Sunday, January 30, 2011
The "Super Hatches" of Spring: "Spiny Crawlers" and "Nemourids"
When I worked with StreamWatch we regularly saw abnormally high numbers of two different taxa in samples done in the spring: "Spiny Crawler" mayflies (family: Ephemerellidae) and "Nemourid" stoneflies (family: Nemouridae). Nemourids, in fact, simply aren't found any other time of the year (at least I haven't seen them), and Spiny Crawlers are found, but in much smaller numbers (and the genus usually differs from the one we find in the spring.)
Spiny Crawlers: According to Barbara Peckarsky, et.al. -- Freshwater Macroinvertebrates of Northeastern North America, pp. 26-29 -- there are six spiny crawler genera we might find in our streams. To date, I've only seen three: Ephemerella, Serratella, and Drunella. Without any question, most of the spiny crawler nymphs that we see in this part of Virginia are genus Ephemerella. Fly fishermen are very familiar with the name of this genus, since it accounts for three major late spring and early summer Eastern hatches: Ephemerella subvaria (the "Dark hendrickson"), Ephemerella rotunda and invaria (the Sulphur Duns), and Ephemerella dorothea (the Pale evening dun -- or P.E.D.). (On the Ephemerella hatches, see, for example, Malcolm Knopp and Robert Cormier, Mayflies: An Angler's study of Trout Water Ephemeroptera, p. 191). (The photos that follow are both genus Ephemerella.)
The feature that allows us to identify spiny crawlers is the location of their gills on top of the abdominal segments; they do not stick out to the sides of the abdominal segments (as they do with all other mayflies that we have discussed-- Small Minnows, Brushlegged mayflies, etc.). Ephemerella nymphs have oval, plate-like gills on top of segments 3-7, but the gill on segment 7 is often totally or partially blocked from view by the gill on segment 6. Thus, when we look at this nymph through a loupe, we normally see only 4 gills on each side of the abdomen -- as in the photo below. (Note the "spiny" edges on the abdominal segments.)
This same gill arrangement is found in the other genera we're discussing as well -- Serratella and Drunella. So how do we distinguish the nymphs of these three different genera one from the other?
Ephemerella and Serratella nymphs look much the same. But, there are three ways to tell them apart.
Number one is a matter of timing. Serratella nymphs start to show up only after the Ephemerellas are gone. All of the spiny crawlers I saw last year in the spring were Ephemerella (or Drunella); all that I saw in the summer were Serratella (Serratella deficiens produces the late summer "Little dark hendrickson" hatch). Number two, at least with the nymphs I have seen, mature Serratella nymphs are quite a bit smaller than mature Ephemerellas, and they tend to be darker in color. A Serratella nymph is pictured below.
The third feature distinguishing Ephemerella and Serratella nymphs from one another is the look of the tails: Ephemerella tails are covered with many fine hairs (intrasegmental setae); Serratella tails simply have whorls of spines. They are pictured in order below.
What of the third genus that we see in our streams, spiny crawlers genus Drunella? Let's take a look at this "odd man out". The "muscular" front arms are a good clue to use for identification (also note how well the gills show up on top of the abdominal segments.)
Since this is a nymph that stream monitors can find confusing, it's worth looking at more than one sample.
This could be mistaken for a small frog! But the mistake a lot of monitors make is to put these nymphs in with the flatheaded mayflies based on the size and shape of the head. The genus Drunella differs from Ephemerella and Serratella -- and all other spiny crawler genera, for that matter -- by the presence of "spines" or "tubercles" on the leading edge of the femurs on the front legs -- pretty obvious in the picture below.
As I peruse the StreamWatch database, I find very few watershed streams that do not have spiny crawlers, and the large numbers in samples always occur in the spring. The only streams where spiny crawlers seem not to be found -- or found in very small numbers -- are the worst streams sampled by StreamWatch; e.g. Meadowcreek, Moores Creek, and the South Fork of the Rivanna. I'm quite sure that almost all of the spinys that show up in samples are genus Ephemerella. My impression is that Drunella and Serratella nymphs live in very few of our streams, and with Drunellas, that would be in pretty good streams. Drunella nymphs have a tolerance value of 1.0 (or less). (TV lists do not agree on what value to assign Spiny Crawlers in general, but VA DEQ and StreamWatch give them a value of "4".) I have found Drunella spinys in the Moormans River, Cunningham Creek, the Lynch River, and Buck Mt. Creek.
Various common names have been proposed for this insect (family: Nemouridae) -- "Spring stoneflies," "Brown stoneflies," "Forest Flies" -- but we find it best to just stick with "Nemourid". I think of "Nemourid nymph season" as running from March 1 - May 30. I'm sure that's a little too rigid, still, that's when I normally find them. Nemourid stonefies are small stoneflies: I doubt they ever exceed 1/2" in length. They are usually dark brown in color, and one can often see a "Y" pattern on top of the head.
But THE defining characteristic for this particular taxon is the presence of "cervical" gills -- i.e. frilly bunches of gills that stick out from the neck. Let's take a look.
And here is a closer look at those gills from the ventral side.
(I keep trying to come up with some clever way to describe these: "whiskers"? "sideburns"? Just not quite on the mark! In any event, they're actually pretty easy to see in the field with minimum magnification -- even with very small nymphs.
But monitors should be aware that we do have two genera of Nemourids that show up in our streams.
The one with the "cervical gills" -- and clearly this is the most common Nemourid we see -- is genus Amphinemura; the genus Nemoura does not have those gills! Here is a photo of that type of nymph.
I have found exactly two of these in the many excursions I've made to streams in the last few years and in the samples I've taken. Still, they're there. And it can be difficult to distinguish this nymph from the large winter stonefly (the wingpads look exactly the same). To do that you need to see the length of the two tarsal segments, for which you need a pretty good microscope.
Although we find a lot of Nemourids in this watershed in the spring, they are not normally found in the Rivanna itself. They prefer small to middle size streams, and with a tolerance value of 2.0, they prefer pretty clean water.