Micronereis nechtochaete larva

This polychaete nechtochaete larva was captured in a plankton sample taken off a dock in Charleston, OR on April 5, 2012. I identified it as belonging to Micronereis nanaimoensis (Fam. Nereidae) (Crumrine 2001). It has three setigers (body segments bearing chaete), and the chaete (or setae) are characteristically compound, i.e. composed of two  "segments". Nereid eggs are typically supplied with a large amount of yolk (in a form of large lipid droplets), and this larva clearly developed from such an egg. The "shiny marbles" in its gut are the lipid droplets. Nereid nechtochaetes, in general, have two anal cirri (leaf-like appendages at the posterior end), and large eyes. Micronereis is unusual among local nereids in that it does not have tentacular cirri (also known as prostomial antennae) decorating its prostomium (the segment anterior to the mouth, which bears eyes). 

On this photograph one can discern a pair of chitinous jaws inside the pharynx, a characteristic of all nereids. You can see the pharynx through the body wall - it is a large pear-shaped structure located just posterior to the eyes between the two anteriormost chaetal bundles and "bissected" by a longitudinal brownish line which corresponds to the lumen. The pharynx ends where the gut (filled with lipid droplets) begins.  The jaws are semi-transparent at this stage. If you look closely you will also notice three transverse ciliary bands (one per setiger) - the cilia are visible posterior to each chaetal bundle. The larva uses these ciliary bands to swim. These larvae do not feed until they exhaust their yolk reserves.

Crumrine, L. 2001. Polychaeta. In: An identification guide to larval marine invertebrates of the Pacific Northwest. Edited by A. L. Shanks. Oregon State University Press. Corvallis.