Morphological Variation in Nerita Gastropods in Northern Gulf of California

1. Punta Pelicana Punto Peñasco Estero Morua Morphological Variation In Allopatric Populations Of Nerita Gastropods In The Northern Gulf Of CaliforniaAllison Gilliland, Shannon O’Hara, Urs Riner, Michael Lorentzen, Ryan Horenziak, Emerald Tataryn, Raena Cota, & Stephen Shuster. Northern Arizona University, Flagstaff, Arizona,86011 Methods and Materials We collected Nertia sp. gastropods from 3 intertidal zones in Puerto Peñasco, Sonora, Mexico, during the week of October 27, 2004 (Fig. 1). Sites included Punto Penasco – 29.98 º, 111.18º , basalt boulders in the upper Intertidal zone (N=40), Punta Pelicana – 31.33º, 113.63º, Granite outcrops in the middle Intertidal zone (N=40) and Estero Morua – 31.28º, 113.41º, coquina limestone island, isolated near the mouth of a sandy estero, terrain of island was very rough and sharp (N=49) (Fig. 2). Live snails were transported to the laboratory where they were measured to the nearest .01 mm using a dissection scope equipped with an ocular micrometer, or using calipers. Measurements included shell height (SH), shell width (SW), aperture height (AH), aperture length (AL), medial tooth width (MT), lateral tooth width (LT), whorl height (WH), and operculum length, (OL) (Fig. 3). To allow direct comparison of the magnitude of these characters for each site, we calculated their averages and 95% confidence limits. To determine whether shells from each site were morphologically distinct, we performed principal components analysis (PCA) on the data using PC-ORD, version 4, 2005. PCA is a mathematical procedure that transforms a larger number of correlated variables into a smaller number of uncorrelated variables called principal components. The first principal component identifies an axis in multivariate space that accounts for the maximum variability in the data. In morphometric analyses, PC1 is usually associated with the “size” of the object under investigation, whereas PC2 is usually associated with object “shape.” Each succeeding component accounts for the remaining variability in the data. Abstract Geographically distant populations are often morphologically distinct. Environmental effects, genetic effects, or an interaction of the two can cause these differences. To address this issue we examined morphological variation in three disjunct populations of Nerita, near Puerto Peñasco, Sonora, Mexico. The three populations included, (1) Estero Morua, where comparatively large snails were isolated on a coquina limestone island near the mouth of the sandy estero habitat; (2) Punta Peñasco, where smaller snails were wide spread on the basalt boulders in the upper intertidal zone; (3) Punta Pelicano, where medium sized snails were clustered on granitic outcrops in the middle intertidal zone. We measured 8 morphological shell characteristics. The characteristics include; shell height, shell width, aperture height, aperture width, medial tooth length, lateral tooth length, whorl height and operculum length. Principal components analysis (PCA) showed significant morphological differences among the three populations (A=.68, P<.001); snails from Estero Morua were markedly larger than the other two populations, which were similar in size. These results suggest that snails in Estero Morua are geographically isolated from the other two populations. Figure 3 Figure 5: Graphical representation of Nerita data produced by PC-ORD Figure 2 Figure 1 Results and Discussion The means and 95% CI for Punto Peñasco and Punta Pelicana overlapped in all 8 variables, suggesting the existence of little morphological differentiation in Nerita morphology between these sites. In contrast, the mean and 95%CI for Nerita in Estero Morua were significantly greater in all cases (Fig. 4). This result was corroborated by our PCA analysis, which showed significant morphological differentiation of Nerita shells among sites (Within group correlation, A = .698, P<<.001; Fig. 5). The first principal component (PC1) explained 95.2% of the total variation in the data (Table 1). PC2 and PC3 explained 1.37% and 1.21% of the total variation, respectively (cumulative variation = 97.8%) with the remaining variation explained by PC4-PC8. As expected from these results, Estero Morua shells were differentiated from Punto Penasco and Punta Pelicana shells primarily along PC1, that axis associated with shell "size." Although some large Nerita were observed at Punto Penasco (N=4) and Punta Pelicana (N=5), Estero Morua shells were clearly much larger in all dimensions. These results suggest either that the Estero Morua population belongs to a species distinct from that found in the other two sites, or that conditions in the Estero favor growth and survivorship to an unusual degree. We plan to test these hypotheses with future genetic analysis of snails from each of these 3 sites. Introduction Members of the genus Nerita are herbivorous, high-tide rock dwelling marine gastropods that are mainly tropical in their distribution. Many populations in the Gulf of California have disjunct distributions due to the existence of large expanses of sand separating rocky habitats. In the northern Gulf of California the most abundant species is N. funiculata whose distribution includes the entire Gulf of California as well as the Pacific Ocean from Baja California Norte to Peru, and the Galapagos Islands (Keen 1971). Considerable variation in Nerita size and shell characters appears to exist in rocky habitats near Puerto Penasco, Sonora. Mexico. To investigate the possibility that different populations of Nerita may be morphologically distinct, and thus perhaps represent genetically isolated populations, we examined 8 morphological characters in 3 populations of Nerita at this location. Our proximate goal in this analysis was to determine whether genetic analyses may be justified. Our ultimate goal was to provide more detailed information on habitat diversity and biogeography of Gulf of California gastropods. Nerita funiculata at Estero Morua References Keen, A.M. 1971. Sea Shells of Tropical West America; Marine Mollusks from Baja California to Peru, Stanford University Press, Stanford, CA. Acknowledgements Department of Biological Sciences, College of Engineering and Natural Sciences, Northern Arizona University and CEDO Contact: raena.cota@nau.edu