Early Life History and Distribution of Pistolgrip (Tritogonia Verrucosa (Rafinesque, 1820)) in Minnesota and Wisconsin

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ABSTRACT.-

We conducted a series of studies to improve our understanding of pistolgrip life history and distribution in Minnesota and Wisconsin. In the St. Croix River, where this species is relatively abundant, we studied animals biweekly from May-Nov. 1997, Apr.-Oct. 1998 and nearly biweekly during May-Jul. 2004-2007 and observed gravid females between late Apr.-Jul. at water temperatures 13-25 C. Females held mature glochidia in a large mantle magazine that was significantly more inflated at night. Fifty-seven pistolgrip glochidia measured using scanning electron microscopy had an average height and length of 119 ± 6 µm and 102 ± 4 µm (±1 SD), respectively. Of 65 fish species (18 families) exposed to pistolgrip glochidia only flathead catfish (Pylodictis olivaris) and brown (Ameiurus nebulosus) and yellow (A. natalis) bullheads were suitable hosts, with flathead catfish showing the strongest host response. Glochidia grew 422 ± 17% while attached to fish. Pistolgrip is found in central and southeastern U.S. but is declining in several locations. Surveys conducted between 1980 and 2009 show the geographic range of pistolgrip has decreased in Minnesota and Wisconsin. It is extirpated from the Minnesota River and nearly so in the Mississippi River. However, we see evidence of a recovering population in a once heavily polluted reach of the Mississippi River downstream of Minneapolis-St. Paul. The largest populations are in the lower reaches of the St. Croix (MN, WI), Chippewa, Black, Wolf and Wisconsin rivers (WI). In light of the apparent close association between pistolgrip and flathead catfish, we recommend pistolgrip conservation efforts include sustainable flathead catfish management and habitat improvement to support expansion of remaining pistolgrip populations.

INTRODUCTION

"The heavy waves will break many of the fine strings on which the young clams are fastened and wash them to the shore. Millions of such little tiny shells are at times piled up to six inches deep on the shores of Lake Pepin (Mississippi River) after a storm." Paul Harder, The History of the Fresh Water Clam or Musselshell, ca. 1908

Understanding the life history and distribution of a rare species are fundamental to returning a rare species to historic levels. Freshwater mussels are among the most threatened groups of organisms in the United States (Master et al, 2000) and there is a growing effort to coordinate freshwater mussel conservation through a national strategy (The National Native Mussel Conservation Committee, 1998). Improving our understanding of biological needs and distribution of mussels are ranked among the highest goals in this strategy.

An important part of understanding the life history needs of unionoid mussels is knowledge of the host requirement(s) of the larvae (glochidia). Most unionoid bivalves attach to a host fish in order to metamorphose from a glochidium into a juvenile (Williams et al, 2008). Several mussel species exhibit elaborate behaviors to draw hosts to their young (Barnhart et al, 2008). Natural glochidia hosts are usually identified through two kinds of studies, (1) laboratory trials where potential hosts are exposed to glochidia to determine if metamorphosis occurs (ZaIe and Neves, 1982; Jones et al, 2004), and (2) identification of glochidia, or preferably juveniles, recovered from naturally infested hosts using scanning electron microscopy (Kennedy and Haag, 2005; Allen et al, 2007) or molecular analysis (Kneeland and Rhymer, 2007). Recently, knowledge of host fish (es) has been used to strengthen conservation efforts through propagating rare mussel species (Jones et al, 2004; Barnhart, 2006; McUeod, 2006).

Culturing rare juvenile unionids for conservation purposes requires knowledge of how and when to find gravid mussels. Currently, glochidia for propagation efforts are obtained from wild stocks (Jones et al, 2004). Most mussel species brood mature glochidia for only a portion of the year (Williams el al, 2008). …