Tuesday, May 17, 2011
Juvenile spicules in bipinnaria
During the 4th week of development of my Evasterias troschelii (a starfish) larvae, I noticed several spicules near the larval stomach. These calcareous structures will eventually become incorporated into the skeleton of the adult asteroid. The spicules began forming near the posterior end of the bipinnaria larva, which is the first larval stage of asteroids. The top picture shows a dark-field view of portion of the larval body (anterior end up). The dark triangle near the top is the larval mouth. The opaque tube right below is the esophagus, which is connected to the stomach via a sphincter. The stomach is the large upside down pear shape at the bottom of the image. You can see six dichotomously branching spicules overlaying the larval stomach. The transparent sack on either side of the stomach and esophagus is the larval coelom (body cavity). The bottom picture is a close up of the same individual. I noticed that most commonly the spicules start off looking like the third one from the top and subsequently grow and branch to create an intricate and beautiful pattern.