The Sargassum fish (Histrio histrio Linnaeus, 1758) is a member of the Histrio genus in the frogfish family (Antennariidae). In June 2013 researchers of the Dongsha Atoll National Park found one in a seaweed bed in a lagoon west of Dongsha Island. Further verification shows that it is a new species recorded in Dongsha Atoll National Park.

The Sargassum fish is about 20cm long at maximum. It usually settles in barriers or floating weeds between the tidal range and sub-tidal range in waters with coral reef. It changes its body into different colors to adapt to the environment. The pectoral fins and pelvic fins of the Antennariidae have evolved into a toe shape for it to climb on weeds and other floating objects or crawl on the seabed. When it crawls, it uses it pectoral fins, pelvic fins and caudal fin like five legs. As this is very special, Taiwanese fishermen commonly call it the “five-leg tiger”.

Although the Sargassum fish looks cute, it is a “killer fish”. Its pelvic fins have evolved into hard kiss tentacles with a threaded end, shaking like a “hook with bait”. When small fish or a crustacean approaches, it suddenly opens its mouth to swallow them. As a true carnivorous fish, it deserves to be called a “tiger” fish.

Both habitat diversity and biodiversity co-exist in Dongsha Atoll National Park. The Sargassum fish recorded this time is the second species of the Antennariidae found in the park. The painted frogfish (Antennarius pictus) previously recorded is mostly found in the broken coral reef area. It has sharp body colors and usually disguises into the color of the coral around it. In contrast, the Sargassum fish basically lives in Sargassum weed and changes its colors into that of the weed and other floating objects. This behavioral variation of both species from the same family is thus worthy of further exploration. How many more members of the “five-leg tiger” there are in the mysterious and immense waters over Dongsha Atoll National Park is awaiting researchers to find it out in the future.