The first picture is that of a copepod nauplius from a plankton tow collected at the mouth of Coos Bay, OR on Saturday, May 5th. The nauplius is the first larval stage of many crustaceans (although many species pass through this stage while still enclosed in the egg capsule). Nauplii are characterized by having three pairs of appendages: a pair of uniramous (not forked) antennules, and a pair of antennae and manidibles, both of which are biramous (i.e. forked). Nauplii are some of the most common organisms one encounters while sorting plankton samples. In Coos Bay we often encounter the nauplii of both barnacles and copepods.
In the second picture we see the nauplius of a barnacle. Typically barnacles go through four to six naupliar stages depending on species. The distinguishing characteristic of a barnacle nauplius is the pair of fronto-lateral horns at the anterior end. Copepod nauplii do not have any such horns. Being able to distinguish between the two kinds of nauplii is important because both have a biphasic life cycle, but copepods are holopelagic (meaning they spend the entire life cycle in the water column) whereas barnacles are benthopelagic (adults are benthic, while larvae are planktonic) so different ecological questions can be addressed by studying these two kinds of organisms.
After the nauplius barnacles go through an additional larval stage called the cyprid (left). The final nauplius stage molts into the cyrpid stage, which is non-feeding. Cyprid larva has a pair of large appendages at the anterior end (left side of this photo) called antennae - one can see these sticking out from the carapace. The cyprid will use these to “walk” around on the substrate while looking for an ideal spot to settle. Once it finds a spot it will cement itself to the substrate and undergo metamorphosis into an adult (permanently attached) stage.