Cnidarian Larvae: True Planulae, Other-Than-Planulae, and Planulae That Don’t Look Like Planulae

Abstract

The life cycle of the common ancestor of Metazoa is a widely debated topic in EvoDevo. This is intimately linked to a number of questions, such as how the larva appeared in the metazoan life cycle and which larval form can be considered ancestral. To approach these questions, we can analyse the life cycles and larval forms of Cnidaria, the basal metazoans that form a sister group to the Bilateria. Almost all cnidarians have a pelagic larva in their life cycle. These larvae are commonly referred to as “planula,” with few exceptions. The planula is a ciliated lecithotrophic larva with epithelial ectoderm and endoderm, a gastric cavity, and an elongated body. The review examines whether the larvae of various Cnidaria fit this description and explores which larval form is ancestral for different cnidarian taxa. It also highlights the enormous diversity of cnidarian larvae, which is still underestimated, and infers the relationship between the evolution of life cycles, reproductive patterns, and larval forms in various phylogenetic groups of cnidarians.