Japan's 'common octopus' not so common after all

Madako, which means 'common octopus,' is a name well known in Japan and, for over a century, scientists have called it Octopus vulgaris.

But according to Ian Gleadall, a British researcher at Tohoku University's Graduate School of Agricultural Science, the Japanese common octopus is a different species, Octopus sinensis.

The name Octopus vulgaris was first used for the European common octopus, which was long thought to be cosmopolitan, with similar-looking octopuses mistakenly identified in many countries -- including Japan -- as just one species distributed globally.

But recent research has shown that the so-called global Octopus vulgaris is actually a mixture of several distinct, though closely related, species. One of these is now recognized as Octopus sinensis, which is found throughout warm seas on the continental shelf of East Asia, including the main islands of Japan.

Octopuses get their name from their distinctive eight arms, and are known for their ability to rapidly change their colour and appearance. These features tend to make all octopus species look rather alike.

The new study therefore set out to look for physical differences between the Japanese and European common octopuses. It found that mature Japanese Octopus sinensis have a relatively broader head and shorter arms with about 80 fewer suckers per arm than the European Octopus vulgaris. The third right arm on mature O. sinensis males, which is used for mating, also has far fewer suckers than that on O. vulgaris (120-140 versus 150-190, respectively).

Modern DNA barcoding, too, can now distinguish clearly between the two species.

The impact of this finding is significant because the growing popularity of octopus as food is putting intense pressure on marine resources.

Octopus flesh has no bones or gristle. It is high in protein and the healthy amino-acid taurine, while also low in fat and carbohydrates. Currently, the world's common octopuses are the most highly prized for their taste, and their popularity has contributed to employment in the fisheries and aquaculture industries.

But a steady fall in catches of common octopuses around the world is causing concern. Imports of Octopus vulgaris have been masking the decline in O. sinensis catches in Japan. And off Europe and Africa, O. vulgaris is in trouble, too. The Moroccan octopus fishery collapsed several years ago, while that off Mauritania continues to be heavily exploited.

Until recently, the declining numbers caused little concern, as it was assumed that all common octopuses belong to a widely available global species. But now that it's been proved that there are several independent species of common octopus within the O. vulgaris group, and that the numbers are limited, Gleadall is calling for more focus on local fisheries management policies, in order to make sure that each species can be fished sustainably.