Like almost all riffle beetles, spider water beetles are aquatic, feeding on algae and decaying wood tissue. However, they can not actively swim. They can be found crawling along or clinging with their claws on boulders or submerged wood in lotic riffles of streams and rivers.
The larvae are exclusively aquatic. They breathe by means of tracheal gills. Spider water beetle adults, like all members of the subfamily Elminae, can also remain indefinitely underwater by means of a plastron, a thin film of gas trapped by hydrophobic bristles (setae) on their body. As the insect breathes, the oxygen concentration in the gas film drops in comparison to the surrounding water, causing new oxygen to diffuse again into the plastron.
Because of their reliance on the plastron for breathing, spider water beetles are restricted to the highly oxygenated environments in moderate to fast-moving permanent running water. They are therefore extremely sensitive to water pollution and are potentially valuable bioindicators for measuring the health of river ecosystems.
Members of the Ancyronyx variegatus species group are mostly found in slightly to moderately polluted (mesosaprobic) rivers, almost always found on submerged wood (with the exception of Ancyronyx yunju which were collected from sand and grass roots). Members of the Ancyronyx patrolus species group, meanwhile, are only found in clean permanent streams, usually among rocks.
Ancyronyx malickyi have been caught using light traps, which might indicate phototaxis.