Genital plate formation in echinopluteus larva

Indirect development often involves a major change in body plan. The larva of the purple sea urchin, Strongylocentrotus purpuratus, has bilateral symmetry, and the adult has 5-fold radial symmetry. As the echinopluteus larva approaches metamorphosis clues to the body-axis shift start to appear. The first image shows a lateral view of the larva. The two vertical white lines are calcareous rods that support the arms of the larva.  They appear to glow because I used polarized light microscopy. Forming around each rod is a honeycomb-like structure which will become one of the genital plates in the adult. After metamorphosis, these plates are located on the aboral side of the adult (the side opposite the mouth) and surround the mouth. They are called genital plates because the gonads (testes or ovaries) open to the outside via little holes (called gonopores) in these plates. The endoskeleton of adult urchin, called the test, is made up of many such closely fitted calcareous plates.

The second picture is a close-up aboral view of the test of an adult urchin. Surrounding the large hole in the center (the anus) are the apical plates. You can also see five smaller openings, the gonopores, in the genital plates. Starting at 12 o’clock is the genital plate 2 or G2. This much larger plate perforated by a large number of small pores is called the madreporite; it connects the water-vascular system of the urchin to the outside. Clockwise from G2 are the other four genital plates in the following order: G3, G4, G5, and finally G1. I think the left and right genital plates on the larval image above correspond to G5 and G3 respectively (Emlet, 1985).

Emlet, R. B. 1985. Crystal Axes in Recent and Fossil Adult Echinoids Indicate Trophic Mode in Larval Development. Science 230: 937-940.