Curious case of the Giant Cuttlefish
Beneath the tropical clear waters surrounding the Australian coastline, the world’s largest coral reef system the Great barrier reef is home to a surprising and elusive individual. The giant Australian cuttlefish (Sepia apama) the world’s largest cuttlefish which can reach up to 1 meter in total length and can weigh in excess of 10.5 kilograms. Like other cephalopod species they have a closed circulatory system, and highly developed nervous system and a large brain. The giant Australian cuttlefish also has the ability like many cephalopods to camouflage itself by changing its skin colour and texture against its surroundings.
Biology and Anatomy
The Giant Australian cuttlefish has eight arms and two extended tentacles, these extended tentacles are used for mating and catching prey. The feeding tentacles when not in use retract into pouches found at the base of the third and fourth arm pairs. The mantle cavity, a distinctive feature in all molluscs, has been adapted to help the giant Australian cuttlefish to avoid predation. The mantle can suck and eject water rapidly creating a simple form of jet propulsion enabling the cuttlefish to make a quick escape from predators.
They are able to remain buoyant within the water due to an internal structure known as the cuttlebone which is made from aragonite a porous structure. The pores provide the buoyancy which the Cuttefish regulates by changing the ratio of gas – liquid in the cutlebone via the ventral siphuncle.
Sepia apama has a closed circulatory system to transport the blood round their bodies. This is done by not one heart but three separate hearts, two branchial hearts pump blood to their gills, one for each set of gills. whilst the remaining heart pumps blood to the remainder of the body. The blood of the Sepia apama is a blue green color, this is due to it containing the copper protein haemocyanin to carry oxygen rather than the iron containing protein haemoglobin.
Camouflage against predation
The Giant cuttlefishes ability to camouflage itself comes from the many thousands upon thousands of color changing cells called chromatophores, located just below the surface of the skin. The center of each chromatophore contains an elasticated sac full of pigment. Nerves and muscles control whether the pigment sac is contracted or expanded. When the sac is subjected to expansion the colors shown are more visible. They also excel at changing the textural appearance, contracting muscles allows them to sculpt and reform their body shape into different textures that can resemble seaweeds, background debris such as corals or boulders. The ability to change color, is also used as a warning towards predators as an intimidation technique. Sepia apama can change their color to resemble an animal who is venomous if provoked or ingested.
All cephalopods especially the cuttlefish species, have large, and extremely developed eyes, allowing the cuttlefish to detect low light levels. Which is vital in hunting prey and avoiding predation at night from marine mammals such as the Bottlenose Dolphin (Tursiops aduncus).
Sepia apama not only use their ability to change color virtually instantaneously to prevent predation, but engage in very frenzied and promiscuous mating aggregations. Which results in multiple paternities and sperm competition. The large dominating males will use their chromatophores in flashing patterns to attract females for mating and deter other males competing for the same female mate. The smaller males that would not be able to compete against the larger male rivals for the females take a more subtle and sneakier way to mate. Smaller males will disguise themselves as females by changing their coloration and hold their tentacles curved more towards their body. Once the dominant males have become distracted the sneaky smaller male can also start mating with the female.This method adopted by the smaller males is known as the ” sneaker male phenomenon” This mix of both sperm from each of the cuttlefish to the female enables for very little inbreeding and a large gene pool to be present within the species. During mating the males will transfer over spermatophores to the females mantle cavity, using a specialized tentacle called hectocotylus. As the female spawns the eggs are fertilized by the spermatophores placed in her mantle near the oviduct. The breeding season although intense and incredibly competitive takes its toll on both male and female Sepia apama, as they do not feed during the breeding season and die after mating season is ended.