Saturday, 26 March 2011

A life on the ocean waves

Fly to Brazil from Europe or Africa and you will pass over thousands of miles of ocean. As you look down on the endless waves you might reflect on life below - perhaps you imagine you see a whale spouting, or a fishing boat trawling for cod. If you've already experienced Brazil and it's teeming millions of insects, you might wonder if there are any of those down there too. In fact, in all the world there are only 5 species of ocean going insects, all from the same genus, Halobates, and only one of these H. micans, is found in the South Atlantic.

Dias & Lopes 2009. Occurrence, distribution and abundance of Halobates micans Eschscholtz, 1822 (Heteroptera, Gerridae) along the southeastern Brazilian coast. Braz. J. Biol. vol 69.

Halobates micans, the Sea Skater, is in the same family as the pond skaters, and you can see why, with it's long legs and antennae. They live in a purely 2 dimensional world, they can't fly and die if immersed under water for long, though they cover themselves with water repellent and are covered with tiny hairs which trap air, so they are very buoyant. Instead of flying or diving they skate across the sea surface at up to 1m per second, hunting for prey. Well, very small prey, as their bodies are only about 5mm long, so plankton, or perhaps fish larvae trapped on the water surface. The weak link is egg laying, for which they need a solid object. This can be hard to find in the open ocean and they will use anything, really anything, seaweed, dead jelly fish, lumps of petroleum. A plastic gallon jug was found to have 70,000 eggs attached!

Zoologische Staatssammlung München Halobates micans; Eschscholtz, 1822; Meerwasserläufer, Wikicommons

Sea skaters seems to be fairly common, though not near the coast, or too far south where the water is cold. But why does he have the whole south Atlantic to himself? After all there are plenty of insects living in fresh water. Well, salinity is a problem, although the salt marsh mosquito, for example, can tolerate 3x this level. Worse maybe is the problem of breathing. Insects need access to air, at least occasionally, and the rough and tumble of the ocean may drive their little bodies too deep into the ocean for them to return. This is maybe why the only insects there are restricted by super-buoyancy to the surface.

The ocean can, from above, appear like a desert, but for Halobates it's actually worse. Most desert animals can burrow into the sand to escape the heat of midday, or predators, but Sea Skaters are stuck on the surface with nowhere to hide. The sun burns down, with levels of UV that would fry most creatures, but Halobates is shielded by a layer of very dark UV protection in it's upper cuticle, which blocks pretty much all UV light. Then there are fish and sea birds attacking from below and above, which Sea Skaters avoid by jumping several centimetres into the air, away from the water surface.

Put all this together and you start to see why, of over 1,000,000 insect species, there is only one in the vastness of the South Atlantic, the little known Sea Skater, Halobates micans.

If you can lay your hands on Antenna, the journal of the Royal Entomological Society, there is an excellent article on Halobates there. Alternatively, there is a good web resource at


Anonymous said...

God, only 5 species! Very surprising!

Rohrerbot said...

That's a really interesting write and I had no idea that there were actually sea skaters. It just shows that certain critters can handle some harsh conditions. Thanks for sharing!