Copepods- Jump for joy

by Matthew Norton

In a world with ocean giants such as sharks, dolphins and whales, it can be easy to forget about the smaller animals and how they play their own roles in the sea machine. Copepods, tiny crustaceans that can only reach lengths of 1-2 millimeters, are one such group of unsung heroes. What they lack in size, they more than make up for in sheer numbers, exceeding the head count of most, if not all animals on earth. Copepods are also found in almost any body of water and are an important food source for many animals. Often bridging the gap between phytoplankton (tiny aquatic plants) and the meat eaters.

Copepods come in a huge variety of shapes and sizes (up to around 2mm long). These are just a few examples among thousands.

But copepods are more than just a selection of light bites. One of their most impressive tricks is their ability to jump at speeds of 2-4 miles per hour to evade predators and move towards food. These speeds would be easy enough for large animals like ourselves, but if you scale it down to copepod size it would be the same as a human being reaching nearly 4,000 miles per hour in a single bound. These powerful jumps are made possible by their streamlined shape, quick reflexes and possession of two types of ‘swimming legs’. Those nearer the head are used for casual movement and feeding strategies that do not require fast jumps. The swimming legs further down the body are reserved entirely for those fast jumps and not tired out by other tasks before the copepod desperately needs them.

You can just about see the feeding/casual swimming legs around the head of the copepod on the left. The jumping legs, found further down the body, can be seen more easily in the side view image of the copepod on the right. Some copepods may even use the latter to jump out of the water if the need arises.

Such powerful jumps will inevitably leave vortex rings, spinning vacuums of water, in their wake which could leave a trail for their predators to follow. But copepods deliberately create two vortex rings per jump that spin in different directions. When combined, these vortex rings (almost) cancel each other out and cover the copepod’s tracks.

Unfortunately for our copepods, these fast jumps are mostly useless against seahorses and other predators that can approach undetected and strike with no warning. Ironically, it is the slow and gentle swimming of a seahorse, combined with their convincing camouflage, that makes them invisible to the copepods. Even their snouts are built to minimise the disturbances they cause in the water and allow the seahorse to get within striking distance.

Seahorses are such effective hunters of copepods, as well as other tiny planktonic animals, that their success rate is far higher than apex hunters such as lions and sharks.

Copepods, and their seahorse predators, are shining examples of what the littles guys and girls in the sea are capable of. If we could somehow take away the size difference, the copepods would likely outshine the ocean giants. Then again, it is just as likely that their small size is the key to their accomplishments. We all live in the same world, but the way it looks to a tiny copepod would be very different from our own experiences. A pond for example, is merely a small body of water with some interesting plants and animals for us. But to a copepod, that same pond is a vast wilderness full of dangerous beast and tempting riches. The rules they have to follow and the challenges they will face are completely different, but so are the opportunities.

From a human perspective

Not all copepods are fast jumpers. Many species have opted for a parasitic lifestyle and stay attached to a host animal for the whole of their adult lives. This can become a problem when they target marine animals that we want to eat. The aquaculture industry in particular have found copepod parasites like the dreaded ‘sea louse’ to be an expensive thorn in their side because no one wants to eat diseased fish.

Parasitic copepods as recovered from a European flounder Platichthys flesus (top left) and the deep-sea fish Pristipomoides filamentosus  as photographed in the 21st century. But copepod parasites have been recognised for some time, as evidenced by the image of an infected brook trout from 1899 (bottom left) and the two sketches of freshwater copepod parasites from the 1910s (bottom right).

Fish are also not the only possible hosts. Shellfish, worms, corals, marine mammals and many others can all be burdened with these parasites.

Direct infection of human beings is unlikely, but copepods can act as unintended carriers (so far as we know) of human diseases that can be transmitted from drinking water, such as cholera and the guinea worm parasite. Wherever there is water, there are probably copepods to make it more difficult to eradicate these diseases. But the key role that they play in transmission could also help us to predict outbreaks and put measures in place to stem the tide of infections. One such early warning system could be to monitor the environment for explosions of phytoplankton growth (called blooms). With more food in the water, there are likely to be more copepods swimming around and therefore more potential carriers of disease. 

One way to avoid contracting cholera is to use certain materials to filter out the Vibrio cholerae bacteria, and the copepods that carry them, from drinking water. Just like the woman on the right is doing.
The cycle of how the guinea worm parasite can get around when it has access to both human and copepod hosts. Presumably, any other human diseases that can be carried by copepods would work in a similar way.

Copepods can also protect us from diseases that are spread by their prey. This applies to diseases spread by insects who spend part of their lives as aquatic larvae before morphing into their adult form and flying away. The more copepods there are in a given water body to pick off the insect larvae, the fewer carriers there are available to spread disease. Using copepods in this way has had some success in controlling the spread of dengue fever and could theoretically be used against other nasty tropical diseases. 

The yellow fever mosquito Aedes aegypt (left) and the asian tiger mosquito Aedes albopictus (right) are both carriers of several nasty human diseases. If copepods are universally effective in controlling their transmission of dengue fever, they could have a similar effect against the zika and chikungunya viruses.

But we must be careful with manipulating nature to help control diseases, or we could end up getting carried away and making things worse. This was the case with the western mosquitofish (Gambusia affinis), a freshwater fish that was introduced in many countries in the early 20th century to combat the spread of malaria. As with copepods, this fish consumes insect larvae during their aquatic larval stage and using it’s natural feeding habits probably made perfect sense at the time. However, the western mosquitofish has outstayed its welcome and has been outcompeting native mosquito eating fish, species that could have done a better job of controlling the spread of malaria. I am not saying we shouldn’t ever make use of our natural resources, but in this kind of enterprise it would be wise to exercise some cautious doubt until we know the full implications of what we are doing.

Also, as helpful as copepods can be for dealing with obvious problems like disease, it is what they do in the background that is the most important and least appreciated. For example, their role in the global carbon cycle is substantial given that they are a major consumer of phytoplankton. Much of this carbon, which the phytoplankton originally extracted from the water as carbon dioxide, will travel up the food chain. But a portion will sink towards the bottom as uneaten copepod corpses and faeces. Given the rapid increases in carbon dioxide in the atmosphere, and dissolved in water, we should hope that the carbon filled remains of these copepods are locked away for a very long time.

Anyone reading this article might feel a bit uneasy about the idea of protecting our planet over protecting ourselves from disease, especially given our current circumstances. But disrupting the balance of nature will have far reaching consequences that include changes in how diseases operate and spread. The coronavirus pandemic may very well be the latest in a long line of high profile and disastrous consequences of our poor treatment of the natural world. Clearly something needs to change, but for truly effective conservation we have to show consideration to the whole environment and everything that lives within it. Even those tiny little copepods.

Thanks for reading

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All other images are public domain and do not require attribution

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