Biotic Inventory
Documenting Diversity at the Katharine Ordway Natural History Study Area
Dolomedes tenebrosus, the Dark Fishing Spider
Taxonomy
Kingdom: Animalia
Phylum: Arthropoda
Class: Arachnida
Order: Araneae
Family: Pisauridae
Genus: Dolomedes
Species: tenebrosus1
Common names: Nursery Web Spiders, Fishing Spiders, the Dark Fishing Spider
Diagnostic characteristics
Whenever identifying a species of spider, the first
place to go to for a distinguishing characteristic is the eyes. While
markings may be useful for distinguishing species, markings can be very
similar between species and even families, often overlapping.
Especially for a species like this, which exhibits no bright colors or
particularly unusual body plans, determining which family of spider
this sample is dependant on certain morphological features, the eyes
being the primary feature. While marking patterns may be very similar,
eye patterns can be very unique to each family of spider.2
This organism, in particular, has a distinguishing
set of eyes. Initially this sample appears to be a member of the Wolf
Spider family Lycosidae due to its marking pattern. However, upon
closer examination we see that this animal’s eye pattern gives it away.
The Wolf Spider’s eyes are arranged in three rows, with two large eyes
at the top, two large eyes in the middle, and four smaller eyes at the
bottom. This individual’s eyes, on the other hand, are arranged in two
rows, with larger eyes at the top. These larger eyes are key to the
identification process, as only the Dolomedes genus of the Pisauridae
family has eyes that follow this pattern.3
Once narrowed to the Dolomedes genus, other traits
of this organism become more pertinant. The size of this animal is very
small – around 7mm. Generally this is too small for a member of this
family. The males are smaller, however this spider’s palps were too
small for a fully developed male. Also, the marking patterns were
relatively indistinct compared to the fully mature Dolomedes genus
member. As a result, it is reasonable to believe that this organism is
a juvinile.
This animal is specifically a member of the species
Dolomedes tenebrosus due to its marking pattern and where it was found.
Most members of the Dolomedes genus are water or swamp dwelling
spiders. This spider, on the other hand, was found in underbrush a
hundred yards from the Mississippi river. Dolomedes tenebrosus fits
this pattern, as it is an opportunistic feeder often found in dry
wooded areas near a water source.4
Ecology
These animals, when fully grown, live off the water.
Instead of creating webs to catch their prey, members of the Dolomedes
genus are hunters who stalk their meals. They habituate in areas near
freshwater, and so those meals consist mainly of aquatic insects and
small fish. They will even run on the water’s surface, using surface
tension to maintain balance and buyancy. These animals are largely
active during the night, hiding under trees or rocks during the day.
Dolomedes tenebrosus, though, is more adapted to land life than other
members of the same genus. They often stray quite far from water, often
living in wooded settings, and have even been reported in basements and
bedrooms.5 Unlike their other Fishing Spider bretheren, they would be
better classified as a tree-dwelling species.6 Their eating plans are
far less specific than most other members of the Dolomedes genus; they
have even been reported to consume slugs.7
Life History
Considering the life history of these organisms, it
makes sense for this individual to be a juvinile. Dolomedes tenebrosus
goes through a period of hibernation as juveniles, spending their time
hidden under rocks and underbrush. They do not reach maturity until
early May.6 Considering this spider was taken during early April, it
makes sense that it would be a juvenile.
Upon reaching maturity when the weather warms up in
the spring, D. tenebrosus most likely immediately attempts to find a
mate and reproduce. This most likely takes place near a body of water,
however mating behavior has only been observed once. That attempt ended
with the female eating the male.8
Egg casings form in June, and are attached to the underside of the
female until the underlings are ready to hatch. Egg casings are large
compared to the organism, about 14 mm in diameter, and can contain up
to 1400 baby spiders. Days before the eggs hatch, the female attaches
the sac to some plant matter, and builds a nursery web around it. The
hatchlings then live in the nursery web for some time afterwards, with
the mother spider keeping guard of the web.4
Distribution
These spiders are unique to North America, but they
have a very large range across the eastern half of the continent. D.
tenebrosus has been found as far south as Florida and as far north as
southern Canada, showing the wide variety of temperatures this species
can survive under. Detailed studies have been performed in the New
England states, particularly Connecticut, though this species has been
documented as far west as Texas and the Dakotas. While there is nothing
in the literature showing identificiations in the state of Minnesota,
these spiders have been documented by amature entomologists in
southeastern Minnesota.9 The discovery of a juvenile specimen at the
Ordway Nature Preserve does establish the ability of these spiders to
breed at these latitudes.
Voucher Information
This organism was found hidden in the underbrush of
the Ordway Nature Preserve, roughly a football field away from the
shore of the Mississippi river. Its larger environment was forested
with a wide variety of oaks and birch trees.
References
1. Species Dolomedes tenebrosus. 2010. http://bugguide.net/node/view/2011
2. Myths about Identifying Spiders. 2003. http://www.washington.edu/burkemuseum/spidermyth/myths/easy.html
3. Spider Eye Arrangements. 2010. http://bugguide.net/node/view/84423
4. The Arthropod Museum: Dark Fishing Spider. 2003. http://www.uark.edu/ua/arthmuse/dolomede.html
5. Carico, J. E. 1973. The Nearctic species of the genus
Dolomedes (Araneae: Pisauridae). Bulletin of the Museum of Comparative
Zoology 144 (7): 435-488.
6. Fishing Spider. 2006. http://ento.psu.edu/extension/factsheets/fishing-spider
7. Kissane, K. C. 2001. Slug eating by the pisaurid spider
Dolomedes tenebrous Hentz, 1843. British Arachnological Society
Newsletter 91: 4-5.
8. Sierwald, P., and J. A. Coddington. 1988. Functional aspects
of the male palpal organ in Dolomedes tenebrosus, with notes on the
mating behavior (Araneae, Pisauridae). Journal of Arachnology 16:
262-265.
9. Large Spider in Rural Southeast Minnesota. 2005. http://bugguide.net/node/view/21350/bgimage
Compiled by Joe Lalli.
Biodiversity & Evolution (BIOL 270), Professor Sarah Boyer. Spring 2010.
Specimen collected at Macalester College's Katharine Ordway Natural History Study Area on April 15, 2010.