Degeneration of Comb Plates during Larval Stages in a Sessile Platyctenid Ctenophore, Lyrocteis imperatoris (Ctenophore, Platyctenida).

In species belonging to the phylum Ctenophore, comb plates consisting of bundles of multiple cilia are a characteristic synapomorphy and are essential for active swimming. However, in most species of the order Platyctenida, even though pelagic larvae have comb plates, adults completely lose comb plates and exhibit a sessile lifestyle. To understand the evolution of platynectids' unique lifestyles, it is necessary to reveal how comb plates are lost during the larval stages. Therefore, in this study, to reveal the process of comb plate degeneration, histological observations on larvae of different post-hatching ages were carried out in the harp comb jelly, Lyrocteis imperatoris (Platyctenida: Lyroctenidae). The observations showed that the number of comb plates decreased along with the larval growth, and they completely disappeared before the larvae settled down. Also, it was shown that the sizes of the ribs (rows of comb plates) in larvae increased until around 30 days after hatching, while relative sizes of ribs with respect to larval body sizes continued to get smaller after hatching. This suggested that, in L. imperatoris, although the ability to disperse away from their parents would be maintained after hatching as long as ribs develop, this ability would significantly decrease after the cessation of rib development. Therefore, in contrast to pelagic comb jellies in which ribs continue to develop throughout their lives, L. imperatoris acquired mechanisms to cease rib development and promote rib degeneration during their post-hatching growth, which are involved in the evolution of novel sessile lifestyles.