Where Insects Fear to Tread

By Josie Glausiusz|Saturday, March 01, 1997
About five-sixths of known animal life is made up of insects. They flourish almost everywhere, from the Antarctic to the Arctic, in caves, lakes, deserts, and rain forests, in hot springs, and even in pools of petroleum. But oddly enough, not in the ocean.

Why not? Jeroen van der Hage, a physicist at Utrecht University in the Netherlands, thinks he may have found an answer. There are few marine insects, he says, because there are almost no flowering plants in the sea. And because the two have evolved together, the absence of flowers made life in the sea impossible for insects.

It’s not as if insects are completely averse to life in water. Some 3 to 5 percent of all insect species live in lakes and rivers--and some have even adapted to the salinity of salt marshes. Yet almost none live beneath the surface of the open sea. A rare exception is Pontomyia, a midge that lives as a larva submerged in Pacific tide pools, but even this unusual insect must emerge--for a brief hour or two--to mate and lay eggs before dying. Five species of water strider skate around the surface of the open ocean and lay eggs on floating debris. And some coastal insects live on sand and seaweed. But none of these species are fully marine.

Previous attempts to explain the dearth of marine insects have all been unsatisfactory, says Van der Hage. Some theories have suggested that physical barriers--waves and salt--prevented an insect invasion; others proffer that predatory fish were a deterrent. Van der Hage points out that such obstacles have not hindered other arthropods, such as arachnids--400 different sea spiders and many mites live happily in the sea.

While spiders and mites thrive, flowering plants, or angiosperms, don’t. The vast majority of plant life in the ocean consists of simple plants like single-celled green plankton and seaweed that lack true leaves, stems, or roots. There are only about 30 marine angiosperms, and all live in coastal regions.

The reason flowering plants, which evolved on land, have been unable to colonize the sea, says Van der Hage, has to do with the movement of particles in a fluid. If a pollen grain is immersed in a fluid of the same density, such as water, then pollen released from an underwater flower will be carried along by water flow. Even if by some chance an animal carried a few pollen grains to a flower’s stigma, flowing water would easily wash them off. But in a fluid such as air, which is a thousand times less dense than water, stigmas can easily capture pollen. That is why flowers are rare underwater.

According to the conventional view, to which Van der Hage subscribes, insects as a group languished for some 250 million years, eking out an existence foraging in detritus. But when flowering plants appeared some 115 million years ago, the fortunes of insects changed dramatically. They exploded across the planet, developing a variety of specialized mouthparts for feeding on pollen and nectar, until most became dependent on some flower for survival. And those insects that didn’t feed on flowers most likely fed on insects that did. Since flowering plants failed to colonize the ocean, insects, says Van der Hage, remained landlubbers.

Unfortunately, his argument fails to convince Smithsonian paleobiologist Conrad Labandeira. Some years ago, Labandeira advanced the idea that insects diversified long before the advent of flowering plants, evolving specialized mouthparts to feed not on flowers but on ferns, cycads, conifers, and other more primitive plants.

Labandeira explains the oceans’ lack of insects very simply: There are no trees in the sea. An average tree contains a multitude of habitats for insects: roots, bark, strengthening tissues, seeds, leaves. By comparison, seaweed often consists of just a few spongy leaflike tissues. What gives terrestrial ecosystems such a unique habitat for insects is the architectural diversity of plants, says Labandeira. In the ocean, that diversity is not there.
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