Day 500: Kwaj hermit crabs and the fly angle

A crabby experiment and an even crabbier video

A few months ago, one of the high school science teachers showed me a bowl full of hermit crabs on a table in the back of her room. Shells bobbed and clicked at the bottom of the bowl as the little crustaceans attempted to climb its vertical sides. On her desk, she also had an article that described an experiment involving three species of hermit crabs and their function within the Kwajalein Atoll ecosystem.

Kwaj hermit crab
Hermit crabs may play a special role in atoll ecosystems.

“I’m going to have the kids identify the crab species, but I’ll also use the article to help them think about experimental methods,” she said. “Then of course,” she added, “there’s the fly angle.”

The fly angle

Published in Pacific Science in 1983, the project in her article set out to document hermit crab feeding patterns and examine whether the little clawed decapods (10-leggers) help reduce the fly population on tropical islands. Although it was published 36 years ago from experiments conducted while a marine research station was active in the Marshall Islands, the study is still directly related to a persistently pesky problem on Kwaj. In the last year, and particularly in July and August, the fly population on Kwajalein has surged, with the many-eyed winged insects swooping around in droves, especially if they catch any whiff of food. 

I used to love sitting on my porch with a cold drink while dinner cooked in the oven or Jake grilled in the yard. Now, when we grill outside, 20-25 buzzing black bodies hover around our porch and door, just waiting for any chance to feed. With Herculean effort, I can ignore the buzzing and dive-bombing, but usually I end up back inside.

Jake has developed a special skill in darting from kitchen to grill and back without letting flies into the house. Since he’s not always successful, we’ve all honed our abilities to swat flies when they land on a window or even thwack them from midair to the ground. 

Granted, the flies typically disappear with the sun, so with darkness comes fly relief. Still, until the sun sinks beneath the watery horizon, we keep the food covered and the flyswatter handy.

Over the last two months, fly numbers have admittedly lessened. But we’re still interested in anything that might curb their numbers even more. So my science teacher friend’s article with it’s “fly angle” was of particular interest to me.

Crabby science

For the project described in the article, two marine biologists from the University of California Santa Barbara’s Marine Science Institute, Mark Page, PhD, and Stewart Willason, PhD, carried out a simple experiment in September and October of 1979 on Mut and Ikuren, both islets located on the southern edge of Enewetak Atoll.

Mut and Ikuren are islets on the southern end of Enewetak Atoll in the Republic of the Marshall Islands.

First, they set up three types of cages containing carrion — either raw chicken or fish — both near the beach and inland. The first type of cage had two completely open sides, so crabs of all sizes (as well as flies) could reach the meat. The second type had small openings cut in the sides to limit the size of crab that could get in while still allowing limitless flies. The final type of cage was enclosed on all sides — a no crab, all fly set up.

This crab was scavenging on the oceanside beach at low tide.

Over the next two days, biologists Page and Wilson counted the number and species of crabs on the chicken or fish every two to four hours. After 48 hours, they estimated the amount of meat left in each cage and counted the number of fly maggots in the carrion. They found large maggots in the partially enclosed and completely enclosed cages that limited the size or number of crabs that could reach the meat. In the completely open cages, however, they found no remaining chicken or fish, which meant no maggots as well.

The two researchers concluded that since flies can develop from egg to pesky adult in less than 10 days, the scavenging behaviors of small crabs and the voracious appetites of large crabs may actually help reduce the fly population of inhabited islands. 

In other words, here on Kwaj, hungry hermit crabs could be making my evenings less fly-filled. And with less annoying flies, I’ll be spending my time back on that patio, cold drink in my hand.

Ever wondered what a wandering hermit crab looks like? Check out the video: