Dock spiders, members of the genus Dolomedes, are large arachnids often associated with aquatic environments. These fascinating spiders are identifiable by their brownish-grey coloration, frequently displaying black and brown markings on their abdomens and striped legs. They possess two rows of eyes and can reach a substantial size; female dock spiders can have a body length of up to 26 millimeters, with a leg span extending to 100 millimeters (about 4 inches), making them among the largest spiders in certain regions like Canada. Their common names, such as fishing spiders or wharf spiders, clearly indicate their preference for living near water.
Their Aquatic Homes
Dock spiders are most commonly found in habitats directly connected to water bodies. Their namesake locations, such as docks, piers, and boat launches, are prime areas where these spiders thrive. They frequently inhabit the edges of freshwater environments, including lakes, ponds, slow-moving rivers, streams, marshes, and reservoirs. Within these aquatic settings, dock spiders utilize specific structures for shelter and hunting. They can be found on pilings, floating debris, and rocks, or concealed within overhanging vegetation directly over or adjacent to the water. Dock spiders often remain motionless on these surfaces, poised to detect vibrations from potential prey.
Beyond the Water’s Edge
While their primary habitats are aquatic, dock spiders are not exclusively confined to the immediate water’s edge. They can also be observed in vegetation bordering water bodies, such as reeds, cattails, and other shoreline plants. This allows them to expand their hunting grounds and find additional cover.
Occasionally, these spiders may venture into human-made structures located close to their preferred watery environments. This includes boathouses, sheds, or even homes, and sometimes swimming pools. They may seek warmer winter shelter in autumn, including under stones, leaves, bark, or inside buildings. Geographically, dock spiders are widely distributed across North America, inhabiting the contiguous United States and southern Canada, and are also found in parts of Europe. In Canada, they are common in provinces like Ontario, Manitoba, and Quebec.
Adaptations for Their Environment
Dock spiders possess several biological adaptations that enable them to thrive in their semi-aquatic environments. One notable ability is walking across the surface of water. This is possible due to surface tension and their specialized, widely spaced legs covered with water-repelling, waxy hairs. Their weight is evenly distributed across their eight legs, preventing them from breaking the water’s surface tension.
These spiders are also capable of diving underwater, a behavior used both for hunting prey and escaping predators. When submerged, they trap an air bubble, which appears as a silvery film, around their abdomen and legs. This trapped air allows them to breathe while underwater, and they can remain submerged.
Dock spiders do not typically spin webs to catch prey; instead, they are ambush hunters. They detect vibrations on the water’s surface using their sensitive legs, then swiftly move to capture their meal. Their diet primarily consists of aquatic insects, small fish, tadpoles, and frogs, and they are known to consume prey up to five times their own size.