Structure and Function of the Holdfast Organ and Lappets of Alaria marcianae (La Rue, 1917) (Trematoda: Diplostomatidae)

Abstract

Adult Alaria marcianae (La Rue, 1917) are attached to the host mucosa by the forebody; the lappets and holdfast organ serve only a secondary role. There is some breakdown of host tissue in contact with the holdfast organ but more occurs in the area of the lappets. Adults apparently feed primarily on this material although some tissue may be ingested directly by the oral sucker. From histochemical staining tests, nonspecific esterases, RNA, protein, and complex mucopolysaccharides are present in the forebody and holdfast organ gland cells; acid phosphatase is present only in the latter cells. It is suggested that certain holdfast organ cells and the forebody gland cells secrete enzymes for extracorporeal digestion, with the latter cells being of more importance. The view that the holdfast organ has been a deciding factor in strigeid host-specificity because of its involvement in extracorporeal digestion is questioned. Adult strigeid trematodes exhibit a relatively high degree of specificity to their hosts, which is not due entirely to an ecological segregation of intermediate hosts (Dubois, 1938, 1953, 1957). These flukes possess a unique structure called the holdfast organ (adhesive organ, organe tribocytique). Dubois (1957) postulated that this structure has been of major importance in determining their specificity because of its role in an unusual mode of feeding (enzyme secretion for extracorporeal digestion). Dubois' view on the role of the holdfast organ in feeding was based on histological studies of the parasites in situ (Szidat, 1929; Baer, 1933). More recently, studies involving histochemical staining techniques, in vitro methods, and electron microscopy have been made to elucidate the function of this structure (Lee, 1962; Ohman, 1965, 1966a, b; Erasmus and Ohman, 1963, 1965; Bogitsh, 1966a, b; Erasmus, 1967, 1968, 1969a, b, c, 1970). These studies have revealed differences among the strigeids on the extent of the holdfast organ's involvement in extracorporeal digestion and also in attachment. Additionally, they have indicated the importance of the lappets and specialized gland cells in extracorporeal digestion and attach-