Ontogeny of gut morphology in the white shrimp Penaeus setiferus (Decapoda, Penaeidae)

Ontogeny of the gut in Penaeus setiferus was investigated by reconstruction of serial sections examined by light microscopy. Development of the gut into the adult form is protracted over several weeks beyond metamorphosis in steps that may be directly related to the unique postlarval life history of Penaeus. The gastric mill is lacking in larval stages of P. setiferus. In protozoeal stages Z₁ -Z₃ , the pyloric ampullae are blind sacs that do not communicate with the midgut. The gland filter first appears in mysis stage M₂ . The gastric mill in early postlarval (PL) stages consists of poorly chitinized lobes with flexible setae. By PL₂₁ the ossicles of the gastric mill are rigid and setae are replaced by spine-like denticles, but even by PL₃₅ the gastric mill is neither as massive nor heavily chitinized as in adults. During the mysis stages and early PL stages, the hepatopancreas communicates freely with both the foregut and the midgut trunk. By PL₃₅ the hepatopancreatic ducts are essentially isolated from the remainder of the midgut by foregut ossicles. The midgut in Z₁ consists of two pairs of simple caeca and the midgut trunk. During larval growth, each of the lateral midgut caeca develops into a number of lobes. After metamorphosis these lobes begin to ramify into small-diameter tubules, and by PL₃₅ have completely ramified into the hepatopancreas of adults. From M₁ to PL₄ , the anterior midgut caeca decrease in absolute size and become a single anterior diverticulum. The posterior midgut diverticulum first appears in PL₂₁ as a simple sac and thereafter increases in size and complexity.