Managed to identify this prawn as Melicertus latisulcatus from this useful resource on food. It’s vernacular is the Lum Bueh (Hokkien) or blue tail. Another vernacular is the Sua Beh or sand horse.
Fishmonger said this has a crunchy texture. They are a pretty sight though and probably why I got it. Do all prawns taste the same? Probably subtly different. $15 for a kilo. Now I wonder if they are sustainably harvested or farmed?
They have a preference for the sand so they can hide in it to avoid predators.
Maybe that’s why the prawns that belong to the Sua Lor (sand prawns) group have a crunchy texture.