Omura's Whale

## Description

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*.

## Appearance

The Omura's whale is a relatively small baleen whale 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. Its appearance resembles the larger Fin whale (thus the alternate common names of Dwarf fin whale and Little fin whale), both having a dark gray left lower jaw, and on the right side a white mandible patch, a white blaze, a dark eye stripe, a white inter-stripe wash, as well as a white chevron on the back, pectoral fins with a white anterior border and inner surface, and flukes with a white ventral surface and black margins. Like fin whales, the Omura's whale also exhibits a white left gape and a dark right gape, a reversal of the asymmetrical pigmentation on the lower jaw.

## Distribution

Omura's whales are found in the Indo-Pacific and the Atlantic Oceans. Their range includes southern Japan, South Korea, China, Taiwan, Hong Kong, the Philippines, Vietnam, Malaysia, Thailand, Indonesia, the Cocos (Keeling) Islands, the Andaman Islands, Australia, the Solomon Islands, New Caledonia, Sri Lanka, the Chagos Archipelago, Iran, Egypt, northwestern Madagascar, Mauritania, Brazil and in the vicinity of Saint Peter and Saint Paul Archipelago. Omura's whales don't migrate and live in tropical and warm-temperate seas. They spend most of their time in shallow waters and rarely venture into deep waters.

## Behavior

Little is known about the behavior of Omura's whales. Their blow is low and diffuse. After surfacing, the dorsal fin is usually not visible until after the head and splashguard have disappeared and they don't fluke when diving. They usually travel alone or in pairs but loose aggregations of as many as a dozen whales could be seen. Omura's whales communicate with amplitude-modulated songs. They sing their melody on average within 9 seconds followed by a tonal call of 4 seconds in duration. These songs are repeated every two to three minutes, sometimes for as long as thirteen hours. Omura's whales may even sing their song in overlapping choruses.

## Diet

Omura’s whales are carnivores (piscivores). They have been seen lunge feeding on krill and it is known that their diet also includes crustaceans and fish.

## Reproduction

Nothing is known about the reproductive habits in Omura's whales. Generally, gestation in baleen whales lasts 11-12 months, so that both mating and birthing occur at the same time of year. Cows give birth to a single calf, which is usually weaned after 6-12 months, depending on the species.

## Population

### Population threats

The main threats to Omura's whales include habitat loss due to pollution and noises, entanglement with fishing gears, and ship strikes.

### Population number

Due to the WDC (Whale and Dolphin Conservation) resource, the total population size of the Omura's whale in the oceans is about 50,000 individuals. Currently, this species is classified as Data Deficient (DD) on the IUCN Red List site.

## Fun Facts for Kids

The Omura's whale was named after Japanese cetologist Hideo Omura.

The Omura's whale belongs to rorquals - the largest group of baleen whales. They include what is believed to be the largest animal that has ever lived, the Blue whale, which can reach 180 tonnes (200 short tons), and the Fin whale, which reaches 120 tonnes (130 short tons); even the smallest of the group, the Northern minke whale, reaches 9 tonnes (10 short tons).

It is suggested that the Omura's whale is more closely related to its larger relative, the Blue whale.

Omura's whales catch their prey by lunge-feeding on bait balls. Lunge feeding is an extreme feeding method, where the whale accelerates to a high velocity and then opens its mouth to a large gape angle. This generates the water pressure required to expand its mouth and engulf and filter a huge amount of water and fish.

Due to a number of anatomical features Omura's whales are able to open their mouths so wide that they would be capable of taking in water at volumes greater than their own sizes.