Hair cell mechanoreception in the jellyfish Aglantha digitale

The jellyfish Aglantha digitale is equipped with clusters of hair cells on the velum and on the tentacle bases. The cells have a central non-motile cilium surrounded by a collar of microvilli. The microvilli are graded in length, from long on one side to short on the other, giving the collar a marked polarity. The hair cells are set in specific orientations in all regions where they occur, as shown by this polarity. Small mechanical displacements of the velum or tentacles in the vicinity of the hair cells evoke bursts of potentials which can be recorded extracellularly from the outer nerve ring. Intracellular recordings from the ring giant axon, which is implicated in escape locomotion, show patterns of facilitating EPSPs correlated with these potentials. Hair cell input may be important in feeding as well as in locomotory behaviour. Three classes of hair cells were studied : those of the tentacle bases (T cells) and those of the velum (C cells and F cells). Direct stimulation of the tentacle bases shows that the T cells respond to slight mechanical displacements. To distinguish the functions of the C cells and F cells, which lie close together, a laser was used to ablate selectively one or other kind. It was found that the C cells respond to small mechanical displacements of the velum but no response could be assigned to the F cells.