High gene flow in oceanic bottlenose dolphins (Tursiops truncatus) of the North Atlantic

Abstract

Despite the openness of the oceanic environment, limited dispersal and tight social structure often induce genetic structuring in marine organisms, even in large animals such as cetaceans. In the bottlenose dolphin, mitochondrial and nuclear DNA analyses have revealed the existence of genetic differentiation between pelagic (or offshore) and coastal (or nearshore) ecotypes in the western North Atlantic, as well as between coastal populations. Because previous studies concentrated on continental margins, we analysed the population structure of bottlenose dolphins in two of the most isolated archipelagos of the North Atlantic: the Azores and Madeira. We analysed 112 samples collected on live animals in the two archipelagos, and nine samples collected on stranded animals in Madeira and mainland Portugal. Genetic analyses consisted in molecular sexing, sequencing of part of the mitochondrial hypervariable region, and screening of ten microsatellite loci. We predicted that: 1/ there is at least one pelagic and one or more coastal populations in each archipelago; 2/ populations are differentiated between and possibly within archipelagos. Contrary to these predictions, results indicated a lack of population structure in the study area. In addition, comparison with published sequences revealed that the samples from the Azores and Madeira were not significantly differentiated from samples of the pelagic population of the western North Atlantic. Thus, bottlenose dolphins occurring in the pelagic waters of the North Atlantic belong to a large oceanic population, which should be regarded as a single conservation unit. Unlike what is known for coastal populations, oceanic bottlenose dolphins are able to maintain high levels of gene flow.

Molecular insight into the population structure of common and spotted dolphins inhabiting the pelagic waters of the Northeast Atlantic

“…In common dolphins of the North Atlantic, lack of population structure within the eastern and western basins suggested that individuals might undergo long-distance migration movements (Mirimin et al 2009). In the Azores, high levels of gene flow were found in bottlenose dolphins of the species T. truncatus (Quérouil et al 2007), and a photo-identification study revealed that some individuals travel between groups of islands and probably come from outside the archipelago ). In the eastern tropical Pacific, capture-recapture data showed that dolphins of the genus Stenella can travel more than 1000 km, and seasonal shifts in distributions suggested that movements could be as wide as 2500 km (Reilly 1990).…”

“…Residency has been shown to be associated with population differentiation in other species (e.g., bottlenose dolphins of the species T. aduncus in Australia, Möller and Beheregaray 2004; spinner dolphins in the South Pacific, Oremus et al 2007). However, in the Azores, no population differentiation was found in T. truncatus despite the existence of known resident individuals (Quérouil et al 2007;Silva et al 2008). The extent of differentiation might depend on the proportion of individuals that are resident.…”

“…For instance, in common bottlenose dolphins, coastal forms tend to be resident and form small groups (Connor 2000), and populations are differentiated over small geographic distances (Sellas et al 2005;Parsons et al 2006). On the contrary, offshore forms of common bottlenose dolphins tend to be transient, form large groups (Connor 2000), and maintain high gene flow over wide geographic areas (Quérouil et al 2007). It is likely that other species exhibit similar patterns, but very little information is available on the population structure of dolphins in pelagic environment, especially in distant offshore regions.…”

“…The most abundant species are the short-beaked common dolphin, Delphinus delphis, Linnaeus 1758, the Atlantic spotted dolphin, Stenella frontalis (Cuvier 1829) and the common bottlenose dolphin (Silva et al 2003;Freitas et al 2004). Whilst the genetic structure of the latter species was studied recently in the Azores and Madeira (Quérouil et al 2007); there is virtually no information available on the stock structure of the former two species in that region. Preliminary data indicate that the common dolphin is resident in the Azores, but its abundance fluctuates seasonally (Quérouil et al 2008).…”

Effective population size of an offshore population of bottlenose dolphins, Tursiops truncatus, from the São Pedro and São Paulo Archipelago, Brazil

“…However, it was not considered as a valid species of Tursiops by the Committee on Taxonomy of The Society for Marine Mammalogy (2016). Despite the broad geographic coverage of all these studies, bottlenose dolphins have mostly been studied in peri-continental and shallow waters, and very little is known about offshore populations (Klatsky et al, 2007;Quérouil et al, 2007;Baird et al, 2009).…”