Learn About Sea Squirts

While on its odyssey, the snail encounters a tunicate or sea squirt, which 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

The response of Styela montereyensis after some
neutral red dye is delivered to the inhalent siphon.

ANIMATION of snail meeting the TUNICATE
© 2010 Thomas Carefoot

To learn about west-coast TUNICATES: select a topic from the Ascidiacea > sea squirt menu at the top of the page

OR: play the ANIMATION of the snail meeting the TUNICATE

OR, if you want to see other animations: follow the snail on its ODYSSEY by CLICKING on any X-marked invertebrate on the map

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 tunicates 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

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

Sea squirts possess the typically chordate features of notochord, dorsal tubular nerve cord, and pharyngeal gill slits, and so are our closest invertebrate relatives.  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 Garstang, 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.  Dawkins 2005 The Ancestor’s Tale.  Houghton Mifflin Harcourt Publ., 688pp. 

NOTE  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’"

NOTE  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 76pp. 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 LEARNABOUT ABALONES & RELATIVES: PREDATORS & DEFENSES: LARVAL DEFENSES/TORSION

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.  The resulting genome contains about 16,000 genes, photograph of tunicate Ciona intestinalissimilar 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 this story.  Dehal et al. 2002 Science 298: 2157. Photograph courtesy the authors.

Solitary tunicate Ciona intestinalis. The hermaphroditic condition of tunicates is evident in the photograph, with the sperm duct (white) parallelling the egg duct (orange)