Tunicates, commonly known as sea squirts, are marine invertebrates that appear simple. Yet, they possess unique biological characteristics and a significant place in the animal kingdom. Classified within the subphylum Tunicata (or Urochordata) under the phylum Chordata, they are unexpectedly linked to vertebrates, including humans. This classification highlights their distinct evolutionary journey.
Defining Features
Adult tunicates have a sac-like body, often attached to a substrate. They feature two siphons: an incurrent (inhalant) siphon for water intake, and an excurrent (exhalant) siphon for expelling water and waste. This design facilitates their filter-feeding, capturing plankton and other particulate matter from seawater. Their outer covering, called a “tunic,” is composed of a cellulose-like material, unusual for animals, providing protection and structural support.
Tunicates are classified as chordates due to features present during their larval stage. The free-swimming, tadpole-like larva possesses a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Most of these chordate traits are lost during metamorphosis to the adult sessile form, but their transient presence in the larval stage confirms their classification.
Life Cycle Transformations
The tunicate life cycle begins with a free-swimming “tadpole larva,” a motile stage seeking a suitable settlement site. This larval form is short-lived, lasting only hours to days, as it does not feed. The larva is equipped with sensory organs, such as an eyespot and an otolith, to help it orient and locate an attachment surface.
Upon finding a suitable substrate, the larva undergoes retrogressive metamorphosis. It attaches, usually by its anterior end, reabsorbing its tail, notochord, and nerve cord. This reabsorption provides energy and materials for adult development. The larva transforms into the sessile adult, losing much of its larval nervous system, a change sometimes colloquially described as the sea squirt “eating its own brain.”
Tunicates reproduce both sexually, as hermaphrodites releasing sperm and eggs, and asexually through budding, particularly in colonial species. Colonial forms bud off new individuals (zooids) that remain connected, forming a larger organism within a shared tunic.
Marine Habitats and Roles
Tunicates inhabit a wide range of marine environments, from shallow intertidal zones to the deep sea. Many species, particularly the ascidians (sea squirts), are benthic, meaning they live on the seafloor, attached to surfaces like rocks, coral reefs, dock pilings, boat hulls, or other marine organisms. Other classes, like thaliaceans (e.g., salps) and larvaceans, are pelagic, living freely in the open water.
As filter feeders, tunicates play an important role in marine ecosystems. They process large volumes of seawater, removing plankton, detritus, and suspended particulate matter. This feeding activity contributes to water clarity and nutrient cycling. They also serve as a food source for marine animals and contribute to the downward transport of organic matter to the deep sea via their fecal pellets.
Ecological and Scientific Importance
Tunicates hold important ecological and scientific significance. Ecologically, some tunicate species are known as invasive organisms, rapidly colonizing and dominating new environments. They can cause biofouling on marine infrastructure (e.g., docks, aquaculture equipment), outcompeting native species and potentially impacting commercial shellfish industries. Their ability to reproduce sexually and asexually, coupled with resilience, contributes to their invasive potential.
From a scientific perspective, tunicates are important model organisms for studying chordate evolution and developmental biology. Their phylogenetic position as the closest living relatives to vertebrates, combined with their simple, transparent embryos, offers insights into the genetic and cellular mechanisms underlying chordate development. Tunicates also show promise in biotechnology, with research exploring unique compounds for potential pharmaceutical applications. Several tunicate-derived compounds, including those with anti-cancer properties, have been investigated in clinical trials.