My punt is Female edible crab abdomen.
Google pictures for 'edible crab abdomen' gave very good matches at aphotomarine http://www.aphotomarine.com/crab_edible_cancer_pagurus.html (=the last ie 9th picture in the list). (& another at http://howto.gd/h_150_how-to-tell-the-difference-between-male-and-female-crab.html )
In contrast here's female spiny spider crab carapace http://www.naturalvisions.co.uk/ImageDetail.aspx?imdet=49913
(but I couldn't match the red colour)