The external morphology and morphological variations of Sepiella inermis vary across regions, necessitating investigation. However, the histological information on the subcutaneous gland has been insufficient to describe it. In this study, specimens were systematically collected and characterized from the Gulf of Thailand. Regarding external morphology, female cuttlebones exhibit greater width and more pronounced curves compared to males, while males feature 17–19 white dots along the fin margins. The presence of the subcutaneous gland was discerned during the embryonic stage at stage 19. A histological study of the subcutaneous gland illustrated the structure and development of the gland in both embryonic and adult stages, with four layers of membranes covering the gland. In the adult stage, trabeculae are dispersed throughout the gland, whereas in the embryonic stage, they form four distinct lines. The morphometric analysis revealed significant differences between males and females (p < 0.05) and the morphological variations within both sexes across the seven study areas exhibit significant differences (p < 0.05). According to the discriminant analysis results, there were significant differences (p < 0.05) between the groups in Surat Thani Province. Examining the length–weight relationship between dorsal mantle length and body weight showed significant differences between the sexes, indicating an allometric growth.

This study aimed to analyze the growth parameters and age-length relationships of the sandbird octopus (Amphioctopus aegina) along the coast of Chao Samran Beach, Phetchaburi Province, Thailand. This included determining reproductive stages, the gonadosomatic index, and size at first maturity. A total of 1436 specimens comprising 782 males and 654 females were collected using octopus traps during January to December 2021. The mantle length and weight of each specimen were measured. Reproductive stages were assessed to determine the gonadosomatic index and size at first maturity, and growth parameters were estimated using the von Bertalanffy growth equation. Results showed that males exhibited a mantle length range of 3.1–7.5 cm (mean: 5.2 ± 0.7) and a weight range of 5.99–94.00 g (mean: 35.34 ± 11.24). Mantle length at first maturity for males was 4.2 cm, with full maturity at 7.0 cm. Females had a mantle length range of 2.8–9.4 cm (mean: 5.7 ± 0.9) and a weight range of 7.70–105.00 g (mean: 45.43 ± 17.00). Mantle length at first maturity for females was 5.9 cm, with full maturity at 7.5 cm. GSI values were highest in both sexes in February, April, May, June and November, suggesting reproduction occurred through much of the year, and females showed a large peak in May. GSI values were lowest in both sexes in September. The smallest recorded mantle length was 2.8 cm in May, and the largest was 9.4 cm in January. The specific values of t₀ for males, females, and both combined sexes were −0.017719, −0.014519, and −0.00672, respectively. These findings provide valuable insights into the growth and reproductive biology of A. aegina, which can inform sustainable management and conservation efforts for this species.