Evolution of water surface locomotion by spiders: A comparative approach

Spiders vary enormously in their behaviour when placed on the surface of fresh water. In some families (e.g. Theridiidae), the spider typically becomes wet and either sinks or is incapacitated by adhesion to the water. In other families (e.g. Agelenidae), the spider remains dry and moves across the water using its legs in much the same way it does on land, with the members of each leg pair moving in alternation with each other. In at least one family (Pisauridae), the spider remains dry and moves across the water using a rowing or galloping gait in which the members of each propulsive pair of legs move in synchrony with each other. While some degree of hydrophobicity is widespread among spiders, the ability to move on water by rowing occurs rarely; it is common only among families in the Lycosoidea, which is a subset of the GST (Grate-Shaped Tapetum) clade. Our mapping of water surface locomotion behaviour of representatives of 42 families of spiders onto cladograms of the Araneae suggests that the ability to row evolved at the base of the clade that includes Trechaleidae, Pisauridae and Lycosidae and evolved independently in some members of the family Ctenidae. Rowing behaviour is seen in all subfamilies of Lycosidae but, unlike in the Pisauridae in which all animals tested showed the rowing behaviour, many individuals that could row did not do so all of the time. Among the 166 non-lycosoid species we have tested, we have found one species of Araneidae and two species of Salticidae that can row. It is evident from our data that, in most spiders, phylogeny trumps recent selection (based on habitat preference) in determining the spiders' locomotor behaviour on the water surface.