Abstract
Adaptations to a terrestrial lifestyle occurred convergently multiple times during the evolution of the arthropods. This holds also true for the “true crabs” (Brachyura), a taxon that includes several lineages that invaded land independently. During an evolutionary transition from sea to land, animals have to develop a variety of physiological and anatomical adaptations to a terrestrial life style related to respiration, reproduction, development, circulation, ion and water balance. In addition, sensory systems that function in air instead of in water are essential for an animal’s life on land. Besides vision and mechanosensory systems, on land, the chemical senses have to be modified substantially in comparison to their function in water. Among arthropods, insects are the most successful ones to evolve aerial olfaction. Various aspects of terrestrial adaptation have also been analyzed in those crustacean lineages that evolved terrestrial representatives including the taxa Anomala, Brachyura, Amphipoda, and Isopoda. We are interested in how the chemical senses of terrestrial crustaceans are modified to function in air. Therefore, in this study, we analyzed the brains and more specifically the structure of the olfactory system of representatives of brachyuran crabs that display different degrees of terrestriality, from exclusively marine to mainly terrestrial. The methods we used included immunohistochemistry, detection of autofluorescence- and confocal microscopy, as well as three-dimensional reconstruction and morphometry. Our comparative approach shows that both the peripheral and central olfactory pathways are reduced in terrestrial members in comparison to their marine relatives, suggesting a limited function of their olfactory system on land. We conclude that for arthropod lineages that invaded land, evolving aerial olfaction is no trivial task.
Highlights
Land-living crustaceans are fascinating animals that adapted during relatively short evolutionary time period to a number of highly diverse terrestrial habitats in which they have become highly successful, and in some cases the predominant life forms (Hansson et al, 2011)
The degree of terrestrial adaptation in crustaceans has been categorized into five classes ranging from T1 to T5 depending on the degree of independence from immersion in water and the animal’s need to access water for reproduction (Hartnoll, 1988; Greenaway, 1999; Powers & Bliss, 1983; but see Schubart et al, 2000 for an alternative classification)
In favor of a consistent terminology, here, we suggest avoiding the term “optic neuropils” (Hanström, 1925; Sombke & Harzsch, 2015) as well as “optic lobes” (Kenyon, 1896)
Summary
Land-living crustaceans are fascinating animals that adapted during relatively short evolutionary time period to a number of highly diverse terrestrial habitats in which they have become highly successful, and in some cases the predominant life forms (Hansson et al, 2011). The degree of terrestrial adaptation in crustaceans has been categorized into five classes ranging from T1 to T5 depending on the degree of independence from immersion in water and the animal’s need to access water for reproduction (Hartnoll, 1988; Greenaway, 1999; Powers & Bliss, 1983; but see Schubart et al, 2000 for an alternative classification) In this traditional classification, several brachyuran taxa have been ranked within the two highest grades of terrestrial adaptation (e.g. Gecacinidae, and some representatives of the Sesarmidae, Potamidae, Gecarcinidae, Potamonautidae, Pseudothelphusidae, and Trichodactylidae) whereas many amphibious freshwater forms and supra-littoral species are ranked in less terrestrialized categories
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