Early Life-History and Conservation Status of Venustaconcha Ellipsiformis (Bivalvia, Unionidae) in Minnesota

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ABSTRACT.-

The ellipse, Venustaconcha ellipsiformis (Bivalvia: Unionidae), was first recorded in Minnesota from the Straight River in 1987, but we knew little of its distribution in the state, brooding behavior, glochidial host fish relationships or status. To examine these questions, we followed standard procedures to conduct an extensive, qualitative statewide mussel survey, described mantle flapping behavior from field and laboratory observations and identified suitable glochidia hosts in the laboratory and from naturally infested fishes. We found extant populations in five Mississippi River tributaries in southeastern Minnesota: the Cannon, Cedar, Root, Upper Iowa and Zumbro rivers. This appears to be the extent of its historic range in Minnesota as no valid records were found elsewhere. Among these, the largest population occurred in the headwaters of the Root River drainage, which was also the only drainage where we observed recent recruitment. Brooding V. ellipsiformis quickly flap a small mantle extension, often in response to passing shadows or jarring of the substrate, or their mantle extensions may be slowly undulated. We identified 11 fish species as suitable hosts for V. ellipsiformis glochidia in the laboratory: brook stickleback (Culaea inconsians), mottled sculpin (Cottus bairdii), slimy sculpin (C. cognatus), logperch (Percina caprodes), mud darter (Etheostoma asprigene), rainbow darter (E. caeruleum), Iowa darter (E. exile), fantail darter (E. flabbelare), Johnny darter (E. nigrum), banded darter (E. zonale) and blackside darter (P. maculata). Rainbow darter, fantail darter and blackside darter were also found naturally infested with V. ellipsiformis glochidia. Venustaconcha ellipsiformis should remain classified as a "Threatened" species in Minnesota and management should include conserving populations within drainages due to its restricted range and likely low dispersal ability.

INTRODUCTION

Rare mussel management is improved with knowledge of species distribution and life history. Venustaconcha ellipsiformis (Conrad, 1836), the ellipse, is a small stout-shelled mussel that generally lives in small to medium sized streams with stable gravel or mixed sand and gravel bottoms (Cummings and Mayer, 1992). This species has a broad extralimital distribution in the central United States; including Arkansas, Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Ohio and Wisconsin. (Baker, 1928; Van der Schalie and Van der Schalie, 1963; Cummings and Mayer, 1992; Oesch, 1995; Obermeyer et al., 1997; J.L. Harris, pers. comm.). Venustaconcha ellipsiformis was not included in earlier accounts of Minnesota unionids (Dawley, 1947). (Davis, 1988) was the first to report the species in Minnesota from the Cannon River drainage, and (Graf, 1997) later reported V. ellipsiformis from the Zumbro River and St. Croix River drainages. Venustaconcha ellipsiformis is currently listed as Extirpated from Ohio, Endangered in Kansas, Threatened in Iowa, Minnesota and Wisconsin; and a Species of Special Concern in Illinois, Indiana and Michigan. As part of this study, we wanted to determine the range of V. ellipsiformis in Minnesota.

Like most North American freshwater mussels, Venustaconcha ellipsiformis larvae (glochidia) are obligate parasites on fishes. Freshwater mussels benefit from this parasitic larvae life stage in several ways, including improved dispersal ability (Smith, 1985). Specialized extensions of the mantle, "mande flaps," have evolved in other North American mussel species, and it is common for these species to flap mantles in a rhythmic motion. This behavior is thought to draw host fishes to females brooding glochidia (Haag and Warren, 2003), and it may serve to suspend glochidia in the water column and increase the likelihood of their encountering fish (Kraemer, 1970). Villosa nebulosa and V. vibex have been shown to release larger numbers of larvae when fishes are present, and V. …