Phylogeography and alpha taxonomy of the common dolphin (Delphinus sp.)

The resolution of taxonomic classifications for delphinid cetaceans has been problematic, especially for species in the genera Delphinus, Tursiops and Stenella. The frequent lack of correspondence between morphological and genetic differentiation in these species raises questions about the mechanisms responsible for their evolution. In this study we focus on the genus Delphinus, and use molecular markers to address questions about speciation and the evolution of population structure. Delphinus species have a worldwide distribution and show a high degree of morphological variation. Two distinct morphotypes, long-beaked and short-beaked, have been considered different species named D. capensis and D. delphis, respectively. However, genetic differentiation between these two forms has only been demonstrated in the Pacific. We analysed samples from eight different geographical regions, including two morphologically defined long-beaked form populations, and compared these with the eastern North Pacific populations. We found high differentiation among the populations described as long-beaked instead of the expected monophyly, suggesting that these populations may have evolved from independent events converging on the same morphotype. We observed low genetic differentiation among the short-beaked populations across a large geographical scale. We interpret these phylogeographical patterns in the context of life history and population structure in related species.     

Criteria for assessing maturity of skulls in the common dolphin,Delphinus sp., from New Zealand waters

Knowledge about the maturity status of specimens included in evolutionary, taxonomic or life history investigations is fundamentally important. This study investigated the use of the degree of cranial suture fusion, the developmental status of cranial bones, and the degree of tooth wear as indicators for cranial maturity status in Delphinus sp. from New Zealand waters. In total, 15 sutures, one joint and three nonmetric characters were assessed on 66 skulls obtained from stranded and bycaught individuals sampled between 1932 and 2011. A suture index (SI) was computed based on 10 sutures, in which degree of fusion was correlated with age and the three misclassification indices (MI), calculated for a given suture, were <50%. In addition to these, five premaxilla‐maxilla fusion and seven tooth wear categories were assessed. Results suggest that New Zealand Delphinus sp. skulls should be regarded as cranially mature if at least two of the following criteria are met: (1) individuals assessed as sexually mature, (2) aged ≥ 11 yr, (3) SI ≥ 8, and (4) premaxilla‐maxilla fusion ≥ 75% of the length of the dorsal side of the rostrum. Presence of any number of rostral teeth worn to the gum line provided further evidence for cranial maturity.     

Comparison of reproductive parameters for populations of eastern North Pacific common dolphins: Delphinus capensis and D. delphis

Reproductive parameters were estimated and compared for eastern North Pacific populations of common dolphins using specimen and photogrammetric data. Age and length data for Delphinus capensis and D. delphis specimens recovered as bycatch or strandings were used to estimate the postnatal growth rates needed to estimate age for calves measured in aerial photographs. Bayesian methods propagated uncertainty among models and revealed that the 2009 cohort of calves had birth dates centered on 6 March 2009 for D. capensis and 12 December 2008 for D. delphis. The evidence for discrete calving seasons suggests a mechanism of reproductive isolation has evolved between species. Photogrammetric data and Bayesian methods were also used to estimate the average length at which calves swim independently: 145.1 cm (≈ 11.1 mo) in D. capensis and 140.1 cm (≈ 14.0 mo) in D. delphis, and the proportion of calves (calves/dolphins counted): 0.045 in D. capensis and 0.069 in D. delphis. The latter parameter was converted to an index of calf production (calf/female dolphin) that was >50% lower than pregnancy rates suggesting few births occurred during the study year. Comparisons of regional differences in calf production suggest variability in habitat use patterns within the study area.     

External morphology of the short-beaked common dolphin, Delphinus delphis: growth, allometric relationships and sexual dimorphism

Growth, allometric relationships and sexual dimorphism are described from measurements of 105 male, 149 female and 38 unsexed specimens of short‐beaked common dolphin, Delphinus delphis, stranded along the Irish coastline (53.8% of the sample) or by‐caught in fisheries (46.2% of the sample), from 1990 to 2003. For each dolphin, 24 external body length measurements were recorded. Ages were determined for 183 dolphins by analysis of growth layer groups in the dentine. Males ranged in total body length (TBL) from 105 to 231 cm and females from 93 to 230 cm, with a maximum age of 25 years obtained for both sexes. Using a single Gompertz growth curve, asymptotic values obtained for TBL were 211.6 cm and 197.4 cm for males and females, respectively. Asymptotic lengths were attained at 11 years in males and 9 years in females. The gestation period was estimated to last approximately 11.5 months. Sexual size dimorphism (SSD) was evident, with males being significantly larger than females for 20 of the characters measured, and an SSD ratio of 1.06 was obtained. Sexual shape dimorphism was lacking, except for the presence of prominent postanal humps in mature males.     

Comparative milk composition of the bottlenosed dolphin (Tursiops truncatus), humpback dolphin (Sousa plumbea) and common dolphin (Delphinus delphis) from southern African waters

1. Gross compositional data for milk samples of Tursiops truncatus, Sousa plumbea and Delphinus delphis are presented and compared with existing cetacean milk values. 2. Two factors appear to govern fat content and calorific value of the milk: (a) habitat, and (b) the stage of lactation. 3. Delphinus milk is unusual in that it has a higher P than Ca content. 4. The Fe content in cetacean milk is higher than that for terrestrial mammals and may be related to improved O2 binding requirements.     

Seascape genetics of a globally distributed, highly mobile marine mammal: the short-beaked common dolphin (genus Delphinus)

Identifying which factors shape the distribution of intraspecific genetic diversity is central in evolutionary and conservation biology. In the marine realm, the absence of obvious barriers to dispersal can make this task more difficult. Nevertheless, recent studies have provided valuable insights into which factors may be shaping genetic structure in the world's oceans. These studies were, however, generally conducted on marine organisms with larval dispersal. Here, using a seascape genetics approach, we show that marine productivity and sea surface temperature are correlated with genetic structure in a highly mobile, widely distributed marine mammal species, the short-beaked common dolphin. Isolation by distance also appears to influence population divergence over larger geographical scales (i.e. across different ocean basins). We suggest that the relationship between environmental variables and population structure may be caused by prey behaviour, which is believed to determine common dolphins' movement patterns and preferred associations with certain oceanographic conditions. Our study highlights the role of oceanography in shaping genetic structure of a highly mobile and widely distributed top marine predator. Thus, seascape genetic studies can potentially track the biological effects of ongoing climate-change at oceanographic interfaces and also inform marine reserve design in relation to the distribution and genetic connectivity of charismatic and ecologically important megafauna.     

Brain lesions in a Pacific white-sided dolphin (Lagenorhynchus obliquidens)

A young, male, free-ranging Pacific white-sided dolphin (Lagenorhynchus obliquidens) was found disoriented and died after being held in captivity for several months. Malacic lesions in several areas of the brain were associated with helminth eggs. The appearance and location of these eggs suggested they were of the genus Nasitrema.     

Influences of past climatic changes on historical population structure and demography of a cosmopolitan marine predator,the common dolphin (genus Delphinus)

Climatic oscillations during the Pleistocene have greatly influenced the distribution and connectivity of many organisms, leading to extinctions but also generating biodiversity. While the effects of such changes have been extensively studied in the terrestrial environment, studies focusing on the marine realm are still scarce. Here we used sequence data from one mitochondrial and five nuclear loci to assess the potential influence of Pleistocene climatic changes on the phylogeography and demographic history of a cosmopolitan marine predator, the common dolphin (genus Delphinus). Population samples representing the three major morphotypes of Delphinus were obtained from 10 oceanic regions. Our results suggest that short‐beaked common dolphins are likely to have originated in the eastern Indo‐Pacific Ocean during the Pleistocene and expanded into the Atlantic Ocean through the Indian Ocean. On the other hand, long‐beaked common dolphins appear to have evolved more recently and independently in several oceans. Our results also suggest that short‐beaked common dolphins had recurrent demographic expansions concomitant with changes in sea surface temperature during the Pleistocene and its associated increases in resource availability, which differed between the North Atlantic and Pacific Ocean basins. By proposing how past environmental changes had an effect on the demography and speciation of a widely distributed marine mammal, we highlight the impacts that climate change may have on the distribution and abundance of marine predators and its ecological consequences for marine ecosystems.     

Population genetic structure and taxonomy of the common dolphin (Delphinus sp.) at its southernmost range limit: New Zealand waters

New Zealand is the southernmost limit of the common dolphin's (genus Delphinus) distribution in the Pacific Ocean. In this area, common dolphins occur in both coastal and oceanic habitats, exhibit seasonal and resident occurrence, and present high morphological variability. Here we investigated the population structure and the taxonomic identity of common dolphins (Delphinus sp.) within New Zealand waters using 14 microsatellite loci, 577 bp of the mtDNA control region, and 1,120 bp of the mtDNA cytochrome b gene across 90 individuals. We found high genetic variability and evidence of population expansion. Phylogenetic analyses conducted to clarify the taxonomic status of New Zealand common dolphins did not show any clustering reflecting geographic origin or morphotypes. The microsatellite analysis showed genetic differentiation between Coastal and Oceanic putative populations, while mtDNA revealed significant genetic differentiation only between the Hauraki Gulf and other putative groups. Our results suggest that differences in habitat choice and possible female site fidelity may play a role in shaping population structure of New Zealand common dolphins.     

Stratification and intra- and inter-specific differences in fatty acid composition of common dolphin (Delphinus sp.) blubber: implications for dietary analysis

Sixty-five fatty acids were quantified in the blubber of common dolphins (Delphinus delphis, D. capensis) incidentally caught off the coast of southern California. Dolphins were grouped by sex, reproductive status and species, and a blubber sample was collected at a mid-lateral site located caudal to the trailing edge of the dorsal fin. Samples were divided horizontally into inner, middle and outer layers and gradients in fatty acid content (mass percent) were observed across the depth of the blubber. Levels of monounsaturated fatty acids were greatest in the outer layer, whereas levels of saturated and polyunsaturated fatty acids were greatest in the inner layer. Degree of stratification was greatest in sexually mature dolphins. Blubber of sexually immature, but physically mature, male dolphins was also highly stratified, suggesting that this difference may be attributed to differences in diet. Classification and regression tree analysis resulted in the fewest misclassifications when dolphins were grouped by species, possibly indicating that these closely related animals forage on different prey species. Dietary-derived fatty acids were typically selected as splitting criteria in classification and regression tree analyses, suggesting that the observed differences in fatty acid composition between the various groups of dolphins may be attributed to differences in diet.     

Complete amino acid sequence of the myoglobin from the Pacific spotted dolphin, Stenella attenuata graffmani.

The complete amino acid sequence of the major component myoglobin from the Pacific spotted dolphin, Stenella attenuata graffmani, was determined by the automated Edman degradation of several large peptides obtained by specific cleavage of the protein. The acetimidated apomyoglobin was selectively cleaved at its two methionyl residues with cyanogen bromide and at its three arginyl residues by trypsin. By subjecting four of these peptides and the apomyoglobin to automated Edman degradation, over 80% of the primary structure of the protein was obtained. The remainder of the covalent structure was determined by the sequence analysis of peptides that resulted from further digestion of the central cyanogen bromide fragment. This fragment was cleaved at its glutamyl residues with staphylococcal protease and its lysyl residues with trypsin. The action of trypsin was restricted to the lysyl residues by chemical modification of the single arginyl residue of the fragment with 1,2-cyclohexanedione. The primary structure of this myoglobin proved to be identical with that from the Atlantic bottlenosed dolphin and Pacific common dolphin but differs from the myoglobins of the killer whale and pilot whale at two positions. The above sequence identities and differences reflect the close taxonomic relationship of these five species of Cetacea.     

Behaviorally measured tactile sensitivity in the common bottlenose dolphin,Tursiops truncatus

Little research has been conducted on the somatosensory system of toothed whales and it remains uncertain how tactile sensitivity varies about their bodies. In this study, tactile sensitivity to high-frequency (250-Hz) displacement of the skin was quantified in three trained adult common bottlenose dolphins (Tursiops truncatus) using a vibratory device (tactor). The magnitude of skin displacement was controlled by varying the voltage to the tactor held against the skin surface with a constant force. Tactile thresholds were determined using an adaptive method of limits in which dolphins reported perception of the tactile stimulus by producing a whistle. Displacement thresholds ranged from 2.4 to 40 μm, with the greatest sensitivity found along the rostrum, melon, and blowhole. Sensitivity decreased caudally along the body, with the dorsal fin and tip of the fluke being the least sensitive locations tested. The results support hypotheses that the follicles on the dolphin rostrum are particularly important for perception. The reduction in tactile sensitivity at the appendages is consistent with their primary role in stabilization and locomotion compared to exploration or environmental sensing.     

Isolation and characterization of novel microsatellite loci for the short‐beaked common dolphin (Delphinus delphis) and cross‐amplification in other cetacean species

Seven tetranucleotide and three dinucleotide microsatellite loci were isolated from the short‐beaked common dolphin (Delphinus delphis). Number of alleles and observed heterozygosity ranged from three to twelve (mean 8.4) and from 0.087 to 0.978 (mean 0.741), respectively. All loci were tested in four other odontocete species and showed polymorphisms in most locus–species combinations. This panel of nuclear markers will be useful for the investigation of population ecology and social interactions in common dolphins and other cetacean species.     

Population differentiation in the Pacific white-sided dolphin Lagenorhynchus obliquidens inferred from mitochondrial DNA and microsatellite analyses

We investigated genetic diversity and differentiation of the Pacific white-sided dolphin (Lagenorhynchus obliquidens) in Japanese coastal waters and offshore North Pacific by analyzing mitochondrial DNA and nuclear microsatellite variation. A total of 519 bp of the mitochondrial control region was sequenced and five microsatellite locus were genotyped for 59 individuals. A high level of haplotypic diversity (h=96.1%), moderate level of nucleotide diversity (pi=1.65%) and average expected heterozygosity (HE=0.66-0.76) were within an extent of those reported for other odontocetes. Consistent genetic difference between the samples from Japanese coastal Pacific-Sea of Japan and offshore North Pacific was indicated by analyses of molecular variance (AMOVAs) based on mtDNA and microsatellite variations, comparison of genetic variabilities, and geographical distributions of mtDNA haplotypes and microsatellite alleles. This result suggests that Pacific white-sided dolphins in each of the above two areas belong to different populations between which gene flow has been severely restricted. The low genetic diversity and mtDNA genealogy of the population in Japanese coastal waters suggest that it originated from a small population that colonized the Sea of Japan or that experienced population reduction when this Sea was isolated from the North Pacific during a glacial period in the Late Pleistocene.