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3. Rhithrogena (species?)

These are photos of a Rhithrogena nymph from the Russian River. They are also found in Mill Creek, a tributary to the Russian River. They are typically in the river and creeks from February until April. The mature ones are from 12-14 mm long (excluding cerci and antennae). I'd like to find the species if possible. The red ones make up about 5% of the population and appear to be the same species as the brown ones.

Hi Mark-

I believe the red ones to be Rhithrogena robusta, and the brown ones to be Rhithrogena morrisoni.

Roger,



What do you base your supposition on? Is there a paper or did you get there through personal experience? Just curious.

Hi Mark-

My supposition is based on several factors.

Mayflies (Ephemeroptera) of the Far Western United States. Part 3: California, by Meyer and McCafferty, 2008, lists mayfly distribution by county. So, I printed (7) copies of the CA Counties map (which was also in the paper) in order to mark the county distribution of each of the (7) Rhithrogena species listed for CA.

R. decora is only recorded in (3) E. CA counties
R. flavianula is only recorded in (5) E. CA counties
R. hageni is widely distributed throughout the state, and is recorded in Mendocino Co., but not in Sonoma Co.
R. morrisoni is widely distributed throughout the state, and is recorded in Sonoma Co.
R. plana is only recorded in (1) E. CA county
R. robusta is mostly recorded in E. CA counties, but is also recorded in several W. CA counties, the closest to Sonoma Co. being Colusa Co.
R. undulata is not recorded as being present in a county anywhere near Sonoma Co.

So I believe R. hageni, R. morrisoni, and R. robusta to be the only likely candidates. In that R. hageni is not recorded in Sonoma Co., and R. morrisoni is recorded in Sonoma Co., I believe that R. morrisoni is more likely to be the species of your brown ones.

And, in that Rhithrogena robusta is reported to have red gills, I believe that it is most likely candidate to be the species of your red ones.

In any event, and for better or worse, that is the basis for my supposition.

Roger,

I agree with the R. morrisoni assessment and your R. robusta is likely to be correct except for one thing. R. morrisoni has a red version that is detailed in Day's paper on "The California Mayflies of the Genus Rhithrogena (Ephemeroptera)". http://www.ephemeroptera-galactica.com/pubs/pub_d/pubdayw1957p1.pdf

I believe it to be the red version of R.morrisoni that is the other nymph here.

Mark

Just for fun, here's a post about red nymphs from little while back:

http://www.troutnut.com/topic/3373/Red-Heptagenia

Mark-

I notice the last sentence of Day's paper (dated January, 1957) says:
"There are but four good species on the genus Rhithrogena that have to date been reported from California those being R. brunnea Hagen, R. decora Day, R. flavianula McDunnough, R. morrisoni Banks."

Day's paper does not mention R. robusta. Do you think what Day was describing as red R. morrisoni may have since been recognized as R. robusta?

In any event, I have fixed your link to Day's paper, so it now works.

Roger-

Day's paper doesn't mention R. robusta. He also doesn't mention raising adults of R. morrisoni from the nymphs he found. It's possible the nymphs of the red variant of R.morrisoni were actually the nymphs of R. robusta. I suppose the only way to tell would be to raise the nymphs that are reddish from this area and seeing whether they key out to R. robusta or R. morrisoni.

Mark

Paul-

Just checked your link to the red Heptageniidae. Didn't realize there were so many Heptageniidae that had the mutation. Fascinating stuff!

Mark

Another possibility is that it is a recessive gene within the same species that comes out frequently in a population. In other words a throwback instead of mutation. There may also be environmental factors as well. It is common enough among heptageniids (and other families as well, though less frequently) that I'm surprised it's not been reported on more or that a study hasn't been done on it - to my knowledge anyway...

I'm quite confident these are all morrisoni based on the concave anterior margin of the mesothorax and the paired dots on the terga.