Brine flies provide a feast for birds and humans

By Jeff Mitton
Driving along a one-lane road winding through the Sand Hills of Nebraska, I spotted a flock of avocets at a small lake.

I turned off the engine, drifted quietly to a stop between the road and lake and grabbed my camera. I walked quietly toward the avocets but was distracted by a soft hissing at the shore. As I approached, my shadow fell on them, causing thousands and thousands of flies to take wing.

The flies flew in a distinct way. They took wing, but they flew low, just a few inches off the ground, just a few feet along the shore before landing. Not a single fly landed on me, or even flew near me. I learned that they did not mind me but were uneasy if my shadow fell over them. This was probably a behavioral mechanism to avoid predation.

Some of the flies were walking on the land, but many were walking on water. In places, flies were so densely packed that they obscured the shore -- I literally could not see the land or water they were standing on. The dense mats of flies extended a few feet out into the lake, but the flies were more sparsely distributed all the way across the lake.

These were brine flies, Ephydra hians, also called shore flies, alkali flies, Mono Lake flies and several other names. Brine flies reach extraordinary densities along the shores of Mono Lake in California and the Great Salt Lake in Utah, but they also occur in many lesser-known salt or alkaline lakes.

They are adapted to saline and alkaline lakes, for they have several mechanisms to live under water in lakes with high concentrations of various salts.

For example, larvae have a lime gland to remove carbonate and bicarbonate ions from the blood and excrete them. They also have an unusual pumping mechanism that moves sulfate from the gut to the blood to the colon and then out. Larvae also have strong prolegs that allow them to anchor to hard surfaces under water and to hold the pupae under water, where predation is rare.

Adult brine flies can walk under water, but they are highly buoyant and cannot breathe under water. So, to get to the bottom of the lake they need to climb down into the water, clinging to the substrate with their feet.

As they go below the surface a bubble is formed around them, perhaps formed and stabilized by the hairs on their body. The bubble allows them to breathe for about 15 minutes while they forage for food or lay eggs. When oxygen in their air bubble gets low, they release their grip on the substrate and pop to the surface like a cork, landing on their feet, perfectly dry.

Saline and alkaline lakes grow a lot of algae. Adult brine flies eat some algae while foraging under water but eat much more of the algae that collects in windrows along the shore.

With an abundance of food and a paucity of competitors, brine fly populations grow to amazing densities.

For example, biologists have estimated that 37 million flies can be found on one linear mile of shore at the Great Salt Lake. When strong winds form whitecaps on the Great Salt Lake, turbulence dislodges larvae and pupae from the bottom, and waves pile them up on the shore. Biologists have reported windrows of pupae and larvae one foot deep on the shore, stretching for miles.

Dense populations of flies attract numerous species of birds. That day in the Sand Hills, avocets were picking them, one at a time, from the surface of the water. A large flock of yellow-headed blackbirds was foraging in the short grass near the lake, probably taking brine flies.

American Indians also harvested brine flies. Paiute Indians in California collected bushels of larvae from Mono Lake and removed the coarse integument by rubbing and rolling them, yielding a tasty kernel of nutritious protein.

To the west of present-day Mexico City, Aztecs harvested brine fly larvae but also collected eggs, from which they made flour.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

January 2012