The callynophore, a eucaridan/peracaridan sensory organ prevalent among the Amphipoda (Crustacea)

The crustacean model Parhyale hawaiensis

Arthropods are the most abundant and diverse animals on earth. Among them, pancrustaceans are an ancient and morphologically diverse group, comprising a wide range of aquatic and semi-aquatic crustaceans as well as the insects, which emerged from crustacean ancestors to colonize most terrestrial habitats. Within insects, Drosophila stands out as one of the most powerful animal models, making major contributions to our understanding of development, physiology and behavior. Given these attributes, crustaceans provide a fertile ground for exploring biological diversity through comparative studies. However, beyond insects, few crustaceans are developed sufficiently as experimental models to enable such studies. The marine amphipod Parhyale hawaiensis is currently the best established crustacean system, offering year-round accessibility to developmental stages, transgenic tools, genomic resources, and established genetics and imaging approaches. The Parhyale research community is small but diverse, investigating the evolution of development, regeneration, aspects of sensory biology, chronobiology, bioprocessing and ecotoxicology.

More than one way to smell ashore - Evolution of the olfactory pathway in terrestrial malacostracan crustaceans

Crustaceans provide a fascinating opportunity for studying adaptations to a terrestrial lifestyle because within this group, the conquest of land has occurred at least ten times convergently. The evolutionary transition from water to land demands various morphological and physiological adaptations of tissues and organs including the sensory and nervous system. In this review, we aim to compare the brain architecture between selected terrestrial and closely related marine representatives of the crustacean taxa Amphipoda, Isopoda, Brachyura, and Anomala with an emphasis on the elements of the olfactory pathway including receptor molecules. Our comparison of neuroanatomical structures between terrestrial members and their close aquatic relatives suggests that during the convergent evolution of terrestrial life-styles, the elements of the olfactory pathway were subject to different morphological transformations. In terrestrial anomalans (Coenobitidae), the elements of the primary olfactory pathway (antennules and olfactory lobes) are in general considerably enlarged whereas they are smaller in terrestrial brachyurans compared to their aquatic relatives. Studies on the repertoire of receptor molecules in Coenobitidae do not point to specific terrestrial adaptations but suggest that perireceptor events - processes in the receptor environment before the stimuli bind - may play an important role for aerial olfaction in this group. In terrestrial members of amphipods (Amphipoda: Talitridae) as well as of isopods (Isopoda: Oniscidea), however, the antennules and olfactory sensilla (aesthetascs) are largely reduced and miniaturized. Consequently, their primary olfactory processing centers are suggested to have been lost during the evolution of a life on land. Nevertheless, in terrestrial Peracarida, the (second) antennae as well as their associated tritocerebral processing structures are presumed to compensate for this loss or rather considerable reduction of the (deutocerebral) primary olfactory pathway. We conclude that after the evolutionary transition from water to land, it is not trivial for arthropods to establish aerial olfaction. If we consider insects as an ingroup of Crustacea, then the Coenobitidae and Insecta may be seen as the most successful crustacean representatives in this respect.

Comparative analysis of the antennae of three amphipod species with different lifestyles

Crustaceans detect chemical stimuli in the environment with aesthetasc sensilla, which are located on their 1st antennae. With the transition to other environments, chemoreception faces physical challenges. To provide a deeper understanding of the relation between the morphology of olfactory organs and different lifestyles, we studied the peripheral olfactory system of three amphipod species, the marine Gammarus salinus, the blind subterranean freshwater species Niphargus puteanus, and the terrestrial Cryptorchestia garbinii. We compared the 1st and 2nd antennae of these species with respect to length and presence of aesthetascs and other sensilla. The females of N. puteanus reveal the longest 1st antennae in relation to body size. G. salinus shows the largest aesthetascs and the same relative length of the 1st antennae as male N. puteanus. C. garbinii has very short 1st antennae and reduced (putative) aesthetascs. Our findings show that the compensation of vision loss by olfaction cannot be generally assumed in animals from dark environments. Furthermore, the behaviour of C. garbinii indicates a chemosensory ability, despite the reduction of the 1st antennae. A comparison with other terrestrial crustaceans suggests that the loss of the olfactory sense on the 1st antennae in C. garbinii might be compensated with chemoreception by the 2nd antennae.

The "amphi"-brains of amphipods: new insights from the neuroanatomy of Parhyale hawaiensis (Dana, 1853)

BACKGROUND

Over the last years, the amphipod crustacean Parhyale hawaiensis has developed into an attractive marine animal model for evolutionary developmental studies that offers several advantages over existing experimental organisms. It is easy to rear in laboratory conditions with embryos available year-round and amenable to numerous kinds of embryological and functional genetic manipulations. However, beyond these developmental and genetic analyses, research on the architecture of its nervous system is fragmentary. In order to provide a first neuroanatomical atlas of the brain, we investigated P. hawaiensis using immunohistochemical labelings combined with laser-scanning microscopy, X-ray microcomputed tomography, histological sectioning and 3D reconstructions.

RESULTS

As in most amphipod crustaceans, the brain is dorsally bent out of the body axis with downward oriented lateral hemispheres of the protocerebrum. It comprises almost all prominent neuropils that are part of the suggested ground pattern of malacostracan crustaceans (except the lobula plate and projection neuron tract neuropil). Beyond a general uniformity of these neuropils, the brain of P. hawaiensis is characterized by an elaborated central complex and a modified lamina (first order visual neuropil), which displays a chambered appearance. In the light of a recent analysis on photoreceptor projections in P. hawaiensis, the observed architecture of the lamina corresponds to specialized photoreceptor terminals. Furthermore, in contrast to previous descriptions of amphipod brains, we suggest the presence of a poorly differentiated hemiellipsoid body and an inner chiasm and critically discuss these aspects.

CONCLUSIONS

Despite a general uniformity of amphipod brains, there is also a certain degree of variability in architecture and size of different neuropils, reflecting various ecologies and life styles of different species. In contrast to other amphipods, the brain of P. hawaiensis does not display any striking modifications or bias towards processing one particular sensory modality. Thus, we conclude that this brain represents a common type of an amphipod brain. Considering various established protocols for analyzing and manipulating P. hawaiensis, this organism is a suitable model to gain deeper understanding of brain anatomy e.g. by using connectome approaches, and this study can serve as first solid basis for following studies.

The aesthetasc concept: structural variations of putative olfactory receptor cell complexes in Crustacea

The structure of the aesthetascs has been investigated in the prawn Macrobrachium rosenbergii (larvae and juveniles), the opossum shrimp Neomysis integer, the euphausid Meganyctiphanes, and in the water-fleas Daphnia magna and D. longispina. The aesthetascs, that are thought to represent olfactory receptors, exhibit a considerable structural variation, ranging from the well known aesthetascs of higher crustaceans (lobster, crab, crayfish) to the corresponding sensilla found in the water-fleas and the males of opossum shrimps. The two following morphological characteristics of the aesthetascs are thought to indicate an olfactory function: the shape of the cuticular hair that is long and essentially hose-shaped, and the thin, loosely arranged cuticle of at least the outer part of the cuticular hair. The presence of other structural elements such as sensory cells, cilia, and enveloping cells are vital for the olfactory function, but the development is variable, which makes their use in the morphological definition of aesthetascs problematic.