Sponges: New Views of Old Animals

Sponges (phylum Porifera) are exclusively aquatic, sedentary, filter-feeding invertebrates, occupying es- sentially all benthic marine and some freshwater en- vironments. With a worldwide fauna of at least 15,000 species (Hooper, 1994), poriferans are among the most diverse of sessile marine taxa. Sponges diverged from other animals earlier in evo- lutionary history than any other known animal group, extant or extinct, with the first sponge-related record in earth history found in 1.8 billion year old sediments, based on a demosponge-specific chemofossil, 24-iso- propylcholestane (McCaffrey et al., 1994). The first morphological sponge fossils are known from the Ear- ly Vendian Doushantou phosphorites in China ( ;580 Milllion years ago; Li et al., 1998) and the Neopro- terozoic (;550 my) Cloudina-Reefs of southern Na- mibia (Reitner and Worheide, 2002). The sponge body plan, comprising tissue-like cell associations, inhalant and exhalant canal systems, and chambers lined by flagellated choanocytes, allows for the efficient filtra- tion of particles from the water column and is so con- strained that little variation has arisen in its core com- ponents during at least 580 Ma of animal history (the fossil record of demosponges, the most diverse sponge ''class'' is reviewed in Reitner and Worheide (2002)). The seemingly simple, homogeneous bauplan and morphology of sponges contrasts with their high com- plexity and diversity at nearly every other biological level (e.g., phylogenetically, ecologically, develop- mentally).