Tunicates, salps and pyrosomes (Subphylum Tunicata)
Tunicates are primitive marine chordates (the group of animals that includes the vertebrates such as fish, birds and mammals).
While tunicates do not have a backbone like vertebrates, they do have a notochord, which is equivalent to a backbone but is a simple cartilaginous rather than bony rod. In most tunicates the notochord is only present in the tadpole-like larval stage and is lost in the adult.
In fact, in an evolutionary sense vertebrates like us may be tunicate larvae that have never grown up, but have become sexually mature as larvae.
Many tunicates are brilliantly coloured. This is Polycarpa aurata, a common species in Australian tropical and subtropical waters. Photo by Nick Hobgood CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=5614889
There are five main groups (clades) of tunicates, three of which were previously classed together and called ascidians or sea squirts. Ascidians live their adult lives attached immovably to rocks and other hard surfaces. Some are solitary and may be quite large, while others are colonial, with many small individuals making up a colony.
The body of the adult is essentially a fragile to very tough sac with an inlet and outlet tube (siphon). Water is drawn in through the inlet siphon, filtered of microscopic organisms, and expelled through the outlet siphon. Sea squirts get their name from their habit, when exposed at low tide, of contracting strongly when disturbed, squirting the water held inside their bodies through the siphons.
The two non-ascidian groups of tunicates float freely in the world's oceans. One group comprises the salps, pyrosomes and doliolids, solitary or colonial tunicates that may be small or, in the case of some colonial forms, very large. As with the ascidians, individuals are sac-like with an inlet and outlet siphon through which they feed by filtering the water they swim through. The inlet and outlet siphons are arranged so that the individuals (or colonies) can swim slowly using jet propulsion. Many pyrosomes are spectacularly luminescent.
The second non-ascidian group are the appendicularians, small, tadpole-like tunicates that resemble the larval forms of other tunicates. Appendicularians, though small, are common and ubiquitous in the world's oceans where they feed on plankton which they concentrate using a complex, secreted bubble of protein and cellulose called a 'house'. Appendicularian 'houses' are discarded frequently when they become clogged or otherwise cease to work, and form a significant portion of the organic matter that gradually sinks from shallow waters to the ocean depths, taking carbon and other nutrients with it.
The ocean-going, colonial salps are remarkable creatures that can grow to more than 15 m long. Salps and their relatives play an important part in the cycling of carbon in the world's oceans. Photo: Oregon Department of Fish & Wildlife - 23_salpchain_frierson_odfw, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=47026390
More than 3,000 species of tunicates are currently known globally. Most of these are ascidians; salps, pyrosomes, doliolids and appendicularians together comprise only a few hundred species.
In Australia, nearly 800 species have been described. As with the global pattern, the vast majority of these (over 700 species) are ascidians. Around 20 species of appendicularians are known from Australian waters, along with nearly 30 species of salps, pyrosomes and doliolids. Taxonomists estimate that around 90% of Australian tunicates have been named.
Collections of tunicates are held in all Australian natural history museums, where they are studied by taxonomists working on marine invertebrates.