Oceanographic drivers of population differentiation in Indo-Pacific bottlenose (Tursiops aduncus) and humpback (Sousa spp.) dolphins of the northern Bay of Bengal

The Bay of Bengal is one of the most productive ecosystems in the northern Indian Ocean and it harbours a rich community of cetaceans, including Indo-Pacific bottlenose (Tursiops aduncus) and humpback (Sousa spp.) dolphins. The taxonomy of these genera has been controversial, but within the Indian Ocean both seem to be divided into phylogenetically discrete units that range from the east to the west. Within the Sousa genus, S. plumbea is distributed in the western Indian Ocean while S. chinensis is distributed in the eastern Indian and western Pacific Ocean. T. aduncus has a discontinuous distribution throughout the Indo-Pacific Ocean and two different phylogenetic units are known to exist, one along the eastern African coast and another one in the eastern Indian and west Pacific Ocean. In this study we investigate the phylogeography of Indo-Pacific humpback and bottlenose dolphins in the northern Bay of Bengal. We sequenced the mitochondrial DNA control region for 17 bottlenose and 15 humpback dolphins and compared the results with previously published sequences within each genus. In both cases, we found that Bangladesh dolphins are genetically different from neighbouring populations. While the Bangladesh T. aduncus seem to be more closely related to the African T. aduncus form than the Pacific form, Sousa spp. seem to be more closely related to individuals from Australia. The genetic uniqueness of these populations has important evolutionary implications, due to their isolation, coastal distribution in a geographic cul-de-sac characterized by an extreme infusion, redistribution and recycling of biological productivity, and conservation implications since their survival is threatened in particular by fatal interactions with fisheries. We suggest that the particular and extreme oceanographic conditions found in the Bay of Bengal may be driving speciation in these dolphins and other marine megafauna.info:eu-repo/semantics/publishedVersio

Biosonar parameters of Irrawaddy dolphins (<i>Orcaella brevirostris</i>) and Ganges river dolphins (<i>Platanista gangetica gangetica</i>).

*<p>Click parameter abbreviations: SL_pp : peak-to-peak source level; SL_RMS : RMS source level within a −10 dB energy window; SL_EFD: Energy flux density source level within a −10 dB energy window; D_-10dB: Click duration (−10 dB energy window); Fc: centroid frequency; Fp: peak frequency; BW: Bandwidth (−3 dB, −10 dB or root-mean-square); Q_RMS: Ratio of centroid frequency to RMS bandwidth; ICI: Inter-click interval.</p

Directionality of Ganges river dolphin biosonar.

<p>Composite horizontal directionality plot of biosonar signals from Ganges river dolphins with original data (black squares) redigitized from Bahl et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059284#pone.0059284-Bahl1" target="_blank">[65]</a>. Gray line is a best fitting piston model of an on-axis click transmitted through a circular piston with a radius of 9.7 cm. The symmetrical -3 dB beamwidth of the fitted piston model is 14.5 degrees.</p

Interclick intervals of Irrawaddy dolphins and Ganges river dolphin echolocation signals.

<p>Histograms show the distribution of interclick intervals for clean series of off-axis clicks from Irrawaddy dolphins (A) and Ganges river dolphins (B). Black interrupted lines show log-normal probability density functions fitted to the data. For Irrawaddy dolphins, median ICI was 30.1 ms (N = 923) while for Ganges river dolphins, median ICI was 27.8 ms (N = 614).</p

Field site and distribution of Irrawaddy dolphins and Ganges river dolphins.

<p>A) Map of the Sundarbans mangrove forest, Bangladesh, including sighting data of Ganges river dolphins (triangles) and Irrawaddy dolphins (circles). Adapted with permission from Smith et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059284#pone.0059284-Smith5" target="_blank">[73]</a>. Inserts show pictures of B) Irrawaddy dolphin, and C) Ganges river dolphin, taken by E. & R. Mansur, WCS.</p

Calibration of the acoustic localization procedure with a vertical 4-hydrophone array.

<p>Top: Localization range (mean ± SD) given by the acoustic localization procedure, as a function of the calibration distance. Precise localization indicated by the dotted line. Bottom: RMS error in the estimated transmission loss as a function of range from the array.</p

Species discrimination based on centroid frequency relevant for passive acoustic monitoring in the Sundarbans.

<p>A: Theoretical normalized probability density functions based on centroid frequency estimates (mean ± SD from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059284#pone-0059284-t001" target="_blank">Table 1</a>) from Ganges river dolphins (grey: PGG), and Irrawaddy dolphins (black: OB) and based on peak frequency estimates from Yangtze finless porpoise (NPA) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059284#pone.0059284-Li1" target="_blank">[42]</a> assuming normally distributed estimates. Abbreviations are for latin species names. Stacked bar plot indicates probability density of centroid frequency estimates for this study. Best separation criterion (stipled lines) provides a theoretical 98.7% correct classification of Ganges river dolphin clicks and 98.9% correct classification of Irrawaddy dolphin clicks. B+C: For off-axis clicks, spectral distortion increases low-frequency energy so centroid frequency estimates decrease (B: Irrawaddy dolphins, and C: Ganges river dolphins), reducing success rate of Irrawaddy classifications to 72.7% (N = 971) with the remainder being misclassified as Ganges river dolphins, and with Ganges river dolphins being classified successfully 99.2% of the time (N = 641).</p

The unique cranial morphology of Ganges river dolphins.

<p>Cranial morphology of a Ganges river dolphin as seen from A) a left lateral and slightly anterior viewpoint, and B) an anterior viewpoint looking back along the anterior-posterior axis. Notice the unusual, highly porous bony maxillary crests that project anteriorly over the rostrum and nearly encircle the melon. Photos by A. Galatius.</p

Source levels of Irrawaddy dolphins and Ganges river dolphins.

<p>A) Estimated RMS source levels (SL) as a function of range between hydrophone array and estimated source position for both Irrawaddy dolphins (black) and Ganges river dolphins (grey). B) Normalized density estimates of the SL from both species, estimated using normal kernels with a 3 dB kernel width.</p

Representative echolocation clicks from Ganges river dolphins and Irrawaddy dolphins.

<p>A: Signal waveform (solid line) and envelope (interrupted line) of a Ganges river dolphin echolocation click. B: Normalized power spectrum of a Ganges river dolphin echolocation click. C: Signal waveform (solid line) and envelope (interrupted line) of Irrawaddy dolphin echolocation click. D: Normalized power spectrum of Irrawaddy dolphin echolocation click. Time-domain signal is shown as the instantaneous source level, corrected for transmission loss and absorption between source position and hydrophone (note the different amplitude scales). Power spectra are constructed from a 32-point rectangular window around the peak of the envelope, and interpolated with a factor 320, for a spectral resolution of 24 Hz.</p