 Thread @-@ sail filefish grow to a maximum adult length of about 30 centimetres ( 12 inches ) . The first dorsal fin is a strong retractable ( folding backwards ) spine . The second dorsal fin and anal fin are soft . They have comparatively small pectoral fins and truncated , fan @-@ shaped tail fins . The dorsal and anal fins are colorless . Their second dorsal , anal and caudal fins rounded . In males , 1 @-@ 3 soft dorsal fin rays extended as filaments ; the first ray has a particularly long thread . The fish have a small abdominal spike . The fish are colored from light brown , to <unk> to light greenish @-@ beige , and are slightly patterned with irregular , broken stripes that range from medium brown to blackish . 
