Ontogenetic Changes in the Digestive Tract of the Larval Gizzard Shad, Dorosoma cepedianum

Abstract

The digestive anatomy of young planktivorous gizzard shad (Dorosoma cepedianum) is described, and development of the digestive system is traced through the adult condition by which time the fish has adopted a bottom-feeding habit. The mouth changes from a supra-terminal to a sub-terminal position to facilitate benthic feeding. The pharynx is modified for straining microscopic particles from the water. Pharyngeal organs possessing both goblet cells and taste buds develop, and esophageal and gastric glands appear. The gizzard acquires a thick muscular wall which facilitates its presumed function as a triturating organ in the adult. Intestinal length increases greatly, presumably in response to adoption of a predominantly herbivorous diet. Larkin (1956) stated that most young fish feed on plankton and later utilize other food items. Gizzard shad, Dorosoma cepedianum (LeSueur), follow this trend by feeding on plankton early in life then adopting a bottom-feeding habit as adults (Baker & Schmitz, 1971). With respect to morphological considerations, Lagler & Kraatz (1944), Miller (1964), and Schmitz & Baker (1969) studied the digestive anatomy of the species in the genus Dorosoma. Bodola (1966), describing D. cepedianum larval development, outlined certain ontogenetic changes at the gross anatomical level in the digestive tract. No investigation to date has dealt with organogenesis of the gizzard shad gut. Presumably, anatomical changes are concomitant with alterations in food preference. I describe such changes in the gross and microscopic gut anatomy at significant stages of larval development. MATERIALS AND METHODS All specimens were killed and fixed in Bouin's fluid (original formula) because this fixative was found to cause minimal tissue shrinkage. Larval specimens were identified according to the criteria of Kersh (1970). Larvae of nine size classes ranging from 16-56 mm total length were measured and dissected under a binocular microscope. For histological studies, the body posterior to the anus was removed from I am grateful to Dr. E. H. Schmitz, Department of Zoology, University of Arkansas, for his assistance throughout this study. I am further indebted to Dr. J. M. Walker, Department of Zoology, University of Arkansas, for his critical review of the manuscript, and to Mr. Alfred Houser and Dr. Larry Aggus, Bureau of Sport Fisheries and Wildlife, United States Fish and Wildlife Service, for providing the specimens used in this investigation. 2 Publication costs, in part, are being met by a grant from the Spencer-Tolles Fund of the American Microscopical Society. TRANS. AM. MICROSC. Soc., 101(3): 262-275. 1982. ( Copyright, 1982, by the American Microscopical Society, Inc. This content downloaded from 40.77.167.94 on Mon, 23 Jan 2017 18:26:53 UTC All use subject to http://about.jstor.org/terms VOL. 101, NO. 3, JULY 1982 smaller specimens. It was necessary to remove most of the body wall from larger specimens, or even to dissect out the entire digestive tract to insure thorough infiltration. All specimens were dehydrated in a graded series of tertiary butyl alcohol (TBA) to absolute TBA, remaining in each change for 2 h, transferred to a series of five changes of paraffin maintained at 56.5?C for a total of 10 h, embedded, and serially sectioned at 8 /tm. Slide preparation followed the procedure described by Schmitz & Baker (1969) in which Haupt's adhesive was used to affix sections to slides; paraffin ribbons were floated and expanded in 0.5% potassium dichromate solution on a warming table maintained at 45?C. Staining involved the use of azocarmine G as a nuclear stain in conjunction with aniline blue, light green, and orange G as counter-stains. Sections were mounted in Permount according to the method described by Schmitz (1967). Histological studies were conducted with an Olympus FHT trinocular research compound microscope equipped with bright-field transmitted light and phase-contrast optics. All drawings were made using the grid method.