It‘s unlikely because decades of experiments suggest so and also because it‘s unlikely that animals with a very basic neural system can have a sense of self. The burden of proof is on those claiming differently.

The burden of proof is on anyone claiming anything because the polarity of propositions is irrelevant to standards of proof, which is why you brought up the studies, because you know that intuitively if not formally.

That's seems backwards. Everything is highly unlikely, a priori. To say that something isn't unlikely is the assertion which requires evidence.

There’s been a lot of research on self identification in many species. One paper I’m aware of did an analysis of 50 years worth of research on over 30 species. So we have a fairly decent idea about the development of theory of mind and self identification in animals. For example self identification has only been found in social animals, and lacking in all the solitary animals studied.

It seems like there’s a progression from simple organisms with sense/response behaviour but little or no cognitive modelling of the environment. Then organisms that can model and reason about their environment in fairly sophisticated ways. Then some animals develop the ability to model the knowledge, intentions and responses of other creatures. That’s what evolutionary psychologists call theory of mind.

Finally there’s development of a sense of self, and then further on the ability to reason about one’s own thoughts, motivations, intentions and cognitive abilities, and act to change those. That’s us. It’s a continuum though, with various species at different points along the line.