Ozyptila praticola Found In Tree Trunk Moss

Deliciously shaggy moss on
alder trunks, Cultus Mtn.

Last week, while collecting spiders with Rod Crawford on Cultus Mountain (Skagit County), I decided to sift some of the luxuriant moss growing on the trunks of alder trees there. One of the spiders I found therein was a male Ozyptila pacifica, a native thomisid. This caught my attention because I had found a female O. pacifica in cascara tree trunk moss near Vader (Lewis County) the previous spring, which happened to be the only other time I had ever sifted tree trunk moss.

Readers of this blog will know that I've used a variety of sampling methods in my search for the introduced thomisid Ozyptila praticola in western Washington. These include tapping fallen conifer cones, sifting leaf litter, placing pitfall traps, and deploying various types of cardboard live traps in shrubs and on tree trunks. But it wasn't until finding that male O. pacifica last week that it occurred to me to search for O. praticola in nature's ready-made live trap: tree trunk moss. Tree trunk moss isn't common in the urban areas I've been searching for O. praticola, but it can sometimes be found in protected pockets of forested parkland, especially near streams. If O. praticola also uses tree trunk moss, I'd have an additional tool in my praticola search toolbox. Time to sample tree trunk moss in the O. praticola zone!

Thin moss on alder trunks
was nearly uncollectable

Knowing that O. praticola is common along a certain stretch of the Burke-Gilman Trail in Seattle (King County), I grabbed my sifting gear and hiked towards a stretch of the trail that traversed a forested slope. The first place I looked was an unnamed wooded draw up-slope from the trail. I found several alder trees that had moss growing on their trunks, but the moss layer was so thin that it was impossible in most places to remove from the bark. The tiny bits of moss I was able to harvest held no Ozyptila, but many tiny theridiids.

Moss on maple trunk thick
enough to harbor O. praticola

From there I crossed the trail and made my way down to the base of the slope to a small creek, a 35 meter drop in elevation. That's where I found what I was looking for: next to the creek stood a bigleaf maple that had a one to two inch thick cushion of moss growing on the side facing the creek. I was only able to collect a few handfuls, but even so that modest volume held two female and several juvenile O. praticola. I sifted a second double handful of moss removed from a nearby log, and again found a female and several juvenile O. praticola.

What I didn't find was any O. pacifica, even though Seattle is theoretically in O. pacifica's range. In fact, I've never found O. pacifica in any place where O. praticola is present. Whether this is due to the presence of O. praticola per se or to other factors is an unanswered question.

Female O. praticola found in moss

Thinking back to the European reports I've read about O. praticola being found under cardboard placed on tree trunks, it makes sense that the species would also utilize tree trunk moss in places where it is available. However, I've never seen mention of this in any published source. Rod Crawford tells me that this is the first observation he is aware of of O. praticola in Washington using the tree trunk moss microhabitat. In any case, it looks like it will be worth my time to sift moss when I'm out looking for O. praticola.

Moss, lovely moss