Honolulu Star-Advertiser © Susan Scott
March 8, 2010
While reading about Midway’s Bonin petrels, I learned that these seabirds eat fish, squid, marine insects and crustaceans. The marine insect part puzzled me. In all my time at sea, I’ve seen only one insect, and that one I brought with me.
About a week after setting sail from Palmyra Atoll for Tahiti, a mud dauber began flying around the cockpit. Mud daubers are black-and-yellow wasps that stick mud nests to walls, lay one egg inside, stock the nest with paralyzed spiders as baby food, seal up the hole, then leave. These non-native, nonaggressive wasps are abundant in Palmyra, and for some reason like to build hidden nurseries on boats.
Mud daubers fly with their long legs dangling below as if dancing on air. Watching that young wasp fly around and around the boat looking for nectar, spiders or a place to land was so heartbreaking my biologist friend and I couldn’t watch. We went below deck, and when we came up a few hours later, that ballerina of insects was gone.
Our pet wasp surely died quickly in the open ocean. Another insect, however, is right at home offshore because it walks on water and eats anything it finds. This is the sea skater, a miniature marine version of the freshwater insect we know as a water strider.
Researchers have identified 47 species of sea skaters in the world’s oceans. Most scoot around on the ocean surface near the shoreline. At least five species, however, thrive in the warm open waters of the Atlantic, Pacific and Indian oceans.
Sea skaters are widespread because they glue their eggs on things that float or fly. These days, the most common drifters are pieces of plastic, but skaters also lay eggs on natural objects such as cuttlefish shells, driftwood and bird feathers. Given the chance, sea skater females also attach their eggs to the tail feathers of live seabirds resting on the water.
Females lay 10 to 20 eggs at a time, an astonishing number since sea skater eggs are about a fifth the size of the female. One egg is about 1 millimeter, or about one-twenty-fifth inch, in diameter. Female skater bodies are about 5 millimeters long, a little less than one-fourth inch, and males are even smaller. (Pond and lake water striders are much bigger, about 1 1/2 inches long.)
No wonder I’ve never see these insects at sea. But I have seen their prey, either on calm days offshore or, when it’s windy, on Hawaii’s beaches. These are Portuguese men of war, by-the-wind-sailors and blue buttons, all wind-driven jellyfish-type creatures that spend their lives drifting on the ocean’s surface. Sea skaters also eat tiny fish eggs, fish larvae and crustaceans that get caught in the film at the ocean’s surface.
Like mosquitoes, sea skaters use a pierce-and-suck technique to eat. (With their long legs, sea skaters look somewhat like mosquitoes, too.) The tip of a skater’s hollow feeding spear contains sensory hairs that help the insect find a suitable spot to stab. At the base of the spear, serrated points act as anchors to hold the mouthparts in place. Once settled in, the insect spits enzymes into its prey, which liquefies its tissue. The skater then slurps up its meal and moves on.
Or not. Blue noddies, Bulwer’s petrels, gray-back terns (the cowfish eaters) and Bonin petrels manage to find and eat the minuscule sea skaters.
Maybe once, one of those foraging seabirds also ate a mud dauber.