Genomics of Population Differentiation in Humpback Dolphins, Sousa spp. in the Indo-Pacific Ocean.

Speciation is a fundamental process in evolution and crucial to the formation of biodiversity. It is a continuous and complex process, which can involve multiple interacting barriers leading to heterogeneous genomic landscapes with various peaks of divergence among populations. In this study, we used a population genomics approach to gain insights on the speciation process and to understand the population structure within the genus Sousa across its distribution in the Indo-Pacific region. We found 5 distinct clusters, corresponding to S. plumbea along the eastern African coast and the Arabian Sea, the Bangladesh population, S. chinensis off Thailand and S. sahulensis off Australian waters. We suggest that the high level of differentiation found, even across geographically close areas, is likely determined by different oceanographic features such as sea surface temperature and primary productivity.

Cutaneous nodules in Irrawaddy dolphins: an emerging disease in vulnerable populations.

The presence of cutaneous nodules is reported in vulnerable populations of Irrawaddy dolphins Orcaella brevirostris from Malaysia (Kuching, Bintulu-Similajau, Kinabatangan-Segama and Penang Island), India (Chilika Lagoon) and Bangladesh (Sundarbans). Approximately 5700 images taken for photo-identification studies in 2004 to 2013 were examined for skin disorders. Nodules were detected in 6 populations. They appeared as circumscribed elevations of the skin and varied in size from 2 to >30 mm, were sparse or numerous and occurred on all visible body areas. In 8 photo-identified (PI) dolphins from India and Malaysia, the lesions remained stable (N = 2) or progressed (N = 6) over months but did not regress. The 2 most severely affected individuals were seen in Kuching and the Chilika Lagoon. Their fate is unknown. Cutaneous nodules were sampled in a female that died in a gillnet in Kuching in 2012. Histologically, the lesions consisted of thick collagen bundles covered by a moderately hyperplasic epithelium and were diagnosed as fibropapillomas. Whether the nodules observed in the other O. brevirostris were also fibropapillomas remains to be investigated. Disease prevalence ranged from 2.2% (N = 46; Bintulu-Similajau) to 13.9% (N = 72; Chilika) in 4 populations from Malaysia and India. It was not significantly different in 3 study areas in eastern Malaysia. In Chilika, prevalence was significantly higher (p = 0.00078) in 2009 to 2011 (13.9%) than in 2004 to 2006 (2.8%) in 72 PI dolphins. The emergence of a novel disease in vulnerable O. brevirostris populations is of concern.

Population differentiation of 2 forms of Bryde's whales in the Indian and Pacific Oceans.

Accurate identification of units for conservation is particularly challenging for marine species as obvious barriers to gene flow are generally lacking. Bryde's whales (Balaenoptera spp.) are subject to multiple human-mediated stressors, including fisheries bycatch, ship strikes, and scientific whaling by Japan. For effective management, a clear understanding of how populations of each Bryde's whale species/subspecies are genetically structured across their range is required. We conducted a population-level analysis of mtDNA control region sequences with 56 new samples from Oman, Maldives, and Bangladesh, plus published sequences from off Java and the Northwest Pacific. Nine diagnostic characters in the mitochondrial control region and a maximum parsimony phylogenetic analysis identified 2 genetically recognized subspecies of Bryde's whale: the larger, offshore form, Balaenoptera edeni brydei, and the smaller, coastal form, Balaenoptera edeni edeni. Genetic diversity and differentiation indices, combined with a reconstructed maximum parsimony haplotype network, indicate strong differences in the genetic diversity and population structure within each subspecies. Discrete population units are identified for B. e. brydei in the Maldives, Java, and the Northwest Pacific and for B. e. edeni between the Northern Indian Ocean (Oman and Bangladesh) and the coastal waters of Japan.

Clicking in shallow rivers: short-range echolocation of Irrawaddy and Ganges River dolphins in a shallow, acoustically complex habitat.

Toothed whales (Cetacea, odontoceti) use biosonar to navigate their environment and to find and catch prey. All studied toothed whale species have evolved highly directional, high-amplitude ultrasonic clicks suited for long-range echolocation of prey in open water. Little is known about the biosonar signals of toothed whale species inhabiting freshwater habitats such as endangered river dolphins. To address the evolutionary pressures shaping the echolocation signal parameters of non-marine toothed whales, we investigated the biosonar source parameters of Ganges river dolphins (Platanista gangetica gangetica) and Irrawaddy dolphins (Orcaella brevirostris) within the river systems of the Sundarban mangrove forest. Both Ganges and Irrawaddy dolphins produced echolocation clicks with a high repetition rate and low source level compared to marine species. Irrawaddy dolphins, inhabiting coastal and riverine habitats, produced a mean source level of 195 dB (max 203 dB) re 1 µPapp whereas Ganges river dolphins, living exclusively upriver, produced a mean source level of 184 dB (max 191) re 1 µPapp. These source levels are 1-2 orders of magnitude lower than those of similar sized marine delphinids and may reflect an adaptation to a shallow, acoustically complex freshwater habitat with high reverberation and acoustic clutter. The centroid frequency of Ganges river dolphin clicks are an octave lower than predicted from scaling, but with an estimated beamwidth comparable to that of porpoises. The unique bony maxillary crests found in the Platanista forehead may help achieve a higher directionality than expected using clicks nearly an octave lower than similar sized odontocetes.