Rediscovering the Missing "Comma" Leech

This "comma" that I found in a small creek is actually a rare leech, Helobdella farmeri.

What comes to mind when you look at the picture above? Maybe you think it looks like a comma-the punctuation mark we all know too well.

Well, that's exactly what some of my fellow teen volunteers at Stulsaft Park thought when I flipped over a rock in Ojo de Agua Creek in Woodside. They had no idea that what they were looking at was actually a rare endemic freshwater jawed leech, Helobdella farmeri.

Grassroots Ecology is a program that I take part in where, once every Wednesday during the months of June and July, I drive down from San Francisco all the way to Stulsaft Park in Woodside to meet up with fellow teenagers from across the Bay Area and adult leaders to weed and care for the native serpentine habitat. This Wednesday, July 8, was creek day. After watering the shriveling native plants in their "test plot," all of the students headed down to explore the creek and learn about macroinvertebrates.

As a macrophotographer and wildlife identifier, I already knew how to identify most of the basic macroinvertebrates they taught us about. Every time I showed the other kids something that I considered "common" because I'd encountered it so many times in the field-water striders, water boatmen, slender salamanders, and so on-they would get incredibly excited. To me, those species had become so familiar that I barely paid attention to them anymore.

What surprised me, though, was that when I brought over something genuinely rare-this leech, which was my first leech ever, found beneath a rock in the flowing creek-the group reacted with exactly the same excitement they had shown over a common water strider.

Another picture of the specimen showing it's minuscule size and brown-tan coloration

"Oh my gosh, it's so cute! Look at its little face. It's just sitting there, completely unfazed by us humans," one steward exclaimed, referring to the broad end of the leech that was actually its rear sucker.

"It looks like a comma," another person said.

Everyone, including me, burst out laughing.

As it turns out, the leech I found belongs to a species that is relatively new to science (at least if you consider 2023 to be recent). When biologist Ulrich Kutschera formally described H. farmeri in 2023, he noted that the species had completely disappeared from its historical Northern California habitats. Surveys conducted between 2007 and 2019 in Golden Gate Park and other Bay Area waterways failed to find a single specimen, leading scientists to believe that wild populations had gone extinct and that the species survived only in the lab.

This new wild observation provides evidence that breeding populations still exist outside of the laboratory.

Historically, wild populations had only been documented in San Francisco and in Matadero Creek on the Stanford University campus in Palo Alto. My observation from Woodside extends the known geographic range of the species and demonstrates that it is successfully inhabiting aquatic ecosystems in the Santa Cruz Mountain foothills.

For context, the first-ever observation of its close relative, H. californica (the Golden Gate leech), was uploaded to iNaturalist only three months earlier, making observations of these endemic Bay Area leeches especially noteworthy.

So, are you convinced now that going out, flipping over rocks in creeks, and taking the time to notice little tan things most people ignore isn't just an obsession? It can actually benefit science.

Many important discoveries begin with someone who is simply curious enough to look closely. Sometimes, all it takes is flipping over one rock.