Octopuses’ arms are important not only for locomotion, but also for hunting. New research results show how precise the control of the eight arms is possible.
When hunting, octopuses adapt the use of their eight arms to the respective prey. Crabs, which are slow to flee, usually overwhelm the animals by using several arms at once – sometimes they jump on the prey like a cat on a mouse. When chasing fast-fleeing shrimp, on the other hand, they use their arms in a more targeted manner and one after the other, reports a team of US researchers in the journal Current Biology. The second arm on the right and left side of the body, viewed from the front, has a dominant function.
If you just look at an octopus for a moment, the movements usually don’t seem repeatable, says study leader Trevor Wardill. “They’re fidgeting and just looking weird in their exploratory movements.” For a more detailed analysis of the movement sequences, the researchers filmed Californian two-spotted octopuses (Octopus bimaculoides) in an aquarium. They added either white leg shrimp or fiddler crabs and watched the octopuses subdue the prey from their hiding place.
The researchers were able to distinguish three approaches strategies used by the octopuses: the animals approached from ambush, pursued the prey or crept up to it cautiously. As the analysis showed, shrimp were more likely to be attacked from ambush or after sneaking up on them, while crabs were more often pursued.
Next, the researchers analyzed the actual attack and how the four pairs of arms – the researchers numbered the arms on the right and left side of the body from 1 to 4 – are used. The slower crabs were more often overpowered by the simultaneous use of two to six arms. The second arm in each case dominated. The faster shrimp were attacked more cautiously and more specifically – often with the second arm first. Once this made contact with the prey, the octopuses secured their prey with the two adjacent arms
The scientists continue to report that the arms on the left and right side of the body are basically equal. The use is often related to the field of vision: If prey animals were perceived with the left eye, the arms on the left side of the body would be used in an attack.
The analysis further showed that the octopuses used only a small part of all theoretically possible arm combinations when hunting. So she seems to be using her arms very purposefully indeed.
