Common Sydney Octopus

Octopus tetricus is normally coloured grey to mottled brown with rufous arm faces that taper towards the tip. They have orange colouration of arms, funnel, and mantle opening. Their eyes are typically white in colour. The skin is granular, and has many small irregularly shaped patches and large warty structures used to make it appear spiky when the octopus camouflages itself as seaweed. The adults typically have an arm span of 2 metres (6.6 ft) and a mantle length of up to 20 cm (7.9 in). The maximum weight recorded for O. tetricus is 2.6 kg (5.7 lb). The second and third arms of O. tetricus are longer than the first and fourth arms, typical of all species in the Octopoda order. The brain, optic lobes, and the highly developed arm nervous system are the three main components of the octopus nervous system. Each arm of the octopus has been described as having its own brain, where individual processing, motor systems, and sensory exploration can occur. This is where the concept that octopus have many brains comes from. The arm nervous system of the octopus contains three-fifths of the octopus's total number of neurons – 350 million neurons distributed equally throughout each of its eight arms.

O. tetricus was originally discovered in New South Wales and was also found along the eastern Australian coastline. It occurs from Lakes Entrance in Victoria to Moreton Bay in southern Queensland. O. tetricus is distributed in the subtropical seas of eastern Australia and northern New Zealand, including Lord Howe Island. A close relative, Octopus djinda, occurs at similar latitudes in Western Australia, from Shark Bay to Cape Le Grand, and was considered to be conspecific with O. tetricus until 2021.The first sighting of O. tetricus in New Zealand was not until 1997, and has since become one of the most common species of octopus found in New Zealand, alongside Pinnoctopus cordiformis and Macroctopus maorum. O. tetricus populations in New Zealand are found along the North-East coast of the North Island, between the Bay of Plenty and Northland. Gene squencing confirms that O. tetricus from Australia and New Zealand come from the same original population. It is proposed that the mechanism for gene flow across the Tasman Sea is through planktonic larvae and/or adults using floating wood or algae as transport. It is hypothesised that warmer waters spreading south of Australia are influencing populations to O. tetricus to disperse further south in response to climate change.

The reproductive cycle of females is tied to seasonal changes, similar to many other species. Females reach maturity around Australia's spring and summer seasons in order to mate and lay eggs. During the mating process the male O. tetricus passes spermatophores to the female in two different ways. The male either wraps his arms and web around the female's mantle or reaches his mating arm from a distance and inserts it into the female's mantle. The males third right arm is the specialised mating arm, and the sperm is moved down a groove on this arm which has a sucker-less tip. Spermatophores are released from the male's "terminal organ", moved by the male's oral suckers, and then the spermatophores are inserted into the female's oviduct. O. tetricus start out as eggs that are laid in large numbers in the octopus's nest, approximately 270,000 eggs per female. The eggs are normally glued to the rock or substrate at the top of the den created by the female octopus. The female usually lays her eggs over several nights in a string formation. The size and number of strings of eggs usually depends on how large the female is and can have between 60 and 200 egg strings. The female then guards the eggs until they hatch. Female O. tetricus have also been known to store viable spermatozoa for up to 114 days. …

Octopus tetricus Gould, 1852 was described by American conchologist Augustus Addison Gould in the 1852 publication, Mollusca of the United States Exploring Expedition under Captain Charles Wilkes. An illustration of O. tetricus was also published a few years later (plate 47). The type specimen is listed as being from Sydney, New South Wales, Australia. The type was one of many thousands of specimens collected by the Wilkes Expedition, however this particular specimen of O. tetricus is thought to be lost. The New Zealand species O. gibbsi is considered to be a synonym of O. tetricus.

The species epithet tetricus means 'forbidding' or 'gloomy'.