Sea squirts, also known as ascidians or tunicates, are marine invertebrates found in shallow waters worldwide, typically attached to surfaces like rocks or docks. Numbering around 3,000 species, these organisms are sac-like and sessile in their adult form, ranging from half a centimeter to about ten centimeters in size. Their common name comes from their tendency to forcefully expel water from their openings when disturbed.
The adult sea squirt is a filter feeder, drawing in seawater to capture tiny food particles. They live a stationary life, existing either as solitary individuals or in dense colonies. Despite their simple appearance, sea squirts possess a unique biological history that places them surprisingly close to humans on the evolutionary tree.
Defining the Sea Squirt’s Chordate Identity
Biologically, the sea squirt’s most astonishing characteristic is its classification within the Phylum Chordata, the same phylum that includes all vertebrates. Chordates are defined by possessing four specific features at some point in their life cycle: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. The adult sea squirt, a stationary, bag-like animal, appears to lack these defining characteristics entirely.
This organism belongs to the subphylum Urochordata (Tunicata), and its chordate status is revealed exclusively in its larval stage. The free-swimming larva, which resembles a tiny tadpole, possesses a temporary notochord and a dorsal hollow nerve cord. It also features a post-anal tail and pharyngeal slits, making it a complete chordate.
Once the larva settles and transforms, it loses the majority of these advanced structures through retrogressive metamorphosis. This transformation involves the degeneration and absorption of the notochord, the tail, and much of the nervous system. The remnants of the nervous system condense into a single cerebral ganglion, earning the sea squirt the common description of “eating its own brain”.
Anatomy and Filter Feeding Mechanism
The adult sea squirt’s body is encased in a thick, protective outer layer called a tunic. This tunic is unique because it is composed largely of cellulose, a polysaccharide typically found in plant cell walls. The tunic is a living tissue that provides a tough, leathery covering for the soft inner body.
The animal uses two prominent openings, or siphons, located on its upper surface to interact with the surrounding water. Water is drawn into the body through the larger incurrent (oral) siphon, flowing into a large, perforated chamber called the pharynx or branchial basket. The pharynx is lined with cilia and covered in a mucous sheet secreted by a groove called the endostyle. This mucous sheet traps suspended food particles as the water passes through numerous small slits in the pharynx wall. The water, now stripped of food, moves into a surrounding cavity called the atrium and is then expelled through the smaller excurrent (atrial) siphon. This continuous flow allows the sessile sea squirt to efficiently filter-feed and obtain oxygen.
The Dramatic Life Cycle Transformation
The life cycle begins with the “tadpole larva,” a short-lived, free-swimming stage. The primary function of this mobile form is dispersal and finding a suitable hard surface for permanent settlement. The larva uses its temporary tail, supported by the notochord, for propulsion. Equipped with sensory organs, including a light-sensing ocellus and a gravity-sensing statocyst, the larva actively searches for an optimal habitat. Upon finding a satisfactory location, the larva attaches itself head-first to the substrate using specialized adhesive papillae. This attachment is irreversible and quickly initiates the rapid metamorphosis into the adult form.
The structures designed for mobility are rapidly broken down and reabsorbed, including the tail and the notochord. The internal organs rearrange themselves to suit the new, stationary, filter-feeding lifestyle. The result is the final sac-like adult form, which remains permanently fixed to that spot.