SCAPPATICCI, A.A.*; KASS-SIMON, G.: Nematocyst Discharge: The Response of Prey Capturing Desmonemes to Chemical and Mechanical Stimulation.

The effects of chemical and mechanical stimulation on the discharge of desmonemes were quantitatively studied in situ, in ablated tentacles and in hypostome-attached tentacles of Hydra. Feeding and locomotion in hydra depend upon the discharge of nematocysts. Nematocysts are specialized organelles which possess a prominent trigger hair, the cnidocil. There are three types of nematocysts, the stenoteles, which pierce and paralyze prey, desmonemes, which ensnare them, and isorhizas, used for locomotion. Cnidocils of desmonemes were stimulated, 4 and 24 hours after tentacle ablation, with piezoelectrically-driven probes vibrating at frequencies ranging between 2 and 80 Hz. The effect of a nematocyst sensitizing agent, mucin, was studied at 6 and 60 Hz. Without mucin, desmonemes discharged in a frequency-dependent manner in 24 hour ablated tentacles, and in 4 and 24 hour ablated hypostome-attached tentacles, with optimal discharge occurring between 15 and 35 Hz. The presence of the hypostome significantly depressed the response by over 30%. There was no frequency differentiation in 4 hour ablated tentacles. Desmoneme discharge was enhanced by the presence of mucin in a dose-dependent manner, with optimal discharge occuring at 1 X 10^-11M and complete inhibition occurring at 1 X 10^-5M. Mucin increased the number of desmonemes discharged at 60 Hz to equal the number discharged at 6 Hz without mucin. 6 Hz is the swimming frequency of laboratory prey. Striking the cnidocil with single contacts of increasingly greater force caused a significant increase in discharge in a dose-dependent manner. Our results suggest that the desmonemal cnidocil responds to the impulsive force of a mechanical stimulus whose threshold is lowered by the presence of mucin.