The fishing spiders of the genus Dolomedes are large, semi-aquatic arachnids often found near freshwater bodies such as ponds, streams, and swamps. Their impressive size, with some females reaching a leg span of over three inches, allows them to be formidable predators in their watery habitats. These spiders have developed unique physical adaptations, which permit them to exploit the water’s surface tension for hunting and locomotion. Their ability to prey on aquatic life gives them their common name.
Primary Land-Based Prey
Despite their association with water, fishing spiders maintain a diet that includes a substantial amount of terrestrial and surface-dwelling prey. They are opportunistic hunters, and insects often make up the bulk of their caloric intake. They readily consume various flying and crawling arthropods, including flies, moths, crickets, and even smaller spiders.
A fishing spider will often position itself on the shore or on vegetation overhanging the water, waiting for land-based prey to come within range. When an insect lands nearby, the spider swiftly runs to subdue it. This generalized feeding behavior ensures survival even when aquatic prey is scarce.
The Specialized Aquatic Diet
The diet that gives the fishing spider its name involves small vertebrates and aquatic invertebrates captured from the water itself. This includes various small fish, such as minnows and guppies, which they subdue through a combination of strength and venom. Larger species of Dolomedes are capable of tackling prey that may be several times their own weight.
Beyond fish, their aquatic menu features amphibians like small frogs and tadpoles. They also prey on aquatic insects, including water striders and dragonfly larvae, capturing them either on the surface or after a brief dive.
Hunting Techniques on Water
Fishing spiders employ a sophisticated hunting strategy that hinges on their ability to sense vibrations across the water’s surface. They rest with their front legs extended onto the water, using the surface film as an extended sensory net. Specialized sensory hairs on their legs, called trichobothria, detect the slightest ripples caused by struggling prey or a fish moving near the surface.
When a suitable disturbance is detected, the spider rapidly moves across the water towards the source. Their hydrophobic body hairs trap air, allowing them to walk or run on the water like a water strider, supported by surface tension. For larger or submerged prey, the spider will plunge beneath the surface. The air trapped by its body hairs creates a silvery, temporary air bubble that allows it to breathe and remain submerged for several minutes while subduing its meal. They use their foremost legs, tipped with small claws, to grasp the prey before injecting venom, which is particularly effective against aquatic life.