Two-millimetre-long gloomy octopus hatchlings move towards a source of light, possibly because it makes it easier to hunt prey
16 December 2021
Tiny, newly hatched gloomy octopuses seek out light – possibly to help them find their minuscule prey.
Adult gloomy octopuses (Octopus tetricus) – also known as the common Sydney octopus – weigh around 5 kilograms and live on the ocean floor around Australia and New Zealand. The sharp-beaked, smooth-skinned cephalopods are covered in textured spots that allow them to disguise themselves as seaweed.
Almost fully transparent, 2-millimetre-long gloomy octopus hatchlings – called paralarvae – are shaped like adult squid. They begin to hunt tiny marine life, such as crab larvae, from the time they leave their latex-like eggs.
Human demand for the animal is on the rise, says Stefan Spreitzenbarth at the University of Auckland in New Zealand. Octopuses are a popular seafood, and the animals’ large neurons, behavioural complexity, problem-solving skills and ability for camouflage make them a sought-after research specimen.
Research from the 1970s suggested that gloomy octopus paralarvae move away from light. But given that light plays an important role in the survival of larvae and paralarvae in other aquatic species, Spreitzenbarth decided to test this out.
In a series of experiments, Spreitzenbarth and his colleague Andrew Jeffs, also at the University of Auckland, placed newly hatched octopuses from wild-caught females in a dark seawater tank. Within half an hour, the paralarvae were spread evenly throughout the tank.
The pair then switched on a white, blue, pale blue or green LED light, aimed at the centre of the water’s surface. Within 15 minutes of turning on a light of any colour, most of the paralarvae congregated in the top third of the tank’s volume. There, light intensity was more than twice as strong as it was in the bottom two-thirds of the tank, says Spreitzenbarth.
Ten minutes later, the majority of the paralarvae were still in the top third. That means the hatchlings quickly made their way towards the light and then stayed there, says Spreitzenbarth.
The researchers aren’t sure why octopuses seek out light. “It might have something to do with where their prey live,” says Spreitzenbarth. “Light would play an important role in detecting potential prey, as octopus paralarvae are predominantly visual predators.”
The finding that the animals responded to all the tested colours of light didn’t come as a surprise, he adds. Adult and juvenile gloomy octopuses see in shades of blue-green. While the paralarvae are too fragile for such vision experiments, they are likely to experience colour in the same way, says Spreitzenbarth.
The researchers hope the finding will enable aquaculture farmers to create better living environments for paralarvae raised in captivity. Such animals rarely make it to adulthood.
“Once paralarvae survival [in captivity] is higher and the juvenile phase is reached more often, science and aquaculture can be independent of wild-caught octopus,” says Spreitzenbarth.
Journal reference: Applied Animal Behaviour Science, DOI: 10.1016/j.applanim.2021.105528
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