Sea squirts are soft-bodied marine animals that look nothing like the vertebrates they are closely related to. Formally called ascidians, they belong to the subphylum Tunicata and sit on the branch of the animal family tree closest to vertebrates, making them more closely related to humans than to the sponges and corals they superficially resemble. What makes them fascinating is the split personality of their life cycle: a free-swimming tadpole larva with a nerve cord and a rudimentary backbone-like structure gives way, through one of the most dramatic metamorphoses in the animal kingdom, to a barrel-shaped adult cemented to the seafloor and siphoning water for a living.
From Tadpole to Sessile Adult
A sea squirt begins life as a tiny tadpole larva, usually no more than a few millimeters long. This larva has a tail stiffened by a notochord, the same flexible rod that appears, at least transiently, in every chordate embryo, including yours. It also has a hollow dorsal nerve cord, a simple eyespot for detecting light, and a gravity-sensing organ called a statocyst. The larva does not feed. Its job is to swim, find a suitable surface, and attach headfirst within hours to a day or two of hatching. The larval dispersal range is short, since the swimming phase is brief and the larvae carry limited energy reserves.
Once the larva cements itself to a rock, dock piling, or piece of shell, metamorphosis begins. The tail, along with its notochord and most of the nervous system, is reabsorbed. The body reorganizes around two openings, an incurrent siphon and an excurrent siphon, that will define the adult’s anatomy. What was once a motile animal with a recognizable chordate body plan becomes a stationary filter feeder. The transformation is so thorough that early naturalists classified adult sea squirts alongside sponges rather than recognizing them as relatives of fish and frogs.
Adult Anatomy and Filter Feeding
An adult solitary sea squirt looks a bit like a rubbery vase or a small barrel. Its body is wrapped in a tough outer layer called the tunic, from which the entire group gets its formal name, tunicates. Two siphons protrude from the top. Water enters through the incurrent (oral) siphon, passes through a large perforated structure called the branchial sac, and exits through the excurrent (atrial) siphon. The branchial sac works as both a gill and a food-collection device: a sticky mucus sheet traps particles as water filters through tiny slits in its wall, and cilia move the loaded mucus toward the gut. Food particles, mostly phytoplankton and bacteria, are digested in a simple stomach and intestine. Waste exits through the excurrent siphon along with the filtered water.
Adults are entirely sessile, meaning they cannot move once settled. They can, however, contract their siphons rapidly when disturbed, squirting a jet of water. This defensive reflex is what earned them the common name “sea squirt.” Beyond that, they have no limbs, no eyes, and only a vestigial nervous system compared with the larva. A small ganglion between the two siphons handles basic coordination of siphon contraction, heartbeat, and the rhythmic beating of cilia. The brain, such as it was in the larva, is gone. This has led to the popular quip that a sea squirt “eats its own brain,” though what actually happens is more of a wholesale demolition and recycling of larval tissues during metamorphosis.
The Only Animals That Produce Cellulose
The tunic is one of the most unusual structures in the animal kingdom. It is composed largely of cellulose, the same polysaccharide that gives plant cell walls their rigidity. Sea squirts are the only animals known to produce cellulose, a trick they apparently picked up through an ancient horizontal gene transfer from bacteria. At some point deep in tunicate evolutionary history, a bacterial cellulose synthase gene was incorporated into the tunicate genome, giving these animals the molecular machinery to build their own plant-like protective coat.
The tunic itself is not just a simple wrapper. It contains proteins, water, and sometimes living cells, and in some species it can be leathery, gelatinous, or even translucent. It serves as structural support, physical protection against predators and abrasion, and a barrier against pathogens. In certain commercially harvested species like the sea pineapple Halocynthia roretzi, the tunic has attracted interest as a source of cellulose nanofibers for biomedical and industrial applications.1ScienceDirect. Structure and composition of the tunic in the sea pineapple Halocynthia roretzi: A complex cellulosic composite biomaterial
A Heart That Reverses Direction
The circulatory system of a sea squirt is open, meaning blood flows through sinuses and tissue spaces rather than entirely through enclosed vessels. The heart is a simple tube that pumps blood by peristaltic contractions, waves of muscle squeezing that move blood along much like you push toothpaste through a tube. What sets the tunicate heart apart from virtually every other animal heart is that it periodically reverses the direction of pumping. In the species Corella inflata, the heart beats about once every 1.4 seconds in one direction for roughly 180 beats, then pauses and reverses, sending blood the opposite way for another stretch.2PubMed Central. Blood circulation in the ascidian tunicate Corella inflata (Corellidae) – Section: Heart structure and function
Why reverse blood flow? The prevailing idea is that it ensures all tissues get adequately perfused. In an open circulatory system without true capillaries, some regions may receive less blood depending on the direction of flow. By switching directions regularly, the heart avoids leaving any part of the body chronically under-supplied. The reversal is not a malfunction or a random event; it is a built-in rhythm, consistent and predictable within each species.
Reproduction, Budding, and Colonial Living
Most sea squirts are hermaphrodites, carrying both ovaries and testes. Depending on the species, eggs and sperm are either released into the water column for external fertilization or brooded inside the parent’s body until they develop into larvae. Despite being hermaphrodites, most species are self-sterile, requiring cross-fertilization with a neighbor to produce viable offspring.3Braz. J. Biol.. Gonadal proliferation and reproductive cycle of the exotic sea squirt Cnemidocarpa amphora
Sea squirts come in two broad lifestyle categories: solitary and colonial. Solitary species, like Ciona intestinalis and Halocynthia roretzi, live as individual animals. Colonial species, like Botryllus schlosseri (the star ascidian), reproduce not only sexually but also asexually through budding. A single founding individual can generate an entire colony of genetically identical zooids, each roughly 2 millimeters long, arranged in star-shaped clusters around shared excurrent siphons. Every zooid in the colony has its own brain, heart, branchial sac, and digestive system, but they are all connected by a vascular network that allows blood cells to circulate throughout the entire colony.4PubMed Central. Contributions from both the brain and the vascular network guide behavior in the colonial tunicate Botryllus schlosseri
That shared vascular network does more than just shuttle nutrients. The epithelial cells lining the blood vessels are electrically excitable and generate action potentials, both spontaneously and in response to touch. They are linked by gap junctions, essentially tiny channels connecting neighboring cells, which means electrical signals can propagate across the colony. This gives colonial sea squirts a form of whole-colony coordination that is partly nervous and partly vascular, a system unlike anything seen in vertebrates.4PubMed Central. Contributions from both the brain and the vascular network guide behavior in the colonial tunicate Botryllus schlosseri
Whole-Body Regeneration from a Blood Vessel Fragment
Colonial sea squirts in the genus Botrylloides possess one of the most extreme regenerative abilities known among chordates. A tiny fragment of a peripheral blood vessel, containing just a small number of blood cells and no intact organs, can regenerate into a complete, functional organism with all three embryonic tissue layers represented. The process appears to rely on totipotent stem cells circulating in the blood, cells capable of giving rise to every tissue type in the body.5PubMed. Whole-body protochordate regeneration from totipotent blood cells
This whole-body regeneration is not instantaneous. The blood vessel fragment first forms a small mass of cells, then gradually organizes into recognizable structures: a siphon, a branchial sac, a beating heart. When multiple regeneration sites develop simultaneously within the same fragment, they compete with each other, with typically one dominant bud outgrowing the others. Researchers studying Botrylloides leachi have identified shared features of this regeneration across several botryllid species, including the systemic induction process and the role of circulating multipotent stem cells.6PubMed Central. From fragment to form: whole-body regeneration in a model urochordate
For biologists interested in regenerative medicine, this is tantalizing. Here is an animal in the same phylum as humans, Chordata, that can rebuild its entire body from a scrap of tissue. Understanding how these stem cells maintain their totipotency, and why vertebrates lost the ability to do something comparable, is an active area of research.
Recognizing Self from Non-Self
When two colonies of Botryllus schlosseri grow until their edges meet, one of two things happens: they either fuse into a single chimeric colony, sharing blood freely, or they reject each other, with tissue at the contact zone dying and forming a visible barrier. This natural transplantation reaction is controlled by a single highly polymorphic genetic locus, the Botryllus histocompatibility factor (BHF), which is the only gene known to predict fusion or rejection outcomes with complete accuracy.7PubMed. Intra-colony divergence and global allele sharing reflect purifying selection and recombination at the Botryllus histocompatibility factor locus
The parallel with vertebrate immune systems is striking. Vertebrates use the major histocompatibility complex (MHC) to distinguish self from non-self tissue, which is why organ transplants require careful matching between donor and recipient. Sea squirts achieve something functionally analogous with a single locus rather than the large multi-gene MHC complex. The system is ancient: the tunicate version of allorecognition evolved independently but shares the same logic of polymorphic self-recognition that vertebrate immunity uses.8PubMed Central. Botryllus schlosseri Allorecognition: Tackling the Enigma
Why would sessile colonial animals need such a precise system? Fusion with a genetically different colony carries risks. A colony that fuses with a competitor might find its own stem cells outcompeted by the other colony’s cells, a kind of parasitism at the cellular level. Rejection protects the genetic integrity of the colony. The high polymorphism at the BHF locus, meaning there are many different versions of the gene in the population, ensures that random encounters between unrelated colonies almost always result in rejection rather than fusion.
Vanadium Accumulators
Several families of ascidians do something chemically bizarre: they concentrate vanadium, a transition metal, in specialized blood cells at levels vastly exceeding what is found in seawater. The champion accumulator is Ascidia gemmata, whose blood cells contain vanadium at a concentration roughly ten million times that of the surrounding ocean.9Coordination Chemistry Reviews. Vanadium accumulation in ascidians: A system overview
What the vanadium is actually for remains one of the more persistent mysteries in marine biology. Proposed roles include defense against predators (vanadium compounds taste terrible and are mildly toxic), antimicrobial protection, and involvement in the synthesis of the cellulose tunic. None of these explanations has been fully confirmed. The vanadium is stored in a highly reduced chemical state inside the blood cells, kept in an acidic environment that prevents it from reacting. The fact that not all ascidian families accumulate vanadium, and that some accumulate other metals like iron or niobium instead, suggests this is an evolved specialization rather than a passive quirk of filter feeding.
A Window into Vertebrate Origins
Sea squirts hold an outsized place in developmental biology and evolutionary genetics precisely because of their phylogenetic position. As part of the sister group to vertebrates, they offer a window into what the common ancestor of all vertebrates might have looked like. The species Ciona intestinalis (recently split taxonomically but still widely referenced by that name) has become a workhorse model organism thanks to a fortunate set of practical advantages: translucent embryos that develop quickly, a compact genome, and strong molecular and genetic toolkits.10PubMed Central. Genetic and Genomic Toolbox of the Chordate Ciona intestinalis
One area where Ciona has proved especially useful is the study of the notochord, the defining structure of chordates. In vertebrate embryos, the notochord appears early and then is largely replaced by the vertebral column, making it hard to study in isolation. In Ciona, the notochord is simpler, with fewer cells, and its formation can be tracked from the very first cell divisions. Researchers have used this system to identify many of the genes and regulatory sequences that control notochord development, building a gene regulatory network that can be compared with what vertebrates use to form the same structure.11PubMed Central. The notochord gene regulatory network in chordate evolution: Conservation and divergence from Ciona to vertebrates
The evolutionary lineage extends deep into the fossil record. A mid-Cambrian fossil tunicate, Megasiphon thylakos, discovered in the famous Marjum Formation of Utah, shows that the fundamental ascidian body plan, including the barrel shape and paired siphons, was already in place shortly after the Cambrian Explosion, over 500 million years ago.12PubMed Central. A mid-Cambrian tunicate and the deep origin of the ascidiacean body plan That makes tunicates one of the oldest recognizable animal body plans still represented by living species today.
Pharmaceutical Compounds and Symbiotic Chemistry
Sea squirts and their microbial partners produce a striking array of bioactive chemical compounds, some of which have entered clinical development as cancer drugs. The didemnins, cyclic peptides originally isolated from colonial ascidians in the genus Trididemnum, were among the first marine natural products to reach clinical trials for cancer. Didemnin B showed strong antitumor activity through mechanisms including inhibition of protein synthesis, induction of programmed cell death, and disruption of the cell cycle. A derivative called plitidepsin (also known as Aplidin) has been developed with improved effectiveness and reduced side effects.13PubMed. Didemnins as marine-derived anticancer agents: mechanistic insights and clinical potential Plitidepsin also gained attention during the early 2020s for potential antiviral activity, though its primary development track remains oncology.
Even edible sea squirts have shown pharmacological promise. Extracts from Halocynthia roretzi, the sea pineapple commonly eaten in Korea and Japan, have demonstrated activity against liver cancer cells in laboratory studies and showed a synergistic effect when combined with the chemotherapy drug doxorubicin, allowing the dose of doxorubicin to be halved while maintaining effectiveness. The active ingredients were identified as a mixture of fatty amides.14PubMed Central. Aqueous Extract of Sea Squirt (Halocynthia roretzi) with Potent Activity against Human Cancer Cells Acts Synergistically with Doxorubicin These are laboratory findings, not clinical treatments, but they illustrate why marine biologists keep returning to ascidians as a source of novel chemistry.
Much of this chemical richness comes not from the sea squirts themselves but from symbiotic microorganisms living inside them. Certain tropical colonial ascidians harbor dense populations of the cyanobacterium Prochloron didemni, which lives inside the tunic and provides photosynthetic products to its host. The relationship is so tight that some host species cannot survive without Prochloron. Beyond basic nutrition, the cyanobacterium synthesizes secondary metabolites of pharmaceutical interest and profoundly influences the lipid composition of the host animal, including producing sterols and an unusual lipid with potential as a biofuel feedstock.15PubMed Central. Complex microbiome underlying secondary and primary metabolism in the tunicate-Prochloron symbiosis
Invasive Fouling and Ecological Impact
For all their scientific value, some sea squirts are serious pests. Several species have become invasive in harbors, aquaculture facilities, and coastal ecosystems worldwide. Adults are filter feeders of suspended particles and can thrive in many marine habitats, particularly rocky shores and artificial hard surfaces, as long as salinities stay above about 25 parts per thousand.16ResearchGate / Journal of Experimental Marine Biology and Ecology. Invasive sea squirts: A growing global problem
Their short-lived, non-feeding larvae would seem to limit natural dispersal, and in an undisturbed setting, sea squirt populations do tend to stay local. The problem is human-assisted transport. Sea squirts hitchhike on ship hulls, in ballast water, and on aquaculture equipment, giving them access to coastlines thousands of miles from their native range. Once established, fast-growing colonial species like Didemnum vexillum can smother mussel lines, oyster beds, and native benthic communities. The economic costs to shellfish aquaculture have been substantial in regions like the northeastern United States, eastern Canada, and New Zealand. Management options are limited: scraping fouled surfaces, using antifouling coatings, and monitoring ports for new arrivals are the main strategies, but eradication of an established population has proved extremely difficult.
Solitary invasive species cause problems too, though their growth rate is slower. Species like Styela clava and Ciona robusta can dominate fouling communities on harbor structures, competing with native invertebrates for food and space. The traits that make sea squirts successful invaders are the same traits that make them successful in general: prolific reproduction, tolerance of a wide range of temperatures and food conditions, and a tough tunic that few predators bother with.