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Copepods are small crustaceans that are ubiquitous in all natural waters, and probably the most abundant multicellular animals on earth. Many of them are planktonic, meaning that they drift with the water because they are so small, typically about 1-5 millimeters long as adults. Planktonic copepods feed on phytoplankton (planktonic algae), protozoans, and sometimes each other. They are fed upon in turn by fish, and most fish depend on copepods for food during their early lives. Thus, copepods are a key element of aquatic food webs. Despite their small size, copepods have a complex life cycle and a wide array of behavioral adaptations to find food and mates and to avoid predators. They are skilled at selecting good food from the myriad of microscopic particles and organisms surrounding them. Adult copepods typically have 11 pairs of appendages, six highly specialized pairs for finding and handling food and detecting predators, and five pairs of powerful swimming legs. Over a thousand studies have been conducted of feeding in adult copepods, including what they eat, how they get it, how fast they feed, and where they feed. Yet, this is a complex topic because of the variety of copepods and their habitats and the constantly changing array of potential food. We still have a lot more to learn. Copepods develop from eggs that hatch into larvae called "nauplii" (singular: nauplius) (Figure 1). These are very small, typically 10-20% of the length and a few percent of the mass of the adults. They look like little mites, with three pairs of unspecialized appendages used in swimming and feeding. How do they get their food? Only a handful of studies have examined feeding by these small animals. Our study was intended to begin closing the gap in knowledge of nauplii, the neglected life stage. We took two very different approaches to feeding by the nauplii of very different copepods. In the first part of the study, we fed nauplii and adults of two very different copepod species with a variety of phytoplankton (planktonic algae) of different types and sizes. We used a high-power microscope to determine whether the nauplii and adults fed on these phytoplankton. When chlorophyll is exposed to blue light it fluoresces a faint red, and with this microscope we can see either the whole animal or just the red light through the copepodsÆ translucent bodies, and infer the amount of chlorophyll and therefore phytoplankton (Figure 2). This allows us to determine how much of that particular species the copepods had eaten. We also measured the feeding rate of adults and nauplii using an instrument that can measure fluorescence in a large number of samples at once. We were surprised to learn that there were greater differences between the two copepod species than between the nauplii and adults of either species. In other words the nauplii, despite their small size and rudimentary feeding appendages, were able to obtain similar foods as the adults, with their complex feeding appendages and strong swimming ability. The second part of our study looked at feeding by a copepod that, as an adult, is predatory on other copepods. This copepod, called Tortanus (Figure 3) has a pair of stout hooks that it uses to grab prey, usually other copepods nearly as large as itself. The nauplius looks much like those of other copepods (compare with Figures 1 and 2). We used a molecular method to find out what these animals were eating in San Francisco Bay. We extracted DNA from adults and nauplii, and used a technique developed by the student working on the project to identify the DNA of potential prey organisms. This showed that the adults were mostly eating copepods, as expected, but the nauplii were eating a wide variety of prey, and their diet was more like what we would expect from a particle-feeding copepod than a predatory one. To understand the role of these important organisms in the waters of the world, we need to understand how the...