Crustaceans may be able to experience some emotions, a study published in the journal Science suggests.
Researchers in France have found that crayfish seem to show anxiety, a feeling previously thought to be too complex for these primitive animals.
It follows a number of studies that suggest that crustaceans can also feel pain.
Some experts say the seafood industry may need to rethink how it treats these creatures.
Dr Daniel Cattaert, from the University of Bordeaux, who carried out the research, said: "Crayfish are primitive, they have been around for hundreds of millions of years.
"The idea that this animal could express some anxiety didn't seem possible, but with our experiments we're more and more convinced that this was the case."
To investigate, the scientists exposed the crustaceans to a stressful situation - in this case an unpleasant electric field.
The creatures were then placed into a cross-shaped tank. Two of the arms of the cross were dark - an environment that most crayfish prefer, while two were light.
Dr Cattaert said: "When you have a naive crayfish (one not exposed to the electric field), you observe that the animal will go in all of the arms, but with a slight preference for the dark arms.
"But when we place a stressed animal in the maze, we observe the animal never goes in the light arms.
"The light arms are perceived as too threatening."
The researchers found that the crayfish produced high levels of serotonin, a chemical that is released by the brain to counteract anxiety.
They also discovered that when they injected the stressed creatures with an anti-anxiety drug, they stopped being so wary and began to explore the light arms of the tank.
"The behaviour observed was reminiscent of anxiety behaviour," said Dr Cattaert.
He said that anxiety could have evolved much earlier than previously thought and that it was a useful feature for survival.
"If an animal has expressed stress, after this, if the animal doesn't change at all its behaviour - it goes on exploring - then it may encounter a predator," explained Dr Cattaert.
"But if the crayfish changes its behaviour, then it will avoid being in a situation where danger may occur. Even if there is no predator, it will be minimising this threat."
The fact that these animals may get anxious adds to a number of studies that suggest crustaceans also feel pain.
Commenting on the research, Prof Bob Elwood, from Queen's University Belfast, said: "This work shows the behaviour is consistent with a state of anxiety.
"But pinning together what the animals are feeling is the impossible thing. We know how we are feeling, and we know the behaviour associated with that - but you cannot ask a crayfish how it feels.
"The crustaceans are showing the behaviour, but some people will say that doesn't mean they feel anxious in the way we feel."
However, he said that if there was a possibility that the creatures feel anxiety and pain, then their welfare should be looked at.
Crustaceans are not considered sentient by bodies such as the European Food Safety Authority, and there are no regulations concerning their treatment.
Prof Elwood said: "I think it must be regarded as a possibility that they experience anxiety and pain.
"And if we consider there is a possibility, then effective safeguards against inflicting pain should be taken just to be on the safe side, and we should also ensure they are killed rapidly."