Chiton trochophore

These are pictures of a chiton trochophore larva, which hatched from an egg picked out from plankton and given to the Embryology class by Dr. Richard Emlet. Because it was colledcted from plankton rather than from a spawning event by a known adult, I don’t know what species this larva belongs to. A trochophore is the larva of a spiralian animal, in this case a polyplacophoran mollusk, or a chiton. The trochophore is characterized by the prototroch, a pre-oral ring of ciliated cells that propels the larva though the water. You can see the long prototroch cilia on both of these pictures. This chiton larva was very opaque, so I used a dark-field technique to make some structures, e.g. eye spots, become more apparent. Chiton trochophores have two eyes located ventro-laterally posterior to the prototroch. While in some trochophores the ciliated band is involved in feeding as well as locomotion, it is not the case in chiton larvae which are lecithitrophic, or non-feeding, and depend entirely on the yolk supplied in the egg.

The second picture is taken using bright-field microscopy, and shows a series of seven bumps on the dorsal side posterior to the prototroch. Each of those bumps will become one of the distinctive shell plates of the adult chiton. I could clearly see the lines that mark the plates run down the side of the trochophore as it swam around. I was told that later another plate will form anterior to the ciliated band to bring the total number to eight, the number found in adult chitons.