Good article on Cris / Koch's perspective on this. A follow up to this article could talk about the views of Mark Solms, Joscha Bach. Both of which would include most animals as conscious, but in different ways. Mark Solms talks about how affect (i.e. feelings) is where consciousness arises from. Living organisms developed homeostatic mechanisms (i.e. maintaining oxygen levels, finding energy, etc), and this mechanism becomes more and more abstract in such a way that they become the "feelings" of the organism. From this perspective, any sufficiently complex nervous system that is maintaining homeostasis probably has feelings and with enough complexity will have some level of consciousness.

Personally, I agree with Joscha Bach who says consciousness is part of the simulation that the brain needs to do in order to model the world and the needs of the body. So any organism that has a simulation of the external world is probably conscious. Crabs would certainly have one, even if it's a very simple model. They probably also have a self-model, but no self-consciousness of that model like in mammals.

My guess is that although we don't know what it's like to be a crab, we might have an idea of what it's like to be a non-human mammal, even a bat. I think when we are infants (1-3 yrs old?) we have this kind of limited consciousness, which lacks self-awareness but we still have experience and emotions. From the computationalist perspective (i.e. Joscha Bach), this makes sense because our world simulation is still being "built", and for non-human mammals like chimps, this could be their computational limit.