WATCHING ANY MARINE-LIFE interaction is fascinating, but the act of reproduction surely has to be one of the most interesting.
On a muck-dive site in the Raja Ampat on which nothing much seemed to be happening, my attention was drawn to two small cuttlefish that seemed to be extremely interested in each other.
Settling to watch the unfolding scenario, I soon realised that these were not the only cuttlefish in the vicinity. The whole area was alive with activity, with at least 10 more animals all with just one thing on their minds – mating!
Most cephalopods are “big-bang” spawners with just a single breeding season, and I seemed to be right in the middle of it.
Cuttlefish are famous for their ability to change the way they look. One minute they are blending into their surroundings, often reinforcing their disguise by pushing up fingers of skin to mimic branches of weed or coral.
The next, they are putting on a firework show of pulsating and flashing skin changes, especially when stalking prey, fighting rivals and courting mates.
These colour changes are made possible by small structures within the skin that contain coloured ink. These can be expanded and contracted to allow cuttlefish to communicate with others through patterns, textures and many colour schemes.
Cuttlefish belong to an order of molluscs with a porous internal shell. This cuttlebone plays an important role in buoyancy control, because they fill the chambers or empty them of gas, much as we do with BCs.
I watched a courting pair where the male, the larger of the two, was following the female’s every move.
They would be actively feeding, then, with a quick flash of colour, would turn to face each other and come together to mate.
With the female opening her arms to reveal her mouth, the pair would lock arms together and the male, using his modified arm, would pass a spermatophore, a waterproof container for storing sperm, into a pouch near the female’s mouth – the “seminal receptacle”.
The transferred spermatophores then burst, and the sperm moves up the oviduct to fertilise the eggs.
Sexually receptive female cuttlefish mate repeatedly, and their choice of mate is not exclusive, which makes the competition between males intense. The male has to protect his valuable sperm investment, and will closely guard “his” female and chase and even fight off would-be suitors.
Mating and reproduction is a one-off affair in cuttlefish, although the process can last for days. Once the eggs have been laid, the female will die, followed soon afterwards by the male.
I continued watching this magical ritual, with the cuttlefish pair feeding one minute and mating the next, and suddenly became aware of a third cuttlefish in the territory.
Another male had arrived, and it was down to the guarding male to ensure that his mating efforts were not in vain.
Smaller males will often mimic females by acquiring their mottled skin-pattern and imitating their egg-laying posture.
This enables them to get closer without raising suspicions from the consort male – sneaking in through the side door, you might say.
Suddenly, with alarming speed and intense colours flashing, the intruding male was chased off to try his luck elsewhere. Soon afterwards the courting couple resumed their mating, totally oblivious to me and my bulky camera.
The term “sperm competition” is used to describe the competitive process between the sperm of two or more males trying to fertilise an egg of a lone female. Images I took on this dive shows evidence of this, with the collection of sperm around the female’s mouth.
The male may have propelled a jet of water into the female’s pocket to expel any previously deposited sperm, but DNA analysis has proven that sperm from more than one male has been used to fertilise a clutch of eggs.
This suggests that the female can draw on any of her sperm stores to fertilise the next generation.
The eggs, typically round or cylindrical, are laid on the seabed, often attached to rocks or coral, developing and hatching within one to two months.
The new-borns are near-perfect replicas of the adult, with a nourishing supply of yolk to keep them alive until they make their first kill. They also carry ink designed to confuse and help them evade any would-be attacker.
If you see more than one cuttlefish on the reef, slow down, approach quietly and watch what must be one of the most beautiful events in the sea.