Abstract

The Coenobitidae (Decapoda, Anomura, Paguroidea) is a taxon of hermit crabs that includes two genera with a fully terrestrial life style as adults. Previous studies have shown that Coenobitidae have evolved a sense of spatial odor localization that is behaviorally highly relevant. Here, we examined the central olfactory pathway of these animals by analyzing central projections of the antennular nerve of Coenobita clypeatus, combining backfilling of the nerve with dextran-coupled dye, Golgi impregnations and three-dimensional reconstruction of the primary olfactory center, the antennular lobe. The principal pattern of putative olfactory sensory afferents in C. clypeatus is in many aspects similar to what have been established for aquatic decapod crustaceans, such as the spiny lobster Panulirus argus. However, there are also obvious differences that may, or may not represent adaptations related to a terrestrial lifestyle. In C. clypeatus, the antennular lobe dominates the deutocerebrum, having more than one thousand allantoid-shaped subunits. We observed two distinct patterns of sensory neuron innervation: putative olfactory afferents from the aesthetascs either supply the cap/subcap region of the subunits or they extend through its full depth. Our data also demonstrate that any one sensory axon can supply input to several subunits. Putative chemosensory (non-aesthetasc) and mechanosensory axons represent a different pathway and innervate the lateral and median antennular neuropils. Hence, we suggest that the chemosensory input in C. clypeatus might be represented via a dual pathway: aesthetascs target the antennular lobe, and bimodal sensilla target the lateral antennular neuropil and median antennular neuropil. The present data is compared to related findings in other decapod crustaceans.

Highlights

  • Besides Isopoda (Harzsch et al, 2011; Kenning and Harzsch, 2013), certain decapod crustaceans have succeeded in meeting the physiological challenges of a terrestrial life style including the need for olfactory sensory systems to function in air rather than in water

  • Lateral and Median Antennular Neuropils (LAN and MAN) Our observations suggest that axons of putative mechanosensory and non-aesthetasc chemosensory neurons innervate the paired lateral and unpaired median neuropils (LAN and MAN, Figures 1A,C, 2, 6) through the antennular nerve branches A1Nv2 and A1Nv3

  • Similarities of C. clypeatus and its remote marine relative, P. argus include the division of the antennular nerve into three bundles: one possibly carries olfactory information from the aesthetascs to the antennular lobe, the others relaying chemical and mechanosensory information from bimodal sensilla to the lateral and median antennular neuropils

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Summary

Introduction

Besides Isopoda (Harzsch et al, 2011; Kenning and Harzsch, 2013), certain decapod crustaceans have succeeded in meeting the physiological challenges of a terrestrial life style (review Hansson et al, 2011) including the need for olfactory sensory systems to function in air rather than in water. Prominent amongst land crustaceans are members of the Coenobitidae (Decapoda, Anomura, Paguroidea), a taxon of hermit crabs that includes two genera with fully terrestrial life styles as adults, Coenobita and Birgus (Greenaway, 1999, 2003; McLaughlin et al, 2007; Drew et al, 2010). Using electroantennogram recordings (EAG) it was possible to establish the response spectrum of olfactory sensory neurons in Coenobita clypeatus (Krång et al, 2012) and Birgus latro (Stensmyr et al, 2005) to various chemical stimuli such as number of simple organic acids, amines, aldehydes. In addition to the behavioral, physiological and genetic aspects, other studies have analyzed the structure of the peripheral and central olfactory pathway in Coenobitidae (Harzsch and Hansson, 2008; Krieger et al, 2010; Brown and Wolff, 2012; Polanska et al, 2012; Wolff et al, 2012; Tuchina et al, 2014)

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