Adaptation for Marine Environments by Locomotive Tunic Structure in Cuttlefish

Cephalopods have higher motility and more widely distributed in the ocean than other marine invertebrates. Especially members of the family Sepiidae have diverse habitats and locomotory modes. There are inner and outer tunics in the mantle, and the mantle muscle is sandwiched by inner and outer tunics. Their mantle is used for locomotion and respiration, and the tunics of the mantle supports its movements. However, the relationship between the tunic structure and habitats/locomotive modes of cuttlefish remains unclear. In this study, we made histological observations of the outer tunics of three species of Sepiidae with different locomotory modes and quantified the percentages of different collagen types (type Ⅰ collagen: contributing tissue elasticity, type Ⅲ collagen: contributing tissue flexibility). The structure and the collagen types of the outer tunic were related to the habitats and locomotory modes of Sepiidae. In Metasepia tullbergi, which walks on the sea bottom in shallow waters, the outer tunic has a reticular structure that mostly consists of type III collagen. This species’ tunic is suitable for walking in shallower water. In Sepia kobiensis, which swims in deeper depth, the tunic has an oriented layer structure composed of type Ⅰ collagen. This structure is suitable for swimming under higher water pressure. In Sepia esculenta, which swims at shallower depths, the tunic has a thick, reticular structure and is made of both type Ⅰ and III collagen. This structure is suitable for swimming in shallower water. This study reveals Sepiidae have various locomotive tunic structure for adaptations for the diverse marine environment.