Omura's whale

Omura's whale or the dwarf fin whale (Balaenoptera omurai) is a species of rorqual about which very little is known. Before its formal description, it was referred to as a small, 'dwarf' or 'pygmy' form of Bryde's whale by various sources. The common name and specific epithet commemorate Japanese cetologist Hideo Omura. The scientific description of this whale was made in Nature in 2003 by three Japanese scientists. They determined the existence of the species by analysing the morphology and mitochondrial DNA of nine individuals – eight caught by Japanese research vessels in the late 1970s in the Indo-Pacific and an adult female collected in 1998 from Tsunoshima, an island in the Sea of Japan. Later, abundant genetic evidence confirmed Omura's whale as a valid species and revealed it to be an early offshoot from the rorqual lineage, diverging much earlier than Bryde's and sei whales. It is perhaps more closely related to its larger relative, the blue whale. In the third edition of Mammal Species of the World, the 'species' is relegated to being a synonym of Balaenoptera edeni. However, the authors note that this is subject to a revision of the genus. The database ITIS lists this as a valid taxon, noting a caveat on the disputed systematics of this species, Balaenoptera edeni and Balaenoptera brydei. The six specimens obtained in the Solomon Sea in 1976 were only noted to be smaller at sexual maturity than the 'ordinary' Bryde's whales caught off New Zealand, whereas the two caught near the Cocos-Keeling Islands in 1978 were not differentiated from the 118 other 'ordinary' Bryde's whales taken in the eastern Indian Ocean, south of Java. As a result of allozyme analysis, their distinctive baleen and small size at physical maturity compared to Bryde's whale, and photographs obtained of the harvested whales (showing their fin whale-like coloration), Shiro Wada and Kenichi Numachi (1991) decided that these eight individuals represented members of a new species of baleen whale. However, due to the lack of a detailed osteological study and the absence of 'conclusive data', the International Whaling Commission decided to consider them only as a regionally distinct group of 'small-form Bryde's whale'. Despite this declaration, the specific status of the Solomon Sea specimens was supported by a mitochondrial DNA study done by Hideyoshi Yoshida and Hidehiro Kato (1999). The identity of these eight specimens was finally resolved in 1998 when an unidentified whale, which had died after colliding with a fishing boat in the Sea of Japan and was towed to Tsunoshima, was examined by Tadasu Yamada, Chief of the Division of Mammals and Birds at the National Science Museum, Tokyo. This specimen closely resembled the individuals caught in the 1970s in external appearance and allowed a complete osteological examination of the putative new species to be conducted. As a result of external morphology, osteology, and mitochondrial DNA analysis of two of the harvested whales and the Tsunoshima specimen, Wada, Masayuki Oishi, and Yamada described Balaenoptera omurai in the 20 November 2003 issue of the journal Nature. In honour of the people of Tsunoshima, who helped remove the flesh from the type specimen, it was given the Japanese vernacular name of Tsunoshima kujira (English: Horn Island whale). The holotype is an 11.03 m (36.2 ft) adult female, NSMT-M32505 (National Science Museum, Tokyo), which stranded at Tsunoshima (34°21′03″N 130°53′09″E / 34.35083°N 130.88583°E / 34.35083; 130.88583) in the southern Sea of Japan on 11 September 1998. It includes a complete skeleton, both complete rows of baleen plates, and frozen pieces of muscle, blubber, and kidney collected by T. K. Yamada, M. Oishi, T. Kuramochi, E. Jibiki, and S. Fujioka. The type locality is the Sea of Japan, which may not be representative of the species’ typical range. The paratypes include the eight specimens (five females and three males), NRIFSF1-8 (National Research Institute of Far Seas Fisheries, Fisheries Research Agency, Shizuoka), collected by Japanese research vessels in the Indo-Pacific in the late 1970s. The longest baleen plate (NRIFSF6 includes 18 more baleen plates), an earplug, and a piece of the sixth thoracic vertebra with associated epiphysis were collected from each individual. Omura's whale has several unique skeletal features that distinguish it from its congeners, namely B. brydei and B. edeni. In B. omurai and B. brydei, the posterior end of the ascending process of the maxilla widens to become squarish, whereas in B. edeni, it is slender and round throughout its length. In B. omurai, this widened posterior portion conceals the premaxilla, which disappears below the maxilla and nasal and does not reach the frontal, whereas in both B. brydei and B. edeni, the premaxilla reaches the frontal. The parietals flare laterally in dorsal view in B. omurai and the Indo-Pacific form of B. brydei, but are invisible in dorsal view in B. edeni and the North Pacific form of B. brydei. B. omurai has two small foramina 'along the suture between the parietal and squamosal in the posterior wall of the temporal fossa', which both B. brydei and B. edeni lack. B. omurai has an oblique ridge on the dorsal side of the maxilla near the base of the rostrum, which is absent in both B. brydei and B. edeni. Unlike B. edeni, the alisphenoid is separate from the squamosal in B. omurai. The head of the first rib is not bifurcated in B. omurai, unlike B. brydei and B. edeni. Omura's whale has a total of 53 vertebrae, including seven cervical (the standard number among mammals), 13 thoracic, 12 lumbar, and 21 caudal. Like all members of its genus, it has only four digits on the manus of each pectoral fin (the third digit is missing). The phalangeal formula is: I-5, II-7, IV-6, V-3.