A Sequential Ranking System for Developmental Stages of an Acanthocephalan, Leptorhynchoides thecatus, in Its Intermediate Host, Hyalella azteca

Abstract

Based on morphological features, 22 stages were defined in the development of Leptorhyn- choides thecatus in its intermediate host, Hyalella azteca. The ranking system allows close comparison of development and growth within or among amphipods, and is amenable to nonparametric statistics. Certain aspects of L. thecatus development are redescribed. Apical nuclei arise from the central, nuclear mass, rather than a cortical nucleus. Lateral, nucleated bands, previously thought to be somatic muscle anlagen, are actually the median longitudinal canals. The body cavity develops in a single step as muscles of the body wall and the median longitudinal canals separate from the proboscis receptacle and genital strand. Developmental anomalies (short, conical proboscis, misaligned hooks, and ventrally shortened trunk) were observed and are thought to be induced by high temperatures. Many factors, such as temperature, rearing conditions, and intraspecific competition, may affect size or developmental rates of parasites. These effects can be identified and character- ized only if developmentally equivalent in- dividuals can be recognized and ranked ac- cording to their developmental condition without regard to their chronological age. Pre- vious descriptions of acanthocephalan devel- opmental stages (Meyer, 1931; Kates, 1943; Van Cleave, 1947; Butterworth, 1969) recog- nized no more than nine ranks and were based on acquisition of major structural fea- tures (e.g., body cavity). They were not suf- ficiently sensitive to changes within these structures for close comparisons. Develop- mental descriptions are usually based on time-specific morphology. Individual varia- tion cannot be quantified, and effects of ex- perimental manipulation cannot be simply or precisely described. A morphologically based, sequential ranking system is desirable. DeGiusti (1949) provided a well-illustrated, comprehensive account of day-to-day changes during development of Leptorhynchoides thecatus in its intermediate host, Hyalella az- teca. Based on this description and examina- tion of many additional specimens, we devel- oped a ranking system which recognizes 22 stages. Each stage is defined by morphologi- cal features, and is assigned a rank number.