Morphology and life cycle of Retractocephalus aulacophorae Haldar et al., 1982 (Apicomplexa: Eugregarinida), revealed by modern microscopy

Abstract Background The genus Retractocephalus Haldar and Chakraborty, 1976 (Apicomplexa: Eugregarinida), known for its retractable globular epimerite, comprises twelve species, with Retractocephalus aulacophorae commonly infecting Aulacophora intermedia . This species was previously described solely using camera lucida drawings. Present study provides a detailed morphological redescription of the life cycle stages of R. aulacophorae using DIC microscopy and Scanning Electron Microscopy alongside of an assessment of its prevalence and seasonal variation. Results Trophozoites are narrowly elliptoid (40.7–397.4 μm), possessing a large globular epimerite and a broadly ovoid protomerite. Association is primarily biassociative and caudofrontal, with occasional multiple linear syzygies involving three gamonts, and epimerite retention in the primite were also observed. Gametocysts are orbicular (146.9–196.4 μm), and spores are dolioform (9.9 × 4.2 μm), released in cohesive chains. SEM reveals longitudinal, undulatory epicytic folds (3.5–4.5 per micron) on gamonts, forming a circular ring at the satellite’s protomerite during association, allowing a firm connection. Out of 235 host individuals, 161 were infected (prevalence 68.51%). Conclusions Retractocephalus aulacophorae is distinguished by diverse syzygy patterns, occasional epimerite retention, lower epicytic fold density, and a distinctive circular ring of elevated epicytic folds for secure attachment during association. A minor revision of the species diagnosis is proposed to include variations in syzygy and epimerite retention in the primite.