Worms With Armor: Tegumental Adaptations of Stomach Fluke, Dinurussp. (Trematoda: Hemiuridae)—Ultrastructural Characteristics

Abstract

The stomach is a hostile environment rarely utilized as a habitat by parasites, but stomach flukes are unique helminths that have adapted to survive in this acidic environment. Unlike the flatworms, stomach flukes have a cylindrical body like blood flukes. We hypothesized that, in addition to suckers, stomach flukes are equipped with structures to prevent them from being ripped loose by peristaltic movements and that they have secretory glands to resist the pH of the niche. In the present study, we collected Dinurus sp. (Trematoda: Hemiuridae) from dolphinfish Coryphaena hippurus to understand the tegumental structure and host–parasite interactions. We used ultrastructural and histochemical techniques to investigate the functions of the tegument. The prosoma (proper body) was equipped with densely packed frill‐like tegumental plications with rough and smooth lobes, while the ecsoma (tail) was without plications. Smooth‐lobed prosomal plications had pores (5 μm) that were secretory in function. Secretory granules were delivered from the underlying type‐2 tegumental cells through cytoplasmic connections. Electron‐dense granules coalesced within these cells and were secreted by holocrine method. They were also periodic acid‐Schiff (PAS) and Alcian blue positive. Similarly, PAS positive mucus cells were observed in the gastric pits of the host stomach. Rough‐lobed plications of the flukes enable them to cling onto the mucin. In Dinurus, a glycocalyx forms the outer limit of the tegument. Below the glycocalyx is a stream of cytoplasm devoid of mitochondria, suggesting the tegument is for protection and not for absorption. Under similar conditions, tegumental pores of the ecsoma secreted serine proteinase, which hydrolases gelatin but is incapable of hydrolysis in the presence of serine protease inhibitors. Although additional studies are needed to confirm these initial observations, our study demonstrated for the first time that tegumental plications of stomach fluke Dinurus sp. have mechanical and secretory, not absorptive, functions. Related helminths, blood flukes (schistosomes) are medically important digeneans that cause human diseases. Further, tegumental enzymes in the native form are too scarce for analysis in blood flukes. Unlike blood flukes, Dinurus are relatively large (few mm). Based on the year‐round availability, larger size, and nature of the tegument, we propose Dinurus sp. as an excellent study model for tegumental investigations, such as serine proteinase enzyme analysis.