 Like other members of the genus Sepioteuthis , bigfin reef squids are easy to distinguish from other squids in that they possess thick and muscular oval fins that extend around almost the entire mantle . The fins extend about 83 to 97 % of the mantle length and are 67 to 70 % of the mantle length in width . Because of these fins , bigfin reef squids are sometimes mistaken for cuttlefish , a fact reflected by their scientific names . A narrow blue or white line is visible at the point of attachment of the fins to the mantle . A fleshy ridge is also present where the fins meet at the back of the squid . 
