Molecular and Morphological Differentiation of Common Dolphins (Delphinus sp.) in the Southwestern Atlantic: Testing the Two Species Hypothesis in Sympatry

Abstract

The taxonomy of common dolphins (Delphinus sp.) has always been controversial, with over twenty described species since the original description of the type species of the genus (Delphinus delphis Linnaeus, 1758). Two species and four subspecies are currently accepted, but recent molecular data have challenged this view. In this study we investigated the molecular taxonomy of common dolphins through analyses of cytochrome b sequences of 297 individuals from most of their distribution. We included 37 novel sequences from the Southwestern Atlantic Ocean, a region where the short- and long-beaked morphotypes occur in sympatry, but which had not been well sampled before. Skulls of individuals from the Southwestern Atlantic were measured to test the validity of the rostral index as a diagnostic character and confirmed the presence of the two morphotypes in our genetic sample. Our genetic results show that all common dolphins in the Atlantic Ocean belong to a single species, Delphinus delphis. According to genetic data, the species Delphinus capensis is invalid. Long-beaked common dolphins from the Northeastern Pacific Ocean may constitute a different species. Our conclusions prompt the need for revision of currently accepted common dolphin species and subspecies and of Delphinus delphis distribution.

Post-glacial habitat release and incipient speciation in the genus Delphinus

“…Across the broader geographic range there may be convergent evolution of morphotype for the longbeaked form, but not at the same stage of genetic differentiation (c.f., Heyning and Perrin, 1994;Rosel et al, 1994;Natoli et al, 2006). Cunha et al (2015) have suggested that beak length could be phenotypically plastic, although this will be difficult to test for a dolphin species. As has been often proposed, it is possible that both plasticity and local adaptation play a role (see West-Eberhard, 2003).…”

“…Elsewhere in the world there are long-beaked populations (of the 'capensis' type) proposed to occur off Peru, Chile, Venezuela, Brazil, Argentina, South Africa, Madagascar, India, Indonesia, China, Korea and southern Japan (see Hammond et al, 2008;Jefferson et al, 2008), although for a number of these locations occurrence is not well documented. Based on the 'rostral index' (rostrum length/zygomatic width ratio, see below), Cunha et al (2015) identified 5 'delphis' and 9 'capensis' types off Brazil and Argentina among the 14 skulls they measured, and Tavares et al (2010) identified 47 of 59 measured skulls from this region as the 'capensis' type. However, Cunha et al (2015) suggest that D. capensis should not be used to refer to the long-beaked form, and instead the lbENP form should again be recognized as D. bairdii and all other populations recognized as D. delphis.…”

“…Based on the 'rostral index' (rostrum length/zygomatic width ratio, see below), Cunha et al (2015) identified 5 'delphis' and 9 'capensis' types off Brazil and Argentina among the 14 skulls they measured, and Tavares et al (2010) identified 47 of 59 measured skulls from this region as the 'capensis' type. However, Cunha et al (2015) suggest that D. capensis should not be used to refer to the long-beaked form, and instead the lbENP form should again be recognized as D. bairdii and all other populations recognized as D. delphis.…”

“…However, independent studies comparing populations elsewhere could not find support for two genetic lineages representing distinguishable 'long-beaked' and 'shortbeaked' species worldwide based on 199 samples analyzed for microsatellite DNA and mtDNA control region sequences (Natoli et al, 2006), mtDNA cytochrome b (cytb) sequences for 343 samples and nuclear sequence data at 5 loci for 90 samples (Amaral et al, 2012) and mtDNA cytb sequences for an additional 37 samples (Cunha et al, 2015). Instead, it was suggested that the long-and shortbeaked phenotypes showed convergent adaptation to coastal and pelagic habitat, respectively (Natoli et al, 2006), divergent selection with incomplete lineage sorting (Amaral et al, 2012) or phenotypic plasticity within the species D. delphis (Cunha et al, 2015). A third morphotype with an even longer rostrum and greater number of teeth was identified in the Indian Ocean and off China, and was nominally identified as a subspecies of D. capensis, D. capensis tropicalis (Jefferson and van Waerebeek, 2002).…”

Lunar cycles affect common dolphin Delphinus delphis foraging in the Southern California Bight

“…Two subspecies are recognized in the SCB: the short-beaked and long-beaked common dolphins (D. d. delphis and D. d. bairdii, respectively;Cunha et al 2015). School sizes of 10s to 1000s of individuals have been observed and have been shown to vary in size seasonally, with the largest group sizes in the SCB observed during winter months (Campbell et al 2015).…”