How Discoveries are Made in the Lab: The Nemourid genus Nemoura

I've commented in several previous entries on my delight in finding this stonefly -- Nemourid, genus Nemoura -- in a number of streams in the last 3-4 weeks.   In the first year or so that I worked with StreamWatch, we occasionally found several of these in winter samples; we were not really sure what they were.  Our feeling was that this was a large winter stonefly, genus unknown.  We were familiar with two genera of large winter stones -- Taeniopteryx and Strophopteryx.   But these could not be Taeniopteryx nymphs since they lacked "coxal gills" (see the entry for 12/16/10), and they could not be Strophopteryx nymphs since they lacked a "triangular apical plate" (see the entry for 12/17/10).   There are other genera of large winter stoneflies, and we (myself and Rose Brown) carefully worked through our dichotomous key (Peckarsky, et.al., Freshwater Macroinvertebrates) to figure this out, but we could not pin it down.  That this stonefly was a Nemourid stonefly, we just figured out about six months ago.

Why did we think this was some genus of large winter stonefly?  Three reasons.  1) We saw these in winter samples -- often those done in February -- and that's when large winters abound.  2) The wingpads on this nymph were almost exactly the same as those we find on large winters.  Some pictures.  First, these are the wingpads of a large winter stonefly (genus Taeniopteryx):

And these are the wingpads of our unknown nymph:

Very difficult to distinguish.  And 3) while we knew that the wingpads of large winter stones and Nemourid stones are almost exactly alike -- this could not be a Nemourid!  We see Nemourids a lot in the spring, and they all look like this:

I.e. they always have "cervical gills," those frilly gills sticking out on both sides of the neck.

It was not until we took a closer look at Peckarsky -- and took a closer look at the legs of our nymph -- that we discovered what we had missed.  And that was two things.  1) As I've noted before, tarsal segments 1 and 2 on large winter stoneflies are the same size: on Nemourid stoneflies they are not, tarsal segment 2 is smaller than 1.  Our unknown nymph had Nemourid tarsi!

Tarsal segments 1 and 2 -- large winter stonefly:

Tarsal segments 1 and 2 -- Nemourid stonefly:

2) The other thing we discovered by carfully re-reading our key was that not all Nemourids had cervical gills!  In fact, cervical gills are only found on the genus Amphinemura.

All that remained to do was to identify the genus of this "new" (for us, anyway) Nemourid.  A close look at the pronotum showed that this was genus Nemoura.  The Nemoura pronotum has "...a distinct, single lateral fringe of spines of equal length..." (Peckarsky, p. 67), and the "pronotum is rounded laterally[with] longer, thinner setae absent from the lateral fringe." (Peckarsky, p. 67)  In the photo below, the yellow arrows point out the fringe of spines.

As I think I've mentioned before, the Amphinemura Nemourids (with the frilly gills) are common in StreamWatch spring samples -- and they can be found in very large numbers, sometimes forming the dominant taxon.  A review of the StreamWatch data shows that 1) most are found in April (some in March, some in May), and they are widespread in the watershed, being found in: both reference streams (in Albemarle County and Fluvanna County), Ballinger Creek, Buck Island Creek, Buck Mt. Creek (both sites), Cary's Creek, Cunningham Creek (both sites), the upper Doyles River, Lickinghole Creek, Long Island Creek, the Lynch River, Mechunk Creek (both sites), and the Roach/Buffalo River.

Clearly, the genera Nemoura and Amphinemura are not in our streams at the same time.  I've found the Nemoura Nemourid in February and March -- and they're already mature, preparing to hatch: I've not yet seen even a tiny Amphinemura (though I suspect they're around).  Amphinemura Nemourids mature in April and May.  I also wonder if they are found in the same streams -- or if they prefer different habitats.  I have found the genus Nemoura in Buck Mt. Creek -- where we also find the genus Amphinemura.  My other sightings have been in Elk Run (a Buck Mt. Creek trib), the Moormans River, an unnamed tributary to the Moormans, and the Rapidan River near Wolftown.    More to be learned, always more to be learned.