Figure 1.

Ciona robusta as a model organism for studying gut-microbial interactions. Ciona is a solitary invertebrate chordate that typically grows in close proximity on suitable substrates. Embryonic development results in a larval stage that attaches to a substrate and undergoes metamorphosis to achieve the adult phenotype. (A) At stage 4 of metamorphosis, translucent juveniles open their siphons and begin to filter seawater, where they initiate feeding; the gut is first exposed to and colonized by microorganisms. (B) The stage 8 juvenile has completed metamorphosis and exhibits the anatomical structures of a young adult. (C) Field-harvested full adult, with digestive tract emphasized by dotted line; water flow (first entering the pharynx) depicted by arrows. (D) Graphical (linear) representation of Ciona gut includes the pharynx, esophagus, stomach, and intestine. Hematoxylin/eosin staining of the stomach (left) and intestine (right) is shown, highlighting the folded organization of the stomach epithelium and the smooth layer of the intestinal epithelium. Ph, pharynx; es, esophagus; st, stomach; int, intestine; solid arrow, oral siphon; dotted arrow, atrial siphon; arrows direction depicts water flow. Asterisk, gut lumen. Scale bars: (A), 100 µm; (B), 500 µm; (D) 100 µm.