The first picture is of a larva of a crab from the family Pinnotheridae collected from the plankton on April 19th, 2012 from the boat basin in Charleston, Oregon. Pinnotherids, commonly known as pea crabs, are crabs that live commensally with a variety of organisms e.g. mollusks, annelids and echinoderms. This zoea has relatively short rostral and dorsal spines in addition to two lateral spines. Studies have shown that lateral spines likely serve to reduce predation by planktivorous fishes. Additionally, one can see one of the two large compound eyes of this larva.Pea crab zoeas are some of the most ubiquitous zoea larvae encountered in plankton tows in this part of the world. However, zoea larvae of many brachyuran crabs are very similar and identification can be tricky. The two bottom pictures highlight the differences between zoea larvae of a pea crab and a crab from the family Cancridae.
In the second picture we see a close up of the telson (terminal segment) and the two preceding abdominal segments excised from a pea crab zoea. The enlarged last abdominal segment (wider than the telson) that you can clearly see on this picture is a the identifying feature of pea crabs in the Northeast Pacific. Furthermore, most pinnotherid zoeas have small narrow telson.
In the third picture we see the telson and last four abdominal segments of a zoea from the genus Cancer. One can see a pair of spines on each of the abdominal segments, however none of the segments is expanded to the point of being wider than the telson. Additionally, the telson of a cancrid zoea is wider than that of the pinnotherid zoea and is forked.