How do mandibles sense? The sensory apparatus of larval mandibles in Palaemon elegans Rathke, 1837 (Decapoda, Palaemonidae).

The mandibles of decapod zoea-I larvae are robustly built masticating mouthparts equipped with several processes and spines. Superficial examination of these sturdy, inflexible structures can suggest that they are lacking sensory receptors. However, detailed TEM analysis of their ultrastructure revealed up to 11 sensillar cell clusters on the gnathal edges of the mandibles of the zoea-I in Palaemon elegans Rathke, 1837. Based on ultrastructural criteria we distinguish 7 types of sensilla: mechanoreceptors, chemoreceptors and mechano- and chemoreceptors. One sensory unit located at the base of the 'lacinia mobilis' exhibits the typical features of a crustacean mechanosensitive sensillum with an external seta and corresponding ultrastructure. Another unit shows features indicating bimodal contact chemosensitivity. A third one is similar to known olfactory chemoreceptors. Using the concept of modality-specific structures we analyse the structure and functional morphology of each sensillum, and give a comprehensive overview of the sensory abilities of zoea mandibles. We take a closer look at the ultrastructure of the 'lacinia mobilis', providing further features to trace its evolutionary history in Decapoda, and thus contributing to a better understanding of malacostracan phylogeny.