Changes in midgut gland morphology and digestive enzyme activities associated with development in early stages of the American lobster Homarus americanus

Abstract

Morphology of the midgut gland was examined in embryos, larvae, and postlarvae of Homarus americanus and correlated with changes in digestive protease, lipase, and amylase activities during development. A previously unreported function of R-cells, storage of lipid derived from yolk metabolism by embryos, is described. R-cells of the earlier stages, embryo through stage III larva, lack large numbers of lipid vacuoles presumably because these stages do not store excess dietary lipids. Lipid vacuoles increase in number during stage IV and by stage VI the R-cells resemble those of adults. F-cells are present a few days prior to the hatch while B-cells develop by the time of hatching. Digestive enzyme activities are low in the embryo and increase slightly by the hatching stage, correlating well with changes observed in the midgut gland cells. Enzyme activities double by intermolt of stage I, whether or not the larvae feed. Enzyme activities increase again by intermolt of stage II but in general there are no significant differences in enzyme activity among the older stages, II through V. This work describes new findings on several aspects of digestion among early life history stages of a marine decapod crustacean and has added to the author's understanding of the functional morphology of the midgut gland during early development