THERMAL REQUISITES FOR GROWTH OF ST. CROIX RIVER WINGED MAPLELEAF MUSSEL 
GLOCHIDIA AND EXCYSTMENT OF TRANSFORMED JUVENILES FROM HOST-FISH.

Mark T. Steingraeber1, Michelle R. Bartsch2, John A. Kalas1, and Teresa J. 
Newton2.  1U.S. Fish and Wildlife Service, 555 Lester Avenue, Onalaska, WI, 
54650; 2U.S. Geological Survey, 2630 Fanta Reed Road, La Crosse, WI, 54602.

The winged mapleleaf mussel (Quadrula fragosa) historically occupied a greater 
range than it does today and now has only one known reproducing population that 
is restricted to a 12-mile reach of the lower St. Croix River (Wisconsin and 
Minnesota).  Federal, state, and university biologists are working cooperatively 
to learn more about the current range, life-history, and genetics of this 
federally endangered species to initiate controlled propagation programs that 
will aid its recovery.  Successful propagation depends on identifying suitable 
host fish for transformation of glochidia into juveniles.  We initiated a series 
of host fish tests at three different temperature regimes on 3 October 2003 that 
later identified blue catfish (Ictalurus furcatus) and channel catfish 
(Ictalurus punctatus) as suitable hosts for this mussel.  During the first test, 
groups of glochidia-infested fish were held at a near constant and unseasonably 
warm water temperature (~19.6°C) to accelerate the rate at which transformation 
might occur.  Meanwhile, additional channel catfish used in the two remaining 
tests were infested with the same stock of glochidia but kept at colder 
temperatures (~12.6°C) for the next 47 days.  Fish used in the second test were 
then supplied with water at a temperature (~19.5°C) similar to that of the first 
test.  Peak juvenile recovery occurred about 6 weeks after the start of the 
first test and 10 weeks after the start of the second.  Comparisons of the time 
and cumulative water temperature units required to recover juveniles from 
channel catfish reared under these thermal regimes allowed us to empirically 
estimate: the minimum daily mean water temperature (9.25?C) required for winged 
mapleleaf glochidia to grow and transform into juveniles; and the cumulative 
water temperature units of net daily growth (~395?C?d) needed to initiate peak 
excystment and recovery of winged mapleleaf juveniles.  Remaining fish used in 
the third test were subsequently maintained in a thermal regime that closely 
followed St. Croix River water temperatures from mid-November 2003 through June 
2004.  Peak recovery of juveniles in this test began 21 June 2004, 261 days 
after infestation (453?C?d of growth) and just 7 days later than we predicted.  
Our test results also suggest that minimum daily mean water temperatures of 17 
to 20?C are likewise needed to trigger peak excystment of winged mapleleaf 
juveniles from the gills of their host fish.  These early life-history findings 
are now being applied in propagation programs to help recover winged mapleleaf 
populations within this species’ historic range.

Keywords: winged mapleleaf mussel, St. Croix River, glochidia, growth, water 
temperature