Oithona: A Keystone Aquarium Copepod Species

In terms of both numbers and biomass, the copepods (Subclass Copepoda) dominate the zooplankton oceanwide. These minute crustaceans are of immense value ecologically because they play a central role in the linear trophic transfer of nutrients and food energy as intermediaries between primary producers (e.g. phytoplankton) and higher animals such as fish.

But it’s usually not quite that simple; that’s because, despite the apparent emptiness of the open waters, planktonic food webs aren’t all that simple. Indeed, there often is (1) multiple copepod species present in the same pelagic ecosystem and (2) an astonishing functional diversity within copepod communities in these seemingly niche-poor environments.

Marine ecologist G.E. Hutchinson spoke of the “paradox of the plankton.” Perplexed by the lack of competitive exclusion between species, he wondered if factors beyond the abundance of phytoplankton allowed for the richness and diversity we so often observe within copepod communities. He accordingly took into account not just the interspecific competition for food (and maybe space), but also stuff like predation and environmental instability. He found that such pressures can actually increase copepod diversity in a given ecosystem!

An increasing amount of attention is being paid to the cyclopoids as of late due to their prominent places in marine trophic food webs. In this article we describe the ecologically important cyclopoid copepod Oithona spp. and discuss some the traits that allow it to flourish in the presence of badass copepod competitors—both in nature and in aquaria.

A place at the table

Zooplankton may compete intensely for phytoplankton. Fast-growing but inefficient grazers enjoy fast population growth, though they are ultimately supplanted by slow-growing but more-efficient grazers. Food specialization decreases competition pressure amongst these phytoplanktivores. For example, although large pods that can exploit food resources of all sizes have an advantage over size-restricted grazers, selective grazers might gain a foothold by focusing on higher quality foods.

Of the three major copepods groups–the harpacticoids, calanoids and cyclopoids–it is the calanoids that rule the seas. Calanoids are particularly dominant in pelagic marine environments. That being said, one of the most abundant and widely occurring copepod genera is a smallish cyclopoid: Oithona spp.

Because of the common occurrence of multiple species from the families Oithonidae and Paracalanidae, copepod diversity is markedly greater in the tropics than in temperate regions. Generally inhabiting open waters (as opposed to well-known bottom dwellers such as Tisbe and Tigriopus), Oithona is found in marine, brackish and freshwater habitats.

Oithona earns its place in these habitats by feeding on miniscule food particle (less than 10 micrometers) such as bacterioplankton. This makes it quite competitive with calanoids in its community, as the latter is much more efficient at grazing on larger items (including young Oithona). To some extent Oithona also preys on microprotozooplankton such as ciliates. This means that huge parts of the standing crop of bacteria and picoautotrophs are trophically transferred to these copepods in only one or two steps; since (1) bactivory and herbivory by microprotozooplankton can greatly exceed that by copepods, and because (2) these protozoa can deplete entire phytoplankton blooms, Oithona has been identified as a “retriever” for its role in scavenging them.

Equally important to food size is food whereabouts. Amongst copepods, positioning in the water column (benthic or planktonic habit, daily vertical migration behavior, etc.) strongly determines levels of interspecific food competition amongst copepods. In other words, if separated spacially, even species with the same food preferences will not compete to exclusion.

We observe similar niche partitioning with respect to diel patterns. That is, nocturnally and diurnally active species tend to stay out of each others’ way and “share” food sources much more effectively.

As do most cyclopoid copepods, Oithona spends the majority of its time in the water column. This habit makes life rather difficult and dangerous for the species; rather than competing with harpacticoids for the typically rich food sources on the substrate, it must move up into the open waters to scour bacteria, phytoplankton and protists from the plankton. These activities increase its vulnerability to predation, hence its predisposition for mainly nighttime feeding. In a somewhat ironic twist of fate, it tends to be ignored by predators which favor the bigger, juicier calanoids.

Tough enough

Exhibiting the competitive edge so characteristic of its congeners, and also tolerant of a broad range of environmental conditions, Oithona colcarva is a predominant member of Western Atlantic copepod communities from the Caribbean to Cape Cod.

It is primarily an estuarine species, though it is only found near rivers in areas of reduced flow and increased salinity. Indeed, its population densities are known to positively correlate with higher salinity. Despite its usual penetration into Northern waters, it is considered to be a tropical species. It is most abundant during summer months (dry season=higher salinity), though its summertime numbers are limited by predation by calanoids (e.g. Acartia tonsa). It prefers shallow waters (0-50 meters), where it recedes to the depths between midday and around 8:30 P.M.

When one considers that O. colcarva is (1) so hardy and so adaptable, (2) naturally so widely distributed and (3) small enough (60-220 micrometers) for the tiniest mouths and finest filter-feeders, it is somewhat surprising that the species hasn’t already become a staple of the marine aquarium industry. Its high nutritional content (high amino and unsaturated fatty acid content) plus a relatively short life cycle (14-16 days) only make it more desirable for both aquarium and aquacultural use.

AlgaeBarn does get this. Hence EcoPods. EcoPods is a live, multi-species product that helps you to build and maintain a large, stable and balanced community of harpacticoid and cyclopoid copepods. The tried-and-true harpacticoid species Tisbe and Tigriopus add to your system’s benthos, cleaning the bottom and providing a valuable food source for finicky small reef fishes such as mandarins, even as their microscopic young contribute to the plankton during the earliest life stages. Pretty much planktonic throughout their entire life cycles, the cyclopoids Apocyclops and Oithona add more substantially to the planktos. Their adults are just large enough to be seen, pursued and captured by planktivorous fishes such as anthias and fairy wrasses.

A more dynamic captive ecosystem

But, really, four pod species? Overkill, maybe? Absolutely not. Just as difference of body size allows for different predator-prey interactions and feeding ecologies between Tisbe and Tigriopus, so it does between Apocyclops and Oithona.

Measuring in at a whopping 70-700 micrometers, Apocyclops spends more time out of the water column to chill out on the glass, rock, sand, etc. (a little bit safer from predators!). Oithona, however, spends more time dancing with death in the open waters and therefore relies on its smaller size and nocturnal nature to minimize encounters with predators. In this way it provides an important food source, but nevertheless isn’t depleted in a moment.

By continuously grazing bacterioplankton, picoautotrophs and microprotozooplankton, O. colcarva controls (1) bacterial blooms that follow aggressive carbon dosing, (2) phytoplankton blooms that follow nutrient spikes and (3) protozoa (e.g. Euplotes) that can cause disease in fish and/or corals.

A diverse community of copepods undoubtedly benefits reef aquaria in many ways. While there are many species that could be included here, Oithona clearly ranks as a top-tier candidate. As such, EcoPods should be your first choice for copepod seed cultures. Even better is the Ultimate Ecopack, which includes the appropriate phytoplankton and bacterioplankton to nourish the pods and your entire reef!