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.

The external morphological characteristics and proportion variability of spineless cuttlefish, Sepiella inermis, were investigated at seven locations along the coastal waters of Thailand for one year. A total of 396 specimens were collected from Trat (30), Chonburi (61), Samut Songkhram (80), Phetchaburi (75), Surat Thani (30), Songkhla Province (45) and Ranong Province (75). The results showed that the dorsal mantle length (DML) measurement ranged from 32–92 mm (59.07 ± 10.51), and the body weight ranged from 8.74–133 grams (39.37 ± 18.40). The sexuality has a significant impact on morphological indices, with statistically significant differences at (p < 0.05). The dorsal mantle length can be used to identify sexuality at (p < 0.05), with a range of 40–49 (45.21 ± 2.41) mm for females and 30–39 mm (37 ± 2.82) for males. The morphometric characters at all stations were found to be correlated; Chonburi station stood out significantly from the other stations. Additionally, the description structure of the subcutaneous gland exterior, which had four lobes and was deeply grooved, were examined using Scanning Electron Microscopy (SEM) in the larvae stage. Internally, the wall of the gland consists of four layers: 1) the outer wall is the epidermis, 2) connective tissue, 3) muscle fibers, and 4) the inner wall is mucosa lines and cuboidal in shape. The basal portion of the gland produced an ink source and had four trabeculae branches retaining melanin granule cells. This study of external morphology offers numerous advantages and represents the first reported description of the subcutaneous gland in Sepiella.