Sea squirts are marine invertebrates, often found attached to underwater surfaces. They belong to the subphylum Tunicata and are also known as tunicates or ascidians. Their common name, “sea squirt,” comes from their tendency to forcefully expel water from their bodies when disturbed or removed from the water. These animals are filter feeders, meaning they draw water through their bodies to extract nutrients and oxygen.
The Tunic: Outer Support
The most distinct feature of a sea squirt’s structure is its outer covering, known as the tunic. This tough, protective layer gives the sea squirt its characteristic sac-like or barrel-shaped form. The tunic is secreted by the animal’s epidermis, which is a thin layer of cells on its inner surface. Unlike the shells of many other marine animals, the tunic is unique among invertebrates for being composed largely of tunicin, a polysaccharide chemically similar to cellulose, which is typically found in plants.
This cellulose-like material provides the tunic with rigidity and durability, acting somewhat like an external skeleton for the sea squirt. The tunic’s composition also includes proteins and calcium salts, enhancing its strength. In some colonial species, the tunics of individual sea squirts can even fuse together to form a single, shared structure, providing collective support. The tunic grows as the animal enlarges, providing continuous support and protection without needing to be shed.
Internal Structures and Water Dynamics
Sea squirts possess two important openings, or siphons, for their water dynamics and form. The incurrent, or oral, siphon acts as an intake, drawing water into the animal’s body. The excurrent, or atrial, siphon serves as the outlet, expelling filtered water and waste products. These siphons can vary in appearance, from noticeable tubes to subtle openings, and their positioning influences the direction of water flow.
Inside the sea squirt, a large pharyngeal basket, also known as a gill sac or branchial basket, occupies much of the internal space. Water drawn in through the incurrent siphon enters this pharyngeal region. The pharyngeal basket is lined with small pores or slits, called stigmata, and is covered with cilia, which are tiny hair-like structures. The coordinated beating of these cilia generates a water current, pushing water through the gill slits into a surrounding chamber called the atrium. This continuous flow facilitates filter feeding by trapping food particles in mucus, and it also aids in oxygen exchange.
Structural Adaptations for a Sessile Life
The structure of a sea squirt is well-suited for its sessile existence. The tunic provides a strong outer barrier, protecting the animal from physical damage caused by strong currents, wave action, or predators. Its ability to firmly attach to substrates ensures stability in its marine environment.
The placement of the incurrent and excurrent siphons allows for efficient filter feeding without movement. Water is constantly drawn in, filtered for plankton and organic debris, and then expelled, optimizing nutrient and oxygen uptake from the surrounding water column. The sac-like body shape, reinforced by the tunic, maximizes the internal surface area for filtration, allowing for effective processing of large quantities of water. This structural design enables sea squirts to thrive as filter feeders while remaining anchored in place.