Estimated thermal development of Higgins eye mussel glochidia during metamorphosis on largemouth bass held in warm and cool water temperature regimes at the Genoa NFH during spring 2006. No net development occured at temperatures below 10.9 C. - FWS photo

Host fish with encysted Higgins eye mussel glochidia are initially maintained for several weeks in temperature monitored raceways at the Genoa NFH. - FWS photo

Mussel bearing host fish are moved to cages that are submerged in rivers shortly before the Higgins eye glochidia complete their metamorphosis into juvenile mussels. - FWS photo

A diver guides a cage containing host fish with encysted Higgins eye mussel glochidia to the bottom of a river where glochidia will complete metamorphosis and release from the fish as juvenile mussels. - FWS photo

In the dead of winter, most aquatic biologists can be found keeping warm in front of computer monitors that glow with data collected the previous field season.  Biologists at the Genoa National Fish Hatchery and the La Crosse Fishery Resources Office have likewise kept warm this winter while determining thermal requirements for early life development of the federally endangered Higgins eye pearlymussel. 

Patterned after similar work conducted in 2003 on the federally endangered winged mapleleaf mussel, Genoa mussel propagation biologist Tony Brady reared Higgins eye glochidia on largemouth bass host-fish that were maintained in two varying temperature regimes at the hatchery during spring 2006.  

Genoa NFH staff recorded the water temperature and enumerated juvenile mussel excystment over metamorphosis periods that ranged from 40 (warmer regime) to 49 days (colder regime).  Using a mathematical model that he developed for the earlier winged mapleleaf work, La Crosse fishery biologist Mark Steingraeber used these Higgins eye data to empirically derive a value of 10.9 C as the minimum (i.e., threshold) temperature required for metamorphosis of largemouth bass-encysted Higgins eye glochidia into juvenile mussels. 

This value was subsequently applied to (i.e., subtracted from) all of the daily water temperature values that exceeded it in each thermal regime.  The ensuing values were then summed for each regime to determine the cumulative temperature units of development (169 C·d) needed for Higgins eye glochidia to complete metamorphosis on largemouth bass from the date of encystment until the start of peak excystment.

This finding represents only the second freshwater mussel for which these biological temperature constants have been reliably quantified and suggests that values for these traits are species-specific.  For example, the developmental threshold temperature derived for the Higgins eye mussel is about 1.6 C greater than that of the winged mapleleaf mussel (9.26 C). 

This indicates that the metamorphosis of Higgins eye glochidia into juveniles, which typically occurs in the spring, is more sensitive to colder water temperatures.  However, Higgins eye metamorphosis requires just a small fraction (43%) of the cumulative thermal energy needed for winged mapleleaf metamorphosis (395 C·d) and occurs more rapidly. 

These thermal criteria can be used in future Higgins eye recovery efforts to: attain targeted rates of metamorphosis in the hatchery by making knowledgeable water temperature adjustments; quantitatively estimate cumulative mussel development at any time; and guide the selection of when to place mussel bearing host-fish into rivers to maximize juvenile mussel production.