Dr. Rob Dillon, Coordinator

Friday, March 1, 2013

The Mystery of the SRALP: A Twofold Quest!

Editor’s Notes – If you're just joining us.  This is the second installment in my 2013 series on the Snake River Physa controversy.  It won’t make any sense unless you back up and read my February installment first.  It might also help to read my essays of March 2008 and September 2010, but the most important thing is to read last month’s post, before trying to read this one.  I’m serious, I mean it.

This essay was subsequently published as: Dillon, R.T., Jr. (2019d) The Mystery of the SRALP: A Twofold Quest.  Pp 173 - 180 in The Freshwater Gastropods of North America Volume 4, Essays on Ecology and Biogeography.  FWGNA Press, Charleston.

The rendezvous was set for 9:00 Sunday morning, September 19, 2010, at a gas station by the interstate near Burley, Idaho.  We were four biologists of strikingly different agendas, drawn together on a twofold quest.  And I suppose I should not have been surprised, but I was.

When I first suggested a field trip to the Minidoka Dam to Mr. Ryan Newman, my Bureau of Reclamation host, I had imagined that I would go alone.  I thought perhaps he’d call the staff on duty for me, maybe get somebody to open a gate on a Sunday morning, and I’d be fine.  I was pleased (of course) to discover that he was willing to accompany me as my native guide, and pleased again to see our good friend John Keebaugh’s email address on the CC line.  John works out of the Orma J Smith Museum at the College of Idaho in Caldwell, about 150-200 miles west, back near Boise.  But the fourth member of our party, chauffeured in by Mr. Newman as John Keebaugh and I stood chatting in the parking lot that morning, was a surprise.  Dr. John B. Burch, all the way from Ann Arbor.

Dillon, Burch & Keebaugh
My agenda on the Snake River that morning was simple – to test the hypothesis I first advanced in my essay of March 2008 [1].  Based on the observations of Rogers & Wethington [2], it seemed likely to me that Physa acuta, or some snail not immediately distinguishable from Physa acuta (which we are calling the Snake River acuta-like Physa, or “SRALP”) should inhabit the shallow backwaters downstream from Minidoka Dam.  This would be consistent with the greater hypothesis, that outliers on the margins of acuta-like populations may have colonized more rapid midstream environments of the Snake River, inducing the phenotype we identify as Physa natricina.

It materialized that my three colleagues, on the other hand, were bent on sampling the deeper waters for additional specimens bearing the natricina phenotype.  The rationale for this behavior escaped me, as the existence of small-bodied physid populations bearing wide apertures in the deeper Minidoka tailwaters had already been well-established by the heroic survey of Gates & Kerans, which prompted this field trip in the first place. One would think, if the little things were indeed elements of an endangered species, we would leave them alone.  But no.

John Keebaugh had brought a long-handled dipper with which, wading into the river waist deep and extending to full length, he was able to retrieve cobbles from some rather great depth.  Ryan Newman and Jack Burch sorted through trays of these dredgings, looking for physids.

Meanwhile, I enjoyed a fresh, sunny morning wading around in the Snake River shallows all by myself, looking for the sorts of ponds and protected backwater areas that one might think of as typical Physa acuta habitat.  And finding slick-rock nothing.

It turns out that the Snake River below Minidoka Dam is a really crappy habitat, for snails or indeed for macroinvertebrate benthos of any sort.  Our team visited three sites, from River Mile 675 just below the spillway to River Mile 670 at the Jackson Bridge.  And throughout that five mile stretch, it was my impression that river levels are terribly impacted by the generation schedule at the dam.  On the September day of our visit we found a couple vertical feet of cobble beach exposed, and I would estimate that the water levels regularly fall another 3-4 vertical feet below that.  So even wading knee deep and squinting as far as I could into the dark, roiling river, I probably couldn’t see to any bottom that hadn’t been dewatered last May, and wouldn’t be dewatered again come December.

I did find a few Physa gyrina in the shallows – mostly on sticks and organic debris – probably washed in from little side tributaries [3].  In fact, the only really nice snail population I saw all morning was the Physa gyrina in a seep near the base of the spillway [4].  But I found no acuta-like Physa in the Minidoka tailwaters [5].  That turned out to be a really, really crappy habitat.

So we ate lunch and I bid my colleagues adieu.  And I hopped back into my rental car and turned my attention toward a (rather poor) roadmap I found in the glove compartment.  And began planning a blitzkrieg survey of the Snake River further downstream, on my return trip west, back toward Boise.
Snake R from the US93 Bridge

At this point a brief orientation might be helpful.  The Snake River runs east to west like a giant smiley-face across the bottom of Idaho, then north along the Oregon border through Hells Canyon to join the Columbia River at Kennewick, Washington, there designated “Snake River Mile 0.”  So Minidoka Dam is in eastern Idaho, at river mile RM 675.

Rogers & Wethington [2] never promised us any Physa acuta as high as RM 675.  They reported that their sample of 211 physids collected between RM 573 and 340 contained 94% of the “Physa acuta group.”  Nor did Dwight Taylor list any localities upstream as high as Minidoka County in his original description of Physa natricina [6].  His natricina localities were in Gooding, Elmore, and Owyhee counties, roughly RM 571 – 525.

It looked to me as though the Milner Dam (located at RM 639) backed the Snake River up almost to the Jackson Bridge, where I sat parked early that Sunday afternoon.  So I rather arbitrarily set my course for the US 93 bridge near Twin Falls, further downstream at RM 611.  The river turned out to be inaccessible from the US 93 bridge, but I was able to follow signs down to a public access at the Magic Valley Hatchery, RM 600.

The Snake River at RM 600 had taken an entirely different character from the flashy, sterile thing I’d waded around in all morning.  It was broad and warm and shallow and rich [7]And the rocks on the quiet margins were covered with SRALP-snails, indistinguishable to my eye from Physa acuta.  It didn’t take me ten minutes to squat down and collect at least 25 – 30 individuals in a drinking cup, which I resolved to carry with me to the meeting in Boise Monday morning.

Thousand Springs area
Then I got back into my rental car and continued driving east, enjoying the lovely weather and the countryside all quite exotic to my eastern eyes.   I made three additional stops, all brief: at the Owsley Riverfront Park (RM 582), at Bliss (RM 565) and at Glenns Ferry (RM 538), just as the sun was setting.  The river looked rich at all these spots, with a nice, diverse pulmonate fauna.  The population of Potamopyrgus (“New Zealand Mud Snails”) at Owsley was strikingly dimorphic, which was interesting [8].  And I enjoyed the big Fluminicola population at Bliss, the first I’d ever seen on the hoof.  And at all three spots I found SRALP-snails at least moderately common.  I picked up a few at each site and kept going.  I saw no Physa gyrina anywhere in my quick tour of the Snake River between RM 600 and RM 538 at all.

Back in Boise that night I slept soundly, my sample of SRALP crawling peacefully around the drinking cup at my bedside.  Surely, I thought, everybody at the big Bureau of Reclamation meeting on the morrow would see the importance of these snails to the answer of our ultimate question.  Does the strange little physid population in the Minidoka tailwaters that Gates & Kerans refer to “Physa natricina” indeed represent an endangered species?  Or might they be ecophenotypic variants of an otherwise common species, nevertheless endemic to the Snake/Columbia system?  Or might the Gates & Kerans sample simply constitute 274 ecophenotypic variants of the invasive pest Physa acuta, found everywhere on six continents?

How naïve could I be?  Join us again next time, for … Dixie-Cup Showdown!


[1] Red Flags, Water Resources, and Physa natricina [12Mar08]

[2] Rogers, D. C. & A. R. Wethington (2007).  Physa natricina Taylor 1988, junior synonym of Physa acuta Draparnaud, 1805 (Pulmonata: Physidae)  Zootaxa 1662: 45-51.

[3] To be as complete as possible.  I did collect approximately 15 - 20 juvenile and subadult Physa from the Minidoka tailwaters that I thought, on the morning of 19Sept10, might be Physa acuta.  I brought them home with me to Charleston, reared them to adulthood, and then dissected them.  Nope, they were all Physa gyrina.

[4] One of the strongest memories I have from my very brief introduction to the Minidoka tailwaters on a bitterly cold morning in December of 2005 was the high density of Potamopyrgus.  But on my return visit in September of 2010 I found exactly N=0 in several hours of effort.  The river levels were significantly higher in 2010, so it’s possible that I simply missed them.  Or has there been a flush/crash?

[5] So my hypothesis of 3/2008 was incorrect.  Physa gyrina seems to be washing into the shallows below Minidoka Dam, not Physa acuta.  My new hypothesis is that the little population of snails that Gates & Kerans are calling “Physa natricina” is the relict of a much larger acuta-like population comparable to those inhabiting the Snake River further downstream, but now extinguished from the shallows by operations at the dam.

[6] Taylor, D. W. (1988)  New species of Physa (Gastropoda: Hygrophila) from the western United States.  Malacological Review 221: 43-79.

[7] Although similar in outward appearances, P. acuta and P. gyrina have diverged strikingly in their life history adaptation.  Populations of P. acuta are weedy or “ruderal” – their rapid growth, quick maturity and high reproductive output (relative to body mass) adapted to exploit rich, although often unpredictable habitats.  Physa gyrina are more stress-tolerant (like a cactus), adapted to nutrient poor but nevertheless predictable habitats.  See my book (Dillon 2000) pp 131-136 and Fig 8.10 for more.  In southern Idaho, these life history differences seem to be reflected in the distributions of the two species – acuta in the main river (further downstream) and gyrina in the tributaries.

[8] The presence of two strikingly different shell forms in a single Potamopyrgus population would seem to suggest sexual reproduction.  The Snake River gastropod fauna really does offer a wealth of opportunity for important scientific research, if serious scientists could get beyond all the politics and confusion.  Such a shame.

1 comment:

  1. Pure reading pleasure, Bob! Now you've got me hanging for installment 3!