Thursday, August 27, 2015

[Cetology • 2015] Sousa chinensis taiwanensis • Diagnosability and Description of A New Subspecies of Indo-Pacific Humpback Dolphin, Sousa chinensis (Osbeck, 1765), from the Taiwan Strait


Figure 5: Typical Indo-Pacific humpback dolphins with unspotted and spotted bodies. Photographs of Indo-Pacific humpback dolphins from the Pearl River Estuary (A, C) and the eastern Taiwan Strait (B, D).
Photographs by J.Y. Wang/FormosaCetus Research and Conservation Group.
Figure 1: Map of the study area. Sampling locations of Indo-Pacific humpback dolphins analysed in this study. The green shaded area represents the known distribution of these dolphins, and the red star and yellow circles show the locations from where the holotype and paratype specimens of the Taiwanese humpback dolphin Sousa chinensis taiwanensis were collected, respectively. Some bathymetric contours in the Taiwan Strait are also shown.

Abstract
Background
Subspecies recognition can affect how people (scientists and non-scientists alike) view organisms and thus has important implications for research on, as well as the conservation of, these entities. Recently, a small group of Indo-Pacific humpback dolphins was discovered inhabiting the waters off central western Taiwan. This geographically isolated population possesses pigmentation patterns that are subtly, but noticeably, different from their nearest conspecifics in the neighbouring waters of the Jiulong River Estuary and Pearl River Estuary of mainland China. Due to this population’s low and declining numbers and the numerous threats it faces, it was assessed as critically endangered by the Red List of Threatened Species of the International Union for Conservation of Nature. The purpose of this study is to examine the degree of differentiation of the Taiwanese population to determine if subspecies recognition is warranted.

Results
Analysis of the degree of differentiation in pigmentation patterns revealed nearly non-overlapping distributions between dolphins from Taiwanese waters and those from the Jiulong River + Pearl River estuaries of mainland China (the nearest known populations). The Taiwanese dolphins were clearly diagnosable from those of the Jiulong River + Pearl River estuaries under the most commonly accepted ‘75% rule’ for subspecies delimitation (with 94% of one group being separable from 99+% of the other). Evidence of geographical isolation and behavioural differences also provided additional support for the distinctiveness of the Taiwanese dolphins.

Conclusions
Together, the evidence strongly demonstrated that the Taiwanese humpback dolphin population is differentiated at the subspecies level and on an evolutionary trajectory that is independent from that of dolphins from adjacent waters of mainland China (i.e. Jiulong River + Pearl River estuaries). As a result, the taxonomy of Sousa chinensis was revised to include two subspecies: the Taiwanese humpback dolphin, Sousa chinensis taiwanensis subsp. nov., and the Chinese humpback dolphin, Sousa chinensis chinensis (the nominotypical subspecies). These subspecies are described, and the holotype and paratype specimens for S. c. taiwanensis are established.


Figure 5: Typical Indo-Pacific humpback dolphins with unspotted and spotted bodies. Photographs of Indo-Pacific humpback dolphins from the Pearl River Estuary (A, C) and the eastern Taiwan Strait (B, D).
Photographs by J.Y. Wang/FormosaCetus Research and Conservation Group.
Figure 1: Map of the study area. Sampling locations of Indo-Pacific humpback dolphins analysed in this study. The green shaded area represents the known distribution of these dolphins, and the red star and yellow circles show the locations from where the holotype and paratype specimens of the Taiwanese humpback dolphin Sousa chinensis taiwanensis were collected, respectively. Some bathymetric contours in the Taiwan Strait are also shown.

Subspecies Descriptions

Order Cetartiodactyla Montgelard, Catzefils and Douzery 1997
Cetacea Brisson, 1762
Odontoceti Flower, 1867

Family Delphinidae Gray, 1821
Genus Sousa Gray, 1866
Sousa chinensis (Osbeck, 1765)

Sousa chinensis chinensis (Osbeck, 1765)

Type locality: The species was first described from observing dolphins swimming in the Canton (=Pearl) River Estuary. Swinhoe’s specimen that was described by Flower (1870) was collected from the waters of Quemoy (presently known as Chinmen Island) in the Jiulong River Estuary, which is roughly 500 km northeast of the Pearl River Estuary.

Etymology: The ‘chinensis’ name refers to the general location from where the species was originally described. To maintain consistency with the scientific name of the subspecies, we suggest the following common names: Chinese humpback dolphin or the Chinese white dolphin (a direct translation of the most common local moniker, ‘Zhonghua bai hai tun’). We recognize that before the present revision, Chinese white dolphin referred to the species. To avoid confusion, we recommend that Indo-Pacific humpback dolphin be retained as the common name for the species (following Jefferson and Rosenbaum 2014) while Chinese humpback dolphin or Chinese white dolphin is used in reference to the nominotypical subspecies S. c. chinensis. This nomenclatural arrangement should facilitate increased local acceptance and usage of the Indo-Pacific humpback dolphin name for the species because it does not negate the local common name for the local animals and would also allow easier (and logical) accommodations for any future subspecies that may be found within S. c. chinensis without the need to change the species’ common name. 

Distribution: The nominate subspecies is found from the Yangtze River Estuary in central China south through Southeast Asia (east to at least Borneo) and extending west throughout the northern rim of the Indian Ocean to at least Orissa, India (Jefferson and Rosenbaum 2014). There appears to be at least eight major concentrations of this subspecies in the coastal waters of China (Jefferson 2000; Jefferson and Hung 2004), but others will almost certainly be found throughout the species’ distribution. Further subspecies divisions are probable.

Sousa chinensis taiwanensis subsp. nov

Type locality: The specimen was found stranded on a beach of Tongshiao, Miaoli County (Taiwan) on 25 September 2009. The specimen’s condition was fresh (code 2 of Geraci and Lounsbury 2005) and highly emaciated and showed clear signs of net entanglement (around the tail stock, flukes, rostrum, flippers and possibly the dorsal fin) that likely resulted in her death.

Etymology: The proposed subspecies name reflects the endemic distribution of the subspecies in the waters of western Taiwan and in the same flavour as other endemic subspecies of Taiwan. Similar to the common names for S. c. chinensis, we suggest Taiwanese humpback dolphin or Taiwanese white dolphin, which is the direct translation of a local common name, ‘Taiwan bai hai tun’.

Distribution: This subspecies is endemic to the shallow coastal waters of central western Taiwan (=eastern Taiwan Strait) including the waters of Miaoli, Taichung, Chunghua, Yunlin, Chiayi and Tainan counties. Dolphins have been reported from as far north as the waters influenced by the Zhonggang and Houlong rivers and south to the waters influenced by the Tzengwen River (near Chigu, Tainan County). With the exception of the intertidal waters inshore of large sandbars of Changhua County, they are generally found less than 3 km from the shore, in waters <30 m deep, and tend to be more commonly observed in and near estuaries (Dares et al. 2014). The known area occupied by this subspecies is about 600 km2 , but the full distribution is likely larger, extending potentially further north to the influences of the Danshuei River Estuary, where suitable habitat appears to be present (Wang et al. 2007a; Ross et al. 2010).


John Y Wang, Shih Chu Yang and Samuel K Hung. 2015. Diagnosability and Description of A New Subspecies of Indo-Pacific Humpback Dolphin, Sousa chinensis (Osbeck, 1765), from the Taiwan Strait. Zoological Studies. 54:36. DOI: 10.1186/s40555-015-0115-x

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