The pilidium larva of the Nemertean or ribbon worm is a transparent member of the zooplankton—little smaller than a grain of sand. It consists of an outer sac of cells lined with cilia (in green) that allow the larva to move around and to draw food to its mouth, a region of muscle (in red), and the developing juvenile body (blue region in the middle). When the time is right, the fully formed juvenile ribbon worm will rupture the larval body and devour the larva in a unique process called “catastrophic metamorphosis” that takes about a minute. Ribbon worms are a diverse group of marine worms related to annelids and molluscs. They live in both shallow and deep seas, and most of them hunt prey such as annelids and crustaceans by using a proboscis that they can flip inside out. Whereas most are small, one specimen has the record of the longest known animal at around 60 m long!
Pilidial larvae were collected in the shallow water off the dock of the Marine Biological Laboratory in Woods Hole, MA. Different stains were used to label various structures in the cells of the pilidium larva and viewed using a confocal microscope: microtubules to see the cilia (in green), actin filaments to mark muscle-like structures (in red), and cell nuclei to see the developing juvenile and other cells (in blue).
Image courtesy of Matthew Clark (graduate student in Chris Doe’s lab.), HHMI, Institute of Molecular Biology, University of Oregon. It was taken as a student during the MBL Embryology Course, Woods Hole, MA (http://mblembryology.stowers.org/). Special thanks to Svetlana Maslakova Ph.D, Oregon Institute of Marine Biology, University of Oregon for advice on Nemerteans.