ber die gehirne der amphipoden und isopoden

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.

No sight, no smell? - Brain anatomy of two amphipod crustaceans with different lifestyles

The brain anatomy of Niphargus puteanus and Orchestia cavimana, two amphipod species with different lifestyles, has been studied using a variety of recent techniques. The general aspects of the brain anatomy of both species correspond to those of other malacostracans. However, both species lack hemiellipsoid bodies. Furthermore, related to their lifestyle certain differences have been observed. The aquatic subterranean species N. puteanus lacks eye structures, the optic nerve, and the two outer optic neuropils lamina and medulla. Only partial remains of the lobula have been detected. In contrast to this, the central complex in the protocerebrum and the olfactory glomeruli in the deutocerebrum are well differentiated. The terrestrial species Orchestia cavimana shows a reduced first antenna, the absence of olfactory neuropils in the deutocerebrum, and a reduction of the olfactory globular tract. The characteristics in defining the hemiellipsoid bodies are critically discussed. Contradictions about presence or absence of this neuropil are due to different conceptualizations. A comparison with other crustaceans that live in dark environments reveal similar patterns of optic system reduction, but to different degrees following a centripetal pattern. Retaining the olfactory system seems a general problem of terrestrialization in crustaceans with the notable exception of terrestrial hermit crabs.

Olfactory projection neuron pathways in two species of marine Isopoda (Peracarida, Malacostraca, Crustacea)

The neuroanatomy of the olfactory pathway has been intensely studied in many representatives of Malacostraca. Nevertheless, the knowledge about bilateral olfactory integration pathways is mainly based on Decapoda. Here, we investigated the olfactory projection neuron pathway of two marine isopod species, Saduria entomon and Idotea emarginata, by lipophilic dye injections into the olfactory neuropil. We show that both arms of the olfactory globular tract form a chiasm in the center of the brain, as known from several other crustaceans. Furthermore, the olfactory projection neurons innervate both the medulla terminalis and the hemiellipsoid body of the ipsi- and the contralateral hemisphere. Both protocerebral neuropils are innervated to a comparable extent. This is reminiscent of the situation in the basal decapod taxon Dendrobranchiata. Thus, we propose that an innervation by the olfactory globular tract of both the medulla terminalis and the hemiellipsoid body is characteristic of the decapod ground pattern, but also of the ground pattern of Caridoida.

Transition from marine to terrestrial ecologies: changes in olfactory and tritocerebral neuropils in land-living isopods

In addition to the ancestors of insects, representatives of five lineages of crustaceans have colonized land. Whereas insects have evolved sensilla that are specialized to allow the detection of airborne odors and have evolved olfactory sensory neurons that recognize specific airborne ligands, there is so far little evidence for aerial olfaction in terrestrial crustaceans. Here we ask the question whether terrestrial Isopoda have evolved the neuronal substrate for the problem of detecting far-field airborne chemicals. We show that conquest of land of Isopoda has been accompanied by a radical diminution of their first antennae and a concomitant loss of their deutocerebral olfactory lobes and olfactory computational networks. In terrestrial isopods, but not their marine cousins, tritocerebral neuropils serving the second antenna have evolved radical modifications. These include a complete loss of the malacostracan pattern of somatotopic representation, the evolution in some species of amorphous lobes and in others lobes equipped with microglomeruli, and yet in others the evolution of partitioned neuropils that suggest modality-specific segregation of second antenna inputs. Evidence suggests that Isopoda have evolved, and are in the process of evolving, several novel solutions to chemical perception on land and in air.

Demonstration of substances immunologically related to the identified arthropod neuropeptides AKH/RPCH in the CNS of several invertebrate species

The occurrence of adipokinetic hormone (AKH) and red pigment concentrating hormone (RPCH)-like neuropeptides in the central nervous system of different invertebrate species other than insects was investigated immunocytochemically with polyclonal antisera to N- and C-terminal regions of the AKH molecule. Substances reacting with the C-terminal specific antiserum code 241 were present in neurons of the pond snail, Lymnaea stagnalis, the sowbug Porcellio scaber, the centiped Lithobius forficatus and the crayfish Astacus leptodactylus. Substances revealed by the N-terminal specific antiserum code 433 were demonstrated in the latter two species. The distribution of immunoreactive substances in neuropile areas of several ganglia suggests that these substances act as neurotransmitter/neuromodulator. Their presence in neurohemal organs such as the periphery of the pedal and visceral nerves in the pond snail and in the sinus gland of the crayfish suggests a neurohormonal role in these species.

The hyperglycemic neuropeptide of the terrestrial isopod, Porcellio dilatatus. II. Immunocytochemical demonstration in neurosecretory structures of the nervous system

By use of an rabbit antiserum raised against the crustacean hyperglycemic hormone (CHH) of the shore crab, Carcinus maenas, CHH-producing perikarya were detected in the brain of the isopod, Porcellio dilatatus, by the double-antibody immunofluorescence technique. The course of the secretory axons to the neurohemal organ, the sinus gland, could be traced. The sinus gland also exhibited strong fluorescence. The reaction was very specific, no other structures gave appreciable immunofluorescence. There are two CHH cells in each hemisphere of the brain, located in the medio-rostral part of the protocerebrum. This area appears to correspond to the medulla terminalis of decapods. The cells are aldehyde fuchsin-positive and have been previously described as beta 1 cells. The immunofluorescence could be inhibited by preabsorption of the antiserum with pure Carcinus-CHH.

Ultrastructure of the sinus gland and lateral cephalic nerve plexus in the isopod Ligia oceanica (Crustacea Oniscoidea)

Isopod crustaceans have two distinct cephalic neurohemal organs: the sinus gland (SG) and lateral cephalic nerve plexus (LCNP). The present study of Ligia oceanica was designed to ascertain the ultrastructure, during the moulting cycle, of the terminals constituting the SG and LCNP, both of which store and release neurosecretory material, and to trace these terminals to their probable origin in neurosecretory perikarya. The SG was observed to contain four types of terminals (I, II, III, and IV) assigned, on the basis of the appearance of their neurosecretory granules, to four types of neurosecretory cells in the protocerebrum (beta 1, beta 2, B1, and Bu). When the same morphological criteria were applied to the LCNP, two types of terminals were found--III' and IV'. Type III' was thought to originate in the Bp plexus cells and in the B2 cells of the suboesophageal ganglion. The origins of Type IV' terminals were believed to be the Bu and Bm cells of this ganglion. Release from both the GS and LCNP occurred by exocytosis. The discussion attempts to relate the ultrastructural variations observed in the SG and LCNP with existing data on the neuroendocrine regulation of the moult. Such regulation involves the two antagonistic hormones (moult-inhibiting and moult-accelerating) which determine the circulating ecdysteroid level. It is also suggested that the plexus cells are the site of synthesis of a factor controlling the release of the exuviation factor.