|dc.description.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
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.||en_US||