Octopus tetricus, the gloomy octopus, the common Sydney octopus, or the peachy octopus, is a species of octopus from the subtropical waters of eastern Australia and northern New Zealand. O. tetricus belongs to the Octopus vulgaris species group. All species within the O. vulgaris group are similar in morphology, behaviour, and physiology. The English translation of O. tetricus (Latin) is 'the gloomy octopus'. It is a significant species in the fishing industry in Australia. They play an important role in energy flux between trophic levels in the marine environment.
Distribution 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.
Anatomy and morphology
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 and a mantle length of up to . The maximum weight recorded for O. tetricus is .
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.
Taxonomy and etymology 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'.
Life cycle
Reproduction and growth 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. The amount of time embryonic development takes varies with water temperature, and newly hatched O. tetricus larvae are about 2.5 mm long and 1.1 mm wide, approximately the size of a grain of rice. These larvae go through a stage called the paralarval stage where they are considered planktonic, or free floating, organisms before they settle to the bottom and grow large enough to hunt for their food. This stage may last around 35–60 days. The females of O. tetricus have been known to cannibalise the males following mating.
Temperature plays a key role in growth of this octopus species. With a good food supply, octopuses that reside in areas with a cooler water temperature tend to grow slower during the key growth phase and when they reach maturity they are generally larger than octopuses that are found in warmer water temperatures.
Lifespan Adult O. tetricus is observed to have a relatively small body size and a lifespan of approximately 11 months, although there is evidence that they can live up to 2 years. They have a very fast growth rate, reaching adult size after approximately 100 days. Female O. tetricus rarely eat or sleep during the protection of the nest, devoting all resources to nourishing its offspring, and die shortly after the eggs hatch. Females are found to mature at a slower rate and become larger than male O. tetricus. This is a semelparous species, meaning each female reproduces once in her lifetime.
Habitat and behavioural ecology
Habitat
Octopus tetricus occurs in the intertidal zone along rocky shores and seagrass beds and in the ocean at depths of up to . It has been suggested that this species is associated more commonly with rocky reefs during the breeding season, although they frequent areas of the sea bed with soft-sediments for much of their life. O. tetricus alter their habitats by digging out dens and using remains from prey, including but not limited to shells. It is very common for scallop beds to be found in close proximity to the excavated dens. The scallops serve as a food source and their shells are part of the shell beds built. One O. tetricus den in Sydney has been observed being continuously used by different individuals since at least 2009. O. tetricus has been observed shifting and transporting sediment and shells to maintain their dens. They tend to have a preference for materials that are free from barnacles and encrustations, although the scale on which this is the case is dependent on foraging success.
This species can be found across a variety of coastal environments all year round, but there are some observations of adults aggregating inshore for breeding purposes during spring and summer. Most individuals have high site fidelity, staying in the same location for an average of 3 months. These movements in and out of coastal areas and deeper ocean depths could potentially correlate with the stages of lifecycle they are currently in. Life cycle stage also influences the position which they sit inside their dens, where brooding females will sit close to the entrance as a means of protecting the eggs compared to non-brooding females that reside near the back of the den. Females must defend their eggs from other males of the same species, but also other marine dwelling organisms, such as eels.
This species of octopus is considered to be an ecosystem engineer. This means that the way they create their habitats influences and builds an ecosystem around their dwellings. They carry their prey, often shellfish, on their back and discard the empty shells at their dens when they have finished eating. The shell beds that are created around the excavated dens attract hermit crabs and fish due to the various hiding places created. Small fish and other small prey species attract larger species and the cycle builds, creating an ecosystem. A solid object can serve as a good den that can also be the start of a new settlement for O. tetricus. Studies show that O. tetricus has higher populations in patch reef habitats than broken reef habitats, and were scarcely found on flat reefs. Adult O. tetricus were also found to occupy coastal reefs in the summer and then disappear around the second week of April, which is the second week of autumn in Australia. Shelters serve a vital role in octopus ecology. When resources are scarce, such as food and dens, they will often attempt to exclude and displace other individuals.
Social behaviour and mating Scientists have observed many different behaviours exhibited by O. tetricus including, signalling, mating, mate defence, and aggression. Some have even observed an octopus evicting another from its den. Occasionally this aggressive behaviour led to physical altercations between octopuses. The likelihood of an aggressive encounter leading to a physical altercation is increased when there is a significance difference in the relative darkness of body patterning between the two individuals. The octopus which is paler in colouration is much more likely to retreat and back down, avoiding a fight. Grappling and physical contact between octopus is much more common when the relative difference in darkness is much smaller. Although uncommon, there is some evidence that occasional male-female aggression occurs during mating attempts. This darkening of skin colour when aggressive has been explained by chromatophore muscle tension. Physical displays of aggression also include seeking higher ground, elevated posture by standing up on extended arms, and spreading webbing and raising arms. In general, the species will avoid aggressive encounters, as they have a high cost to reward ratio and can also result in physical damage to their body.