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

Abstract

The development of the digestive system, specifically in relation to the midgut gland (hepatopancreas). in early life history stages of the American lobster Homarus americanus is examined in the present study by means of two separate but complementary approaches. A histological study on the light microscope level details the morphological changes which take ·place among the different cell types which form the midgut gland. Included in this portion of the study are scanning electron micrographs of the midgut gland of stage IV larva. Secondly, a biochemical study correlates changes in the activities of digestive protease, lipase, and amylase with the observed changes in the cells of the midgut gland. A sensitive method for detection of crustacean lipase was developed for this study. In order to optimize the assay conditions for measurement of digestive enzyme activities, a series of control tests was performed to determine the effects of several physical and chemical factors. The developmental stages of the lobster which are examined in these studies include: well-advanced embryos at approximately three days prior to hatching, hatching prelarvae, newly hatched and intermolt stage I larvae, intermolt stage II, Ill, and IV larvae, and intermolt stage V and VI postlarvae. Particular attention is paid to specific transitional stages during development. These include: (1) the changeover from yolk metabolism by the embryos to the dependence on exogenous food by newly hatched larvae; (2) the alteration in body form which occurs at the molt from larval stage III to IV; and (3) the change in habitat from the plankton to the benthos which occurs at the molt from stage IV to V. Results of the present study demonstrate that the R-cells (resorptive cells) of the earliest developmental stages, embryo through larval stage III, do not show the characteristic morphology classically ascribed to this cell type. Presumably because young lobsters do not begin to store excess lipids derived from the diet until stage IV, the R-cells do not contain large numbers of lipid vacuoles. By stage VI, however, the i-cells achieve the classic appearance. A previously unreported function for R-cells, that of storage of lipid derived from yolk metabolism in the embryo, is described. F-cells (digestive enzyme synthesizing cells) are present in the midgut gland of embryos a few days prior to the hatch while B-cells (enzyme secreting cells) have developed by the time of hatching. Thus, morphologically the stage I lobster larva appears to possess digestive capabilities prior to ingestion of the first meal. Changes in digestive enzyme activities during early development correlate well with the morphological changes observed in the midgut gland. Activities of protease, lipase, and amylase are very low in the well-advanced embryo and increase slightly by the hatching stage. Enzyme activities more than double by the time the stage I larva attains intermolt, regardless of whether the larva is fed or fasted. Digestive enzyme activities increase further by the time the stage II larva reaches intermolt; in general there is no significant difference in the levels of enzyme activities measured in the older stages (II through V). The results of a more detailed time course examination of the levels of digestive enzyme activities in relation" to first feeding by the stage I larvae are presented. Although there is a trend for slightly increased protease and amylase activities as the stage I larvae get older. the lipase activity is constant. Lobster larvae normally hatch in late spring and early summer but larvae can be induced to hatch during the winter by maintaining the eggs at higher than ambient water " temperatures. There is no consistent difference in digestive enzyme activities measured for larvae which hatch during the summer compared to those which hatch during the winter. The influence of molt stage on the activity of digestive enzymes in wild caught stage IV larvae is also discussed. Only minor variations in lipase and amylase activity are detected during intermolt and premolt and in general there is no significant effect. Protease activity is significantly greater at Do. This work describes new findings on several aspects of digestion among early life history stages of a marine decapod crustacean and has added to our understanding of the functional morphology of the midgut gland during early development.