As the name implies, finless porpoises have no dorsal fin, and this is their most distinctive characteristic. In some ways, they resemble small, slender beluga whales. The head is beakless; the rounded forehead rises steeply from the snout tip. The body shape, in general, is more slender than in other porpoises. The bodies of at least some finless porpoises are soft and mushy, and the neck is very flexible. Instead of a dorsal fin, the finless porpoise has an area of small bumps or tubercles on its back, running from just forward of mid-back to the tail stock. This dorsal ridge ranges in width from 0.2 to 12.0 cm. There are narrow-ridge forms in Japan and northern China (including the Yangtze River) and wide-ridge forms throughout the remainder of the species’ range. The trailing edge of the flukes is concave and the flippers are large, ending in rounded tips. Regional differences in body size and morphology have been documented, with separate stocks recognized based mainly on skull morphology.
The common name that was used in the past, "finless black porpoise," apparently resulted from descriptions of dead animals, after post-mortem darkening. In most areas, finless porpoises are gray in color, often with lighter areas on the throat and around the genitals. The wider-ridge form is light gray at birth, and darkens to dark gray or nearly black as adults. The narrow-ridge form has the reverse pattern young are dark gray, and lighten as they age. In Japan and northern China, the adults are a light creamy gray.
Tooth counts range from 15-22 in each tooth row. The teeth are small and slender.
Adults of this species can reach somewhat over 2.0 m in length, although many populations are much smaller (males tend to be slightly larger than females). Finless porpoises are apparently about 75-85 cm at birth.
Can be Confused With
The smooth back of the finless porpoise should make it easy to distinguish from other cetacean species, such as humpback, bottlenose, and Irrawaddy dolphins, baiji, and susu, which share parts of its range. Finless porpoises are actually most likely to be confused with dugongs, where they overlap in tropical waters. The double-nostrils on the snout tip, and very different mouth shape should serve to distinguish dugongs.
Tropical to subtropical coastal waters of the Indo-Pacific, both fresh and marine, are home to the finless porpoise. The range runs from central Japan to the Persian Gulf, including a few rivers in the Asian subcontinent. One of the best-known populations is found in the Yangtze River of China.
Ecology and Behavior
Finless porpoises are generally found as singles, pairs, or in groups of up to 20, although aggregations of up to about 50 have been reported. Like other porpoises, their behavior tends to be not as energetic and showy as that of dolphins. However, contrary to what their body shape might suggest, they are fast and agile swimmers. They do not ride bow waves, and in some areas appear to be shy of boats. Mothers have been reported to carry calves on the denticulated area on their backs, but this behavior has not been confirmed. In the Yangtze River, finless porpoises are known to leap from the water and perform "tail stands."
Reproduction has been studied in Japanese and Chinese waters. Sexual maturity occurs at 3-6 years of age, and calving occurs at different times of year in different regions (most often either spring/summer or winter).
Feeding and Prey
Small fishes, squids, and crustaceans form the diet of finless porpoises. Some animals have also ingested some plant material, probably incidentally. Common prey species are fishes of the families Apogonidae, Carangidae, Clupeidae, Sparidae, and Engraulidae; cephalopods of the families Loligonidae, Sepiidae, and Octopodidae; and panaeid shrimps.
Threats and Status
Finless porpoises are not known to be directly killed in large numbers anywhere, but they are incidentally killed in fishing gear throughout their range. Gillnets appear to represent a particularly serious threat in nearly all areas of the range. Reasonable numbers have also been live-captured in Japan, China, and the Gulf of Thailand. This coastal species suffers from serious problems of habitat loss and degradation, vessel strikes, and environmental contamination. The Yangtze River population, a unique subspecies, is apparently declining in numbers.
Currently, they are listed as ‘Endangered’ (IUCN - Yangtze River population), ‘Data Deficient’ (IUCN - all other populations), and ‘Not Listed’ (ESA).
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