The common blanket octopus or violet blanket octopus (''Tremoctopus violaceus'') is a large octopus of the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Tremoctopodidae found worldwide in the

epipelagic zone The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological pro ...

of warm seas. The degree of

sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most an ...

in this species is very high, with females growing up to two meters in length, whereas males grow to about 2.4 cm. The first live specimen of a male was not seen until 2002 off the

Great Barrier Reef The Great Barrier Reef is the world's largest coral reef system composed of over 2,900 individual reefs and 900 islands stretching for over over an area of approximately . The reef is located in the Coral Sea, off the coast of Queensland, ...

. Individual weights of males and females differ by a factor of about 10,000 and potentially more. Males and small females of less than 7 cm have been reported to carry with them the tentacles of the

Portuguese man o' war The Portuguese man o' war (''Physalia physalis''), also known as the man-of-war, is a marine hydrozoan found in the Atlantic Ocean and the Indian Ocean. It is considered to be the same species as the Pacific man o' war or blue bottle, which is ...

. It is speculated that these tentacles serve both as a defensive mechanism and possibly as a method of capturing prey. This mechanism is no longer useful at larger sizes, which may be why males of this species are so small. The web between the arms of the mature female octopus serves as a defensive measure as well, making the animal appear larger, and being easily detached if bitten into by a predator. The complete

mitogenome Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, such as adenosine triphosphate (ATP). Mitochondrial DNA ...

of ''T. violaceus'' is a circular double-stranded DNA sequence that is 16,015 base-pairs long. File:Tremoctopus violaceus2.jpg, Ventral view of large female File:Tremoctopus violaceus3.jpg, Dorsal view of large female File:Tremoctopus violaceus.jpg, Dorsal view of female File:Tremoctopus violaceus5.jpg, Lateral view of adult male with

hectocotylus A hectocotylus (plural: ''hectocotyli'') is one of the arms of male cephalopod A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusive ...

Mating behaviour

The third right arm in male blanket octopus is called the

, which has a sperm-filled pouch between the arms. When the male is ready to mate, the pouch ruptures, and sperm is released into the arm. He then cuts this arm off and gives it to a female. It is likely that the male dies after mating. The female stores the arm in her mantle to be used when she is ready to fertilize her eggs. She may store several hectocotyluses from different males at once.

Sexual dimorphism

The common blanket octopus exhibits one of the highest degrees of sexual size-dimorphism found in large animals. There are several theories as to why this developed. It is advantageous for females to be big; their large eggs take a lot of energy to maintain. The bigger a female is, the more eggs she can carry, and she can birth more offspring that could potentially survive to adulthood. Sperm does not require much energy or space to maintain, so males do not face the same pressure to be big. The use of the Portuguese man o' war tentacles is effective only in smaller animals. So

selection Selection may refer to: Science * Selection (biology), also called natural selection, selection in evolution ** Sex selection, in genetics ** Mate selection, in mating ** Sexual selection in humans, in human sexuality ** Human mating strat ...

may have favoured males to remain small in order to continue to utilize this defence mechanism.

Defense mechanisms

Predators of the common blanket octopus include the

blue shark The blue shark (''Prionace glauca''), also known as the great blue shark, is a species of requiem shark, in the family Carcharhinidae, which inhabits deep waters in the world's temperate and tropical oceans. Averaging around and preferring c ...

tuna A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae ( mackerel) family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max le ...

, and the

billfish The term billfish refers to a group of saltwater predatory fish characterised by prominent pointed bills (rostra), and by their large size; some are longer than . Extant billfish include sailfish and marlin, which make up the family Istio ...

. Female blanket octopuses can roll up and unfurl their webbed blanket as needed. The blanket serves to make the octopus look bigger, and the females can also sever the blanket off to distract predators. The male blanket octopuses was first observed to be using Portuguese man o' war stingers in 1963. The octopus has Portuguese man o' war tentacles attached to its four dorsal arms. It is unknown whether the blanket octopus is immune to the toxins in the stingers or if they only hold the stingers at insensitive tissue. The use of these tentacles could serve both as defensive and offensive mechanisms. The toxin from the Portuguese man o' war wards off predators, but it could also be used to catch prey.

Mating behaviour

Sexual dimorphism

Defense mechanisms

References

Further reading