A new scientific study of octopuses worldwide is revealing previously unknown levels of variability amongst geographically dispersed populations – a fact that may be masking the decline of the species in some areas, including Japan.
Professor Ian Gleadall studies octopuses at Tohoku University in Japan. He has contributed to a study that has collected evidence showing that several closely related species of octopus may have been lumped together under the single scientific name, “Octopus vulgaris.”
As octopuses lack bones, and have the ability to change shape and color, it is difficult to spot differences among populations that would mark them as separate species, especially when processing further alters their appearance. As the “common octopus” has a worldwide distribution, it has been previously assumed that they are all the same. According to Gleadall, after closer study, O. vulgaris is what he terms a “catch-all” species – and further, this misclassification presents a conservation problem.
Gleadall’s own research, published in May 2016, indicates the East Asian population is a separate species, for which he has proposed the name Octopus sinensis. This species, Gleadall said, has declined due to fishing pressure. He feels that this is not easily noticed in Japan, since the decrease has been masked by increased imports.
Current trade statistics and assessments of the vulnerability of species still treat Octopus vulgaris as a single species, and even when a local population declines drastically, it is assumed that a species with such wide distribution cannot be threatened. But if the vulgaris “complex” were broken up into several distinct species – as Gleadall proposes – some of them might be found to require new protections.
“All species in this complex (particularly O. vulgaris and the East Asian species here identified as O. sinensis) are highly vulnerable to overfishing, so recognizing O. sinensis as a species distinct from O. vulgaris is an important step towards improving sustainable fisheries management policies for each species in the this group of commercially valuable octopuses,” Gleadall wrote in his research.
Gleadall has proposed “East Asian common octopus” as the common name for O. sinensis. This is the same octopus that Japanese call “madako.” He proposes renaming the Mediterranean and Atlantic populations of O. vulgaris in Japanese to "chichukai madako" (literally, "Mediterranean common octopus").
To support separate species status, he cites recent molecular genetic analysis (Guerra et al. 2010, Kaneko et al. 2011) and goes on to find physical features that can be used to discriminate between the East Asian common octopus and its Atlantic relative, the real Octopus vulgaris. It should be noted that the above-cited analysis revealed as many as five possible species within the O. vulgaris complex in the Atlantic alone, so O. sinensis is not the only prospective new species.
Gleadall said that it is often debatable whether the differences are great enough to warrant species or sub-species designation, but for O. sinensis, the differences are clear enough for species designation.
In his study, Gleadall compared morphological and meristic features (for example, arm length and counts of suckers) to find identifiable differences. And he found some significant differences, mainly in the males. For example, the number of suckers on the third right arm of the male is fewer in the East Asian common octopus, and he number of suckers on the other arms is also comparatively fewer in O. sinensis. In addition, O. sinensis also has generally shorter arms than O. vulgaris, and the two enlarged suckers of mature males on O. vulgaris are usually positioned in the range of sucker numbers differently than on examples of O. sinensis.
The more recent paper to which Gleadall contributed (Amor, et al. 2016), mainly by contributing O. sinensis specimens and tissue samples, used molecular genetic analysis of several populations within the O. vulgaris complex and closely related species and found shared mutations that indicate the evolutionary relationships among six phylogenetic clades. A “clade” is a group of organisms that share a common ancestor, so by using this information, one can draw an evolutionary tree, or “cladogram.” The authors found that within the O. vulgaris complex, there are several populations that are closely related, but found in different biogeographic regions and with consistent differences detectable by DNA barcode technology.
They then tried to separate the specimens by physical features into their correct biogeographic region, but were only partially successful. They found that the males, which have distinctive differences in their sexually adapted third right arm, could be classified most of the time, but that it was difficult to discriminate among the females.
According to Gleadall, the waters around Japan aren’t the only area where octopus populations appear to be in decline. The stock of O. vulgaris off North Africa is also in poor condition, with the stock collapsed in Morocco and classified as overfished in Mauritania. Moreover, data is lacking for much of South America, Africa, and the Indian Ocean.
In his future research, Gleadall said he hopes to find sufficient data on imports and domestic catches over recent decades to show that imports are masking the decline of the East Asian common octopus, and that octopus populations worldwide should be studied with closer scrutiny.
