Monday, May 9, 2011
Molt of a larval barnacle
On Wednesday, April the 13th, our embryology class collected plankton from the mouth of the Coos Bay in Oregon. Amongst the microscopic organisms there were lots of beautifully transparent naupliar molts. Nauplius is the planktotrophic (feeding) or lecithotrophic (nonfeeding) larval stage of many crustaceans. Pictured here is a naupliar molt of a barnacle (characterized by the two frontal horns) in dark-field. I identified this one as likely belonging to either Balanus glandula or Balanus improvisus. In adult barnacles, thousands of fertilized eggs are held in pouches, called ovisacs. Nauplii hatch from the ovisacs. Once in the plankton, they grow and undergo a series of up to six molts shedding their exoskeleton. The molts are often found in the plankton; they retain the external features of the larva, so one can identify which stage and species it belongs to. This molt is a stage IV. Stage VI nauplius undergoes the final molt and metamorphoses into a bean-shaped nonfeeding cyprid larva, which later settles onto the substratum and develops into the adult barnacle. These two pictures are of the same molt, at slightly different focal planes, to show different features. The top picture shows the fronto-lateral horns, and the dorsal thoracic spine located at the posterior end. Also in focus is a pair of mandibles (posterior-most bifurcated appendages) and a pair of antennae (bifurcated appendages anterior to the mandibles). These appendages bear bristles called setae, and along with the antennulae (anterior-most appendages, in focus on bottom picture), are used for feeding and locomotion. The nauplius has a single naupliar eye which you cannot see in the molt. The two-pronged furcal ramus at the posterior end (bottom picture) is a feature commonly used to identify the nauplii of different species. One can also see the labrum, a posteriorly-directed fold of tissue which extends over the mouth, located in the center of the cephalic shield, and the two small spikes in between the antennulae, called frontal filaments.