Cigalas are in fact dublin bay prawns,a cross between a langostine and a shrimp ,and look like a crayfish but with much longer claws.

http://www.bbc.co.uk/food/glossary/d...blin_bay_prawn

Virtually all cigalas served in Spain are caught in the British Isles (mainly Scotland) and shipped frozen to Spain (passing trucks carrying fruit & veg coming the other way).Their main benefit to paella is to add flavour-there is no claw meat and the edible body is tiny compared to the overall size.
Go to any coastal town at weekends in Spain and the restaurants are full of Spanish (or Catalan) at 3pm tucking into plates of freshly prepared Paella.