Blue-ringed octopus classification

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Classification

All blue-ringed octopuses belong to the genus Hapalochlaena and are characterized by their small body size, considerable reduction of the ink sac, and distinctive patterns of iridescent blue rings and/or lines on their dorsal surfaces and arms. All are venomous and have been reported to be responsible for human fatalities. There are currently four recognized species, although the systematics of this group is poorly understood. I will describe how to recognize the three commonly encountered species. The fourth, Hapalochlaena nierstraszi, was described from a single preserved specimen from the Bay of Bengal and its status is uncertain.

The Greater Blue-ringed Octopus, Hapalochlaena lunulata
This is the species most commonly imported for the aquarium trade into the United States and Europe. It is found from northern Australia,Papua New Guinea and the Solomon Islands, throughout the Philippines and Indonesia and as far west as Sri Lanka. It reaches a maximum mantle length (distance from between the eyes to the posterior tip of the mantle) to 55 mm, although most sexually mature individuals that I have seen are 25 to 40 mm. The background body color is dark to tan-brown to grey. The surface of the animal is often covered with numerous papillae, giving it a rough texture. Large iridescent blue rings cover the dorsal surfaces of the mantle and web and extend out the arms. Rings on the mantle are up to 8 mm in diameter and number fewer than 25. Each iridescent ring is set in a broader ring of dark chromatophores, although the center of the rings are the same brown body color. There is a characteristic short, horizontal iridescent blue line that runs through the eye. When at rest, faint, thin blue rings are usually visible, but when the animal is agitated, the rings become much thicker and brighter, often pulsating. The background color of the body also usually changes to a golden yellow or deep orange and the skin has a smoother appearance. The name "greater blue-ringed" refers to the size of the rings, not the body size.

The Lesser Blue-ringed Octopus, H. maculosa
This species is found only in the temperate waters of southern Australia, from southern Western Australia to eastern Victoria. It is also a small species, mantle length to 57 mm. The dorsal surface of the mantle usually has a rough appearance covered by numerous irregularly arranged wrinkles. There are also fine, unevenly sized tubercles covering the head and mantle. While resting, the background color is a uniform grey to beige, with large, light brown patches or maculae - thus the name H. maculosa. On the dorsal mantle, 10 maculae form a pattern of brown chevrons. Smaller patches dot the web and base of the arms. All eight arms are marked with approximately 10 evenly spaced brown patches that form a bands running down the arms. The blue rings are usually not visible in animals at rest. When the octopus is agitated, the brown patches darken dramatically, and iridescent blue rings or clumps of rings appear and pulsate within the maculae. The blue rings are small, generally less than 2 mm in diameter; hence, the name "lesser blue-ringed". Typically 50 to 60 blue rings cover the dorsal and lateral surfaces of the mantle.

The Blue-lined Octopus, Hapalochlaena fasciata
This small octopus has a mantle length to 45 mm and is found in southeastern Australia from southern New South Wales to southern Queensland. The dorsal surface is covered with a pattern of fine, scattered tubercles and ridges. When at rest, the background color is a uniform light grey to beige. The maculae and blue lines and rings are usually not visible. When active, the background color darkens to a deep charcoal, brown or slate grey and the maculae become very dark. On the dorsal and lateral mantle they form streaks in a diagonal pattern similar to that found in H. maculosa. However, on the dorsal mantle and head intense iridescent blue lines rather than rings pulsate within each dark macula. Single blue rings or clusters of rings do appear within the brown patches on the web and arms.

It is easy to distinguish these three species of blue-ringed octopus simply by observing the pattern of iridescent blue markings on the dorsal mantle and head when the animals are agitated. H. lunulata has a few large blue rings and a distinct blue stripe running through the eye. The rings are surrounded by dark chromatophores, but there is no pattern of dark diagonal streaks. H. maculosa and H. fasciata both have maculae on the dorsal mantle forming a chevron pattern, but H. maculosa has many small blue rings embedded in these dark patches while H. fasciata has distinct blue lines.

There are undoubtedly other blue-ringed octopuses to be described. The female with eggs that my daughter encountered on Lizard Island closely resembled H. maculosa, but is probably another species. Blue-lined octopuses have been reported from southern Japan and Hong Kong. I have seen a smooth, pinkish species from northern Australia that resembles H. lunulata. Several cephalopod experts are currently working on this group and hopefully more complete descriptions will be available soon.

Go back to part II | Part IV: Blue-ringed octopus biology

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