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Finally in our Marine Mammal Highlights series on porpoises, we have the very unusual finless porpoise. What’s it’s unique feature?! We’ll let you guess…

Finless Porpoise

Though originally just categorized as one species, the world of porpoises now has two unique species: the Indo-Pacific and Narrow Ridged finless porpoises! A self-describing name, these porpoises appear incredibly similar with their lack of a dorsal fin and general morphology, yet key differences do exist between them that finally led (just recently in 2008!) to the division of a singular species to these two species. Overlapping in distribution in the Matsu region of China, one theory suggests that during the glacial maximum in the ice age, finless porpoises may have been separated by a land bridge connecting Taiwan to China, thus potentially inhibiting reproduction of the separated populations and leading to the speciation.

A surfacing Indo-Pacific finless porpoise. Credit: https://au.whales.org/whales-dolphins/species-guide/indo-pacific-finless-porpoise/

With an incredibly self-describing name, Indo-Pacific finless species famously lacks the dorsal fin and is found throughout the Indo-Pacific, ranging from Korea all the way west to the Persian Gulf along the coast in shallow water. This species feeds primarily on smaller bottom dwelling fish, as well as cephalopods and occasionally small crabs and other crustaceans. These animals typically live in groups of three to six. With a size of up to 7.5 feet and 160 pounds, the distinctive missing dorsal fin is instead replaced by a flat or concave dorsal surface, followed by a ridge small ridge extending to the tail. This dorsal surface is surrounded by a patch of tubercles, appearing as broad bumps or similar to spines with the thought of functioning as a sensory system for the porpoise.

Tubercle patches found on the Indo-Pacific (above) and narrow ridged (below) finless porpoise. Photo credit: John W. Wang

The narrow-ridged finless porpoise is strikingly similar in both appearance and behavior, but key morphological and geographical differences make it distinguishable. Ranging from Taiwan to Korea (including a population living exclusively in the Yangtze River), the narrow-ridged porpoise is slightly lighter in coloration to that of the Indo-Pacific. Also featuring tubercles, this patch is vastly narrower than the Indo-Pacific and far less numerous. The dorsal ridge following this patch is also much more distinct, rising higher than the Indo-Pacific and almost appearing as a fin. As for the nitty-gritty differences, this species also has a larger skull with a narrower and longer rostrum.

cindy.elliser@pacmam.org

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