In his Correspondence article (), DeForest Mellon, Jr argues that the type of seta in our report (Monteclaro et al., 2010) was erroneously identified. In response, we would like to make the following remarks.
Stimulation of an antennule that was either seta-less or without plumose setae was performed by shaving and cutting setae under the microscope. Cognizant of the fact that a few plumose setae are present in the distal half of the flagellum, we shaved and cut the remaining plumose setae after cutting the proximal half to remove a large proportion of procumbent and standing plumose setae. Examination of these flagella using SEM revealed the presence of medium simple setae and long simple setae on both flagella, approximately two to six procumbent plumose setae on each flagellum, and aesthetascs and associated setae on the lateral flagella. Fig. 1 shows recordings from a seta-less (i.e. all setae removed) medial flagellum during sinusoidal stimulation. Stimulation failed to elicit a response from the flagella, suggesting that the increase in spike discharges resulted from deflection of setal shaft.
Our work did not detail the physical and ultrastructural organization of putative mechanoreceptors. In identifying mechanoreceptive afferents in the crayfish antennules, we depended largely on published literature that reported mechanosensitivity of antennular setae (Chichibu et al., 1978a; Chichibu et al., 1978b; Humphrey and Mellon, 2007; Mellon and Humphrey, 2007; Mellon and Christison-Lagay, 2008). Although we did describe the medium simple setae to be the most dominant setal type present on the dorsal half of both lateral and medial flagella, we did not specifically identify this setal class as the receptor with neurons that responded to our sinusoidal stimulation. In fact, the same impression can be gathered in the papers of Mellon and Humphrey (Mellon and Humphrey, 2007) and Humphrey and Mellon (Humphrey and Mellon, 2007) on the hydrodynamic responses of neurons in the crayfish antennules to fluid flow, and their reference to simple setae as the most numerous setal type in the lateral flagellum (although our experience showed that this distinction belongs to the procumbent plumose setae). Similarly, these authors also stopped short of identifying medium simple setae as the actual receptor in the fluid dynamics test.
Response of mechanoreceptor neurons in a seta-less medial flagellum when stimulated sinusoidally at 10 Hz with water velocity 26.7 μm s–1 r.m.s. Horizontal line indicates duration of the stimulus.
We do esteem and appreciate the comments of Dr Mellon, who has worked a great deal on the subject of crayfish sensory transduction. Our recording technique, i.e. the use of suction electrodes, required us to remove the carapace on the proximal aspect of the flagellum and, consequently, the aggregation of large standing feathered setae that are present in the same area. The removal of these large setae would leave the relatively smaller-sized standing plumose setae on the other segments of the flagellum. Whether these smaller-sized standing plumose setae would respond similarly to the larger-sized counterparts has yet to be reported. In addition, a comparison of the mechanosensitivity of all three putative mechanoreceptor setae – medium simple setae, long simple setae and standing plumose setae – has yet to be shown. Notwithstanding the absence of the actual identity of specific neurons that responded to the mechanical stimulation, we feel that this does not affect any of the conclusions of our article, such as the general mechanosensitivity of both lateral and medial flagella, the relevance of the mechanoreceptors on the antennules during antennular depression, or our attempt to compare crustacean mechanosensitivity with that of the fish mechanosensory system.
