Fishing (Dolomedes)

Physical description and identification

Dolomedes is a genus of large, long-legged hunting spiders in the family Pisauridae, the nursery web spiders. Around a hundred species are currently recognised. They share an unmistakable build — a low, streamlined body, powerful legs held out to the sides, and a dense pile of water-repellent hairs — and a habit no other big spider group matches: hunting on and under fresh water.

One point in older accounts needs correcting. The genus is not mainly Asian. Diversity is high in Asia, but Dolomedes occurs on every continent except Antarctica, with well-known members in Europe, Africa, the Americas and New Zealand. Australia's large fishing spiders, once placed here, have largely been moved to related genera such as Megadolomedes and Ornodolomedes.

Adults

• Size: most species are large. Females of the biggest reach about 26 mm in body length with a leg span near 90 mm (3.5 in) — not the 1.2 cm span that some sources print, which is a unit error. Males are consistently smaller and lighter, often barely half the female's length.
• Colour: camouflage tuned to the habitat. Bark and rock dwellers are mottled grey and brown; reed and marsh species carry crisp pale stripes along the sides of the body; the arboreal D. albineus can be mossy green. Most species have banded legs and a row of pale chevrons or spots on the abdomen.
• Eyes: eight eyes in two rows of four, the rear row strongly recurved, with no enlarged pair. That is the fastest way to rule out wolf spiders, which have two big forward-facing eyes.
• Distinctive features: three claws per foot for gripping wet stems, stiff bristles on the legs that spread the spider's weight over the surface film, and a body hair coat that traps a layer of air underwater.

Eggs and egg sacs

Females of the whole genus build one large spherical sac holding several hundred to well over a thousand eggs and carry it clamped in the chelicerae, cradled by the pedipalps. A carrying female cannot feed. This is a reliable field character, because a wolf spider tows her sac on the spinnerets instead.

Spiderlings

A few days before the eggs hatch the mother fastens the sac into vegetation and spins a dense nursery web around it — the structure that gives the family its name — then guards it from outside. Spiderlings emerge, moult once inside the tent, and disperse after roughly three to seven days, on foot or on short silk lines. Most species overwinter as part-grown juveniles and mature the following year; our guide to the spider life cycle sets out the stages.

The web

No Dolomedes builds a capture web. The nursery tent is a crèche, not a trap. In place of silk the spider uses the water: it rests its front legs on the surface and reads the ripples an insect makes, exactly as an orb weaver reads its radii, a comparison our guide to spider webs explains in more detail. Species living away from water do the same thing on bark, leaf litter or a wall.

Where fishing spiders live

Dolomedes is close to cosmopolitan. Species occur across Asia, Europe, Africa, North and South America and New Zealand, in climates from boreal bog to tropical swamp. Two European species are widely known: D. fimbriatus, the raft spider of acid bogs and wet heath, and D. plantarius, the great raft spider of lowland fens, which survives at only a handful of sites in the United Kingdom. New Zealand has D. minor, whose conspicuous nursery webs in gorse and scrub are a familiar summer sight, and the rare Rangatira spider D. schauinslandi on the Chatham Islands.

In North America the well-known species are the six-spotted fishing spider of pond and marsh margins, the striped fishing spider of streams and rivers, and the large swamp species Dolomedes okefinokensis of the southeastern coastal plain.

The usual habitat is the margin of still or slow fresh water with emergent plants to anchor to: ponds, lakes, marshes, bogs, ditches, docks and quiet stream edges. But the genus is not uniformly aquatic. The white banded fishing spider is an arboreal specialist of southeastern swamp forest, and the dark fishing spider lives mainly in woodland and buildings, often far from any water at all.

Are fishing spiders venomous?

Every species in the genus is venomous in the ordinary sense — venom is how a spider kills prey — and because some fishing spiders take tadpoles and small fish, the venom works on small vertebrates. None of that makes them a danger to people. No Dolomedes anywhere in the world is considered medically significant, no antivenom exists or is needed for the genus, and no death has ever been attributed to one. Fishing spiders are unrelated to the recluse and widow groups and carry none of the necrotic or neurotoxic components that make those spiders a real medical problem.

Can fishing spiders bite?

They can, but almost never do. A fishing spider's response to a large animal is to run, drop into the water or slip beneath the surface, and it will exhaust those options long before it considers biting. Recorded bites come from spiders that were grabbed, sat on or pinned inside clothing, and the largest females are the only ones with fangs long enough to break adult skin easily.

The result is a sharp, localised pain broadly like a bee sting, with redness and slight swelling that fade within a few hours. Wash the site with soap and water, apply a cold pack, and take a simple painkiller if needed. Seek medical attention if the pain is severe or increasing, if swelling spreads well beyond the bite, or if hives, dizziness or difficulty breathing develop, since those point to an allergic reaction rather than to the venom itself.

Ecological importance and behavior of fishing spiders

Fishing spiders hunt at the boundary between water and air, a niche almost no other spider exploits, and they have a remarkable set of movements for it. They walk on the surface film; they row, driving dimples into the water with the middle legs to cross open stretches at speed; they gallop when startled; and in a breeze they raise their front legs and abdomen and sail downwind like a dinghy.

Diet: mainly aquatic and shoreline insects — water striders, mayflies, damselflies, beetles and anything that falls in — plus tadpoles, small frogs and small fish, often heavier than the spider itself. Terrestrial members of the genus take crickets, moths, beetles, slugs and other spiders instead.

Diving: a threatened fishing spider walks down a stem and submerges, breathing from the silvery film of air trapped in its hair coat. That air makes it buoyant, so it must grip vegetation to stay under, sometimes for half an hour or more.

Natural predators: waterbirds, frogs, snakes, lizards, shrews, dragonflies, fish and spider-hunting wasps; ants and parasitoid wasps attack the egg sacs.

Reproduction: sexual cannibalism is common in the genus, but the popular claim that every male is eaten is wrong, and the mechanism varies. In D. triton females often kill males before mating takes place. In D. tenebrosus the male dies spontaneously the instant he inserts a pedipalp, a genuine, automatic death that the female does not cause; she usually eats the body afterwards. In other species males survive and mate more than once.

Relationship with humans: harmless and quietly useful. Fishing spiders need clean, insect-rich water, so their presence around a pond is a reasonable sign of a healthy wetland. Where they enter houses, as the dark fishing spider does, they are easily caught under a glass and released.

Species in the genus and similar spiders

Quick facts

Did you know?

• The name comes from the Greek dolomēdēs, meaning wily or crafty, from dolos, guile. It is sometimes wrongly linked to Diomedes, a hero of the Iliad, but the two words are unrelated.
• A submerged fishing spider looks silver because it breathes from a film of air held against its body by water-repellent hairs, on the same principle as a diving bell.
• A worldwide survey of spiders eating fish found records on every continent except Antarctica, with Dolomedes accounting for more of them than any other genus.
• Dolomedes plantarius is one of the very few spiders in Britain with legal protection, and captive-bred spiderlings have been released to re-establish it at former fenland sites.
• New Zealand's Dolomedes minor breaks the family's own habit by building its nursery webs conspicuously in gorse and shrubs, well away from water.
• The genus builds no snare, so despite the family name, no fishing spider has ever caught a meal in silk.

In summary, Dolomedes is the group of spiders that solved the problem of hunting without a web by borrowing one: the surface of the water. Its hundred or so species are large, fast, superbly waterproof and, for all their size, entirely harmless to people.