Not Your Average Drifter – Plankton Part II

Last week we met some of our most important New England residents – the phytoplankton. Now, we are happy to introduce their animal counterparts – the zooplankton. These animals drift around in the sea in truly astonishing number and form . If you missed our post on their plant partners – the phytoplankton – you can find it here. Go ahead and read it, I’ll wait.

Okay? Well, those phytoplankton are extremely productive, and they’re eaten by many animals, most of which fall into the category of “zooplankton.”

Microscopic or massive, if you’re an animal that can’t swim against the current you’re part of the zooplankton. Some of these animals drift in the water their entire lives. These are what we scientists (who enjoy inventing and using large words) call the holoplankton (holo = entire, plankton = wanderer). The copepods are a perfect example of this.

Copepods may be the most abundant animal group on the planet, and although they contain considerable diversity, most of them are holoplanktonic. They are also usually gonorchoristic (I know, again with the huge words), which means they come in both the male and female variety. When two of them get together, so to speak, the fertilized egg will develop through many larval stages, until they finally metamorphose into the adult form. But all the way through this life cycle – egg to adult – the copepod will remain drifting along in the water.

The same is true for countless other animals, from the familiar jelly to the bizarre Phronima. All of them spending a life adrift, in a world that seems more like science fiction than reality.

Contrast this to members of the meroplankton (meros = partial), who spend only a portion of their lives in the water column. These animals may not be so foreign to you, as most of the meroplankton are the larval forms of animals that we know and love (and love to eat!). These animals drift around as larvae until they metamorphose and become large enough to swim against the current (at which point they are said to be “nektonic” – like a fast-swimming fish), or until they settle to a life on the sea floor (these animals are “benthic” – like a snail or mussel).

Most fish have larvae, as do barnacles, urchins, lobsters, mollusks, and many others. Some have giant spikes coming out of their heads, others look like flying saucers. But the fact that free-living larval stages exist in most marine animals means that they are (or were) evolutionarily important. Perhaps they evolved for dispersal – to avoid competition or inbreeding. Or maybe larvae evolved as a means to temporarily avoid predation on the sea floor… in truth, we don’t know for sure why the larval form evolved.

What we do know is that they are extremely abundant, and together with the holoplankton they make up an undeniably important and enormous group of animals. If the phytoplankton are at the base of the food chain, then the zooplankton are at the first rung. They are so massive in number that they can sustain huge populations of larger animals, some as large as our own North Atlantic right whales, which filter copepods, krill, and other zooplankton out of the water. But some zooplankton are eaten by their tiny buddies (other carnivorous plankton, like some fish larvae), which can make the marine food web a bit complicated.

And though they can’t swim against the current, they’re on the move. Their ecological importance makes the news of a study showing that climate change has caused dramatic shifts in the distribution of many planktonic species troubling. In the study, the investigators found that phytoplankton and zooplankton were two of the groups whose distribution was changing the quickest. As the authors’ of the study state, “species’ interactions and marine ecosystem functions may be substantially reorganized at the regional scale, potentially triggering a range of cascading effects.”

Translation: As the great drifter Bob Dylan said, “The times they are a changin’.” But that doesn’t mean you have to sit idly by… “If your time to you is worth savin’” then find out how you can help.

Up next in our plankton series – we’ll talk about a really cool citizen science plankton project you can get involved in using little more than your smart phone. Stay tuned!

Casey Diederich is a 5th year PhD candidate in Tuft’s University’s Biology Department, and is conducting his research on slipper-shell snails. We are thrilled to have Casey guest blogging for us about some of the more fascinating plants and animals in our ocean. – Ed.

Images copyright of Dr Richard Kirby, Plymouth University. These and other images can be found in the book on plankton, “Ocean Drifters, a secret world beneath the waves.”