--- title: "Plankton Size: How Big Is Plankton, From Viruses to Siphonophores" url: https://thesea.org/plankton-size/ date: 2026-11-13 modified: 2026-11-13 lang: en author: "Blane Perun" description: "TheSea.Org▸Bioluminescence & Plankton▸Plankton▸Plankton Size How Big Is Plankton? — From Viruses to 40-Metre Siphonophores Bioluminescence & Plankton By Blane Perun November 13, 2026 9 min read How big is plankton?..." categories: - "Coral Reef" word_count: 1847 --- # Plankton Size: How Big Is Plankton, From Viruses to Siphonophores [TheSea.Org](/)▸[Bioluminescence & Plankton](/bioluminescence/)▸[Plankton](/plankton/)▸Plankton Size # How Big Is Plankton? — From Viruses to 40-Metre Siphonophores Bioluminescence & Plankton By [Blane Perun](/about-blane-perun/) November 13, 2026 9 min read How big is plankton? Plankton is defined by how it moves, not by its size, so it covers an enormous range. The smallest plankton are **marine viruses under 0.2 micrometres** across; most phytoplankton are **2–200 micrometres**; copepods and krill are **about 1 mm to 6 cm**; and the largest drifters, jellyfish and siphonophores, can trail tentacles or colonies **more than 30 metres long**. Quick facts SmallestViruses, about 0.02–0.2 µm Most abundant photosynthesiser*Prochlorococcus*, under 1 µm Typical phytoplankton2–200 µm Typical copepodAbout 0.5–2 mm Antarctic krillUp to about 6 cm Largest planktonSiphonophores and jellyfish, 30 m+ Size classesFemto- to megaplankton (7 classes) Defining traitCan’t swim against currents Ask most people how big plankton is and they will say “tiny”. That is true for most of it, but it misses the point of the word. “Plankton” comes from the Greek for “drifter” or “wanderer”, and it describes any organism that cannot swim strongly enough to go against the currents. A virus is plankton. So is a diatom, a copepod, a krill swarm and a lion’s mane jellyfish with tentacles longer than a bus. The plankton world spans about nine orders of magnitude in length, from objects you would need an electron microscope to see to animals longer than a blue whale. This page breaks that range into the size classes scientists actually use. For the basic split between drifting plants and drifting animals, see our [plankton](/plankton/) guide. The plankton size classes Oceanographers sort plankton into size classes, each roughly ten times larger than the one before. The scheme most often used was laid out by Sieburth and colleagues in 1978. The classes are named with familiar metric prefixes, though the boundaries do not line up exactly with the units the prefixes suggest. They are useful because size, more than species, often decides what an organism eats, what eats it, how fast it sinks and how quickly it grows. A plankton net with a given mesh size will catch one class and let smaller ones slip straight through, so researchers studying different classes use very different gear. | Size class | Size range | Examples | | ---------- | ---------- | -------- | | Femtoplankton | 0.02–0.2 µm | Marine viruses | | Picoplankton | 0.2–2 µm | Bacteria, *Prochlorococcus*, *Synechococcus* | | Nanoplankton | 2–20 µm | Coccolithophores, small flagellates, small diatoms | | Microplankton | 20–200 µm | Most diatoms and dinoflagellates, ciliates, forams | | Mesoplankton | 0.2–20 mm | Copepods, larvae, small jellies, arrow worms | | Macroplankton | 2–20 cm | Krill, comb jellies, salps, pteropods | | Megaplankton | Over 20 cm | Large jellyfish, siphonophores, pyrosomes | The smallest plankton: viruses and bacteria At the bottom of the scale are viruses. A litre of seawater typically holds billions of virus particles, most of them infecting bacteria and algae. They are far too small to see with a light microscope, and they were badly underestimated until researchers began counting them with electron microscopes and fluorescent stains in the late 1980s. Viruses matter because they burst open the cells they infect, releasing nutrients back into the water and steering which microbes thrive. One step up are the bacteria and the smallest photosynthetic cells. *Prochlorococcus*, a cyanobacterium less than a micrometre across, is thought to be the most abundant photosynthetic organism on Earth, and it was only discovered in the 1980s. Together with its relative *Synechococcus*, it does a large share of the photosynthesis in warm, nutrient-poor open ocean. These cells are so small that they slip through almost any filter used before modern flow cytometry, which is why such an important group went unnoticed for so long. Phytoplankton size: why small is often better Most of the phytoplankton people picture, the diatoms and dinoflagellates, fall in the micro- and nanoplankton range, from about 2 to 200 micrometres. A few large diatoms reach a millimetre or so, and some colonies and chains are visible to the naked eye, but individual cells are microscopic. Being small helps a single cell. A small cell has a lot of surface area relative to its volume, so it can absorb scarce nutrients more efficiently and it sinks more slowly. That is why the tiniest cells dominate in the nutrient-poor blue water of the open tropics. In rich coastal water and upwelling zones, larger diatoms can grow fast and take over, producing the blooms that turn water green or brown; see [phytoplankton blooms](/phytoplankton-bloom/). Size also decides who eats whom. Tiny cells are mostly eaten by single-celled grazers, while large diatoms are big enough for copepods and krill, which is why diatom-based food chains tend to be short and efficient and feed so many fish and whales. Under 1 µm*Prochlorococcus*, the smallest common photosynthetic cell ~6 cmMaximum length of an Antarctic krill 30 m+Tentacles of a large lion’s mane jellyfish Zooplankton size: copepods to krill Zooplankton cover a wider span than phytoplankton. At the small end are single-celled grazers such as ciliates and flagellates, some only a few micrometres long. The mesoplankton, from 0.2 to 20 millimetres, is where copepods live, and copepods are the workhorses of the ocean. Most adults are between half a millimetre and two millimetres long, about the size of a grain of rice or smaller, though a few deep-water species reach a centimetre or more. Many fish larvae, crab and barnacle larvae, and arrow worms fall in the same range. Krill sit one class higher, in the macroplankton. Antarctic krill grow to about 6 centimetres and live for several years, which is large and long-lived for a planktonic crustacean. Swarms can be dense enough to colour the water and stretch for kilometres, and they are what feed many of the great whales. See [krill](/krill/) and [zooplankton](/zooplankton/) for more on how these animals live. The biggest plankton: jellies and siphonophores The largest plankton on Earth are not whales or sharks, which swim well enough to choose their direction, but gelatinous drifters. The [lion’s mane jellyfish](/lions-mane-jellyfish/) can have a bell more than two metres across in exceptional cases, with tentacles reported at 30 metres or more. Siphonophores, colonies of specialised units that work together as one animal, can be even longer. *Praya dubia*, the giant siphonophore, is often cited as reaching 40 metres or more, and in 2020 researchers off Western Australia filmed a spiral-shaped *Apolemia* colony that they estimated at roughly 45 metres along its outer ring. Our [giant siphonophore](/giant-siphonophore/) page covers how these colonies are built. Ocean sunfish, which can weigh more than two tonnes, are sometimes described as planktonic because they drift a lot of the time, but they can swim against currents when they choose, so most biologists class them as nekton (strong swimmers). The definition is about ability, not habit. Plankton that change size class as they grow Size classes describe organisms at a moment in time, and many animals pass through several of them. Biologists split zooplankton into holoplankton, which drift for their entire lives, and meroplankton, which drift only as larvae or eggs. Copepods, krill, arrow worms and most jellies are holoplankton. Meroplankton include the larvae of crabs, lobsters, barnacles, sea urchins, corals, clams and most reef and coastal fish. A coral larva, called a planula, starts as a fuzzy, swimming speck well under a millimetre long, drifts for days to weeks, then settles and builds a colony that may eventually weigh tonnes. A tuna egg is about a millimetre across and drifts as mesoplankton; the adult can be three metres long and cross oceans. Even the largest jellyfish start life as tiny larvae and then pass through a stage fixed to the seafloor before budding off the drifting medusae. So when someone asks how big plankton is, the honest answer is also “it depends on when you look”. A single plankton tow in spring can hold the young of dozens of animals that will never be plankton again once they grow up. Why size matters so much Size determines almost everything about a plankton organism’s life. Small cells grow and divide quickly, often once a day or faster, while large animals take weeks or years to mature. Large particles sink faster, so the size of the plankton in an area affects how much carbon is carried from surface waters to the deep sea, a process called the biological pump. Big diatoms and the faecal pellets of copepods and krill sink fast; tiny bacteria and their remains tend to be recycled near the surface. Size also shapes food webs: in general, predators eat prey about a tenth of their own length, so each class tends to feed the next. Our [ocean food web](/oceans-food-web/) guide follows that chain all the way up to whales. My take: size is what matters in the tank too In my own reef tanks, kept since 1996, size is the first thing I think about when it comes to feeding filter feeders. A coral, a clam or a feather duster can only use food in the size range its polyps, gills or tentacles can capture. Many soft corals and gorgonians take in very small particles, while large-polyp stony corals will grab bigger zooplankton. Dosing phytoplankton or live copepods only helps if the particle size matches the animal you are trying to feed, and too much of the wrong size just ends up as nutrients feeding algae. If I were setting up a new filter-feeder system, I would start by reading up on what size food each species can actually catch, then choose foods to match. Our [phytoplankton for reef tanks](/phytoplankton-reef-tank/) page goes into which cultures are commonly used and who eats them. It is the same rule the ocean runs on: what you can eat depends on how big you are and how big your food is. Frequently asked questions How small is plankton? The smallest plankton are viruses around 0.02–0.2 micrometres across, far too small for a light microscope. How big is phytoplankton? Most phytoplankton cells are between about 2 and 200 micrometres; some cyanobacteria are under 1 micrometre. How big is zooplankton? From a few micrometres for single-celled grazers to several centimetres for krill, and metres for jellyfish. Can you see plankton with the naked eye? Yes, larger plankton such as copepods, krill, larvae and jellyfish are visible. Most phytoplankton are not. What is the largest plankton? Siphonophores and the lion’s mane jellyfish, which can stretch 30–40 metres or more. Are whales plankton? No. Whales swim against currents, so they are nekton, not plankton. Sources & further reading - Sieburth, J.McN., Smetacek, V. & Lenz, J. (1978). Pelagic ecosystem structure: heterotrophic compartments of the plankton and their relationship to plankton size fractions. *Limnology and Oceanography* 23(6): 1256–1263. - Chisholm, S.W. et al. (1988). A novel free-living prochlorophyte abundant in the oceanic euphotic zone. *Nature* 334: 340–343. - NOAA Ocean Service: What are plankton? - Schmidt Ocean Institute: Ningaloo Canyons expedition (2020). Continue reading More about plankton [PlanktonPlankton Under a MicroscopeWhat diatoms and copepods look like. ](/plankton-under-microscope/) [CrustaceansKrillThe small crustaceans that feed whales. ](/krill/) [Deep SeaGiant SiphonophoreThe longest drifter in the sea. ](/giant-siphonophore/) BP Last updated November 13, 2026 Written by Blane Perun Blane Perun has been exploring Earth’s oceans and marine life for over 25 years. Founder of TheSea.Org in 1999, underwater photographer, coral aquaculture pioneer, and explorer of 80+ countries. [About Blane Perun →](/about-blane-perun/) [More on plankton →](/plankton/)