What IS plankton?

Any documentary or article on ocean life is bound to mention plankton at some point. It’s a fundamental part of our ocean ecosystems that sustains countless billions of invertebrates, fish, seabirds and marine mammals- and by extension, us! Yet we seldom talk about plankton outside its role in sustaining larger, flashier organisms. Plankton never asked to get eaten by a whale, but it rarely gets its time in the spotlight unless it’s being swallowed by something. As a result, many people aren’t all that sure what plankton actually IS.

It's this guy... right?

In fact, plankton is so little-discussed that you could be forgiven for thinking that it’s the name of a single, tiny and very abundant species floating in our oceans. As a child, my mum described plankton to me as “a mixture of plants and animals”, and I therefore imagined it as the lovechild of a shrimp and Bulbasaur, with clusters of green bulbs on its back and rows of tiny legs. Yet in reality, when a whale takes an enormous mouthful of plankton-rich seawater, it isn’t the equivalent of swallowing a tonne of homogeneous steak. This is more of a varied and colourful stew, with countless ingredients from multiple kingdoms of life. “Plankton” is not a biological classification like “mammal” or “rodent”: it’s a title earned through lifestyle. If you are a living thing that drifts in the water column, then congratulations: you are plankton. There are some very strange organisms under this vast umbrella term that really do deserve a closer look… although you may need to pack a microscope.

Plankton both directly and indirectly support some of the largest animals on our planet. But it’s not just big creatures that eat plankton- there’s an entire food web within the planktonic world itself, and photosynthesising organisms, or phytoplankton, are at the bottom of it. However, you won’t find anything particularly leafy floating in the soup: the oceans are the realm of algae. We often think of algae as simply being tiny plants, but they actually belong to an entirely different kingdom of life to the terrestrial plants we are familiar with. All land plants- mosses, trees, flowers, conifers, grasses- evolved from an ancestral aquatic alga that adapted to dry land around 450 million years ago, and very few of its descendants have made the transition back to the sea. Under the microscope, algae have an enormous diversity of shapes and forms compared to their larger terrestrial cousins.

Algae. It's all green and same-y.

Some exist as single cells. Others form bizarrely-shaped colonies. Some are bioluminescent, and light up the sea during large nocturnal gatherings. While most drift benignly in the water column, other groups use flagellae (tails) to propel themselves through the water, and may even supplement their photosynthesising by hunting smaller microorganisms. This is no homogeneous field of daisies.

Yep, allllll green and same-y. 

But algae aren’t the only tiny sun-worshippers in the sea. Some forms of bacteria- called cyanobacteria- can also photosynthesise. In fact, they pretty much invented photosynthesis, and they changed the world forever by doing so. About 3 billion years ago, oxygen was almost non-existent in Earth’s atmosphere, and any that appeared was quickly taken up by the vast amounts of iron in the crust, creating iron oxide, or rust. As cyanobacteria evolved and became more and more successful, their photosynthesis produced vast amounts of oxygen. This caused mass rusting of the planet’s iron reserves, creating huge depositions of iron oxide that appear as striking bands of bright orange in the Earth’s crust. Once the iron sinks were exhausted, oxygen began to accumulate in the atmosphere-- great news for our very distant, microscopic ancestors, as it led to the evolution of mitochondria (our cells’ powerhouses) and bigger, more complex bodies. However, a vast number of oxygen-intolerant species were lost forever in one of Earth’s largest mass extinction events, and the planet underwent a reverse greenhouse effect, becoming swamped in snow and ice for the next 300 million years. Not bad for an organism that looks like a Smurf’s nose hairs.

Feeding on this garden of freaks is a whole host of animals and single-celled organisms, collectively called zooplankton. Some are instantly familiar to us: many kinds of tiny crustaceans live as plankton, including krill, the staple food of the blue whale. Others, however, are downright weird. Amongst many examples are tiny flying slugs known as sea angels…

…amoebae with shells…

…the adorably-named tintinnids…

… and gigantic strings of clear jelly called salps.

Fun fact: the two creatures in this picture are more related than you'd think

But whilst all of these creatures typically spend their whole lifecycles as planktonic nomads, many others come along for the ride exclusively during their youth. You see, childhood works a little differently in the oceans. Land animals have had to invent all sorts of ingenious ways to stop their developing young from drying out, mostly by confining their most vulnerable growth phases to a nice damp waterproof capsule, such as an egg or uterus, until they can survive in the outside world. However, to marine creatures, the seas are just one big bath of amniotic fluid into which they can cast their offspring without a care. Many therefore undergo bizarre planktonic larval phases that just aren’t visible in their land-dwelling relatives, simply because they would usually be confined to eggs or their pregnant mothers.

Take snails, for example. Land snails hatch as miniature shelled versions of the adults, but their marine cousins go through multiple larval stages that look nothing like the creatures they will eventually become. The second of these incarnations is called the veliger. 

Um...what?

It flies through the water on hairy wings, and undergoes a painful-sounding developmental process in which all of its organs twist 180 degrees relative to the rest of its body, so that it can fit into its developing shell. As a result, its anus ends up in a fairly inconvenient place...

All right when you're stretched out, not so great when you're napping inside...

The larvae don’t join the plankton crowd as a kind of self-discovery gap-year experience before they settle down. Having a tiny, mobile larval stage is a great way for creatures that are slow or fixed to the sea floor to exploit new parts of the ocean. This specialised dispersal phase means that some species apparently regress to a simpler form upon reaching adulthood. For example, larval sea squirts bear a striking resemblance to tadpoles, with eyes, rudimentary “brains” that control their movement, complete but non-functioning digestive tracts, and a spine-like structure called a notochord that marks them as close relatives of the vertebrates. But when they find a suitable place to live, they plant themselves headfirst into the ground, and absorb all of these complex features to begin life as simple, filter-feeding adults, leaving no trace of their relatively advanced place on the tree of life. 

Pretty, but brainless

Meanwhile, some larvae show a remarkable amount of ingenuity when searching for a new home. Rather than drifting aimlessly and risking settlement in an unsuitable spot, coral larvae listen for the sounds of distant reefs with the fringe of fine hairs (cilia) on their bodies. They can distinguish the typical sounds of a busy reef from other ocean noise with astonishing precision, allowing them to swim towards their nearest friendly neighbourhood.

Who knew mouldy gherkins were so smart?

 For many larvae, navigating towards a suitable permanent home is a race against time: while some are equipped to feed, many carry a limited supply of yolk for energy, and will eventually die if they stay adrift for too long. Fish, for example, undergo a larval stage in which they lack fins, scales or swim bladders, and rely on a distended belly full of yolk for sustenance. 

At least you can sort of TELL that they're fish.

As with other plankton, they must navigate a whole suite of dangers during this time, including adult fish, jellyfish and indiscriminate filter feeders like basking sharks. Unsurprisingly, creatures with planktonic offspring must produce a vast number of eggs if any are to survive their dangerous childhood in the ocean’s great microscopic buffet.

Plankton might not have the dangerous sex-appeal of a great white shark. Tourists seldom spend hundreds of pounds to go on boat trips to see them. Doctors’ waiting rooms don’t contain tanks of exotic and colourful plankton to calm the nerves of their patients. Although in our heart of hearts we know that they’re by far the most important inhabitants of the ocean, sometimes it’s hard to appreciate plankton beyond their supporting roles as snack bar and crèche to our favourite ocean creatures when they’re so incredibly tiny. Even so, their unfathomable yet practically invisible diversity is surely worth celebration of its own. Each teaspoon of seawater is home to an otherworldly soup of nomadic baby animals, alien algae and single-celled oddities, as different from each other as you are from the banana in your lunchbox or the bacteria on your skin. If you find yourself struggling for inspiration, just remember: if you don’t have a microscope handy to picture them, your wildest imagination might be just as accurate.