Copepods are Nature’s trophic intermediaries. That is, they
transfer nutrients and food energy from lower (e.g. plants) to higher (e.g.
carnivores) levels in the food chain. This also happens to make them nearly indispensable
in aquarium systems, where they (1) consume unwanted material such as nuisance
algae and detritus and (2) are eaten by zooplanktivores ranging from fish
(anthias, mandarins, etc.) to corals. Their young, quite notably, generally are
phytoplanktivores, eating free-living microalgae from the water column during
their planktonic larval stages.
As it is comparatively more nutritious, pods prefer algal
material to detritus. They mainly consume soft microalgae. This means that they
typically do not eat crustose forms such as coralline algae, nor do they eat
macroalgae (aside from dead, decaying, softened material). They love green algae and diatoms; these are both available in a benthic form as thin
films (for adult harpacticoid copepods) and in a planktonic form as single
cells (for larval harpacticoid copepods and all cyclopoid copepods). As such, a
diverse community of copepods helps to keep the aquarium water clear and the
rocks, sand and glass free of green or brown films.
Being so beneficial, aquarists generally seek to culture as
high of numbers of these critters as possible. Unsurprisingly, their population
densities are limited by food availability. This means that if you add pods to
remedy a preexisting algae plague or detritus pool, you might observe steady
increases during the exponential growth phase, only to see numbers drop as food
supplies are gradually depleted.
Sometimes a pod population will decline before the
nuisance algae and/or detritus is gone, or worse yet, they never really take
off at all. Where losses to heavy mechanical filtration (e.g. filter socks) or
UV sterilization can be discounted, this usually due to the starvation of
larvae. Remember, their “babies” need phyto! Those aquarists who dose phyto on
a regular basis maintain larger pod populations (can’t have lots of adults
without lots of larvae). And, when raised on a nutritionally balanced live
phyto product such as OceanMagik, your pods will be more nutritious for the
animals that prey on them!