Copepods

Copepods are small crustaceans found in both marine and freshwater environments. Calanoids, cyclopoids and harpacticoids are of meticulous curiosity to reef keepers, since most species of these orders generally form the first link in the aquatic food chain after phytoplankton. They are the second largest source of protein in the oceans, next only to krill.

The huge mass of the fishes in the hobby are planktivores whose natural diet would be profoundly dependent upon copepods, along with other small prey items such as fish and invertebrate eggs and larvae. Smaller sized copepods, copepodites, and copepod larvae are well suited to feeding larval fishes and difficult to feed corals such as: Goniopora species, sea fans and Gorgonians, along with many other small mouthed coral species.

The main thing to remember with copepods is that it’s not a suitable for all kind stuff, the more diverse species that can be included in your feeding system, the better your reef and your fishes will do.

Ostracods

Ostracods are small crustaceans belonging to the class Ostracoda, sometimes referred to as seed shrimp or mussel shrimp on account of their appearance. In the reef aquarium they form part of the meiobenthos, those tiny benthic organisms between around 5 mm and 0.1 mm in size living on or amongst the substrate. This includes some the smallest species to be seen unaided by the human eye and comprises organisms such as nematodes, foraminiferans, and smaller crustaceans such as copepods, gammarids, mysids, ostracods etc.

In the reef aquarium ostracods are most commonly seen close to the viewing pane moving around on and in the top layer of the substrate. The ostracods encountered in the reef aquarium tend to be small, rarely longer than around one millimeter in length. There are thousands of species of ostracods making classification very tricky. Ostracods have been with us for a very long time, varying little in appearance since the Cambrian Period.

They mostly feed on microorganisms and organic debris and may be filter feeders, although some species may predate on smaller invertebrates, and a few are parasitic. In the reef aquarium they are a useful food source for benthic fishes, such as Dragonets or Mandarin fishes that pick at the substrate, and for fishes that sift through the substrate, sleeper gobies, and even form part of the diet of larger herbivores where they are taken incidentally when grazing.

Mysids

Mysids are a group of small, shrimp-like crustaceans; their common name of ‘opossum shrimps’ comes from females having a brood pouch, or marsupium, located in the thoracic segments between the legs. Mysids are found in marine waters throughout the world, but are also present in some fresh or brackish ecosystems in the Northern hemisphere where they are harvested for the aquarium industry.

Mainly benthic but exhibit irregular pelagic behavior characterized by a kind of figure-of-eight swimming pattern, juveniles can often be seen swimming in the water column in the lee of rock work. Mysids are the organism most likely to be mistaken for “baby fishes” or “baby cleaner shrimp” by inexperienced reef keepers.

Species seen in the reef aquarium tend to be around five to ten millimeters long, and are pale in color, nearly transparent, but may have patches of red to orange pigmentation. Most mysids are free-living but a few species are commensal and associate with sea anemones such as Bartholomea annulata, the Corkscrew Anemone, from the Caribbean.

The majority of Mysids are omnivores, feeding on algae, detritus, and zooplankton. A possible candidate for culture but cannibalism is common, with adults preying on newly hatched juveniles.

Tanaids

Most Tanaids encountered in the reef aquarium are tiny organisms that have a body length ranging from around two to five millimeters with a pair of antennae almost the same length again.  They are perhaps more insect-like in appearance and less shrimp-like than other members of the Peracarida, such as Mysids and Gammarids.

Mainly appear to be filter feeders but some species may prey on smaller organisms as their sole food source or in addition to filter feeding. Mostly seen on the viewing pane of the aquarium or along the substrate, they may also construct burrows in fine substrate.

As with other members of the Peracarida, females possess a brood pouch or marsupium. There is no larval stage; eggs hatch directly into a juvenile resembling a smaller version of the adult organism.

Gammarids

Gammarids are small, shrimp-like crustaceans with laterally compressed bodies; those seen in the reef aquarium tend to have a curved dorsal surface and are generally in the range of five to ten millimeters in length.

Usually seen through the front viewing pane of the aquarium moving along the top of the substrate or actually on the viewing pane itself, some species construct burrows or tunnels in the substrate.

Mature females have a brood pouch, or marsupium, where the eggs are fertilized and then held until the young are ready to hatch. Gammarids don’t have a larval stage; eggs hatch directly into a juvenile form having the appearance of a miniature adult. Adulthood, along with sexual maturity, generally occurs after six months.

Gammarids are predominantly detritivores, while some species may graze on algae and others may prey on smaller crustaceans, in the reef aquarium it’s probably best to regard them as opportunistic omnivores.