Common dolphin morphotypes: Niche segregation or taxonomy?

Abstract

In the extensive geographical distribution of the common dolphin, several morphotypes of uncertain taxonomic status, identified by the relative length of their rostra, have been established. We investigated variation in skull morphometrics and isotopic signatures of carbon and nitrogen (d 13 C and d 15 N) in individuals from the subtropical waters of the North Atlantic Ocean, in order to assess population structure and taxonomic status. The distribution of the relative rostral lengths (RL) of individuals followed a cline with no subgrouping. Both d 13 C and d 15 N showed high variability, which suggests that individuals use habitat heterogeneously. d 15 N correlated with RL, indicating that longer beaked individuals either feed at a higher trophic level and/or inhabit waters located further offshore than shorter beaked animals. Although d 13 C and d 15 N were correlated, RL and d 13 C failed to show any correlation, possibly because the incremental effect of trophic level on d 13 C has been offset by the potential allopatric distribution of the morphotypes. We conclude that both the long-beaked and short-beaked forms of common dolphin do occur off Mauritania but, in contrast to other areas, the existence of more than one species in the region is questioned because both stable isotopes and skull morphometric appear to reflect differential use of habitat rather than taxonomy. Even though proposed previously by some authors, this is the first time that skull differentiation in common dolphins has been demonstrated to be likely due to niche segregation and not to speciation. This reveals that caution is needed when considering that long-beaked and shortbeaked common dolphins from outside the eastern North Pacific fall into the taxonomic model described for this region.

Variation in δ15N and δ13C stable isotope values in common dolphins (Delphinus spp.) worldwide, with particular emphasis on the eastern North Atlantic populations

“…The long‐beaked common dolphin from the Gulf of California (GC) showed the highest δ 15 N values followed by the north‐west African population (ZIII) due probably to the presence of the long‐beaked morphotype, which is present in both areas. Consistent with this, previous studies have shown that individuals with longer beaks tend to feed at higher trophic levels than animals with shorter beaks . The short‐beaked common dolphins from Areas ZI+ZII+GAL showed the third highest δ 15 N values of the populations analysed, consistent with the published TLs for the coast of Galicia.…”

“…Only adult animals were used in this study in order to avoid variability in stable isotope abundances due to ontogenic development. The physical maturity of the specimens was established as described by Pinela and co‐authors …”

“…Conversely, when the same differentiation was described for the eastern Atlantic Ocean, the long‐beaked morphotype revealed a tendency for an offshore/pelagic distribution compared with the short‐beaked morphotype . Decades later, the taxonomical status of these morphotypes is still a matter of debate and it is unclear whether the observed diversification is determined genetically or ecologically, or by both causes . Natoli and colleagues proposed that the long‐beaked morphotype originated independently in different regions and suggested that adaptation to local environments was driving local speciation.…”

“…Thus, the long‐beaked morphotype would be mostly of coastal or neritic distribution, while the short‐beaked would be of both inshore and offshore distribution . Conversely, when the same differentiation was described for the eastern Atlantic Ocean, the long‐beaked morphotype revealed a tendency for an offshore/pelagic distribution compared with the short‐beaked morphotype . Decades later, the taxonomical status of these morphotypes is still a matter of debate and it is unclear whether the observed diversification is determined genetically or ecologically, or by both causes .…”

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

“…Phenotypic differentiation among populations and species inhabiting coastal and offshore aquatic environments is well established, and sometimes clearly associated with adaptive differentiation (see Schluter, 2000). For the genus Delphinus, there are coastal populations worldwide that show morphological differentiation associated with longer beak length among other characteristics, although there are also some short-beaked populations found nearshore (for example, in the Mediterranean Sea), and stable isotope analysis of beach-cast specimens was used to question the distribution of longbeaked forms off North Africa (Pinella et al, 2011). However, as we confirm here, only the long-beaked coastal population off southern California and Baja California is strongly genetically differentiated (Figure 2 and Supplementary Table S4).…”