Characterization of the Digestive Tract of Greenlip Abalone, Haliotis Laevigata Donovan.I. Morphology and Histology

Australasian abalone such as the greenlip abalone, Haliotis laevigata, prefer a diet of red algae (Rhodophyta); whereas, abalone from elsewhere more commonly prefer brown algae (Phaeophyta). Because of this feeding preference, the structure of the digestive tract of H. laevigata was investigated using histological and scanning electron microscopy (SEM) techniques. The digestive tract of both starved and fed adult H. laevigata revealed the presence of ciliated, mucus, and secretory cells throughout the digestive tract. The esophagus contained secretory, ciliated, and large mucous cells, with fragmentation spherules also present. The crop extended from the esophagus to the stomach. It was surrounded by thin muscularis and consisted mainly of secretory cells, although some phagocytes were present. The stomach possessed mainly secretory cells, although some ciliated cells, mucous cells, and phagocytes were present. The style sac differed from the stomach, having more ciliated cells. In intestinal regions I to III, the epithelium was shorter than in previous regions. Few cilia were present on the ridges, although many were observed in the gutters. Intestinal regions IV to V contained more mucous cells than intestine III, and more bacteria were observed associated with the fecal string than in other regions. The low incidence of bacterial association with the gut epithelium was attributed to the occurrence and number of mucous cells, common throughout the digestive tract. Spherical bodies present in the lumen are believed to be fragmentation spherules involved in waste removal and enzyme release. Starved abalone contained fewer mucous cells in the esophagus, had less pronounced staining reactions in the stomach, contained large amounts of granular inclusions in the style sac, and had fewer phagocytes in the intestines.