Ultrastructure and potential significance of cerebral light-refracting bodies of Stenostomum virginianum (Turbellaria, Catenulida)

Summary

Light-refracting bodies, possibly photoreceptors, occurring in the posterior lobes of the brain are considered characteristic for most species of catenulid turbellarians of the freshwater genus Stenostomum. In S. virginianum, each of two light-refracting bodies consists of a single cup-shaped granule (3–4 μm) situated in the perikaryon of a single nerve cell (the ‘clear vesicle’ of earlier papers). TEM reveals each granule as an enlarged and folded mitochondrion with a dense matrix inclusion of undetermined composition. Cristae are well-developed and there is a dense granular extramito-chondrial layer of uniform thickness (100 nm) along the posteromedial surface of the mitochondrion. The perikaryon is packed with ribosomes, β-glycogen granules and 60–100 nm dense-cored vesicles. A neurite extends from the perikaryon into the neuropile of the brain. Experimental data indicate an absence of phototaxis and photokinesis and an absence of ultrastructural modifications with light- and dark-adaptation. An ultrastructural comparison is made of the light-refracting bodies of S. virginianum with those of a second species. Hypotheses regarding the role of light-refracting bodies as photoperiodic receptors and/or specialized neurosecretory cells are advanced.