While on its odyssey, the snail encounters a tunicate or
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.
ANIMATION of the snail's odyssey © Thomas Carefoot 2020
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
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
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 this story.