Comparative structure of the larval apical sensory organ in gastropods and hypotheses about function and developmental evolution

Abstract

Summary Larval gastropods have a distinctive sensory structure located at their anterior end. This apical sensory organ typically consists of neuronal somata clustered around a medial neuropil region (the apical ganglion) and cells giving rise to two or three long tufts of cilia. The neuronal nature of the tuft cells has not been firmly established. Putative sensory neurons within the apical ganglion have a dendrite-like extension and one type has an invaginated pocket filled with cilia. Another type of putative sensory neuron shows serotonin-like immunoreactivity. Serotonin-like antigenicity may also be expressed by non-sensory neurons within the apical ganglion and is invariably expressed by neurites within the neuropil, and by peripheral neurites extending toward the peripheral, prototrochal ciliary band of the velum. I review evidence suggesting that the apical ganglion is a sensory-motor ganglion that coordinates activity of velar prototrochal cilia during the free-living larval stage and during encapsulated developmental stages. Recent evidence also implicates the apical sensory organ in the reception or integration of sensory cues that initiate metamorphosis. Finally, I map two traits of the larval sensory organ on a phylogenetic hypothesis for relationships between major gastropod clades to infer patterns of evolutionary change for this highly conserved larval structure.