The ocean’s foundation rests on phytoplankton, microscopic, plant-like organisms including diatoms and cyanobacteria. These primary producers act as the aquatic equivalent of land plants. Through photosynthesis, phytoplankton convert sunlight and carbon dioxide into organic matter, forming the initial energy source for almost all marine life and generating at least half of the oxygen in Earth’s atmosphere. Understanding the animals that consume this vast food source reveals the intricate structure of the marine environment.
Primary Consumers: The Zooplankton Layer
The most abundant consumers of phytoplankton are zooplankton, the animal component of the plankton community. These “animal drifters” cannot swim against ocean currents and graze on the microscopic plants floating around them. Zooplankton include diverse organisms like copepods (tiny crustaceans) and krill, which form dense swarms.
This layer also includes meroplankton, the temporary larval stages of many larger invertebrates and fish that feed on phytoplankton before maturing. Zooplankton typically feed through suspension feeding, using specialized appendages to draw water and suspended phytoplankton toward their mouths. Microzooplankton alone consume a substantial portion of the ocean’s daily primary production.
These small grazers act as a crucial intermediary link, transferring the energy stored in phytoplankton up to the next level of the food web. Their collective biomass represents the largest single food source in the ocean, fueling everything from small fish to the largest animals on the planet.
Specialized Sessile and Coastal Filter Feeders
Stationary and slow-moving animals in coastal and benthic environments also feed directly on phytoplankton. These sessile feeders actively pump and filter large volumes of water to extract suspended food particles. Bivalve mollusks, such as oysters, mussels, and clams, are prominent examples of this feeding strategy.
These invertebrates draw water in through an incurrent siphon and pass it over their gills. The gills are coated in mucus that traps phytoplankton and other organic matter. Tiny cilia then transport this food-laden mucus to the mouth before the filtered water is expelled through an excurrent siphon. An individual oyster can filter up to 50 gallons of water per day.
Other organisms like sponges and tunicates, or sea squirts, also employ similar filtration methods using specialized pores and internal structures to capture phytoplankton. This feeding activity plays an important role in localized water quality by removing suspended particles and preventing excessive phytoplankton blooms.
Apex Filter Feeders
Apex filter feeders are gigantic marine animals that harvest vast quantities of plankton. Baleen whales, such as the Blue and Humpback Whales, are the most famous examples. They use comb-like baleen plates made of keratin that hang from their upper jaws.
These whales engulf massive volumes of water and use their large tongues to push the water back out through the baleen plates, trapping dense aggregations of zooplankton, primarily krill. Although these massive whales do not feed directly on individual phytoplankton cells, their existence is sustained by the zooplankton layer fueled by phytoplankton.
Other ocean giants, like the Whale Shark (the largest fish species) and Manta Rays, employ different filtration mechanisms. Whale sharks use filtering pads in their pharynx to strain plankton and small organisms from the water, often feeding by ram filtration as they swim forward with their mouths open.
Manta rays use specialized flaps near their mouths, called cephalic lobes, to funnel water and plankton toward their wide mouths. They then filter the food through gill rakers, which are bony or cartilaginous projections inside their gills. These enormous animals follow seasonal phytoplankton blooms and the resulting zooplankton swarms.