Plankton refers to a vast and diverse collection of organisms that drift in water, unable to swim against the prevailing currents. This group is not a single species, but an ecological category encompassing life forms from bacteria to large jellyfish. The majority of these organisms are microscopic. Understanding what plankton looks like requires clarifying how this diverse group appears under high magnification.
The Fundamental Division
The most basic distinction is between the plant-like and the animal-like forms, which dictates their visual nature. Phytoplankton are the primary producers, similar to plants on land, using photosynthesis to create food. Under a microscope, they often appear green or brownish-yellow due to chlorophyll and accessory pigments like fucoxanthin. They are generally single-celled, though they frequently join together to form chains or simple colonies.
Zooplankton are the consumers, the animal component of the plankton community. They feed on phytoplankton and other smaller zooplankton, giving them a more complex, mobile profile. Many zooplankton are translucent or nearly transparent, helping them hide in the water column. When viewed, they exhibit movement, often a jerky or twitching motion, which differentiates them from the passive drift of most phytoplankton.
Size and Scale of Plankton
Plankton covers an immense size range, spanning multiple orders of magnitude. The smallest groups, like picoplankton (less than 2 micrometers) and nanoplankton (2 to 20 micrometers), are mostly bacteria and small protists. These require powerful compound microscopes for clear observation and remain visually indistinct as specks, even under moderate magnification.
Microplankton (20 to 200 micrometers) represents the size class most commonly observed with standard laboratory microscopes, including many diatoms and dinoflagellates. Beyond the microscopic realm are the mesoplankton (0.2 millimeters to 2 centimeters), which includes organisms like copepods and krill larvae. These organisms are visible to the unaided eye as tiny, moving specks, but still require a dissecting microscope for detail.
Distinct Visual Characteristics
Under a microscope, the complexity of individual plankton reveals intricate structures and striking geometry. Diatoms, a major group of phytoplankton, are defined by their glass-like cell walls made of silica, called a frustule. These frustules feature elaborate, symmetrical patterns, appearing as geometric shapes like circles, triangles, or rods covered in fine, repeating lines known as striae.
Dinoflagellates, another common phytoplankton type, often possess a more armored, less symmetrical appearance, with a cell wall made of cellulose plates. Many feature spines, wings, or characteristic “horns,” and nearly all have two whip-like flagella used for controlled movement, making them appear to spin or tumble under magnification. Their internal contents frequently appear brownish because of their mixture of photosynthetic pigments.
Among the zooplankton, copepods are arguably the most abundant, resembling miniature, transparent shrimp. They exhibit a segmented body, a single eye spot, and long, feather-like antennae that extend dramatically from their body. Their movement is distinct, characterized by sudden, darting, or jerky swimming motions. Meroplankton, the larval stages of adult sea creatures, can be seen as simplified, recognizable forms of crabs, worms, or snails, drifting temporarily until they settle into their adult lives.
Plankton Phenomena Seen by the Naked Eye
While individual plankton are usually invisible, their sheer numbers can create dramatic visible phenomena. When conditions are right, phytoplankton can multiply explosively, resulting in a dense accumulation called an algal bloom. This collective density visibly colors the water across vast areas, often appearing as murky green, brown, or intense red, commonly referred to as a “red tide.”
Another visual effect is bioluminescence, the ocean’s glow, created by certain dinoflagellates like Noctiluca scintillans. These organisms emit a flash of blue light when physically disturbed by a wave, boat, or swimmer. The flash is thought to be a defensive reaction, startling predators or attracting larger consumers. Large aggregations of macroplankton, such as krill, can also become visible as immense, dense swarms near the surface, sometimes appearing as a reddish, cloudy patch of water.