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Freshwater copepods (e.g., Cyclops)
These tiny freshwater relatives of the marine versions seen in virtually any coral reef aquarium often provide a simpler and cheaper alternative food than their marine cousins for feeding to planktivorous marine fishes. They are also usually devoured by coral reef invertebrates such as cleaner shrimps, gorgonians, soft and hard corals, brittle stars and sea cucumbers as well, making them a highly desirable food item. Although marine copepods are not easily available as a commercial food product, freshwater copepods can be harvested from local ponds or lakes, and there are a couple of commercial sources for either frozen or freeze-dried copepods as well. The reason that I am emphasizing these little "bugs" (no they are not really bugs, but that is what most people consider them to be) is that copepods are among the most nutritious of all food items in the ocean. Fortunately, even if the fish do not naturally feed on copepods, many plankton feeders (even picky feeders like pipefish and Anthias) in the sea readily accept freshwater copepods as a suitable alternative.
Given that copepods are a commonly offered and readily accepted by most plankton-feeding marine fishes, the easy availability of freshwater species makes them a simple option for feeding your fish. But remember, that I said there were two important things to consider for any food: 1) the right size/taste, and 2) the nutritional value of the food. So, I've said that the size, shape and taste of these tiny crustaceans appear to be acceptable for most plankton-feeding marine fishes, what about the nutritional value? Well, it turns out that copepods are one of the most nutritious plankton foods in the ocean, and a variety of studies have shown that feeding them can significantly decrease the rate of disease and death among juvenile fishes raised in captivity. For example, a recent presentation by Todd Gardner at the International Marine Aquarium Conference (IMAC) in Chicago showed that feeding baby seahorses on a diet of copepods for a few days before switching them onto enriched baby brine shrimp led to dramatic increases in the rate of survival compared to treatments in which the juvenile seahorses were fed only on enriched brine shrimp. Results such as Todd reported (Gardner 2003) are becoming more common, both in the hobby and in aquaculture efforts. In fact, copepods are so highly sought-after that many commercial aquaculture facilities are actively pursuing techniques to raise copepods in sufficient numbers to use as a reliable food supply for juvenile fishes. There are also a number of popular fishes (such as the Mandarin Goby or Scooter Blenny) that actually specialize on these little crustaceans. Unfortunately, there are currently no commercial sources of live copepods for either the aquaculture or aquarium industries, but there are a couple of suppliers of freshwater copepods in frozen or freeze-dried forms. In addition, there is always the tried-and-true method of going out to harvest your own, and whether fresh- or saltwater, adding copepods to the feeding regimen of your marine aquarium is sure to be a benefit to many of your pets.