The sea squirt, or tunicate, is a marine animal that often goes unnoticed, appearing as a simple, sac-like organism attached to docks and rocks. This creature, which can range from half a centimeter to over 10 centimeters in size, lives a sessile life, meaning it is permanently fixed to a surface once it matures. Its unassuming adult form belies a complex and bizarre life history featuring one of the most radical transformations in the animal kingdom. The sea squirt’s existence is a biological paradox, starting life with features shared with humans, only to discard them for a stationary, filter-feeding existence.
What is a Tunicate
Sea squirts belong to the subphylum Tunicata (Urochordata), placing them within the phylum Chordata alongside vertebrates like fish and mammals. The adult body is generally cylindrical or rounded, characterized by a tough, protective outer layer called a tunic. This tunic is structurally unique because it is largely composed of tunicin, a substance chemically similar to cellulose, which is typically found only in plants.
The body features two distinct openings known as siphons. Water is drawn into the body through the larger incurrent siphon and expelled through the excurrent siphon. The common name “sea squirt” comes from the animal’s defensive habit of forcefully contracting its muscular body wall when disturbed, squirting a jet of water from these siphons. While many species live solitary lives, others form colonies made up of numerous small individuals, or zooids, embedded within a shared tunic.
The Metamorphosis from Larva to Adult
The life cycle begins with a free-swimming “tadpole larva.” This larval stage possesses a notochord, a dorsal nerve cord, a tail, and a light-sensing eye spot, features that classify it as a chordate and link it to vertebrates. The larva’s primary function is dispersal; it is non-feeding and must quickly find a suitable hard surface to attach itself within a few days.
Once a favorable substrate is found, the larva adheres to it using specialized adhesive papillae at its anterior end. This attachment triggers an irreversible and rapid transformation known as retrogressive metamorphosis, where the complex larva transforms into a simpler adult. The larva’s tail and the notochord are reabsorbed by the body, and the entire locomotor apparatus is dissolved since it is no longer required for sessile adult life.
The transformation involves the nervous system, as the dorsal nerve cord and the cerebral ganglion (primitive brain-like structure) are broken down. This reabsorption of nervous tissue, which is not needed for a stationary existence, is the source of the popular concept that the sea squirt “eats its own brain.” The remaining nervous tissue forms a much simpler nerve center, and the body rearranges to orient the siphons correctly for filter feeding.
How Sea Squirts Feed and Impact Marine Habitats
As adults, sea squirts are efficient sessile filter feeders, drawing a constant stream of water through their bodies to capture microscopic food. Water enters the incurrent siphon and flows into the pharynx, a large internal chamber that acts as a specialized feeding organ. The pharynx wall is perforated with numerous small openings (stigmata) covered in a mucous net continuously produced by the endostyle.
Microscopic particles, including phytoplankton, algae, and detritus, are trapped in this sticky mucus as the water is driven across the pharynx by ciliary action. The mucus net and captured food are rolled into a cord and directed into the digestive tract for absorption. Filtered water and waste are expelled out of the excurrent siphon, allowing the animal to process a large volume of seawater daily.
Sea squirts play a significant role in marine ecology, particularly in nearshore environments where they are abundant. They are considered biofouling organisms, readily attaching and growing on man-made structures like ship hulls, docks, and aquaculture equipment, sometimes causing problems for marine industries. Their ability to accumulate high concentrations of certain trace elements, such as vanadium, from seawater is a unique physiological characteristic.
Evolutionary Importance
The life cycle of the sea squirt holds a unique position on the evolutionary tree, providing insights into the origins of vertebrates. Although the adult form is simple, the presence of a notochord and dorsal nerve cord in the larval stage firmly establishes tunicates within the phylum Chordata. This means they share an ancient common ancestor with all vertebrates, including humans.
Scientists utilize sea squirts as model organisms in developmental biology and genomics due to their compact, fast-evolving genomes and transparent embryos. Studying the metamorphosis, particularly the genetic mechanisms governing the loss of chordate features, helps researchers understand the ancestral chordate body plan. Tunicates represent an evolutionary crossroads, offering a simplified model to explore the genetic pathways linking invertebrates to the more complex vertebrate lineage.