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| Dolomedes | |
|---|---|
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| Raft spider (D. fimbriatus) female with juvenile spiders | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Subphylum: | Chelicerata |
| Class: | Arachnida |
| Order: | Araneae |
| Infraorder: | Araneomorphae |
| Family: | Dolomedidae |
| Genus: | Dolomedes Latreille, 1804[1] |
| Species | |
| Diversity[1] | |
| c. 101 species | |
| Synonyms | |
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Dolomedes/dɒləˈmiːdiːz/ is agenus of largespiders of the familyDolomedidae. They are also known asfishing spiders,raft spiders,dock spiders orwharf spiders. Almost allDolomedes species aresemiaquatic, with the exception of the tree-dwellingD. albineus of thesoutheastern United States. Many species have a striking pale stripe down each side of the body.
They hunt by waiting at the edge of a pool or stream, then when they detect the ripples from prey, they run across the surface to subdue it using their foremost legs, which are tipped with small claws; then injecting venom with their hollow chelicerae to kill and digest the prey. They mainly eat insects, but some larger species are able to catch small fish. They can also climb beneath the water, when they become encased in a silvery film of air. "Dolomedes" is derived from the Greek wordδολομήδης which means wily, deceitful.[2]
There are over a hundred species ofDolomedes throughout the world; examples includeDolomedes aquaticus, a forest-stream species of New Zealand, theraft spider (D. fimbriatus), which lives in bogs in Europe, and thegreat raft spider (D. plantarius), which lives infens, also in Europe. Many species are large, some with females up to 26 mm (1.0 in) long with a leg span of 80 mm (3.1 in).[3]
Dolomedes spiders are covered all over in short, velvety hairs which arehydrophobic. This allows them to usesurface tension to stand or run on the water, likewater striders. They can also climb beneath the water, and then air becomes trapped in the body hairs and forms a thin film over the whole surface of the body and legs, giving them the appearance of fine polished silver. Like other spiders,Dolomedes breathe withbook lungs beneath their abdomens, and these open into the air film, allowing the spiders to breathe while submerged. The trapped air makes them very buoyant and if they do not hold onto a rock or a plant stem they float to the surface where they pop onto the surface film, completely dry.
If any of this species are seen without context, one may confuse them with the familyLycosidae, otherwise known as wolf spiders. They can be differentiated from wolf spiders by their smaller posterior median eyes, and eye arrangement of eight eyes in two rows as opposed to three rows.[4] If this is insufficient, one can further differentiate them thanks to their aquatic adaptations.
Rather than hunting on land or by waiting in a web, these spiders hunt on the surface of the water itself, preying onmayflies, other aquatic insects, and even smallfish.[5][6] For fishing spiders, the water surface serves the same function as a web does for other spiders. They extend their legs onto the surface, feeling for vibrations given off by prey.
Dolomedes are nocturnal hunters, feeding when birds, their main predators, are sleeping. The method they use to fish for insects is to hold on to the shore with their back legs while the rest of their body lies on the water, with legs stretched out.Dolomedes species tend to be robust with thickset legs that allow them to tackle prey larger than themselves. They stretch out their front legs and wait, as if listening. Their front legs feel the vibrations carried on the water, just as other spiders feel the vibrations in a web. They are able to tell what is causing the vibrations that the water is carrying – to distinguish the drawn-out, erratic vibrations of a struggling insect from the one-off vibrations caused by falling leaves or the background noise of the wind or the flow of the water around rocks and other obstacles. As well as identifying the source of the vibrations, the spiders are also able to discern the distance to and direction of the source. To this end they have a range of vibration-detecting organs, including very sensitive hairs (trichobothria) on their legs and feet. Their eyes play a secondary role; experiments on related species show that touch is the main sense these spiders use to catch their prey. Their eyes are of little use for nocturnal hunting. These vibration detectors also serve to warn the spider of predators such astrout.
As soon as the vibrations reveal that there is a floundering insect within range, some fishing spiders may take direct action – they run at pace across the surface of the water and grab the insect before it extracts itself from the water and flies to safety. Some fishing spiders usesilk draglines to prevent themselves from speeding past the prey.
Fishing spiders' main prey in New Zealand is aquatic insects,[7] but they are opportunistic feeders and will eat anything suitable that happens within range. There are reports in Australia ofDolomedes, including aDolomedes facetus, eatinggoldfish and a report in the United States of aDolomedes consuming an immaturesmallmouth bass, but more common prey may beGambusia species,sticklebacks,Elassoma and other small fish depending on locale.[8]
The main predators of fishing spiders are birds and snakes.Dragonflies have also been observed catching young spiders. Species parasitic on the spiders include a wasp of thePompilidae family, commonly called the Spider Wasp,[9] that stings the spider to paralyze it before carrying it off and laying an egg in its abdomen. The larvae of the wasp hatch and proceed to eat the spider from the inside out. One escape technique the spiders use is to disappear beneath the surface tension of the water. However, some wasps, such asAnoplius depressipes, are able to be underwater for a few minutes to sting the spider and drag it out of the water.[10]
The males outnumber the females 3:1 suggesting a male-biased sex ratio. Mating in one North American species (D. tenebrosus) always results in the obligate death of the male, with no obvious involvement from the female.[11]
As of November 2021[update], theWorld Spider Catalog accepted the following 101 species:[1]
The approximately 100 species ofDolomedes have a worldwide distribution. The largest number of species are found in Asia, with particularly high species diversity in South-east Asia, from China and Japan toNew Guinea. The second largest number of species occur in tropical Africa. South America has only four species.[1][12]
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Nine species ofDolomedes exist in North America.[1] The six-spotted fishing spider (D. triton) lives primarily in small lakes and ponds. This spider consumes mostlywater striders (pond skaters), but like allDolomedes, it is an opportunistic ambush hunter that will eat anything that it can capture. Other species include the bog-dwellingD. striatus, and four species living by streams:D. scriptus,D. vittatus,D. gertschi andD. holti. Two North American species,D. tenebrosus andD. okefinokensis, exhibit female giganticism and/or male dwarfism, with their males being less than half the size of the females. The ninth species is thearborealD. albineus.
TwoDolomedes species occur inEurope (excluding Russia).[1] ThePalearcticraft spider (D. fimbriatus) is widespread on the surface ofbog pools and in boggygrassland.[13] Thegreat raft spider (D. plantarius) lives infens, and is listed asendangered inGreat Britain and is globallyvulnerable.[14]
Four endemic species ofDolomedes occur inNew Zealand, three on the mainland and one on theChatham Islands.[15] Two are widespread:D. aquaticus of open riverbanks and lakeshores, andD. dondalei or New Zealand forest fishing spider (once referred to asDolomedes III), which specialises in forested riverbanks.[7] The largest New Zealand species,D. schauinslandi or the Rangatira spider, occurs on rodent-free islands in the Chathams where running water is rare. The fourth and most common species,D. minor, is found in scrubland, grassland, and wetlands. It mostly hunts on the ground, but is still capable of catching aquatic prey. Known as the nursery web spider, it makes white nursery webs on shrubs.[15]
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