ABSTRACT Marine gastrotrichs inhabit sediments with limited available light, yet evidence suggests that photoreceptor organs are present throughout the phylum Gastrotricha. Here, we confirm this proposition with ultrastructural descriptions of photoreceptive organs in two species of Xenotrichulidae (Order Chaetonotida) that lack pigmented ocelli. Xenotrichula intermedia possesses a pair of dorsal, extracerebral cells that fit the description of photoreceptor cells in other gastrotrichs. Both sensory cells are bipolar with an apical dendritic end of microvilli‐like projections that protrude into a thin, cup‐like extension of a sheath cell. Neither the sheath cell nor surrounding cells possess shielding pigments. Both sensory cell bodies contain highly granular cytoplasm with a nucleus, inflated rough endoplasmic reticulum, membrane‐bound vesicles, and several microtubule‐like structures. The photosensory cells extend into the cerebral ganglion, apparently bypassing a cellular capsule around the brain. The presumed photoreceptive organs of D. tesselata were incompletely documented, but possess extracerebral, dendritic projections encapsulated by non‐pigmented sheath cells in identical positions to those of X. intermedia . The present data support the proposition that chaetonotid gastrotrichs possess photoreceptive organs with a similar morphology to those of Order Macrodasyida, thus confirming that all gastrotrichs, even those without pigmented eyespots, may possess photoreceptive organs.
Araújo et al. (2026) studied this question.