Cilia and the life of ctenophores

Ctenophores, or comb jellies, are a distinct phylum of marine zooplankton with eight meridional rows of giant locomotory comb plates. Comb plates are the largest ciliary structures known, and provide unique experimental advantages for investigating the biology of cilia. Here, I review published and unpublished work on how ctenophores exploit both motile and sensory functions of cilia for much of their behavior. The long-standing problem of ciliary coordination has been elucidated by experiments on a variety of ctenophores. The statocyst of ctenophores is an example of how mechanosensory properties of motile cilia orient animals to the direction of gravity. Excitation or inhibition of comb row beating provides adaptive locomotory responses, and global reversal of beat direction causes escape swimming. The diverse types of prey and feeding mechanisms of ctenophores are related to radiation in body form and morphology. The cydippid Pleurobrachia catches copepods on tentacles and undergoes unilateral ciliary reversal to sweep prey into its mouth. Mnemiopsis uses broad muscular lobes and ciliated auricles to capture and ingest prey. Beroe has giant smooth muscles and toothed macrocilia to rapidly engulf or bite through ctenophore prey, and uses reversible tissue adhesion to keep its mouth closed while swimming. Ciliary motor responses are calcium-dependent, triggered by voltage-activated calcium channels located along the length (reversed beating) or at the base (activation of beating) of ciliary membranes. Ciliary and muscular responses to stimuli are regulated by epithelial and mesogleal nerve nets with ultrastructurally identifiable synapses onto effector cells. Post-embryonic patterns of comb row development in larval and adult stages are described and compared with regeneration of comb plates after surgical removal. Truly, cilia and ctenophores, like love and marriage, go together like a horse and carriage.