Development and senescence of control of ciliary locomotion in a gastropod veliger.

The rhythmical ciliary arrest behavior characteristic of the veliger larvae of the prosobranch Calliostoma ligatum develops in a predictable sequence of events. Spontaneous, small-amplitude (1-3 mV) postsynaptic potentials (PSPs) are first recorded intracellularly from prototrochal ciliated cells at about 45 h after fertilization. Prototrochal ciliated cells, which are precursors of the locomotory, preoral ciliated cells of mature veligers, are electrically coupled to each other. Cilia beat continuously and erratically at this stage. PSP amplitude and duration gradually increase with age, and at about 56 h, preoral ciliated cells become electrically excitable. A single regenerative action potential first occurs at this time and causes a velum-wide, ciliary arrest. Between 56 and 72 h, the duration of the depolarizing phase of the preoral ciliated cell action potential decreases, the amplitude increases, and the hyperpolarizing undershoot develops. Preoral ciliated cell action potentials appear to be Ca2+-dependent throughout development. Shortening of the action potential duration and development of the hyperpolarizing undershoot may be due to activation of later developing K+ channels. As veligers become competent to metamorphose, the preoral velar cells and their connections with the body deteriorate.