A tunicate or sea squirt is a type of chordate.  Sea squirts are familiar to all SCUBA divers by the size and general conspicuousness of the subtidal forms, but may go un-noticed by casual beach-walkers because most of the intertidal forms are compound types and are flat, generally inconspicuous, and have a superficial resemblance to sponges.

NOTE "sea squirt" is a common name for tunicates/ascidians and is derived from their tendency to squirt seawater periodically from the main branchial siphon to back-flush sediments, other indigestible matter, and small animals from the filtering basket (about 10 times per hour in some species). Squirting is more commonly seen in the larger, solitary species

Styela montereyensis responds to a small amount of neutral red dye delivered to its inhalent siphon with a small exhalent squirt
Dawkins   2005   The Ancestor’s Tale.  Houghton Mifflin, Harcourt Publ

ANIMATION of the snail's odyssey © Thomas Carefoot 2024
map used by the snail in A SNAIL'S ODYSSEY

To navigate through the ODYSSEY:

  • Select a TOPIC from the menu at the top of the screen
  • OR: play the animation to the left
  • OR: follow the snail's ODYSSEY by CLICKING on any X-marked invertebrate on the map above

Phylum Chordata (lit. “string-like” G.) referring to the segmented backbone – a diagnostic feature of vertebrates).

Subphylum Tunicata (lit. “tailed chordate” G.)

Class Ascidiacea (lit. “bag-like” G.), including about 1250 worldwide species of tunicate1 or sea squirts.

APLOUSOBRANCHIA, including several families of colonial tunicates such as Clavelina huntsmani (light-bulb tunicate), Aplidium spp., Didemnum spp., and others

PHLEBOBRANCHIA, including several families of mostly solitary tunicates such as Ciona spp., Chelyosoma productum, Corella willmeriana, Ascidia spp., and others

STOLIDOBRANCHIA, including several families of colonial and solitary tunicates such as Botryllus schlosseri (colonial), Botrylloides spp. (colonial), Cnemidocarpa finmarkiensis (solitary), Metandrocarpa spp., Styela spp. (solitary), Boltenia villosa (solitary), Halocythia spp. (solitary), Pyura spp. (solitary), Molgula spp. (solitary), and others

Sea squirts possess the typically chordate features of notochord, dorsal tubular nerve cord, and pharyngeal gill slits, and so are our closest invertebrate relatives2.  It took scientists some time to realise this because these features are found only in the larva and are lost at metamorphosis.  In his wonderful book, The Ancestor’s Tale, the well-known evolutionary biologist Richard Dawkins reviews the theories relating to origin of chordates (animals with cartilaginous-like notochords or boney vertebrae).  One of these theories, credited to Walter Garstang3 is that a tadpole larva (or larvae) of a sea squirt underwent early onset of reproductive maturity, a process known as neoteny.  This new genetic line with its array of chordate features came to evolve into the vertebrate groups of fishes, reptiles, birds, and mammals.  The original genetic line carried on as sea squirts.  A second theory, credited to Charles Darwin, is that a pelagic tadpole larva-like ancestor of sea squirts split into two evolutionary lines.  One branch of its descendants stayed tadpole-like and evolved into fishes, while the other settled to the sea bottom and evolved into the present line of sea squirts, retaining ancestral adult characteristics only in its tadpole larval stage. Regardless of the way it happened, Dawkins estimates the time at about 565 million years ago or, in his words, at the approximate time of our 275th million-greats-grandparent. 

NOTE1 in the ODYSSEY the terms ascidian, tunicate, and sea squirt are used more or less interchangeably, for no other reason than convenience

NOTE2 our ancestral relationship with ascidians is nicely summed up in the anonymous ballad: The ancestor remote of Man, says Darwin, is th’ Ascidian and, even more pithily, in a statement credited to the 19thC biologist, W.A. Herdman: "For Man was once a leather bottl’"

NOTE3  among many other contributions during his productive tenure as Leeds University Professor of Zoology, Garstang fashioned many of his evolutionary theories relating to marine-invertebrate larvae into humorous verses, published in 1962 as Larval forms with other zoological verses Basil Blackwell, Oxford.  Although the book contains nothing directly relating to tunicates, one delightful poem about the evolution of torsion in gastropods is given in its entirety at MOLLUSCA>ABALONES & RELATIVES>PREDATORS & DEFENSES>LARVAL DEFENSES/TORSION

Dawkins   2005   The Ancestor’s Tale. Houghton Mifflin Harcourt Publ., 688pp.
Fig. 1.  Solitary tunicate Ciona intestinalis. The hermaphroditic condition of tunicates is evident in the photograph, with the sperm duct (white) parallelling the egg duct (orange), leading to the exhalent siphon

In a somewhat more practical approach, a world consortium of molecular biologists has collaborated to produce a preliminary listing of protein-coding genes in the solitary tunicate Ciona intestinalis (Fig.1). The resulting genome contains about 16,000 genes, similar in size to other invertebrates, but only half that of an average vertebrate (and equal to only 5% the size of the human genome).  Genes in Ciona common to those in vertebrates are ones relating to cell signaling and development, while uncommon ones include, most notably, those relating to cellulose metabolism similar to those in bacteria and fungi.  Cellulose is a principal component of tunicin that forms the tough, outer covering in tunicates.  The tadpole larva of Ciona is small (only 2500 cells), develops to the adult relatively rapidly and, of course, features prominently in several evolutionary theories relating to the origin of chordates and vertebrates.  It should lend itself well to future genetics studies.  As expected, based on earlier identification of  iodine-containing substances in the tunicate endostyle, genes for synthesis of thyroid hormones are identified in Ciona, as are regulatory genes for development of the heart similar to ones known for vertebrates.  The publication is a major tour de force in the field of genetics, development, and evolutionary origins of the vertebrates, and holds much promise for elucidating the role of tunicates in the story.

Dehal et al.   2002   Science 298: 2157