Sea squirts, also known as tunicates, are marine invertebrates that attach themselves to underwater surfaces such as rocks, pier pilings, and boat hulls. These sac-like organisms are found in all seas, inhabiting environments from shallow intertidal zones to deep ocean waters. Their unique feeding strategy is central to their survival and plays a significant role in marine ecosystems.
Their Microscopic Diet
Sea squirts primarily consume plankton, microscopic particles suspended in the water column. This diet includes phytoplankton (tiny marine plants like algae and diatoms) and zooplankton (small animals such as larvae and protozoa).
In addition to living organisms, sea squirts also feed on detritus, which is dead organic matter. Near coastal areas, this detritus can form a substantial part of their diet. In deeper waters, plankton becomes a more prominent food source. Some larger species can capture larger food particles, including small jellyfish, using specialized tentacles.
The Unique Filter-Feeding Process
Sea squirts are efficient filter feeders, drawing water through their bodies to capture food particles. This process begins as water is pulled into the sea squirt through an opening called the incurrent siphon. Inside the body, the water passes into a specialized structure known as the pharynx, which contains numerous gill slits.
A mucous net within the pharynx traps microscopic food particles as water flows through the gill slits. Cilia, small hair-like structures lining the pharynx, beat rhythmically to create the water current and move the food-laden mucus to the stomach for digestion. Filtered water and waste products are then expelled from the sea squirt’s body through a second opening, the excurrent siphon. Sea squirts can process a significant volume of water, with some adults filtering up to three liters per hour.
Ecological Role Through Feeding
The filter-feeding activities of sea squirts impact marine environments. By consuming suspended particles, they improve water clarity. This constant filtering also plays a role in nutrient cycling, as they remove excess nutrients like nitrogen and phosphorus from the water column.
Sea squirts transfer energy from the water column to the seafloor through their feeding and waste production. Their ability to accumulate pollutants, including tiny plastic particles, makes some species useful as bio-indicators for assessing environmental health. These organisms thrive in both clean and industrialized waters, demonstrating their adaptability.