Planktivore
A planktivore is an aquatic organism that feeds on planktonic food, including zooplankton and phytoplankton. Planktivorous organisms encompass a range of some of the planet's smallest to largest multicellular animals in both the present day and in the past billion years; basking sharks and copepods are just two examples of giant and microscopic organisms that feed upon plankton.
Planktivory can be an important mechanism of top-down control that contributes to trophic cascades in aquatic and marine systems. There is a tremendous diversity of feeding strategies and behaviors that planktivores utilize to capture prey. Some planktivores utilize tides and currents to migrate between estuaries and coastal waters; other aquatic planktivores reside in lakes or reservoirs where diverse assemblages of plankton are present, or migrate vertically in the water column searching for prey. Planktivore populations can impact the abundance and community composition of planktonic species through their predation pressure, and planktivore migrations facilitate nutrient transport between benthic and pelagic habitats.
Planktivores are an important link in marine and freshwater systems that connect primary producers to the rest of the food chain. As climate change causes negative effects throughout the global oceans, planktivores are often directly impacted through changes to food webs and prey availability. Additionally, harmful algal blooms can negatively impact many planktivores and can transfer harmful toxins from the phytoplankton, to the planktivores, and along up the food chain. As an important source of revenue for humans through tourism and commercial uses in fisheries, many conservation efforts are going on globally to protect these diverse animals known as planktivores.
Plankton and planktivory across taxonomic classes
Phytoplankton: prey
are defined as any type of organism that is unable to swim actively against currents and are thus transported by the physical forcing of tides and currents in the ocean. Phytoplankton form the lowest trophic level of marine food webs and thus capture light energy and materials to provide food and energy for hundreds of thousands of types of planktivores. Because they require light and abundant nutrients, phytoplankton are typically found in surface waters where light rays can penetrate water. Nutrients that sustain phytoplankton include nitrate, phosphate, silicate, calcium, and micronutrients like iron; however, not all phytoplankton require all these identified nutrients and thus differences in nutrient availability impact phytoplankton species composition. This class of microscopic, photosynthetic organisms includes diatoms, coccolithophores, protists, cyanobacteria, dinoflagellates, and other microscopic algae. Phytoplankton conduct photosynthesis via pigments in their cells; phytoplankton can use chlorophyll as well as other accessory photosynthetic pigments like fucoxanthin, chlorophyll c, alloxanthin, and carotenoids, depending on species. Due to their environmental requirements for light and nutrients, phytoplankton are most commonly found near continental margins, the equator, high-latitudes, and nutrient-rich areas. They also form the foundation of the biological pump, which transports carbon to depth in the ocean.Zooplankton: predators and prey
are generally consumers of other organisms for food. Zooplankton may consume either phytoplankton or other zooplankton, making them the smallest class of planktivores. They are common to most marine pelagic environments and act as an important step in the food chain to transfer energy up from primary producers to the rest of the marine food web. Some zooplankton remain planktonic for their entire lives, while others eventually grow large enough to swim against currents. For instance, fish are born as planktonic larvae but once they grow large enough to swim, they are no longer considered plankton. Many taxonomic groups are zooplankton at some point in their lives. For example, oysters begin as planktonic larvae; during this stage when they are considered zooplankton, they consume phytoplankton. Once they mature to adulthood, oysters continue to consume phytoplankton. The spiny water flea is another example of a planktivorous invertebrate.Some of the largest communities of zooplankton exist in high latitude systems like the eastern Bering Sea; pockets of dense zooplankton abundance also exist in the California Current and the Gulf of Mexico. Zooplankton are, in turn, common prey items for planktivores; they respond to environmental change very rapidly due to their relatively short life spans, and so scientists can track their dynamics to understand what might be occurring in the larger marine food web and environment. The relative ratios of certain zooplankton in the larger zooplankton community can also indicate an environmental change that may be significant. For instance, an increase in rotifer abundance in the Great Lakes has been correlated with abnormally high levels of nutrients.
Vertebrates: predators and prey
Many fishes are planktivorous during all or part of their life cycles, and these planktivorous fish are important to human industry and as prey for other organisms in the environment like seabirds and piscivorous fishes. Planktivores comprise a large component of tropical ecosystems; in the Indo-Australian Archipelago, one study identified 350 planktivorous fish species in one studied grid cell and found that 27% of all fish species in this region were planktivorous. This global study found that coral reef habitats globally have a disproportionate amount of planktivorous fishes. In other habitats, examples of planktivorous fishes include many types of salmon like the pink salmon, sandeels, sardines, and silvery lightfish. In ancient systems, the Titanichthys was an early massive vertebrate pelagic planktivore, with a lifestyle similar to that of the modern basking, whale, and megamouth sharks, all of whom are also planktivores.Sea birds can also be planktivores; least auklets, crested auklets, storm petrels, ancient auklets, phalaropes, and many penguins are all examples of avian planktivores. Planktivorous seabirds can be indicators of ecosystem status because their dynamics often reflect processes affecting many trophic levels, like the consequences of climate change. Blue whales and bowhead whales as well as some seals like the crabeater seal are also planktivorous. Blue whales were recently found to consume a vast amount more plankton than was previously understood, representing an important element of the ocean biogeochemical cycle.
Feeding strategies
As previously mentioned, some plankton communities are well-studied and respond to environmental change very rapidly; understanding unusual plankton dynamics can elucidate potential consequences to planktivorous species and the larger marine food chain.One well-studied planktivore species is the gizzard shad which has a voracious appetite for various forms of plankton across its life cycle. Planktivores can be either obligate planktivores, meaning they can only feed on plankton, or facultative planktivores, which take plankton when available but eat other types of food as well. In the case of the gizzard shad, they are obligate planktivores when larvae and juveniles, in part due to their very small mouth size; larval gizzard shad are most successful when small zooplankton are present in adequate quantities within their habitat. As they grow, gizzard shad become omnivores, consuming phytoplankton, zooplankton, and larger pieces of nutritious detritus. Adult gizzard shad consume large volumes of zooplankton until it becomes scarce, then start consuming organic debris instead. Larval fishes and blueback herring are other well-studied examples of obligate planktivores, whereas fishes like the ocean sunfish can alternate between plankton and other food sources. Facultative planktivores tend to be more opportunistic and live in ecosystems with many types of food sources. Obligate planktivores have fewer options for prey choices; they are typically restricted to marine pelagic ecosystems that have a dominant plankton presence, such as highly productive upwelling regions.
Mechanics of consuming plankton
Planktivores, whether obligate or facultative, obtain food in multiple ways. Particulate feeders eat planktonic items selectively, by identifying plankton and pursuing them in the water column. Filter feeders process large volumes of water internally via different mechanisms, explained below, and strain food items out en masse or remove food particles from water as it passes by. "Tow-net" filter feeders swim rapidly with mouths open to filter the water, whereas "pumping" filter feeders suck in water via pumping actions. The charismatic flamingo is a pumping filter feeder, using its muscular tongue to pump water along specialized grooves in its bill and pump water back out once plankton have been retrieved. In a different filter feeding process, stationary animals, like corals, use their tentacles to grab plankton particles out of the water column and transfer the particles into their mouth. There are numerous interesting adaptations to remove plankton from the water column. The phalaropes use surface tension feeding to transport particles of prey to their mouth to be swallowed. These birds capture individual particles of plankton held in a droplet of water, suspended in their beaks. They then use a sequence of actions that begin with a quick opening of their beak to increase the surface area of the water droplet encasing prey. The action of stretching out the water droplet ultimately pushes the water and prey to the back of the throat where it can be consumed. These birds also spin around at the water surface, creating their own eddies that draw prey up closer to their beaks.Some species actively hunt plankton: in certain habitats such as the deep open ocean, as mentioned above, the planktivorous basking shark track the movements of their prey closely up and down the water column. The megamouth shark, another planktivorous species, adopts a similar feeding strategy that mirrors the movement in the water column of their planktonic prey. Similar to active hunting, some zooplankton, like copepods, are ambush hunters meaning they wait in the water column for prey to come within range and then rapidly attack and consume. Some fishes change their feeding strategy throughout their lives; the Atlantic menhaden is an obligate filter feeder in early life stages, but matures into a particulate feeder. Some fishes, like the northern anchovy can merely modify their feeding behavior depending on the prey or environmental conditions. Some fishes also school together when feeding to help improve contact rates of plankton and simultaneously prevent themselves from predation. Some fishes have gill rakes, an internal filtration structure that assists fishes with capturing plankton prey. The amount of gill rakes can indicate planktivory as well as the typical size of plankton consumed, showing a correlation between gill rake structure and the consumed plankton type.