Squids and octopuses appear similar at first glance since both are soft marine creatures with eight sucker-lined arms and high levels of intelligence. However, their last common ancestor lived around 300 million years ago, leading them to develop distinct evolutionary paths and unique adaptations.
As per Science Alert, recent research has revealed a key difference between the two: while octopuses can taste their surroundings through specialized receptors in their suckers, it was previously believed that squids lacked this ability. It turns out that, like humans, these sea creatures also have distinct tastes.
Octopuses More Attuned to 'Greasy, Sticky Molecules'
New research in two papers in Nature has shed light on the substances that octopuses' receptors are sensitive to. The study has also revealed that squids have taste sensors in their suckers, but their receptors are tuned to a different kind of flavor more suitable for their lifestyle.
In a previous study, scientists discovered that chemotactile receptors in the suckers of California two-spot octopuses allow them to taste "greasy" and insoluble molecules known as terpenes.
To gain a better understanding of these proteins, Science News reported that the researchers employed the use of cryoelectron microscopy to examine the protein's three-dimensional structure. During this process, they found a large molecule stuck in a special pocket designed to detect specific chemicals.
However, the molecule was not the type of molecule that binds to the neurotransmitter receptors from which chemotactile receptors evolved, as it was bulky and greasy, unlike small and soluble neurotransmitters.
The octopus receptors were tested with a range of molecules collected from neighboring labs. The results presented in the first paper showed that the receptors could detect various "greasy, sticky molecules" that do not dissolve in water. But that does not mean that they find all greasy and sticky molecules to be tasty, as shown in one experiment.
Since octopuses feel around for their prey, it is logical that their taste receptors would have evolved to detect molecules that remain stuck to underwater surfaces, such as crab shells or their eggs, rather than small chemicals that quickly diffuse in water.
Squids Can Detect Bitter Compounds
Octopuses have 26 genes that encode slightly different chemotactile receptor proteins. When combined in five groups, these proteins can detect a wide range of molecules. On the other hand, the second paper showed that genetic analyses of squid and cuttlefish revealed that these cephalopods also have modified neurotransmitter receptors in their suckers.
However, while some of the squid and cuttlefish receptors detect the same greasy molecules as octopuses, others detect bitter compounds that can diffuse in water. The latter is thought to signal that food is spoiled or poisonous, and squid will avoid eating prey soaked in bitter compounds.
Octopuses use their chemotactile receptors to explore underwater environments and to hunt in the dark. Squid, on the other hand, rely on their eyesight and are ambush predators that use bitter receptors to determine whether to eat prey once it is in their grasp. Researchers have not yet identified the sorts of molecules detected by the other type of chemotactile receptor found in octopuses.
The researchers believe it will take years to fully understand what all the cephalopods' receptors detect and how they influence the animals' behavior. Nonetheless, they have made significant progress in understanding these receptors' evolution and their role in cephalopod biology.
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