Today’s featured spiders for Spider Week are the fishing spiders (also known as raft spiders) in the genus Dolomedes. I suspect the reason they are so often mistaken for brown recluse spiders is that they are (a) brown, and (b) often very large. Brown recluse spiders aren’t particularly large, but folks seem to (erroneously) associate size with danger when it comes to spiders.
Fishing spiders are members of the family Pisauridae, commonly known as nursery web spiders. Female spiders in this family make excellent mothers. They carry their large silken egg sacs around in their chelicerae (jaws) until the spiderlings inside are just about ready to emerge. Presumably this means that females don’t eat at all during this time!
The spider then builds a nursery web in vegetation and suspends the egg sac inside. She stands guard until the spiderlings emerge. They remain in the nursery web for a while, undergoing one moult before setting out on their own.
The large fishing spiders, including Dolomedes tenebrosus and Dolomedes scriptus are also sometimes called dock spiders or wharf spiders. They are typically found on or near water – often on human-made structures.
As their common name suggests, fishing spiders make a living hunting for fish, tadpoles, and aquatic invertebrates. They can walk on water and even sail across the water’s surface either by lifting their front legs, or by standing up on ‘tip-toe’ to catch the wind.
While hunting, fishing spiders typically rest with their back legs on floating wood or vegetation, and their front legs resting lightly on the water’s surface. This way they can detect surface waves on the water, allowing them to locate potential prey. If the spider detects a fish under the water, they use their back legs to push off and dive after it. Dolomedes triton also dives under water when disturbed – this may be a good way to avoid predators such as birds (or a scary human trying to catch them, which is how I first observed this behaviour). And they can stay under water for up to half an hour! They are able to breathe underwater because spider lungs are located on the abdomen, which is covered with fine hairs that trap air, forming a sort of diving bell.
Notes on Identification
Fishing spiders are most likely to be confused with wolf spiders (family Lycosidae). The best way to tell them apart is the eye arrangement. Wolf spiders have three rows of eyes, with the forward-facing pair in the middle row (the posterior median eyes) very large, and the first row of four eyes in a straight line or slightly procurved (curved downwards).
Fishing spiders have only two rows of four eyes each. Both rows are slightly recurved (curving upwards or toward the back end of the spider) and the posterior median eyes are not that much larger than the rest.
References and further reading:
A dedicated mother (with fantastic photos!) by Alex Hyde
Canada’s largest spider by Chris Buddle
Pisauridae: Nursery web spiders by Africa Gomez
Adams, R. J. (2014). Field Guide to the Spiders of California and the Pacific Coast States (Vol. 108). University of California Press.
Bradley, R. A. (2012). Common Spiders of North America. University of California Press.
Carico, J. E. (1973). The Nearctic species of the genus Dolomedes (Araneae: Pisauridae). Bulletin of The Museum of Comparative Zoology, 144:435-488.
McAlister, W. H. (1960). The diving and surface-walking behaviour of Dolomedes triton sexpunctatus (Araneida: Pisauridae). Animal Behaviour,8(1-2), 109-111.
Nyffeler, M., & Pusey, B. J. (2014). Fish predation by semi-aquatic spiders: a global pattern. PLOS ONE, 9(6), e99459.
Suter, R. B. (1999). Cheap transport for fishing spiders (Araneae, Pisauridae): The physics of sailing on the water surface. Journal of Arachnology, 27:489-496.
I’ve seen plenty of (what I believe to be) fishing spiders in the North Maine Woods. On more than one occasion, I’ve reached for a juicy blueberry near a lake in August only to be startled by a mother on her nursery web. They are of impressive size. But I’ve also seen another spider with an interesting swimming behavior that doesn’t seem to coincide with the locomotion methods you described for fishing spiders. I was wondering if you might know what sort of spider stands on the water surface with the outer 4 legs and swims or strokes it’s way along the surface with the inner 4 (pairs on either side are moved together in unison, and stroked like oars). Either way, thanks for the informative post!
I am not aware of any spiders that locomote like that on water – were they water striders perhaps?
No, I’m quite sure it was a spider, and a fairly large one at that. My arachnid taxonomy may be no good, but my ability to identify insects is well-developed (I trained a bit in entomology, and have published on Lepidopteran flight). It took me a bit, but I dug out my old field notes. Since I can’t include an image in this reply, I’ve posted my old notes on my blog at http://www.ratiocination.org/blog/. My handwriting is likely to be impenetrable, but the diagram might be helpful. The body was apparently 1/2 to 3/4 inches long. It was brown with light highlights giving a mottled appearance. Like many of the spiders in your images, legs were light brown with light, transverse bands. Perhaps it simply was a fishing spider!
Very cool observations! Yes I think it must be a fishing spider, and I was wrong about this not being a standard method of locomotion! Here’s a paper that has a diagram of what you describe: http://jeb.biologists.org/content/jexbio/200/19/2523.full.pdf
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