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Phenotypic Variation in Dorsal Fin Morphology of Coastal Bottlenose Dolphins ( Tursiops truncatus) Off Mexico

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Phenotypic Variation in Dorsal Fin Morphology of Coastal Bottlenose Dolphins ( Tursiops truncatus) Off Mexico

Eduardo Morteo et al. PeerJ.

Abstract

Geographic variation in external morphology is thought to reflect an interplay between genotype and the environment. Morphological variation has been well-described for a number of cetacean species, including the bottlenose dolphin (Tursiops truncatus). In this study we analyzed dorsal fin morphometric variation in coastal bottlenose dolphins to search for geographic patterns at different spatial scales. A total of 533 dorsal fin images from 19 available photo-identification catalogs across the three Mexican oceanic regions (Pacific Ocean n = 6, Gulf of California n = 6 and, Gulf of Mexico n = 7) were used in the analysis. Eleven fin shape measurements were analyzed to evaluate fin polymorphism through multivariate tests. Principal Component Analysis on log-transformed standardized ratios explained 94% of the variance. Canonical Discriminant Function Analysis on factor scores showed separation among most study areas (p < 0.05) with exception of the Gulf of Mexico where a strong morphometric cline was found. Possible explanations for the observed differences are related to environmental, biological and evolutionary processes. Shape distinction between dorsal fins from the Pacific and those from the Gulf of California were consistent with previously reported differences in skull morphometrics and genetics. Although the functional advantages of dorsal fin shape remains to be assessed, it is not unlikely that over a wide range of environments, fin shape may represent a trade-off among thermoregulatory capacity, hydrodynamic performance and the swimming/hunting behavior of the species.

Keywords: Adaptations; Polymorphism; Population discrimination; Stepping stone model.

Conflict of interest statement

The authors declare there are no competing interests.

Figures

Figure 1
Figure 1. Study areas.
(1) Pacific Ocean: EN, Ensenada, Baja California; SQ, San Quintin, Baja California; BM, Bahia Magdalena, Baja California; MZ, Mazatlán, Sinaloa; BB, Bahia Banderas, Jalisco; PE, Puerto Escondido, Oaxaca; (2) Gulf of California: UG, Upper Gulf of California, Sonora; SJ, Bahia San Jorge, Sonora; BL, Bahia de los Angeles, Baja California; BK, Bahia Kino, Sonora; SM, Bahia Santa Maria, Sinaloa; LP, La Paz, Baja California Sur; (3) Gulf of Mexico: TA, Tamiahua, Veracruz; NA, Nautla, Veracruz; VR, Veracruz Reef System, Veracruz; AL, Alvarado, Veracruz; TB, Tabasco, Tabasco; TL, Terminos Lagoon, Campeche; HO, Holbox, Quintana Roo.
Figure 2
Figure 2. Reference points and measurements computed by FinShape software to acquire morphological landmarks.
A = Tip; B = Base. Points C5, C10, C20 and C30 indicate angles (degrees) relative to line AB. Point D is the intersection of a line departing from C30 with the fin’s leading edge, this line is also perpendicular to line AB. Point O is the intersection of lines AB and C30D. Surface area (shaded) was calculated considering line C30B as the limit.
Figure 3
Figure 3. Median dorsal fin contours by study areas ( N = 533 dorsal fins).
Regional divisions are grouped in columns: (A) Pacific Ocean, (B) Gulf of California and (C) Gulf of Mexico. Contours reflect median values of shape and do not represent any particular dorsal fin. Study area codes follow those in Fig. 1, and sample size is shown in parenthesis; error bars and ellipses show variability expressed as quartiles (50% of data). Measurement AB (i.e., from the tip to the anterior insertion into the body) is the same for all fins (10 relative units).
Figure 4
Figure 4. Dorsal fin morphological variability and sample size effect within the 19 study areas (N = 533 dorsal fins).
Figure 5
Figure 5. Dendrogram based on morphometric distances among study areas (N = 533 dorsal fins).
Study area codes follow those in Fig. 1. Values are proportions based on the maximum Squared Mahalanobis distance (Table 3). Major branches are somewhat consistent with the regional division of the study areas: (1) Pacific Ocean (bold lines), (2) Gulf of California (dashed lines), (3) Gulf of Mexico (thin lines).

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Grant support

This research was part of the M.Sc. thesis of the lead author who benefited from a CONACyT (Consejo Nacional de Ciencia y Tecnología) graduate fellowship. Financial aid was also provided by a SNI (Sistema Nacional de Investigadores) scholarship provided by Saúl Álvarez-Borrego, and a thesis grant issued by the Department of Marine Ecology at CICESE. Logistics for the Gulf of California surveys were partially funded by Sergio Flores at UABCS and Drew Talley at UC-Davis. There was no additional external funding received for this study. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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