host equally specialized parasites, including beetles, wasps, and even other flies, the larvae of which develop by devouring larval seaweed flies. Again, such parasite-host relationships are typically very specific, with each parasite species using one genus or species of fly as its host.

In other words, as surprised as I was to find them, the flies I photographed on New Year’s Day were not unusual, and they were not alone. They’re critical pieces in a rich little ecosystem that has evolved around a humble but plentiful resource: washed-up seaweed.