Monday, May 30, 2011
This is a picture of a larva of spionid polychaete (Fam. Spionidae, class Polychaeta, phylum Annelida). It was collected in a plankton tow off the F dock in Charleston, OR on April 13, 2011. Spionids have a small prostomium (segment anterior to the mouth) without antennae. Adult spionids are tube-dwelling worms that feed by extending two grooved tentacles called palps out of their tube into the water. On this picture you can see the primordial palps - the two appendages at the anterior end of the larva (up) as well as four eye spots. Some species of spionids shed eggs directly into the seawater; others keep eggs in their tubes during the early development. In spionids that brood only a few embryos develop, the others are called “nurse” eggs, and are eaten by the developing embryos. Spionids hatch at the stage of 3 setigers (segments bearing chaete). They can spend up to two months in the plankton, but usually they settle within three weeks. This larva already has over a dozen setigers. It is referred to as the nectochaete larva. In the plankton spionid larvae feed on unicellular green and golden-brown algae. Pigment derived from algal food is what likely colors the gut - the long tan-colored tube inside the larva you can see on this picture. Seen in the second picture is a close up of the segments to show the transverse ciliated bands found on many of the segments. These ciliated bands are used to propel the larva through the water.