Earlier this week I spent three lovely days in the Bay Area with Dave Kavanaugh, and on Monday we went to collecting on the slopes of Mount Tamalpais. There, along Cataract Creek, we found a series of small “Bembidion curtulatum” from one patch of fine gravel and coarse sand, in the foreground in the picture below:
I’ve had two previous posts about the Bembidion curtulatum species group, which are small, flat, dark Bembidion subgenus Plataphus that are common on gravel and coarse sand creek and river shores. In the western USA and Canada, these little beetles are currently all going under the name Bembidion curtulatum, but by February of this year, I suspected there were at least three species hiding within that name; by March I was convinced there were four.
Well, the counter keeps on ticking.
I picked up one of the Cataract Creek beetles, looked at it with a hand lens, and realized that it was too shiny and iridescent (which implied the microsculpture on its elytra consisted of fine, transverse lines) to be the same species as the Bembidion curtulatum that is found throughout much of California.
It turns out that I had collected one of those in Portola State Park in June, but hadn’t looked at it closely under the microscope – I had presumed it was a standard B. curtulatum. We sequenced it because it was the southernmost coastal mountain specimen I had, but I hadn’t looked at it or the DNA data carefully until yesterday. It has the exact same 28S sequence as standard B. curtulatum, and a very similar COI. But its CAD and topoisomerase is rather different – in fact, it has an amino acid difference from standard B. curtulatum. We will get sequence data back from the Mt Tam specimens in a week or so to confirm, but I am pretty confident now (based mainly on the microsculpture and the CAD sequences) that there is a fifth western species in “B. curtulatum” that seems to be primarily in the Bay Area of California.
The five species are allopatric, as far as known, and the morphological data have not been thoroughly examined, so I’m not as confident as I might be. But there are morphological differences, and the DNA sequences are very distinct, so I feel comfortable considering them distinct species. The distribution of the species formerly known as “Bembidion curtulatum” is shown below. Except for the pink squares, all of the dots on this map represent specimens whose DNA we have sequenced – I have not yet plotted onto the map specimens identified using their structures.
Although I have yet to examine the type specimens of the available names (I will do so in October), based upon geographic distributions and type localities it appears as if the most widespread California/Oregon/British Columbia species in B. curtulatum proper, and that the high-elevation species in the eastern Rockies is B. decrepitum, and that species 3 (Idaho, Montana, Nevada) and species 4 (Alaska) are undescribed (it may be that the Alaskan species has a name in Asia). The Bay Area species might end up being Bembidion flebile Casey, which would be pleasing, as all of these beetles used to be called B. flebile before Terry Erwin chose as first reviser to switch the name to B. curtulatum Casey (these two names were described in the same paper, and it was up to Terry to choose which one to use). I should find out in late October when I examine the type of B. flebile, and the other Casey types, in the Smithsonian Institution.
This reminds me of the situation with the subgenus Pseudoperyphus, a subgenus centered in New England. In that group, beetles considered to be two species, B. chalceum and B. honestum, were revealed (through a combination of chromosome, DNA, and morphological studies – a link to the PDF is on this page) to be a complex of seven species, with up to five of them living together on a single gravel bar. They are very difficult to tell apart, unless you look at the flagellum of the male genitalia, or DNA sequences, or chromosome number. I wonder how many other groups of Bembidion will turn out to be like this – an explosion of cryptic species.
Update: Added paragraph about Pseudoperyphus.