Validation of Cuttlebone Daily Increments and Estimation of Growth Parameters for the Spineless Cuttlefish, Sepiella inermis (Van Hasselt, 1835)

Accurate age estimation is crucial to understand the growth patterns and population structure of short-lived Cephalopods. This study validated the daily periodicity of cuttlebone increments formation in Sepiella inermis, a commercially important cuttlefish from southwest coast of India and applied the validated approach to describe natural growth patterns in wild populations. Egg masses were collected from Aruthinkal, Kerala (9°39&apos;24"N, 76°17&apos;29"E) at depth of 10-20 m during 2023 and reared under controlled conditions, yielding a hatching success of 97%. Hatchlings measured 3.31 ± 0.20 mm dorsal mantle length (DML) and 0.014 g wet weight. By day 50, juvieniles reached 14.98 ± 2.27 mm DML and 1.036 ± 0.039 g, corresponding to mean daily growth rates of 0.28 mm in DML and 0.029 g in body weight. Both DML and body weight increased significantly with age (p < 0.05). Cuttlebone microscructure revealed approximately five increments at hatching, followed by nearly daily deposition, with juveniles recording 55.2 ± 1.10 increments by day 50. Increment-age and increment-DML relationships in reared specimens were best described by sigmoid models (R2 = 0.97, 0.98). Whereas wild individuals showed a linear relationships between DML and increment count (R2 = 0.78). Length-weight relationships for reared and wild individuals indicated negative allometric growth (b < 3). These findings confirm the reliability of cuttlebone increments as daily age markers and provide baseline growth parameters relevant to fisheries assessment and management of S. inermis,K