Ozyptila praticola In Tree Trunk Moss On The Tolt River

Site location map. Pin and squares indicate cone and moss
samples, respectively. Blue, yellow and red markers indicate
adult O. praticola, juvenile O. ?praticola, or no O. ?praticola
found, respectively.

Since learning a few months ago that the European crab spider Ozyptila praticola  (Thomisidae) can be found in tree trunk moss, I've been working my way up a few river valleys, sifting moss, in my ongoing search for the edges of O. praticola's local range. I just completed one such river series along Tolt River. Tolt River is a tributary to the Snoqualmie River, and runs along the southern edge of Carnation, Washington in King County.

Artifacts indicate past land
use as home sites.

I visited three Tolt River sites on 12 May, then an additional three sites on 18 May. The sites spanned a distance of approximately 5.3 river miles (squares on map, above). The stretch of the river valley I sampled is lightly peppered with currently occupied homes interspersed with former home sites that are being restored as part of the Tolt River Natural Area salmon habitat protection initiative. As far as I could determine from historical aerial photos on Google Earth, many of my sample sites had homes on them as recently as three to seven years ago.

I had tapped O. praticola from cones in Carnation in 2016 (blue pin in map above), so I knew that the species was in the neighborhood. But I didn't know if it had spread into the more natural habitats found along the Tolt River. Rod Crawford didn't find any O. praticola in 2009 when he collected spiders on the west side of Snoqualmie River at Tolt River–John MacDonald Memorial Park.

From parking lot, a view through a
strip of riparian forest to the Tolt River.

Male O. praticola sifted from
tree trunk moss at first site.

My first, most downstream site was located in a wooded riverside parking area near where Route 203 crosses Tolt River. I sifted about a sweep net full of moss from the trunks of black cottonwood (Populus trichocarpa) and bigleaf maple (Acer macrophyllum) and collected 1 female, 2 male and 40 juvenile O. praticola. Ozyptila praticola was by far the most numerous species present in the sample.

Mossy riparian cottonwoods at third
site, where a home formerly stood.

Male O. praticola sifted from
tree trunk moss at third site

The moss I sifted at the next site upstream produced a dozen juvenile O. probably-praticola, but no adults. Moss at the third site, however, mirrored the first in producing 2 female, 2 male, and 20 juvenile O. praticola.

Mossy maple at final up-
stream site.

Female O. praticola sifted from
tree trunk moss at most upstream
site.

Although young thomisids were present at the fourth and fifth sites, I didn't find any spiders that I could with any confidence identify as O. praticola. But at the final, most upstream site, I sifted 1 female and 2 juvenile O. praticola from moss I harvested from bigleaf maple and black cottonwood trunks.

Clearly, O. praticola is present along the Tolt River for at least the first 5.3 river miles from its confluence with Snoqualmie River. This finding then raises the question, has it spread even farther upstream? If the species is simply following the riparian forest upstream, it seems reasonable to anticipate that it may have spread even farther upstream than where I stopped sampling. However, if it is present at these sampling sites only because it was inadvertently introduced by former homeowners, then it may not have spread upstream beyond the homesteaded areas much if at all. The only way to know for sure is to continue sampling farther upstream. But unfortunately, road access is restricted farther upstream. Anyone have a canoe?

The effort to revegetate former housing sites is under way.