External morphological characteristics and histological structures of needle cuttlefish (Sepia aculeata) larvae from Thai waters

The internal and external anatomies of cuttlefish larvae in Thailand have rarely been studied.This information is valuable not only for understanding their early development but also for applications in cephalopod distribution and abundance surveys.Needle cuttlefish (Sepia aculeata) eggs were collected from Phetchaburi Province, Thailand.Sixty hatchlings were prepared to observe the external morphology (30 specimens) and internal histology (30 specimens).A notable external morphological characteristic of the larvae is a broad, sac-like mantle.The hatching gland (Hoyle organ) and Klliker organs are not visible on the skin surface.Chromatophores are distributed across the body, head, arms, and tentacles but are absent from the fins and funnel.White spots are present on the dorsal side, not on the ventral side.Lateral lines appear on the head and arms.The fin length is 80% of the mantle length (ML), the eyes are large and round, and the funnel is stout, with a length 30% of the ML.The arm formula is IV>I>III>II.The optic lobe is large, rounded, and rectangular, filled with medullar neuropil surrounded by pericaryal islands.It is separated into the cortex and medulla.The cortex consists of three layers: outer granule cell layer, outer neuropil layer, and inner granule cell layer.The eye comprises a circular lens, iris, ciliary body, and curved retina.The obtained data provide essential insights into the early developmental stage of S. aculeata and serve as fundamental information for advancing research on cephalopod development, lifecycle, and phylogenetic relationships, particularly within Thai marine ecosystems.