Ultrastructural characteristics of the infective juveniles of Steinernema spp. (Rhabditida: Steinernematidae) with reference to their motility and survival

The ultrastructure of infective juveniles of Steinernema feltiae, S. bibionis and S. glaseri was compared under SEM and TEM to obtain structural evidence for differences in their motility and survival abilities, both of which are crucial to infectivity under natural conditions. The arrangements of the labial/cephalic papillae and amphidial apertures of these nematodes were similar, however, the amphidial aperture with its connecting large nerve bundle was more conspicuous in S. feltiae. The lipid droplets were deposited deep in the cells of the lateral chord and intestine of which microvilli had degenerated. The arrangement of thick and thin myofilaments was clear in somatic muscle cells which always contained many glycogen granules and mitochondria with developed christiae. For all nematodes examined, the cuticle consisted of external cortical (ECL), internal cortical (ICL) and basal layers (BL); the median layer was not clearly differentiated from ICL. The three layers were also recognized in the ensheathing cuticle. The osmiophilic ECL and striated BL were developed to the greatest extent in S. feltiae which also showed the highest nictating activity and leaping behavior. These structural features may explain the differences in motility and survival ability in the infective juveniles of the three species of steinernematid nematodes.