Spider Sunday: Six-spotted Fishing Spider

Among my most favorite spiders are the fishing spiders in the genus Dolomedes, family Pisauridae. They are elegant ambush hunters, and perhaps none more so than the Six-spotted Fishing Spider, Dolomedes triton. This species is widespread in North America, but not seen that frequently, being especially scarce in the Midwest and Rocky Mountains.

These are not small spiders, mature females attaining a body length of 17-20 millimeters, and males 9-13 millimeters. Their extensive legspan makes them appear larger still. They can vary considerably in color and pattern from one individual specimen to another, but the “standard” form sports two rows of white spots on the abdomen. They don’t always add up to the spider’s namesake six, though.

Fishing spiders are famed for their feats of predatory strength in killing small fish, tadpoles, and frogs. Indeed, D. triton is capable of preying on aquatic vertebrates four to five times the spider’s own weight. The spider reacts to the concentric waves created by a surfacing organism. The spider cannot see well, but its sense of touch more than makes up for any visual deficiency. It is able to pinpoint the epicenter of waves with extraordinary precision, and strike with lightning speed. Still, it rarely goes after large prey, and succeeds less than 10% of the time (unless the prey item actually bumps into the spider, in which case the success rate increases to 16%). Most items on the spider’s menu are terrestrial or aerial insects that get blown onto the water’s surface, or fall from overhanging vegetation.

D. triton can pursue prey underwater, but that is a rare behavior. It is more likely to dive to avoid its own predators, namely the spider wasp Anoplius depressipes. Fine hairs covering the spider’s body trap a layer of air against its breathing holes when the spider goes under. Again, this is usually a last ditch attempt at escape when all else fails.

The spider normally rests motionless on emergent vegetation or floating objects in the quiet water of a pond, lake, or river backwater, maintaining contact with the water surface with the first two pair of legs. This is not to say that the spider can’t propel itself across the surface of the water if it wants to. It can, in fact, do so passively or actively, and slowly or rapidly.

Fishing spiders can raise their front pair of legs into the wind and be blown by the most imperceptible of breezes. This form of locomotion is called “sailing,” and the spider has little control over its speed. The most common form of movement over the water executed by Dolomedes is called “rowing.” The spider actually makes the water surface itself do the work. The tip of each leg creates a dimple on the water surface, and by stroking the third and second pair of legs backwards (in that order), the dimples act as oars, pushing water toward the rear of the spider and moving it forward. This is a rather leisurely mode and speed of travel.

When a fishing spider wants to flee a predator or pursue fast prey, it can shift to “gallop” speed. In this case, the spider abandons the dimple propulsion strategy and simply slices the water rapidly with the tips of its legs. Its body is elevated above the surface when galloping, and at certain points the animal is completely airborne. This method of locomotion can result in astonishing maximum speeds of three feet per second. The spider cannot sustain that velocity for very long, however.

Male D. triton spiders have the unenviable task of convincing females that they are not an appetizer. So, they may initiate courtship by literally being “jerks,” using their legs to generate an unmistakable series of rhythmic surface waves that contact the female spider. Alternatively, he may follow the female’s silken dragline, then engaging in rapid leg-tapping as he approaches her more closely. The two spiders touch legs in an apparent identity-verification dance.

Mated females create a spherical egg sac that they cart around in their jaws and pedipalps until just before the spiderlings are likely to hatch. At that point the mother spider spins a “nursery web” and suspends the egg sac inside. She will then guard the egg sac, and the spiderlings that hatch from it. Once the spiderlings molt again, they will disperse and the female will resume her normal hunting lifestyle.

Sources: Zimmer, Carl. 2000. “Walking on Water,” Natural History, vol. 109, no. 3, pp 30-31 (April, 2000).