Six-spotted Fishing (Dolomedes triton)

Physical description and identification

A six-spotted fishing spider on open water is unmistakable: a large, long-legged spider sitting dead still on the surface with the front three pairs of legs spread wide and the rear pair anchored to a leaf or stem. Out of the water, among reeds, it is a handsome brown spider with two bold pale stripes running the whole length of the body.

Adults

• Size: females measure 15–20 mm in body length, males 9–13 mm — roughly half again as long as the male, and noticeably heavier. With the legs extended a large female covers 60–70 mm, about the width of a drinks coaster.
• Colour: brown to brownish grey overall, with a broad white or pale cream stripe along each side of the carapace that continues down the sides of the abdomen. The upper surface of the abdomen carries roughly a dozen small pale spots and short white dashes, and the legs are often faintly banded.
• The six spots: a name that misleads almost everyone. The spots are not on the spider's back but on its underside — six dark marks on the sternum, the plate between the leg bases. You have to turn the spider over, or photograph it from beneath a glass surface, to see them.
• Eyes: eight eyes in two rows of four, the rear row strongly curved backwards, none of them enlarged. Combined with the long, evenly tapering legs this separates the species at a glance from the wolf spiders that share the same shoreline.
• Distinctive features: the legs and body are clothed in short, dense, water-repellent hairs that trap air. These allow the spider to stand on the surface film without breaking it and to carry a silvery air layer when it submerges.

Eggs and the egg sac

Mating and egg production run through the warm months, most often from June to September. The female spins a large, pale spherical sac — conspicuously big compared with her own body — and carries it clamped in her jaws, held clear of the ground by her pedipalps. She cannot feed during this period, which can last two to three weeks.

Spiderlings

A few days before hatching she fastens the sac to emergent vegetation and builds a dense nursery web around it, then stands guard on the outside. The spiderlings emerge into that tent, moult once, and stay about a week before dispersing along the shoreline. Part-grown juveniles overwinter under bark, in bark crevices, under stones and in leaf litter, and reach maturity the following summer.

Hunting instead of a web

No member of this family builds a snare, and the nursery web is the only silk structure in the species' life. Instead the spider treats the water surface as its web: it rests its front legs on the film, and ripples from a struggling insect travel to those legs exactly as vibrations travel along the radius of a true web. Our guide to spider webs explains the contrast with snare builders. When prey registers, the spider sprints across the surface, seizes it and carries it back to a stem or stone to feed.

Where the six-spotted fishing spider lives

Dolomedes triton is a New World species with an exceptionally wide range. It occurs from southern Canada — as far north as the boreal lake country — south through virtually every US state, including Florida and the arid south-west wherever permanent water exists, and on into Mexico, Central America and parts of northern South America. It is one of the most widespread large spiders in North America.

Habitat is narrower than range. The species needs still or slow-moving fresh water with emergent vegetation: pond and lake margins, marshes, swamps, ditches, beaver ponds, boat docks and the quiet edges of slow streams. Cattails, sedges, water lilies and overhanging grasses give it the anchor points it uses while fishing. It rarely strays far inland, which sets it apart from the mainly terrestrial dark fishing spider of the same genus, and it is active in daylight rather than after dark.

Are six-spotted fishing spiders venomous?

Yes. Like almost all spiders it produces venom and uses it to immobilise prey, and in this species the prey can include animals with backbones, so the venom is effective against small vertebrates. That has no bearing on people: the quantity is tiny, the compounds are not adapted to mammals, and there is no antivenom because none is needed. The species is not related to the recluse or widow spiders and has never been linked to tissue damage or systemic poisoning in humans.

Can six-spotted fishing spiders bite?

They can, but they seldom get the chance. A six-spotted fishing spider that senses an approaching person runs across the water, drops beneath the surface or slides behind a stem, and that escape response happens long before biting becomes an option. Bites are recorded almost exclusively from spiders that were handled or pressed against skin.

Where a bite does occur it produces sharp local pain of about the intensity of a bee or wasp sting, with redness and mild swelling that clear within a few hours to a day. Clean the site with soap and water, apply a cold pack, and use a simple painkiller if you need one. Get medical advice if pain is severe or worsening, if swelling spreads beyond the immediate area, or if hives, dizziness or breathing trouble appear — those indicate an allergic reaction, not venom toxicity.

Ecological importance and behavior of the six-spotted fishing spider

This is one of very few spiders that hunts at the boundary between two ecosystems, and it does so with a set of specialised movements. It walks on the surface film like a pond skater, rows with its legs to cross open water at speed, gallops when startled, and even sails: by raising its front legs and abdomen it turns its body into a sail and lets the wind carry it across a pond. Vertical jumps off the surface are used to dodge attacks from below.

Diet: mostly aquatic and semi-aquatic insects — water striders, mayflies, damselflies, beetles, midges and anything that falls in — plus tadpoles, small frogs and small fish. Fish predation by Dolomedes is genuine and well documented, though it is opportunistic rather than the daily routine the name suggests. Prey is often larger than the spider itself. See our guide to what spiders eat.

Diving: when threatened the spider walks down a stem and submerges, held under by gripping vegetation because its air-trapped hairs make it buoyant. It can stay down for half an hour or more, breathing from the silvery film of air held against its body.

Natural predators: herons, kingfishers and other waterbirds, frogs, snakes, dragonflies and predatory aquatic insects, along with parasitoid and spider-hunting wasps. Fish take spiders that lose their grip.

Reproduction: females are famously dangerous to their suitors. Unlike its relative the dark fishing spider, in which the male dies automatically at mating, cannibalism here is driven by the female and often happens before mating rather than after, so a courting male approaches with great caution and retreats repeatedly.

Seasonal behaviour: adults are seen from late spring through early autumn; juveniles overwinter in sheltered crevices, as described in our guide to where spiders go in winter. Females live about a year to eighteen months; males die soon after mating.

Relationship with humans: entirely benign. Because the species needs clean, insect-rich water, a healthy population beside a pond is a fair sign that the water body itself is in good condition.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The six spots that give the spider its name are on its belly, not its back. The pale marks most people count on the upper abdomen usually number about twelve.
• The species name triton refers to the Greek sea god, a fitting label for a spider that walks on water.
• A six-spotted fishing spider can hold its position underwater for more than thirty minutes. It must grip a stem to do it: the air trapped in its hair coat makes it so buoyant that letting go pops it straight back to the surface.
• Given a breeze, the spider raises its legs and abdomen and sails across open water — one of the few documented cases of wind-powered travel by an animal that is not flying.
• The whole nursery web spider family is named for the maternal silk tent, not for a hunting web. None of its members catches prey in silk.

In summary, the six-spotted fishing spider is a large, striped, daylight hunter of North American pond margins that turned the water surface into a web and added diving, rowing and sailing to the repertoire. It is harmless to people, it is a useful indicator of clean water, and its famous six spots are hidden on the underside where almost nobody looks.