Zooplankton

The ocean is home to so many amazing creatures. Compared with sleek and streamlined sharks, majestic whales, or colorful coral reefs, tiny drifting organisms might seem a little boring or insignificant. However, these microscopic organisms are incredibly diverse, exhibiting a variety of strange forms and strategies for surviving in an unforgiving environment where they compete fiercely for food and struggle to avoid being eaten. 

Plankton are organisms that cannot swim against currents. Instead, they move with bodies of water throughout the oceans. Some plankton have a limited ability to swim; several species are known to migrate from deep waters, where they hide from predators, to shallow waters, where they feed. Most of the time, though, they are at the mercy of currents. The term plankton is derived from the Greek word planktos, which means drifter or wanderer. 

In general, plankton are divided into two major groups: the phytoplankton and zooplankton. Phytoplankton, like plants and trees on land, are autotrophic; they make their food by photosynthesizing, using energy from the sun, carbon dioxide, and nutrients in their environment. Zooplankton are heterotrophic, which means they do not make their own food. They prey on other organisms in the oceans, including phytoplankton or other zooplankton. Zooplankton are an important component of marine food webs and healthy marine ecosystems. Over the next several weeks, On the Ocean will describe several species of zooplankton, their role in the world’s oceans, and the challenges they face to survive.

Living in the ocean are tiny marine sculptors known as Tintinnids. Tintinnids are a group of zooplankton that build shells around themselves using primarily organic material, such as proteins, but often incorporating minerals and sediment. Tintinnid shells, known as loricae, are marine works of art, taking many different shapes but most often resembling a vase or bell. Scientists typically classify tintinnids based on the shape of the lorica; within the Tintinnopsis complex, there are roughly 100 described species. The ability of these organisms to gather particles and form shells is currently their only unifying feature; identifying other traits is difficult, as the organisms are obscured by their loricae. 

Zooplankton belonging to the Tintinnopsis complex are present in coastal areas of North America, Europe, South America, and Eastern Asia, where sediment is plentiful for them to cover themselves with. Tintinnids can be found in surface waters to depths of over 300 meters. They are commonly present at densities of roughly 100 per liter but can reach densities of thousands per liter. Tintinnids are grazers, feeding on phytoplankton and bacteria. At one end of the lorica, the Tintinnid beats tiny hair-like appendages known as cilia, creating a current that draws prey toward the cell. These cilia are also used to move the cell through the water. 

Tintinnid abundance is directly related to prey abundance, light and nutrient availability, temperature, and salinity, or the abundance of salt in marine waters. In the spring, the greatest number of Tintinnid species are present, but tintinnid biomass is greatest in the summer months, usually due to the success of one or a few specific species. 

Why did the mushroom get invited to all the parties? Because he’s a fun guy! The star of today’s show, the heterotrophic zooplankter Globigerina bulloides, is not actually a type of fungus, but it does look like a bunch of mushroom caps squished together. Globigerina bulloides belongs to a group of zooplankton called foraminifera, which are characterized by their chambered, often intricate, calcium carbonate shells or tests. These tests have pores through which the cell projects extensions of itself, known as pseudopodia, to capture food, attach to a surface, or swim. 

Globigerina bulloides can be found globally in the photic zone, where sunlight is available throughout the water column. They are most abundant in the Southern Hemisphere, specifically the Southern Ocean. Upwelling in the Southern Ocean ensures availability of nutrients for Globigerina bulloides, particularly in the spring when they are most abundant. 

Globigerina bulloides tests are like tiny history records, preserving indicators of Earth’s climatic conditions in the past. The calcium carbonate in their tests contains oxygen, of which there are isotopes, or atoms with different numbers of neutrons. Oxygen 18, with more neutrons, is heavier than oxygen 16. This difference in mass means the rate of exchange between the ocean and atmosphere for oxygen 18 is different from oxygen 16, both varying with temperature. Measuring the ratio of oxygen 18 to oxygen 16 in Globigerina bulloides tests allows scientists to determine what the climate was like throughout Earth’s history. When there is a greater abundance of oxygen 18 relative to oxygen 16 in calcium carbonate tests, the climate was cooler. When the ratio was lower, the climate was warmer. 

Despite being no larger than your pinky finger, the Antarctic krill, Euphausia superba, is able to sustain the largest animal on Earth, the blue whale. The Antarctic krill is a keystone species in the Southern Ocean, which means it is a species that plays a critical role in the function of its ecosystem. In addition to whales, these tiny crustaceans are preyed upon by seals, squid, and birds.

To sustain so many hungry predators, and maintain their population, there must be a staggering amount of krill in the oceans! The estimated biomass of Antarctic krill is around 500 million tons, making them one of the most abundant animals on Earth, with likely the greatest biomass of any multicellular animal. Krill tend to group together in congregations known as bait balls, with densities reaching 30,000 individuals per cubic meter. These bait balls can often be seen and tracked from space! 

Marine predators are not the only ones chasing after krill. Krill oil is becoming a popular supplement for humans who want a healthy dose of omega-3 fatty acids. Demand for krill oil has driven the development of a commercial fishery for these animals, capturing over 300,000 tons annually to produce several products including krill oil and animal feed. There is concern regarding the potential overfishing of Antarctic krill due to the importance of krill as a keystone species and their role in removing carbon dioxide from the atmosphere. Krill eat phytoplankton, which take up CO2 from the atmosphere. But krill are sloppy eaters, and the pieces of phytoplankton they drop sink to the bottom of the ocean along with their feces. The feeding activity of krill moves substantial amounts of carbon to the deep ocean; overfishing may limit this process, with consequences for Earth’s climate.

If phytoplankton are the grass of the sea, then grazers like the marine ciliate Strobilidium are the cows. Phytoplankton commonly cause harmful algal blooms when they grow to very high densities, becoming detrimental to marine ecosystems and human health. Organisms that feed on phytoplankton deter runaway growth and prevent harmful blooms.

Strobilidium is a naked ciliate; a single-celled organism with tiny hair-like projections called cilia arranged in a spiral around the cell’s mostly spherical body. They are called naked because they do not have a shell or other protective covering. Cells of Strobilidium are 40-60 microns in diameter, or about 2 thousandths of an inch, which is still bigger than the phytoplankton they eat!

Zooplankton like Strobilidium are important members of marine food webs. They are effective grazers, responsible for 30 – 50% of grazing activity on phytoplankton in some environments. Additionally, they are important prey for larger organisms, such as fish. 

Currently, scientists are unsure how grazers like Strobilidium, and marine food webs in general, will respond to climate change. Research has indicated that rising ocean temperatures can promote the growth of Strobilidium, which means more intense grazing on phytoplankton populations. Increased grazing could help prevent harmful algal blooms in the future. However, phytoplankton will also respond to rising temperatures, possibly with increased growth rates of their own. The impact of climate change on marine food webs remains uncertain.

Only slightly larger than a paperclip, the northern krill Meganyctiphanes norvegica might be small in stature, but it plays a big role in marine ecosystems. Meganyctiphanes norvegica is a small, shrimp-like creature found primarily in the western Mediterranean Sea and North Atlantic ocean between 30 and 80 degrees north latitude. These crustaceans grow to between 22 and 45 mm in length, and feed on other tiny organisms like phytoplankton as well as decomposing material. Typically, meganyctiphanes norvegica can be found between 100 and 400 m depths in the ocean, but they have been caught at depths as great as 1500 m. Meganyctiphanes is known to migrate vertically, moving to surface waters at night to feed. 

Meganyctiphanes norvegica is an important prey species for many marine animals. Fish, seabirds, squid, and whales all feed on Northern krill. Unfortunately, populations of Northern krill have been declining in recent years. The cause of the decline of Meganyctiphanes norvegica populations is unknown, but could be related to human activity, climate change, or increased predation. Scientists are currently trying to understand what is driving this change, and how marine ecosystems that depend on Meganyctiphanes norvegica will be impacted. One major issue facing meganyctiphanes norvegica is climate change. Meganyctiphanes norvegica is dependent on ice. Ice offers both protection from predators and an all-you-can-eat buffett in the form of algae and other small plankton growing on the underside of ice sheets. As temperatures rise and ice melts, Meganyctiphanes norvegica struggle to find food and avoid predators.

Lurking in the cold arctic depths, an ambush predator hunts for its next meal. Detecting motion with the tiny hairs along its body, it lunges forward and seizes its target with the sharp, serrated hooks on the side of its head. Having captured and devoured its prey, the creature meanders back to the depths. 

Chaetognaths, which means bristle-jaws, are planktonic, predator marine worms. They are more commonly known as arrow-worms. Arrow worms range in size from 2 to 33 mm, approximately the size of a bobby pin. Their bodies are long, narrow, and transparent, separated into 3 sections: a head, trunk, and tail. Additionally, arrow worms can see using two compound eyes on their heads. On the sides of the arrow-worm’s head are between 4 and 14 spiny bristles used to capture and immobilize prey with neurotoxins. Arrow worms typically feed on small marine crustaceans known as copepods. In addition to being voracious predators, arrow worms are often prey for other, larger marine creatures like jellyfish, seabirds, or even other arrow worms. 

Though they are known to be globally distributed, arrow worms are most commonly found on Arctic and sub-Arctic shelves, living alone or in small colonies. A shelf is a shallow area of the ocean attached to continental landmasses, distinct from the much deeper sea floor in large ocean basins. Currently, there are roughly 120 described species of arrow worms, 20% of which are benthic, which means they live on the bottom of the ocean, attaching to rocks or sediments, instead of in the water column.

It is difficult to imagine how small plankton are. Often, the size of these organisms is measured in micrometers. For perspective, a piece of paper is 100 micrometers, or microns, thick. The subject of today’s show, marine plankton known as Strombidium, are only between 30 and 60 microns long. 

Let’s zoom in on what makes Strombidium so important in the oceans. Strombidium are single-celled marine ciliates, meaning they are covered in tiny hair-like projections that help them swim and eat, with elongate, oval shapes. In contrast with other ciliates, the cilia on much of the surface of Strombidium cells have been lost or shrunk to short bristles, while the cilia around the oral cavity of the cell are large. Strombidium are important members of marine food webs because they eat and repackage extremely small phytoplankton, known as picoplankton, for other, larger predators like copepods to consume.

Strombidium may also be an important indicator of ecosystem health and response to climate change. A study in the western Arctic Ocean during the summer sea-ice reduction period found differences in ciliate community structure within their sampling region associated with environmental conditions and nutrient availability, specifically ciliate abundance and species diversity. These results led the authors to propose monitoring ciliate diversity and abundance could be an effective way to monitor ecosystem health and response to climate change. 

 Imagine a chain of identical sextuplets, linked together and wrapped around their parent; an odd family, to say the least, but that is how the salp Thalia democratica lives. A salp is a gelatinous marine organism that moves by pumping water through its tube-shaped body, filtering-feeding on phytoplankton as it moves.

The salp Thalia democratica is commonly found in continental shelf waters, but is most abundant in the Tasman Sea, between Australia and New Zealand, especially during the austral spring season. The most iconic feature of this zooplankter is its ability to produce long chains of identical buds, which may be as long as 6 meters and containing as many as 350 individuals! 

Thalia democratica are well-adapted for filter feeding; a system of 5 muscular rings propels them forward by pumping water through their tube-like bodies. Due to their ability to form dense aggregations and chains, they are capable of outcompeting other common zooplankton like copepods. Thalia democratica is also commonly preyed upon by sea turtles, marine birds, fish, and sea jellies. The importance of salps like Thalia democratica to marine food webs is still being investigated. 

Thalia democratica is capable of reproducing both sexually and asexually. Buds are formed at the beginning of Thalia democratica’s sexual cycle and wrap around the body of the parent. These buds develop into chains, which are released into the water before transitioning to a female form. Sperm released by male Thalia democoratica fertilize the females’ eggs. Following fertilization, aggregates may develop testes and produce a single male offspring, which releases sperm into the water before dying.

Have you ever wondered what the most abundant animal on earth is? The answer is tiny crustaceans known as copepods. The subject of our show today is one species of these tiny but numerous organisms, Calanus finmarchicus. These zooplankton grow to be roughly 3 to 4 mm long. They are widely distributed, but most abundant in the North Atlantic Ocean, specifically the North and Norwegian Seas, off the coast of Canada, and in the Gulf of Maine. Calanus finmarchicus reside in surface waters but have been found at depths of 4000 meters, and they typically live for 1 to 3 years. 

The diet of Calanus finmarchicus consists of several different species of phytoplankton, including coccolithophores, diatoms, and dinoflagellates. Their life cycle is dependent on the seasonal patterns of their food; Calanus finmarchicus spawns in the spring when waters are warming and phytoplankton bloom, providing abundant food for these copepods. Though spawning typically occurs at night, a behavior that likely reduces predation, female Calanus finmarchicus are the most common predator of their own eggs! So much for nurturing parents. 

Within marine food webs, Calanus finmarchicus is an important prey species for fish like herring and mackerel, as well as other invertebrates. They are high in protein, and rich in omega-3 fatty acids. Calanus finmarchicus is also a commercially relevant species to humans. These copepods are harvested to produce calanus oil, a supplement similar to fish or krill oil, or turned into fish feed for aquaculture or home aquariums.