This a traditional dish from Catalunya. It is based on rice and is made on an iron pot (the meaning of cassola in Catalan language).
It was a popular dish of the farmers and fishermen usually eaten on Thursdays. With the pass of the time became more sophisticated and you can find it in good restaurants any day of the week. Noted that can be also made with meat instead of fish.
In this case I think was used rice from Pals that provides a firm texture of the grain absorbing the different flavors. For being on a village that is on the coast, Palamos in Costa Brava (Spain), the ingridients were different seafood stuff (crayfish, shrimps, calamaries and mussels ). One of the secrets for this dish is the fish stock made with the best and fresh fishes.
The final texture of the rice is smooth something half way in between a Spanish Paella and an Italian rissoto. For me a delightful taste of sea.