Meeting Abstract
The pilidium larva is an idiosyncrasy defining one nemertean clade, the Pilidiophora. It forms the juvenile from a series of isolated rudiments called imaginal discs, then erupts through and eats the larval body in a catastrophic metamorphosis. A typical pilidium is planktotrophic and looks like a hat, but pilidial diversity is much broader than previously imagined. One of the most intriguing recently discovered larval types is the lecithotrophic, superficially trochophore-like pilidium nielseni of an undescribed species. Pilidium nielseni bears two circumferential ciliary bands evoking the prototroch and telotroch of a trochophore larva, found in related Spiralian phyla. One might interpret pilidium nielseni as a reversion to the ancestral condition, or as an example of convergence upon a successful body plan. The typical pilidium is already a highly derived larval form within Nemertea, so a reversion seems unlikely. To more definitely establish pilidium nielseni as an instance of convergence and determine how much of the pilidial developmental pattern is conserved, we describe and illustrate its development with confocal microscopy and compare it to that of a typical planktotrophic pilidium. Their development is strikingly similar; the juvenile develops via three paired imaginal discs and two unpaired rudiments, then emerges in a catastrophic metamorphosis. Pilidium nielseni even develops transient lobes and lappets in early stages, re-creating the hat-like appearance of a typical pilidium. It does diverge from typical pilidial development in the orientation of the anteroposterior (AP) axis of the juvenile to the AP axis of the larval body, and juvenile development is markedly accelerated. This is the first modern description of development of a pilidiophoran with a free-swimming lecithotrophic larva.
