Tunicates are filter feeders that survive almost entirely on phytoplankton, marine bacteria, and tiny organic particles suspended in seawater. They pump water through their bodies and trap anything between roughly 1 and 1,000 micrometers in size, though their filtration system can capture particles far smaller than that. A few rare deep-sea species break the mold entirely and eat live prey.
Phytoplankton and Bacteria Make Up Most of the Diet
The bulk of a tunicate’s diet is microscopic. Phytoplankton (single-celled algae) and marine bacteria are the primary food sources, with the exact mix depending on what’s available in the surrounding water. Tunicates are not selective eaters. They don’t hunt or choose specific prey. Instead, they consume whatever the water delivers, as long as it fits through their intake siphon and sticks to their internal filter.
Some organic detritus, dissolved particles, and even viruses end up caught in the filter as well, though the smallest particles (under 0.05 micrometers, like viruses and colloids) are captured at less than 2% efficiency. The system really hits its stride with particles above 1.2 micrometers, where capture efficiency reaches 100%. That size range covers most bacteria and phytoplankton perfectly.
How the Mucus Net Filter Works
Tunicates feed by contracting muscles around their body to pump seawater in through an oral siphon (an opening at the top) and out through a second siphon. Inside, the water passes through a structure called the pharynx, which houses the animal’s remarkably fine filtration system.
A groove running along the floor of the pharynx, called the endostyle, secretes a continuous sheet of mucus that forms a mesh net. This net has fibers only about 0.1 micrometers thick, with openings roughly 1.5 micrometers wide and 6 micrometers long. For perspective, a human hair is about 70 micrometers across, so the mesh openings are roughly 50 times smaller than a single hair. Rows of tiny cilia (hair-like structures) help shape the mucus into this precise lattice pattern and move it upward across the pharynx, where it collects particles from the passing water. The loaded mucus is then rolled into a strand and directed into the digestive tract.
The rough tunicate, a common species studied in lab settings, filters an average of about 3 liters of water per hour. That means a single animal can process over 70 liters of seawater in a day. Filtration rates vary with conditions: when exposed to higher concentrations of food particles, individual tunicates showed rates ranging from under 1 liter to over 3.5 liters per hour within the same experiment. But across different food concentrations, the average stayed remarkably consistent at around 3 to 3.5 liters per hour, suggesting tunicates maintain a steady pumping rhythm regardless of how much food is available.
Deep-Sea Predatory Tunicates
Not all tunicates are passive filter feeders. A small family of deep-sea species called Octacnemidae have evolved into active predators. The best-known example, Megalodicopia hians, lives in deep submarine canyons and looks nothing like a typical sea squirt. Its oral siphon has ballooned into a large hood with two fleshy lips that can snap shut around small crustaceans crawling along the seafloor. These animals sit motionless with their hood open, waiting for prey to wander inside.
Direct observations of these predatory tunicates are rare because they live at extreme depths, but their anatomy makes the feeding strategy clear. Several related genera share the same oversized lip structure, suggesting predatory feeding has been successful enough in the deep sea to evolve multiple times within this family.
Their Role in Moving Carbon to the Deep Ocean
What tunicates eat matters beyond their own survival. Pelagic tunicates like salps and pyrosomes (free-swimming, open-ocean species) play a surprisingly large role in the ocean’s carbon cycle. After filtering and digesting phytoplankton, they produce dense fecal pellets that sink approximately ten times faster than typical marine detritus. This rapid sinking carries carbon from surface waters, where it was captured from the atmosphere by photosynthesis, deep into the ocean before it can be broken down and re-released.
NOAA modeling research found that this fast-sinking detritus from tunicates and fish increases deep carbon storage across much of the world’s oceans, with the strongest effects in upwelling zones and high-latitude waters. The process is somewhat counterintuitive: incorporating fast-sinking material actually decreases total carbon export from the surface layer while increasing how much of that carbon gets locked away at depth. In other words, tunicates don’t necessarily move more carbon out of the surface, but they deliver it deeper and more permanently.
Where Tunicates Sit in the Food Web
Tunicates occupy a unique position as consumers of the very smallest organisms in the ocean. By efficiently capturing bacteria and phytoplankton that are too small for most other filter feeders to catch, they convert microscopic food into animal tissue that larger predators can eat. That said, tunicates have relatively few predators. In the Gulf of Maine, for example, they serve as a secondary food source for skates, crabs, and sea urchins, but no major predator depends on them as a primary diet item. Their tough outer covering, called a tunic (which gives them their name), likely makes them a less appealing meal compared to softer-bodied alternatives.