SEM study of hooks in the Acanthocephala with emphasis on structural-functional relationships

Abstract

The retractable proboscis of acanthocephalans is equipped with hooks for attachment to the intestine of the definitive host. Throughout their evolutionary history, acanthocephalans have developed a variety of ways to maximize their anchoring to host gut and to avoid dislodgement. Hooks vary in their size and shape along the longitudinal axis of the proboscis, texture,  structure, and hardness as well as in their contribution to the absorption of nutrients. Hooks also vary in their chemical composition, especially calcium, phosphorus, and sulfur, contributing to their hardness. Hook roots are paramount in anchoring them to the cuticular and subcuticular layers of the proboscis. Roots vary in size and shape and are often simple and directed posteriorly but often have anterior manubrial or may be vestigial or absent especially posteriorly. The core layer of roots is usually continuous with that of the hook. Hooks often, but not always, maintain a similar pattern in families. Because of the inconsistencies and inadequacies in the description of hooks, especially in line drawings, in various groups of acanthocephalans, we have decided to provide the largest assortment of morphological and anatomical variabilities among the many species that we have studied over the years. We are, thus, reporting the SEM of hooks of 30 selected species of acanthocephalans in 13 families in an attempt to elucidate patterns and trends characteristic of acanthocephalan families.