Yes, fish do eat plankton, and these microscopic organisms form the foundational energy source for nearly all aquatic life. Plankton is a collective term for a vast and diverse group of organisms, both plant-like and animal-like, that drift within the water column because they are too small or too weak to swim against the current. Despite their size, plankton represents the largest biomass on Earth, serving as the first link in the complex aquatic food chain. This primary food source sustains everything from the smallest fish larvae to the largest marine vertebrates.
The Two Main Types of Plankton
Plankton is broadly categorized into two primary groups based on their biological function. Phytoplankton are plant-like, single-celled organisms that perform photosynthesis, converting sunlight and carbon dioxide into energy and organic matter. These organisms, including diatoms and dinoflagellates, are the primary producers in the ocean, similar to plants on land, and form the base of the marine food web.
Zooplankton, conversely, are animal-like consumers, often comprising tiny crustaceans like copepods and krill, or the larval stages of larger marine animals. These organisms obtain energy by feeding on phytoplankton or smaller zooplankton. Fish consume both types, though their preference often changes as they grow from larvae into adults.
Mechanisms of Plankton Consumption
Fish that consume plankton have developed specialized anatomical structures, primarily related to filter feeding, to efficiently capture these tiny particles from the water. The primary adaptation is the gill raker, which are bony or cartilaginous projections located on the inner surface of the gill arches. These structures function as a sieve, trapping food particles as water passes over the gills.
The morphology of the gill rakers is directly correlated with the fish’s plankton diet. Fish that feed mainly on minute phytoplankton possess numerous, tightly packed, and often hair-like gill rakers that create a fine mesh to strain the smallest particles from the water. Species that target larger zooplankton, such as copepods, typically have shorter, more widely spaced rakers.
Many planktivorous fish use a technique called ram feeding, where they simply swim forward with their mouths open, allowing water to flow continuously over their gill rakers. This process allows them to process massive volumes of water to capture the small, dispersed food items. The trapped plankton is then consolidated and swallowed.
Key Examples of Plankton-Eating Fish
The plankton diet supports a wide range of fish species, from small schooling fish to some of the ocean’s largest inhabitants. Small forage fish like sardines, anchovies, and herring are classic examples of planktivores that form enormous schools. These species rely heavily on zooplankton, such as copepods and krill, using their fine gill rakers to filter huge quantities as they swim.
A single school of Atlantic menhaden can filter a significant volume of water daily, playing an important role in controlling phytoplankton blooms. Moving to the massive scale, the largest fish in the world, the whale shark, is also a plankton feeder. Whale sharks are ram feeders that swim with their mouths open, filtering both phyto- and zooplankton through specialized dermal filter pads near their gills.
Similarly, the giant manta ray, a cartilaginous fish, uses specialized lobes near its mouth to funnel plankton-rich water into its mouth, where it is filtered through branchial filter plates. These large filter feeders demonstrate the efficiency of a plankton-based diet, as they can grow to immense sizes solely by consuming microscopic or near-microscopic organisms.
Plankton’s Central Role in the Marine Food Web
Plankton is the foundation that drives the entire marine ecosystem. Because phytoplankton are primary producers, they represent the first trophic level, converting solar energy into organic energy. Zooplankton then occupy the second trophic level as the primary consumers of this initial energy.
Fish that consume plankton act as the middle link, transferring energy from these lower levels to higher predators. This energy transfer supports the populations of larger fish, marine mammals, and seabirds, creating a vast and interconnected food web. The productivity of commercial fisheries, such as those targeting cod, tuna, and salmon, is intrinsically tied to the health and abundance of plankton populations.
When plankton populations decline, the effects ripple upward, potentially causing collapses in fish stocks and impacting the entire marine biome. The ability of fish to consume plankton is a foundational ecological process that sustains biodiversity and global food security.