Ultrastructure and functional organization of mouthpart sensory setae of the spiny lobster Panulirus argus: New features of putative mechanoreceptors

In comparison with other decapods, the Caribbean spiny lobster Panulirus argus has little diversity in the external morphology of the setae on the mouth apparatus. In mouthpart areas that frequently touch food items only two types of setae can be distinguished: simple setae and cuspidate setae. Simple setae are by far more numerous. The ultrastructural data presented here show that both types of seta are bimodal, in that they both contain mechano- and chemosensory cells as indicated by morphological features. The morphological features divide the sensory cells into three types: type 1, which has a mechanosensory appearance; type 2, which has a chemo-sensory appearance; and type 3, which is believed to be a mechanoreceptor due to desmosomal connections to a scolopale. All three cell types were found in all examined setae. In an earlier study the simple setae were found to contain two types of mechanosensors: bend-sensitive cells and displacement-sensitive cells. The morphological arrangement of the outer dendritic segment described in the present study cannot explain this division. Instead, it is suggested that the difference in sensitivity is caused by a differential arrangement of their stretch-sensitive ion channels. This hypothesis also provides an explanation for the earlier observation that only bend cells respond to changes in osmolarity.