Answer: Fish
Notes: All the species listed are water-dwelling creatures. However, a fundamental difference is that fish belong to the group of vertebrates, meaning their body possesses a spinal column. In contrast, crabs, prawns, and snails are invertebrates, denoting that they lack a backbone. Specifically, crabs and prawns are crustaceans, within the larger class of invertebrates, characterised by their exoskeleton and segmented bodies. Snails are grouped as molluscs, also a class of invertebrates, known for their soft unsegmented body. Hence, in this context, fish are different from the others.
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