Zoea larva of a brachyuran crab

Among plankton sampled on April 16, 2011 by our Embryology class, I found a few crustacean larvae in the zoea stage. The top picture is a regular bright-field photomicrograph, and the bottom one is a dark-field image. A zoea is a larval stage in the development of crabs and other decapod crustaceans. This stage follows the nauplius stage, which (in most decapods) is passed in the egg, and precedes the post-larval planktonic megalopa stage. Zoea larvae swim using thoracic appendages, (maxillipeds and pereopods), which distinguishes them from both earlier and later stages of development. The nauplius larva uses cephalic appendages to swim (antennulae, antennae and mandibles), while the megalopa swims using abdominal appendages called pleopods. I watched my specimens swim—they would swim for a short while, stop, and then continue swimming in another direction.

Zoea larva of a brachyuran crab (shown here) has a dorsal spine, a rostral spine (the anterior-most spine), and two lateral spines, which all extend from the carapace. These spines are thought to aid the larva in directional swimming, and could also be used as a defense against predators. A zoea feeds using the endopodites (inner branch) of bi-ramous maxillipeds and, depending on the species, can be carnivorous, phytoplanktivorous or omnivorous. The zoea has two stalked compound eyes that are relatively large, compared to the rest of the body. You can see on these photos that the diameter of the eye is nearly one third of the diameter of the carapace (not including spines).  These photos also show two maxillipeds (the two long appendages visible between the rostral and lateral spines). The anterior-most maxilliped contains the endopodites that are used for feeding. In the adult, the maxillipeds (including a third pair, which is not present in this stage) are associated with the mouth. Sadly, the abdomen is turned, so only one of the developing pleopods can just barely be seen on the dark-field image.