Hull flotation function in the eggs of Mopalia ciliata (Chitonida: Mopaliidae) and swimming of its larvae through ontogeny

Abstract

The free-spawning species of chitons produce eggs that are enclosed in a capsule known as the hull. Particularly in the group of Chitonida, this outer coat is formed by spine projections. Previous studies have identified the function of the hull as a "sperm guide" that facilitates the fertilization of the eggs, and one has suggested that this hull also serves to slow down the sinking rates of the eggs. In this study, I present concrete evidence showing that the hull does reduce the sinking rates of eggs in the chiton species Mopalia ciliata, acting as flotation device, as a consequence of its lower density compared to sea water and the egg without this structure. Additionally, changes during ontogeny in behavior, swimming velocities, and body shape of the larvae of this species were analyzed. Over time, the larvae decrease their tendency to swim up and prefer to stay in the bottom, and the body becomes elongated. These changes may be related to preparation for settle and metamorphosis into a juvenile. This study shows the importance of the hull structure in the sinking rates of the chitons eggs, and the ontogenetic changes in behavior and form that the larvae present, encouraging further studies to evaluate the ecological impact of these aspects in the persistence of the species.