Invertebrates
Water Boatman
Corixidae · also called water boatman, lesser water boatman
A water boatman is the calm, algae-grazing cousin of the fierce backswimmer. Where the backswimmer rows upside down and stabs its prey, the water boatman rows the right way up and grubs quietly on the bottom for algae and ooze. It looks harmless, and it is — but one two-millimetre species holds an outsized record: relative to its size, it is the loudest animal alive.
Overview
Water boatmen are the water-bug family Corixidae, more than 500 species in still and slow fresh water worldwide. They swim belly-down, rowing with hair-fringed oar-like hind legs, and feed mostly on algae, diatoms, and organic detritus sucked up with a short beak — a few also take tiny prey. They breathe air, carrying a silvery bubble underwater and rowing hard to stay down.
The loud one
The male of Micronecta scholtzi, a water boatman about 2 mm long, sings by rubbing a ridge on its abdomen against its body, reaching nearly 100 decibels at close range. Scaled to body size, it is the loudest sustained sound known in any animal, and much of it is lost crossing from water to air, so the true output is higher still. Passers-by on a riverbank can hear it.
Everyday abundance
Water boatmen fly well and are among the first colonists of any new pond, water butt, or even a swimming pool, whose flat reflective surface they mistake for water. They can be enormously abundant, and they are a major food for fish, ducks, and wading birds. In central Mexico their egg masses are harvested as ahuautle, an ancient food sometimes called Mexican caviar.
Status
Least Concern — abundant, widespread, and tolerant even of brackish and saline water. Only a few salt-lake specialists could be at local risk.
Water Boatman: common questions
What is the difference between a water boatman and a backswimmer?
They are related water bugs that are constantly confused, but they differ in three easy ways. Orientation, in that a water boatman swims the right way up and a backswimmer swims upside down. Diet, in that a water boatman mostly eats algae and detritus and is harmless while a backswimmer is a predator that hunts other animals. And bite, in that a backswimmer will give you a painful, bee-sting-like jab if handled while a water boatman does not bite people. In colour, water boatmen are usually flatter and finely barred brown, while backswimmers often have a pale, keeled back.
Do water boatmen bite?
Not people, in practice. Water boatmen feed with a short beak adapted for sucking up algae, plant juices, and fine organic matter, and while a few species take small prey, they are not equipped or inclined to bite a person, and handling one is harmless. This is a key difference from the backswimmer, its upside-down relative, which is a predator and will readily stab a finger. If a water bug in a pond bites you, it was almost certainly a backswimmer or a much larger water bug, not a water boatman.
How can such a tiny insect be the loudest animal for its size?
The record belongs to Micronecta scholtzi, a water boatman only about 2 millimetres long. The male sings to attract females by drawing a tiny ridge on his abdomen across a patch of his body, a process called stridulation, similar in principle to a cricket. The sound reaches close to 100 decibels measured at a short distance, roughly as loud as a passing train, and much of it is lost passing from the water into the air, so the insect is producing even more at the source. Scaled to its body size, no other animal makes a sustained sound that loud.
Why do water boatmen turn up in swimming pools?
Adult water boatmen fly well and disperse by air, often at night, searching for new water. They find water partly by sight, homing in on flat, reflective, horizontally polarised surfaces, and a swimming pool looks exactly like a large, inviting pond from the air. They land, discover the water is chlorinated and has no food, and either die or fly on. Ponds with algae, garden water butts, and even wet greenhouse floors attract them the same way. They cannot breed in a clean pool, so they are a nuisance rather than an infestation.