Invertebrate Biology 123(1): 1–22. 2004 American Microscopical Society, Inc. Choanoflagellates, choanocytes, and animal multicellularity

Abstract. It is widely accepted that multicellular animals (metazoans) constitute a monophy-letic unit, deriving from ancestral choanoflagellate-like protists that gave rise to simple choanocyte-bearing metazoans. However, a re-assessment of molecular and histological evi-dence on choanoflagellates, sponge choanocytes, and other metazoan cells reveals that the status of choanocytes as a fundamental cell type in metazoan evolution is unrealistic. Rather, cho-anocytes are specialized cells that develop from non-collared ciliated cells during sponge em-bryogenesis. Although choanocytes of adult sponges have no obvious homologue among meta-zoans, larval cells transdifferentiating into choanocytes at metamorphosis do have such homologues. The evidence reviewed here also indicates that sponge larvae are architecturally closer than adult sponges to the remaining metazoans. This may mean that the basic multicel-lular organismal architecture from which diploblasts evolved, that is, the putative planktonic archimetazoan, was more similar to a modern poriferan larva lacking choanocytes than to an adult sponge. Alternatively, it may mean that other metazoans evolved from a neotenous larva of ancient sponges. Indeed, the Porifera possess some features of intriguing evolutionary sig-nificance: (1) widespread occurrence of internal fertilization and a notable diversity of gastru