Fishing spiders, primarily of the genus Dolomedes, are large, robust, semi-aquatic predators known for their unique relationship with water. Some females can reach impressive sizes, with a body length up to 26 millimeters and a leg span exceeding 80 millimeters. Their specialized anatomy allows them to walk effortlessly across the water’s surface. They hunt by detecting vibrations, a method that gives them their common name. Their survival depends entirely on the presence of suitable freshwater bodies.
Geographic Range: A Global View
The Dolomedes genus is widespread, with approximately 100 species documented across the globe, inhabiting nearly every continent except Antarctica. This distribution highlights the genus’s adaptability to various climates, provided a freshwater source is present. The greatest species diversity is concentrated in Asia, particularly Southeast Asia, where numerous forms have evolved in tropical ecosystems.
North America is home to several well-known species, including the widespread Six-spotted fishing spider (D. triton), found from Canada down to the Gulf Coast. In Europe, species such as the great raft spider (D. plantarius) occupy fens and bogs. South America has a notably lower diversity, with only a few species recorded globally.
Defining the Semi-Aquatic Environment
The environment Dolomedes spiders prefer is defined by the presence of fresh, calm water, which is essential for their unique hunting strategy. They are most commonly found near the edges of still or slow-moving water sources, such as quiet ponds, lake margins, marshes, bogs, and sluggish streams. The lack of strong currents is important because their primary hunting tool is the detection of surface ripples created by prey, a sensitivity diminished in fast-flowing water.
These spiders use microscopic hydrophobic hairs on their legs to leverage the surface tension of the water, allowing them to glide across it. When a ripple is detected, the spider runs across the water to ambush the source, which can be an aquatic insect, tadpole, or small fish. If threatened or pursuing deeper prey, they can dive below the surface and remain submerged for extended periods, sometimes up to 45 minutes. They achieve this by trapping a silvery film of air around their bodies for respiration.
Specific Shelters and Resting Spots
While the water surface is their hunting ground, fishing spiders rely on specific micro-habitats within the shoreline for resting, molting, and reproduction. They seek out dense, complex structures that provide camouflage and protection from predators. These shelters often include tangled shoreline vegetation, such as reeds and tall grasses, as well as floating debris, logs, and flat rocks that overhang the water.
For instance, the Dark fishing spider (D. tenebrosus) is frequently found on tree trunks, in rock piles, or beneath loose bark in wooded areas near water. The most specialized structure is the nursery web, which a female constructs when her eggs are ready to hatch. She suspends her large egg sac within a protective tent made of silk and woven leaves or grass, located above the water line to prevent flooding. The mother remains nearby, guarding the nursery web until the spiderlings emerge and disperse.