Evolution of Metazoan Life Cycles and the Origin of Pelagic Larvae

Abstract

The problem of the origin of the metazoan life cycle is analyzed on the base of various hypotheses on the origin of multicellular animals. According to the Gastraea theory and the Phagocytella theory, the ancestral metazoan life cycle was holopelagic. In the framework of the hypotheses of the primary sedentarity, the metazoan ancestor had a pelago-benthic life cycle with floating larvae, the synzoospores. In accordance with this hypothesis, Eumetazoa originated from the progenetic larvae of the sedentary ancestor. The primary life cycle of Eumetazoa (i.e., metazoans with a nervous system, musculature, mouth, and gut) was holopelagic. Particularly this life cycle is typical for recent Ctenophora, which is the earliest branch of the eumetazoans. Cnidaria and Bilateria are sister groups. Their last common ancestor acquired the pelago-benthic life cycle de novo. The pelagic part of the life cycle of cnidarians is comprised of the blastula and gastrula stages only. Some anthozoans still maintain the planktotrophic gastrula larvae in their life cycles. Planulae of Medusozoa are simplified lecithotrophic larvae that had lost the function of spread because of the appearance of the medusa stage in the life cycle. In triploblastic Bilateria, the prolongation of the pelagic stage of the life cycle occurred due to the appearance of the ciliated bilaterally symmetrical larvae, which are actually the juveniles raised into the water column. This phylogenetic modus can be designated by the special term “larvalization.” Thus, the ciliated pelagic larvae of all bilaterians have a common origin from the juvenile stages of the last common bilaterian ancestor. Their ciliated bands came from the modified ciliated tentacular apparatus of juvenile stages of the last common bilaterian ancestor. Homologous elements in the ciliated bands of trochozoan and deuterostomian larvae are traced.