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Morphometric Analysis of Sexual Dimorphism In The Federally Endangered Dwarf Wedgemussel, Alasmidonta heterodon

N. E. Baginski, K. M. O’Brion, K. L. Richardson, P. M. Larson, and B. J. Wicklow

Department of Biology, Saint Anselm College, Manchester, NH , USA 03102

Small and declining populations are at risk of stochastic events that may Small and declining populations are at risk of stochastic events that may reduce genetic diversity and effective population size leading to reduced reduce genetic diversity and effective population size leading to reduced recruitment and local extinction. Sex ratios are fundamental to understanding recruitment and local extinction. Sex ratios are fundamental to understanding the potential for these populations to persist. The dwarf wedgemussel has been the potential for these populations to persist. The dwarf wedgemussel has been extirpated from or has declined to small, insular populations throughout much extirpated from or has declined to small, insular populations throughout much of its former range. Although sexual dimorphism was noted in early of its former range. Although sexual dimorphism was noted in early descriptions of the dwarf wedgemussel, there are no validated studies that descriptions of the dwarf wedgemussel, there are no validated studies that define characters that can be used to differentiate sex. define characters that can be used to differentiate sex.

Length-width ratios differed significantly between male and female Length-width ratios differed significantly between male and female mussels (Figure 5), (T-test,mussels (Figure 5), (T-test, p p < 0.001). Discriminant Function Analysis < 0.001). Discriminant Function Analysis predicted with 95.2% certainty the sex of each specimen based on the 4 predicted with 95.2% certainty the sex of each specimen based on the 4 linear measurements (Figure 6). This means that DFA correctly classified linear measurements (Figure 6). This means that DFA correctly classified the sex of 95.2% of our specimens based on their shell morphology as the sex of 95.2% of our specimens based on their shell morphology as compared to the actual sex as confirmed by examination swollen marsupia compared to the actual sex as confirmed by examination swollen marsupia or gonadal fluid. Results of relative warps analyses of shell shape also or gonadal fluid. Results of relative warps analyses of shell shape also showed clear sexual dimorphism in all three views (lateral, dorsal, and showed clear sexual dimorphism in all three views (lateral, dorsal, and posterior).posterior).

We took caliper measurements of length, width, height, umbo to anterior and We took caliper measurements of length, width, height, umbo to anterior and recorded shell shape attributes. Gravid females were identified by the recorded shell shape attributes. Gravid females were identified by the presence of swollen marsupia. Gonadal fluid was extracted from non-gravid presence of swollen marsupia. Gonadal fluid was extracted from non-gravid individuals to discriminate between females and males (Figures 1 and 2).individuals to discriminate between females and males (Figures 1 and 2).

Our results show that Our results show that A. heterodon A. heterodon is sexually dimorphic and that key is sexually dimorphic and that key measurements and analysis of photographs of mussel shells can be an measurements and analysis of photographs of mussel shells can be an effective, non-invasive method of determining individual sex. effective, non-invasive method of determining individual sex. We plan to apply these methods to determine sex ratios in wild populations We plan to apply these methods to determine sex ratios in wild populations and review data from past monitoring studies to compare changes in sex and review data from past monitoring studies to compare changes in sex ratios over time, thus providing critical information for the management of ratios over time, thus providing critical information for the management of this endangered species.this endangered species.

LOGO

We collected 50 dwarf wedgemussels from the Connecticut River at We collected 50 dwarf wedgemussels from the Connecticut River at Lancaster, NH. Our aim was to use geomorphic morphometric analysis to Lancaster, NH. Our aim was to use geomorphic morphometric analysis to differentiate male and female mussels based on shell shape characteristics. differentiate male and female mussels based on shell shape characteristics. This method could then be used as a noninvasive technique for male and This method could then be used as a noninvasive technique for male and female identification in the field. We hypothesized, based on preliminary female identification in the field. We hypothesized, based on preliminary results, that specific male and female measurements differed significantly.results, that specific male and female measurements differed significantly.

We photographed individual mussels in lateral, dorsal and posterior views We photographed individual mussels in lateral, dorsal and posterior views then overlaid a fan with 28 digitized landmarks on each image using then overlaid a fan with 28 digitized landmarks on each image using MakeFan 6 (Figures 7a and 8a). Landmarks were imported into Tpsdig MakeFan 6 (Figures 7a and 8a). Landmarks were imported into Tpsdig (http://life.bio.sunysb.edu/morph). We ran a relative warps analysis of the (http://life.bio.sunysb.edu/morph). We ran a relative warps analysis of the geometric morphometric data (sliding outline landmarks) in TpsRelw. Scores geometric morphometric data (sliding outline landmarks) in TpsRelw. Scores on the first two relative warp axes were plotted in NCSS. Finally, the four on the first two relative warp axes were plotted in NCSS. Finally, the four linear mesurements were anlyzed in NCSS via a discriminant functions linear mesurements were anlyzed in NCSS via a discriminant functions analysis (DFA). We used a T-test to determine differences in mean length-analysis (DFA). We used a T-test to determine differences in mean length-width ratios of male and female mussels.width ratios of male and female mussels.

Figure 1. Figure 1. A. heterodon A. heterodon sperm cell stained with sperm cell stained with methylene blue (see Saha and Layzer, 2008)methylene blue (see Saha and Layzer, 2008)

Figure 2.Figure 2. Gravid female displaying. Glochidia Gravid female displaying. Glochidia are present just inside the exhalent aperture.are present just inside the exhalent aperture.

Figure 3. Figure 3. Male relative landmark displacement Male relative landmark displacement computed from a consensus of 42 superimpositions.computed from a consensus of 42 superimpositions.

Figure 4. Figure 4. Female relative landmark displacement Female relative landmark displacement computed from a consensus of 42 superimpositions.computed from a consensus of 42 superimpositions.

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Figure 5. Figure 5. Box plot results for length-width Box plot results for length-width ratios of male and female musselsratios of male and female mussels

Figure 6. Figure 6. NCSS plot of discriminant NCSS plot of discriminant regression scoresregression scores

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Figure 7. Figure 7. Male specimen in Lateral Male specimen in Lateral (A), Dorsal (B) and Posterior (C) views (A), Dorsal (B) and Posterior (C) views

Figure 8. Figure 8. Female specimen in Lateral Female specimen in Lateral (A), Dorsal (B) and Posterior (C) views (A), Dorsal (B) and Posterior (C) views

Introduction

Objectives

Methods

Results

Conclusions