Hair bundle mechanoreceptors detect vibrations          produced by swimming prey
Thousands of hair bundle mechanoreceptors cover tentacles of certain sea anemones including Haliplanella luciae and the model sea anemone Nematostella vectensis.  The image at right was taken of Haliplanella using scanning electron microscopy.

Anemone hair bundle mechanoreceptors detect movements of suitable prey swimming in the vicinity of the tentacles.  As you might expect, small swimming animals including planktonic crustaceans move rhythmically as they swim (see the recordings  below taken of individual brine shrimp swimming in small drops of seawater, taken from Watson and Hessinger 1989).  Accordingly, disturbances in the water are vibrations.  Vibrations are ideal stimuli for hair bundle mechanoreceptors.
The surprising finding was that anemones do not readily discharge nematocysts into test probes vibrating at frequencies produced by calmly swimming suitable prey.....unless such movements are accompanied by chemical stimuli such as N-acetyleated sugars including NANA.  Exposure to NANA induces the hair bundles to significantly eleongate while also causing the stereocilia (finger-like projections from the cell surface that constitute the 'hairs' that form the bundle) to untwist into a more elongated array.  Thus, activated chemoreceptors change the shape of mechanoreceptors.

The result of such morphological changes to the hair bundles is to shift responsiveness to lower frequencies that overlap those produced by swimming prey.
In seawater alone, anemones discharge nematocysts maximally into test probes vibrating at frequencies higher than 50 Hz (panel A in the figure shown at the right hand side of this page).  However, in the presence of NANA (panel B), anemones discharge nematocysts maximally into test probes vibrating at frequencies lower tha 50 Hz.  These are frequencies that overlap the swimming movements of calmly swimming prey.

In other words, sea anemones employ a sensory system in which chemoreceptors modulate the responsiveness of hair bundle mechanoreceptors.  If suitable chemical and mechanical stimuli are detected, the anemone is sensitized to discharge the maximum number of nematocysts into it.
To learn more about the structure of hair bundle mechanoreceptors in anemones, please follow this link...