Plankton, a diverse collection of microscopic organisms, forms the foundation of marine life. This group includes phytoplankton, which are tiny plant-like organisms, and zooplankton, which are small animal-like organisms. Phytoplankton are primary producers, converting sunlight into energy through photosynthesis, similar to plants on land. Zooplankton consume phytoplankton or other smaller zooplankton, making them primary or secondary consumers in the ocean’s food web. These organisms, though often invisible, have immense biomass and are essential for sustaining nearly all other ocean life.
Tiny Takers: Zooplankton and Small Invertebrates
Zooplankton consume plankton, particularly phytoplankton. These diverse animal-like organisms, such as copepods, are abundant crustaceans that graze on diatoms and other phytoplankton. Krill, another well-known type of zooplankton, are shrimp-like creatures that feed on phytoplankton and are a significant food source for many larger marine animals. Foraminifera and radiolarians are single-celled protozoans that also consume smaller planktonic life.
Beyond permanent zooplankton, many marine animals begin their lives as temporary plankton, known as meroplankton. Larval stages of various marine animals, including fish and crabs, drift with ocean currents and feed on smaller plankton during their early development. This consumption by zooplankton and larval forms creates a crucial link, transferring energy from phytoplankton to higher trophic levels in the marine food web. Their numbers and grazing activity play a fundamental role in energy flow in ocean ecosystems.
Marine Giants: Filter-Feeding Fish and Mammals
Some of the ocean’s largest inhabitants sustain themselves by filtering vast quantities of water to capture plankton. Baleen whales, such as blue whales, humpback whales, and fin whales, possess specialized baleen plates in their mouths instead of teeth. These keratinous plates act like a sieve, allowing whales to take in water and prey, then pushing the water out while trapping krill. A blue whale, the largest animal on Earth, can consume up to four tons of krill daily during feeding periods, engulfing krill in a single gulp.
Humpback whales employ various feeding strategies, including “bubble-net feeding,” where groups of whales cooperatively release bubbles to create a net-like barrier that corrals prey, making it easier to engulf. Other baleen whales, like right whales, are “skim feeders” that swim slowly with their mouths open through plankton-rich waters, continuously filtering food. These filter-feeding mechanisms allow these mammals to efficiently harvest plankton, supporting their size and energy requirements.
Large filter-feeding fish, such as whale sharks and manta rays, also consume plankton. Whale sharks, the largest fish species, swim with their mouths open, drawing in water filled with plankton and small fish. They use specialized structures called gill rakers, which function like a sieve to trap food particles while expelling water through their gill slits. Manta rays similarly use their wide mouths and gill rakers to filter plankton from the water column. This filtration system allows them to capture even small particles.
Stationary and Slow Movers: Bivalves and Other Filter Feeders
Many less mobile marine invertebrates also consume plankton through filter feeding. Bivalves, a class of mollusks including clams, mussels, oysters, and scallops, are common examples. These organisms draw water into their bodies through an incurrent siphon, passing it over their gills. Cilia on the gills create currents that trap food particles, such as phytoplankton, in mucus, which is then transported to the mouth. A single mussel or oyster can filter several liters of water per hour, contributing to water purification.
Sponges, among the simplest multicellular animals, are sessile filter feeders that pump water through their porous bodies. Specialized cells within the sponge’s interior, called choanocytes, have flagella that create water currents, drawing in plankton, which are then absorbed. Tunicates, also known as sea squirts, are another group of stationary filter feeders that draw water in through an incurrent siphon and expel it through an excurrent siphon. Their pharyngeal basket filters plankton and other organic particles from the seawater, contributing to water quality and nutrient balance.