Finless porpoises are named for and distinguished by the absence of a dorsal fin. Instead of this fin, there is a small ridge on their backs that starts just behind their blowhole and extends as far as the tail flukes. Small circular bumps called tubercles cover the ridge. They have unfused neck vertebrae, which enables unrestricted head movement, and they have a small, curving mouth. About half of them have pink eyes. Their streamlined body is blue-gray, though in northern China and Japan adults are light gray.
The Finless porpoise inhabits the coasts of eastern and southern Asia from the Arabian Gulf eastwards to Japan and southwards to Java, Indonesia. It inhabits warm temperate and tropical coastal waters, preferring waters above sandy or soft bottoms, such as mangroves, shallow bays and estuaries. It is also found in large rivers.
The Finless porpoise usually forms small schools and is hardly ever seen in groups larger than 4 individuals. Generally it is solitary or in a mother-offspring group. Some of these groups (mainly near Japan) are migratory. They live in the Inland Sea in the spring, and between late summer and mid-winter they migrate to the Pacific coast. Finless porpoises are active animals, swimming just below the water's surface with sudden, darting movements. They disturb the water little when they break the surface, and they usually roll onto their sides as they do so. They have been seen spyhopping, but rarely breaching. Finless porpoises tend to be shy and they avoid boats. Calves are known to ride on their mother's back, gripping onto the dorsal ridge, coming up out of the water as the mother comes up to breathe.
Finless porpoises are polygynous, which means that one male mates with multiple females. Breeding takes place from late spring to early summer. Young are born in winter, spring, or summer, depending on their location, after a gestation from ten to eleven months. Finless porpoises give birth to a single calf. Feeding continues from 6 months to more than a year. The mother is usually the caregiver and swims with her calf until it has fully matured. Males gain sexual maturity between four and six years old, and females between six to nine years
The major threat faced by Finless porpoises is entanglement with fishing nets, especially gillnets. Further threats include hunting, human disturbance, live capture for display, habitat degradation, collisions with boats, and noise and chemical pollution.
The total number of the Finless porpoise population is unknown for today but there are estimates available for certain areas: Japanese waters (estimates for 4 subpopulations): 3,807 porpoises in Ariake Sound/Tachibana Bay; 289 porpoises in Omura Bay; 3,743 porpoises in Ise/Mikawa Bay and 3,387 porpoises in Chiba/Sendai Bay. Near Korea there are 21,532 finless porpoises in offshore waters and 5,464 porpoises in inshore waters. In the Yangtze River there are 1,800 porpoises. Numbers of Indo-Pacific finless porpoises in Hong Kong and adjacent waters are estimated to be at least 217 individuals; in the coastal waters of Bangladesh there are 1,382 porpoises. Overall, Finless porpoise numbers are decreasing today and they are classified as Vulnerable (VU) on the list of threatened species.