When heads are not homologous: the coronae of larval and adult collothecid rotifers (Rotifera: Monogononta: Collothecaceae)

Abstract

Rotifers are diverse and abundant aquatic micrometazoans that rely on their ciliated apical end (corona) for locomotion and feeding. In order Collothecaceae, which includes mostly sessile species, larval rotifers go through a complex metamorphosis after settlement wherein they replace their corona with an unusual cup-shaped head that functions exclusively in food capture. This new head, called the infundibulum, consists of morphological elaborations such as lobes or tentacles that develop precociously in the larval stage; in adults they function in ambush and sit-and-wait predation. Here, we provide evidence from brightfield and transmission electron microscopy that the infundibulum of collothecid rotifers is derived from the larval foregut and not the larval corona, suggesting that the adult head of collothecids is a morphological novelty and therefore not homologous with the rotifer corona as classically defined. The wide variety of morphologies of the infundibulum suggests that selection to maximize foraging may have driven the evolution of head form in these sessile species, and that future studies of collothecid rotifers should consider the infundibulum as a unique form of food-collection device that is evolutionarily distinct from the rotifer corona.