Nutrition Part 4: Copepods.
The fourth in the series of articles on breeding marine fishes continues on the subject of larval nutrition. This article looks at Copepods, describing what they are, their role in larviculture, and presents a basic protocol to allow you to culture them yourself.
What are Copepods?
Copepods, often referred to as ‘pods, are small crustaceans featuring a number of very distinct life stages. They start of from an egg as a nauplius, go through six nauplius stages (stages N1 to N6), followed by (usually) six copepodid stages (stages C1 to C6) before reaching the adult stage when the sexes can be identified. At each stage there’s growth and change of body shape – from a larviculturist’s point of view this has the potential of providing us with a graded series of live food enabling us to match the required size of prey to the gape size of the larval fish that we’re trying to raise. Nauplius can range in size from perhaps as small as 50 microns but more usually from around the 100micron mark, giving us a smaller and more nutritious alternative to rotifers.
For the reef aquarist, supplementing the copepod population of a reef aquarium helps provide additional food for fishes throughout the day, particularly important for Mandarin fishes and other species of Dragonets, that are dependent on benthic zooplankton of a small size and for species of coral requiring food of a small particle size. In addition to directly providing food for corals, copepods all so feed corals and other small invertebrates through the production of faecal pellets of a small size and high nutritional value.
Given the number of copepod species potentially available the protocol described for culturing them can only be generalist in nature. Use it as a starting point and then fine-tune it to the species that you are working with.
Currently there are a limited number of copepod species commercially available: a couple of unnamed harpacticoid species, Reef Scientific Zoo mix containing Tigriopus Japonicus (along with rotifers), and Tigriopus californicus. In addition, there’s the option of isolating the species that occur naturally within the reef aquarium to start off a culture or for those near the sea, collecting from the wild.
A Practical Guide to Culturing Copepods.
The process is very similar to the one I’ve described previously for rotifers. You can culture copepods in containers from 0.5 litre upwards, 2 litre drinks bottles are one of the preferred culture vessels given their ubiquity and their disposable nature. If you need larger cultures, 15 to 20 litre plastic aquaria or old salt buckets are good choices. Always run multiple cultures so that you have a back up should your main culture crash.
To start your copepod culture you’ll need a couple of 2-litre clear lemonade bottles, live copepods as a starter culture, and a phytoplankton culture or a supply of phytoplankton for feeding the copepods. This can be live, frozen, or preserved; live is preferable as it will have a higher nutritional value and it’s cleaner to work with i.e. less maintenance is required. In addition, a set of sieves for grading and for copepod collection, is a very useful tool for fine-tuning ‘pod production and for sorting nauplii by size for larviculture.
Previously, in the other articles on culturing live food, the protocol has included the use of aeration via an air pump. This is not strictly needed with copepods, you may include one if you wish but I’d emphasise keeping aeration to a minimum.
Temperature and Salinity.
Many copepod species will tolerate a wide range of temperatures and salinity. For convenience you can successfully culture at room temperature but for optimal results culture at the salinity and temperature required according to your application: if you’re culturing ‘pods for your reef do so at full salinity, if you’re growing food for larval culture match salinity to that of the larval raising tank.
Commercially available ‘pods may vary in salinity according to producer so initially match this salinity then gradually adjust it to the salinity that you require over time.
Split your starter culture between two 2-litre clear lemonade bottles, at room temperature.
Make up 1- litre of water to a salinity and temperature matching the starter culture and split evenly between the two bottles. Dechlorinated tap water is OK for this application. Add enough phytoplankton to each bottle to tint the water green while still allowing you to see through the culture bottle.
Loosely insert a piece of filter floss into the mouth of the bottle; this helps to limit the possibility of contamination. Add phytoplankton daily, maintaining a green tint to the water until both the bottles are full. Once a 2-litre bottle is full, it can be used to start a culture in a larger vessel if required or you can start harvesting.
Once you start harvesting, discard water to waste as you sieve the pods, depending on productivity I’d suggest around 250 ml every couple of days a starting point but you’ll need to judge the quantity according to the performance of your own culture.
The water level in the bottle will gradually reduce until you’re back to the volume of your original starter culture, at which point re-start the culture in a fresh bottle, repeating the entire process whilst starting to harvest from your backup culture.
As with the other foods cultured in this series of articles, to help prevent contamination it’s good practise to sterilise all containers before use, and to sterilise make-up water in the microwave, allowing it to cool before using. This may not be strictly necessary for hobby use but for aquaculture or if culturing copepods from the wild or from reef tank populations, it’s considered essential. In these last two cases, it’s important to observe your cultures for contamination by other organisms that could be detrimental to the culture. Other organisms can out compete your ‘pods for food affecting the productivity of your culture; although for the larviculturist this may offer the opportunity of new finding potential new live foods such as ciliates.
With the exception of Tigriopus japonicus, rotifers should be excluded from copepod cultures.
More is not better! Too dense an algal culture will slow copepod reproduction and raise the pH, possibly to a lethal level. Try to maintain a consistent green tint to the water.
Copepods don’t require any special lighting, keep them maintained in an area that’s lit for around twelve hours a day, say next to your aquarium or on a windowsill.
Copepods vary both species to species and according to developmental stage, in whether they are attracted to or repelled by light.
Higher temperatures will accelerate reproduction but the downside is more feeding and cleaning will be needed.
If the culture develops an unpleasant odour, it indicates bacterial contamination. Depending on severity you may need to discard the culture.
If productivity starts to decline, pour the culture through your sieve, being careful to just collect copepods, leaving any accumulated muck behind. Use these copepods to start off a new culture in clean bottles.
Periodically you’ll need to either clean your culture bottles with a bottlebrush to remove green slime build up or replace them with fresh ones, restart your culture as above.
You can control copepod density depending on your needs, by manipulating salinity, temperature, and feeding you can slow down or increase the productivity of a culture as required.
Rinse collected copepods with a little tank water before using and avoid getting culture water into reef aquaria or larval raising tanks.
Culture density will affect the productivity of the culture; in some species, adult females may inhibit the growth of the culture when the density reaches a certain point. Aim for a density of no more than one adult per ml until you can judge the affect of density on productivity for the species being cultured.
Different species of phytoplankton have different nutrient profiles i.e. different proportions of protein, fat, and carbohydrate; as adult fish of different species have different dietary requirements, so do their larval forms. For best results with a particular species of fish, it may be necessary to experiment with different strains of phytoplankton before finding the optimal nutritional profile. For general reef tank usage, don’t worry over much about the phytoplankton species used.
For further information about copepods, see: Copepods for the Reef Aquarium. Part 1.