LARVAL PLANKTOTROPHY - A PRIMITIVE TRAIT IN THE BILATERIA

The concept of Gosta Jagersten of a primary biphasic metazoan life-cyc le, consisting of a planktotrophic larva and a benthic adult, forms th e basis for several theories on metazoan phylogeny. In this paper the assumed planktotrophic life-style of the larva is critically analyzed and reconsidered. It is shown, in particular for the Mollusca, that a biphastic life-cycle with a lecithotrophic larva is probably the plesi omorphic condition. Character distribution and structural data suggest a parallel evolution of the downstream collecting system used in plan ktotrophic larvae or filter feeding adults of gastropods, bivalves and other spiralian or aschelminth taxa. In the basic metazoans (Parazoa, Placozoa, coelenterates) direct or lecithotrophic development dominat es by far. For the acoelomate (Platyhelminthes, Gnathostomulida) and p seudocoelomate taxa direct development is probably the plesiomorphic c ondition. The structural similarities of the upstream collecting syste m in tentaculate and deuterostome phyla may also be explained by paral lel events of heterochrony out of an ancestor with adult filter-feedin g. The main conclusion of this survey is that larval planktotrophy is likely to be secondary and not a plesiomorphic condition among the Bil ateria. Accordingly, theories which are based on the assumed plesiomor phy of larval planktotrophy of the Bilateria, need careful reevaluatio n.