Monday, April 25, 2011
Exciting Discoveries at Buck Mt. Creek
Let's begin here. This is a Spiny Crawler mayfly (family: Ephemerellidae).
And "this" is a Spiny Crawler mayfly (family: Ephemerellidae). They don't look a whole lot alike. But the difference is a matter of genus. The first one is genus Drunella; the second is genus Ephemerella. I found both nymphs today -- they were on the same rock in Buck Mt. Creek. Ephemerella spiny crawlers are common and prolific right now in our streams; Drunella spiny crawlers -- around here -- are rare. I have seen the genus Drunella in only four of our streams: Buck Mt. Creek, the Lynch River, the Moormans River, and Cunningham Creek (in Fluvanna County). The genus Drunella is less tolerant of stream impairment than the genus Ephemerella. In the "Regional Tolerance Values" for the Southeast (NC), there are seven Drunella species: they range in TV from 0 to 1.3. The same chart has seven species of Ephemerella spiny crawlers: the range there is from 0 to 4.
While many stream monitors can recognize Ephemerella nymphs as "spiny crawlers," very few know what to do with Drunella. Because of its "squat" shape and "muscular" forearms, it is often mistakenly listed as a flathead. This is why it's always important to look for the gills with a loupe: do they stick out to the sides of the abdomen (flatheaded mayfly)? or are they on top of the abdomen (spiny crawler mayfly)? Here's another look at our nymph in which the gill location is pretty clear.
Drunella spiny crawlers and Ephemerella spiny crawlers both have "Lamellate gills present on abdominal segments 3-7" (Peckarsky, et.al., Freshwater Macroinvertebrates, p. 26), even though the gill on segment 6 often covers the gill on segment 7. The defining features for the genus Drunella are the "prominent tubercles or spines present on forefemora" (Peckarsky, ibid.). If you click on the photos above and look closely, the tubercles are easy to see. Here's a microscope photo to help even more.
I knew there were Drunella spinys in Buck Mt. Creek: it's the main reason I went there today, so I was not disappointed. However, I came away with two other treasures: both caught me by surprise.
First, I saw a number of these.
This is a flatheaded mayfly, genus Rhithrogena. This is the only stream in which I've found this particular genus (I found some here last year as well). This genus is similar to Epeorus in having large, "suction cup" gills that seem to encircle the abdominal segments (click on the photo to enlarge). The two differ in that Epeorus nymphs have only two tails -- Rhithrogena have three -- and the gills on Rhithrogena form a complete oval on the ventral side of the nymph. (I regret that I did not bring this bug home with me so I could show you a microscope view of that shape: next time.) The other thing that I think is distinctive about Rhithrogena flatheaded mayflies is the color: very yellow. I'll keep an eye on that feature in future trips to this stream -- but it was certainly true of every nymph that I saw today. Neat!
My third surprise eluded me until I came home and looked at what I had preserved. I thought I had found a small minnow mayfly -- and sure enough I had. But, whereas all the small minnows that I found through the winter -- in this stream, and every other stream for that matter -- were genus Baetis, this was genus Heterocloeon. Here's a look at the specimen (I'll get some live shots when they get bigger.)
Like the Baetis small minnows we saw in the winter, this genus has only two tails. The distinguishing feature is found in the gills, where "the center region" has "a large pigmented area" (Peckarsky, et.al., Freshwater Macroinvertebrates, p. 33). This microscope close-up should help us to see that characteristic.
What else did I find? There were still a few black flies around, and they are still genus Prosimulium (elsewhere, I'm now finding Simulium larvae). And I found several Perlodid stoneflies, both Diploperla and Isoperla. Looking at photos of them is a nice way to close.