Salinity and Egg Shape Variation: A Geometric Morphometric Analysis
The harvest and incubation of American Alligator (Alligator mississippiensis) eggs is an important component to the commercial alligator harvest industry in the southeastern United States. As a result, various methodologies have been used to monitor alligator populations including abundance counts, stress quantification, and nesting surveys. Past studies have dismissed the importance of egg shape in crocodilians, Squamates, and turtles and deemed egg shape in birds and other amniotes as similar, in relation to functionality. The complexity of crocodilian eggs has been examined, and both turtle and Squamate eggs have been regarded recently as physiologically more intricate than bird eggs. This study introduces a physiological approach to monitor alligator populations in freshwater and low salinity environments by quantifying egg shape in correlation with varying salinity. We introduce a fractional semilandmark-shape template method to quantify egg shape within a geometric morphometric framework. This approach is beneficial because it allows for the quantification of shape for curved structures, such as eggs, which lack homologous landmarks. The results from this study suggest that alligator egg shape is correlated with varying salinity levels, such that variation in alligator egg shape at low salinities changes in gradient-like fashion, whereas salinities high enough to be deemed stressful result in reversion back to a low salinity egg shape or desiccation. This study elucidates a correlation that can be implemented in management and breeding techniques and opens the door to in-depth physiological examination of the system.Abstract
Map showing the study sites. Las Conchas Marsh near Slidell, Louisiana, is highlighted to the east, whereas J. D. Murphree WMA and Anahuac NWR are highlighted to the west.
The photographic setup used to collect egg-shape data. Nest and egg identification and scale bar are in picture.
The fractional semilandmark shape template overlaid on an alligator egg displaying all 38 semilandmarks at the intersection of the eggs circumference with horizontal centerline and 19 perpendicular verticals.
Within site canonical variance analyses depicted similar trends in correlation between egg shape and nest-site salinity. (A) Canonical variance analysis performed in MorphoJ using 2009 and 2010 data from J. D. Murphree WMA with one nest at 14 ppt omitted to elucidate gradient-like change in egg shape between varying salinities. (B) CVA performed in MorphoJ using 2009 and 2010 data from Las Conchas Marsh.
Transformation grids depicted in TPSRegr at 10× exaggerated shape change from average consensus egg. The grids show the elongate narrow shape of eggs at lower relative salinities (A) and the shorter wider shape of eggs at higher salinities (B). Arrows indicate the directionality of shape change for each landmark in eggs from a specific salinity.
(A) Canonical variance analysis using 2009 data from both study sites with five salinity classifiers: <1 ppt, 1–2 ppt (1.2), 3–4 ppt (3), 4–5 ppt (4), and 14–15 ppt (14). The ordination reveals separation between each salinity cluster and continuous shape change in a gradient-like fashion from 0 to 5 ppt with 14 ppt being less similar to every other group than all other groups are to each other. (B) Canonical variance analysis of 2009 and 2010 data reveal separation between 0–2 ppt (including all nests from 0 ppt to 2.99 ppt) and 3–5 ppt. These two clusters are oriented near each other with minimal overlap. A 14-ppt egg shape cluster is noticeably different from all other egg shapes at all other salinities. Eggs at 9 ppt overlapped both the 0–2-ppt and 3–5-ppt clusters and were diverse in egg shape.
A. Multidimensional scaling (MDS) using partial warp scores from the first five designated landmarks. Note the separation between eggs at 14 ppt and eggs at all other salinities as well as overlap between eggs at 9 ppt and 0–1, 1–2, 2–3 ppt. (B) Multi-dimensional scaling based on data from the first 20 partial warp scores (top half of egg) from all eggs. Eggs at 0–1 ppt, 1–2 ppt, and 2–3 ppt group out similarly to each other along some axis, whereas those at 3–4 ppt, 4–5 ppt, and 9 ppt group similarly to each other along a similar axis as the lower salinities.
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