There are thousands of marine invertebrate animals in the ocean, from small to large, and in all shapes, sizes and colors. Some of these, like the shrimps and snails, are well known to us, but many invertebrate groups are not familiar to most people. One of these invertebrate groups—the tunicates—should be of special interest, however, because they are our closest invertebrate relatives.
About 3,000 tunicate species are found in salt water habitats throughout the world. Although tunicates are invertebrates (animals without backbones) found in the subphylum Tunicata (sometimes called Urochordata), they are part of the Phylum Chordata, which also includes animals with backbones, like us. That makes us distant cousins.
The most common tunicates are sometimes called sea squirts because when touched or alarmed by a sudden movement, their muscles contract and the water in the animal shoots out. They are sessile after their larval stage, meaning that they remain attached to a hard substrate, such as dead coral, boat docks, rocks or mollusk shells, all of their adult lives. The name “tunicate” comes from their outer covering, called the tunic, that protects the animal from predators, like sea stars, snails and fish. Unlike the sessile sea squirts, other kinds of tunicates float in the water their entire lives. The salps and pyrosomes are mostly transparent tunicates that look a bit like jellyfish floating freely—some pyrosomes have be known to reach 60 feet (18 m) in length. Much smaller but still visible to the naked eye are the larvaceans—tiny tadpole like creatures that live inside a small house that they build and regularly replace.
What unites these diverse groups and makes them our relatives? All animals in the Phylum Chordata have a notochord, a flexible backbone like structure, at some point in their lives.
Sea squirts have a notochord only in the larval stage which they use to swim and find an ideal place to attach—one that is bathed in particle-rich waters, since like all tunicates they are filter feeders and rely on water currents for food and nutrients. Once a good location is found, the larva attaches with a suction-like structure and metamorphosis begins. The notochord shrinks and gets absorbed into the body as the animal changes into an adult, and the tunic forms as the transformation occurs. The animal will then spend its days feeding on tiny particles from the water, primarily bacteria.
There are two types of sea squirts: solitary and colonial. The solitary animals live separately all of their lives inside of their tunics. Each has two siphons—the oral siphon that receives the nutrient rich current and the atrial siphon that excretes the waste. Colonial species share a common tunic and sometimes also share the atrial siphon. Colonies of sea squirts are formed as a result of budding—when the larva settles and changes into the adult form, it then splits (or buds) to produce new individuals, called zooids. Colonies can be a few centimeters to several yards wide depending on food availability and predation.
Sea squirts don’t look much like us as adults on the outside, but they have a digestive system similar to ours—with an esophagus, stomach, intestines and a rectum. But there are plenty of other differences. Unique to the benthic tunicates is a heart that reverses its beat periodically. It’s still a mystery to researchers why the tunicate heart will circulate blood through the heart in one direction and then switch to the opposite direction, or if the ability gives them some sort of advantage.
On land, we don’t encounter sea squirts that often, although they are increasingly eaten by some Mediterranean, Asian and South American countries. Not only is the soft body inside of the tunic eaten, but the tunic itself can be pickled and enjoyed later. Compounds from several tunicate species could be useful in medical treatments for diseases ranging from cancer to asthma. Tunicates act as ocean purifiers, since they consume bacteria, and they can send a message that heavy metals are present in ecosystems where they are found, since they absorb metals like zinc and vanadium. Because they like to attach to hard surfaces, sea squirts are often found on the underside of boats, or inside motors, where they can wreak havoc on equipment, and some have become invasive species after being transported from their native ranges. Their relatives the pyrosomes, also called sea pickles, sometimes wash up in large numbers on the shore and are known for their bioluminescence.
Like with many a large family, most of us don’t know about these distant relatives found in the ocean, but that doesn’t mean they aren’t worth keeping an eye on.
FAQs
Although tunicates are invertebrates (animals without backbones) found in the subphylum Tunicata (sometimes called Urochordata), they are part of the Phylum Chordata, which also includes animals with backbones, like us. That makes us distant cousins.
Do tunicates have spines? ›
(a.k.a. tunicates or ascidians)
Sea squirts get their nickname from their tendency to "squirt" out water when they are removed from their watery home. And while they may look like rubbery blobs, they are actually very advanced animals--close to humans on an evolutionary scale. That's because they have a spine.
What makes tunicates different from all other chordates? ›
Tunicates have a notochord only during their larval stage. The notochord supports the whole tail of the larva but just part of its body. This arrangement gives the subphylum its name, Urochordata (Greek root words uro meaning tail and chord meaning string).
What do tunicates lack? ›
Tunicates lack the kidney-like metanephridial organs typical of deuterostomes. Most have no excretory structures, but rely on the diffusion of ammonia across their tissues to rid themselves of nitrogenous waste, though some have a simple excretory system.
What is the major difference between invertebrates and vertebrates? ›
Invertebrates are animals without spines, while vertebrates have a spine. Invertebrates are sometimes (mistakenly) thought of as primitive because of their lack of developed organs.
Do tunicates reproduce asexually? ›
Tunicates encompass a large group of marine filter-feeding animals and more than half of them are able to reproduce asexually by a particular form of nonembryonic development (NED) generally called budding.
What are some interesting facts about tunicates? ›
Cool facts
Predatory tunicates are simultaneous hermaphrodites — each animal produces both eggs and sperm. If conditions are poor or there are no other tunicates nearby, each tunicate can reproduce by itself.
Which features are unique to the tunicates? ›
External features
The animal is covered with a thick tunic, which consists of some cells, blood vessels, and a secretion of a variety of proteins and carbohydrates, including cellulose, which, although abundant in plants, is unusual in animals.
Are tunicates alive? ›
Tunicate, commonly called a sea-squirt, is a primitive marine animal that spends most of its life attached to docks, rocks, or the undersides of boats. A tunicate is built like a barrel. The name “tunicate” comes from the firm, but flexible body covering, called a tunic.
Do tunicates have a brain? ›
Interestingly, these filter-feeding animals form star-shaped networks called colonies that share blood with each other through a system of connected blood vessels. However, 'every [adult tunicate] is a filter-feeding individual with a brain and nervous system, heart and digestive system', explains Thompson.
Tunicates eat plankton and live by drawing seawater through their bodies.
What is the major difference between tunicates and humans? ›
Tunicates can be quite amorphous in shape and slimy to the touch. Tunicates are animals that bridge the gap between invertebrates (do not have a backbone) and vertebrates (have a backbone). Humans are vertebrates; we have a spinal cord encased in a hard, protective vertebral column.
Which feature is missing from tunicates? ›
Although tunicates are classified as chordates, the adults do not have a notochord, a dorsal hollow nerve cord, or a post-anal tail, although they do have pharyngeal slits and an endostyle.
Are tunicates edible? ›
Tunicates (Ascidiacea) are primarily consumed in Asia, Chile and the Mediterranean, where product is sourced from both the wild and, in the case of the Halocynthia and Styela species in high demand, from cultured populations. All edible species are solitary stolidobranchs.
Which four features do humans share with tunicates? ›
Short Answer. During embryonic development, humans share several features with tunicates and lancelets, which are invertebrate chordates. These shared features include the presence of a notochord, a dorsal hollow nerve cord, pharyngeal slits or clefts, and a post-anal tail.
Do tunicates have organs? ›
A tunicate tadpole larva contains several chordate features, such as the notochord, dorsal nerve cord, and tail. These features are lost, however, as the larva metamorphoses into the adult form. The tunicate larva has special organs of sense and attachment, which it uses to find and occupy a suitable habitat.
Do tunicates have a mouth? ›
During the larval stage, the tunicates does not have a mouth and doesn't feed. After several days, the larval will attach to a surface using anterior adhesive papillae. As the larva metamorphoses into an adult, the larval tail is resorbed, providing food to the adult tunicate.
Do tunicates have an exoskeleton? ›
Tunicates are the only animals able to produce cellulose. They use this structural polysaccharide to build an exoskeleton called a tunic.
