Abstract

Mitochondria are central to cellular energy metabolism, and their internal ultrastructure, particularly the organization of cristae, is closely linked to the cell's metabolic demands. This study undertakes a detailed morphometric analysis of the internal ultrastructure of mitochondria within the muscle tissue of the horsehair worm, *Gordionus alpestris* (Nematomorpha). The objective was to quantitatively assess the mitochondrial compartment in these specialized muscle fibers, which are characterized by high energy requirements for locomotion. Using transmission electron microscopy and advanced stereological methods, key parameters such as the volume density of the inner membrane, the surface density of cristae, and the cristae-to-volume ratio were determined. The findings reveal a highly developed inner membrane system with a significantly high surface density of cristae, suggesting an exceptionally high capacity for oxidative phosphorylation. This structural organization is characteristic of cells with intense energy-consuming processes, providing new insights into the bioenergetic adaptations of the Nematomorpha musculature. The results contribute to a deeper understanding of the relationship between mitochondrial morphology and cellular function in this understudied phylum, with implications for comparative cell biology and muscle physiology.