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Morphological characteristics and the relationship between beak dimensions and body size of marbled octopus, Amphioctopus aegina (Gray, 1849) were determined using specimens collected from the Gulf of Thailand. A total of 408 individuals, 207 males and 201 females, were measured and weighed. Their body weights ranged from 7.30 to 120.00 g (39.78±19.05), mantle length (ML) from 32 to 80 mm (54.39±8.97) and total length (TL) from 114 to 305 mm (210.04±32.19). The rostral tips of the upper and lower beaks were brown to black in color, slightly curved, short, and with a small hood. The upper beak of this species has a short wing with large lateral wall. The ratios among upper beak dimensions were 1:2.95 for upper hood length (UHL):upper crest length (UCL), 1:3.64 for upper rostral width (URW):UCL, and 1:3.05 for upper rostral length (URL):UHL. The lower beak has a long wing and narrow lateral wall. The ratios of lower beak dimensions were 1:3.44 for lower hood length (LHL):lower crest length (LCL), and 1:1.25 for lower rostral width (LRW):LCL. Regressions of body size on beak dimensions were statistically significant (p<0.05). The best equations for determining mantle length (ML) and total length (TL) were ML = 0.531+2.108UHL (R2 = 0.704, MSres = 0.238) and TL = 3.574+2.539UCL (R2 = 0.731, MSres = 2.791), respectively.
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