Morphology and dynamic behavior of anal pores during defecation in the ctenophore Pleurobrachia pileus

Abstract

Defecation in the lobate ctenophore Mnemiopsis leidyi was recently shown to occur periodically with an ultradian rhythm through a single transient anal pore which suddenly appeared, expelled waste, and disappeared afterward. To discover whether this novel method of defecation occurs in other kinds of ctenophores, I examined individuals of Pleurobrachia pileus and Beroe cucumis. Both ctenophores were found to have two identical and permanent anal pores as described in the scientific literature and textbooks. In P. pileus, both anal pores commonly participated in a defecation, but they did so asynchronously with independent and irregular opening and closing kinetics. Individuals of P. pileus defecated with an ultradian rhythm. Closed anal pores in P. pileus and B. cucumis consisted of a continuous ectodermal epithelium overlying a continuous endodermal epithelium with a cup-shaped group of thickened endodermal cells bearing a tuft of cilia which beats into the anal cavity. The rims of opening or closing pores were smooth and uniform without encircling muscles or fibers. This morphology and the continuity of the epithelial layers between defecations suggest that anal pores may not operate by a contractile sphincter, but by a reversible ring of tissue fusion between apposed ectodermal and endodermal epithelia to create an adjustable hole to expel waste.