Functional change in the swimbladder with fish size in Coryphaenoides acrolepis

One common macrourid fish, Coryphaenoides acrolepis, was analyzed to examine the relationship between the length of the rete mirabile (RmL) and water depth with fish size. The relationships differed when specimens were divided into two size classes based on pre-anal fin length (PAF). The RmL in specimens with PAF >15 cm became longer with greater depth, while in specimens with PAF <15 cm it was not affected by depth. On the other hand, the RmL became longer with fish size in the small-size class, while in the large-size class it was not affected by fish size. These differences suggest that the function of the swimbladder in C. acrolepis changes with fish size. The swimbladder of larger specimens would provide more buoyancy than that of smaller ones, and the RmL of C. acrolepis would become longer with growth when PAF is <15 cm. This functional change in the swimbladder at a PAF of 15 cm was in agreement with results on both changes in feeding habits and the start of reproduction. Our findings suggest that C. acrolepis experiences a decisive life cycle change when PAF reaches 15 cm. This study also suggests the possibility that a functional change in the swimbladder might be widespread among deep-sea demersal species in which the diet changes with fish size.