Male-specific structures in the olfactory system of mysids (Mysidacea; Crustacea)

First report of the order Mysida (Crustacea) in Antarctic marine ice caves, with description of a new species of Pseudomma and investigations on the taxonomy, morphology and life habits of Mysidetes species

SCUBA diving explorations of three islands off Dumont d’Urville Station at the coast of Adélie Land, East Antarctica, enabled the observation of marine ice caves. Sampling in this unusual habitat yielded a total of three species of Mysidae, altogether previously poorly known or unknown to science. Pseudomma kryotroglodytumsp. nov. is described, based on the structure of the antennal scale, telson and on cornea-like lateral portions set off against the main body of eyeplates. Mysidetes illigi is re-established at species level after almost a century in synonymy. Re-descriptions are provided for M. illigi and M. hanseni, based on types and ice cave materials. Keys to the Southern Ocean species of Pseudomma and to the world-wide species of Mysidetes are given.

Phylogenetic trees are provided for the genera Pseudomma and Mysidetes. 18S rDNA sequences of P. kryotroglodytum differ from GenBank sequences of other Pseudomma species. First sequence data are given for species of the genus Mysidetes: 18S differs between the two examined species and COI is quite diverse between and within species.

We found previously unknown, probably sensorial structures in these ice cave species: in P. kryotroglodytum, the basal segment of the antennula shows a pit-like depression with striated pad on the bottom and a median cyst, connected with the bottom of the eyeplate cleft. M. illigi shows a female homologue of the appendix masculina bearing a field of modified setae. Subsequent investigations demonstrated these structures also in species from other habitats.

The feeding apparatus and stomach contents of the three ice cave species point to brushing of small particles (detritus, microalgae) from available surfaces, such as sediment, rock and the ice surface. Differences in the feeding apparatus are very subtle between the two Mysidetes species. The high content of fat bodies in M. hanseni could help it to survive periods of starvation. The large storage volume of the foregut in P. kryotroglodytum points to the collection of food with low nutritional quality and could help to balance strongly fluctuating food availability.

Summer specimens of M. hanseni showed a bimodal frequency of developmental stages in the marsupium and bimodal size-frequency distribution of free-living stages. The females with younger brood (embryos) were, on average, larger and carried more marsupial young than those with older brood (nauplioid larvae). All examined incubating and spent females showed (almost) empty foreguts and empty ovarian tubes, suggesting possible semelparity and death following the release of young. The absence of juveniles and immature females from summer samples suggests that growth and accumulation of fat and yolk occur outside ice caves, while such caves could be used by fattened adults as shelter for brooding. A provisional interpretation proposes a biannual life cycle for M. hanseni, superimposed with shifted breeding schedules, the latter characterised by early breeding and late breeding females, probably in response to harsh physical and trophic conditions along the continental coast of Antarctica.

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.

Crustacean olfactory systems: A comparative review and a crustacean perspective on olfaction in insects

Malacostracan crustaceans display a large diversity of sizes, morphs and life styles. However, only a few representatives of decapod taxa have served as models for analyzing crustacean olfaction, such as crayfish and spiny lobsters. Crustaceans bear multiple parallel chemosensory pathways represented by different populations of unimodal chemosensory and bimodal chemo- and mechanosensory sensilla on the mouthparts, the walking limbs and primarily on their two pairs of antennae. Here, we focus on the olfactory pathway associated with the unimodal chemosensory sensilla on the first antennal pair, the aesthetascs. We explore the diverse arrangement of these sensilla across malacostracan taxa and point out evolutionary transformations which occurred in the central olfactory pathway. We discuss the evolution of chemoreceptor proteins, comparative aspects of active chemoreception and the temporal resolution of crustacean olfactory system. Viewing the evolution of crustacean brains in light of energetic constraints can help us understand their functional morphology and suggests that in various crustacean lineages, the brains were simplified convergently because of metabolic limitations. Comparing the wiring of afferents, interneurons and output neurons within the olfactory glomeruli suggests a deep homology of insect and crustacean olfactory systems. However, both taxa followed distinct lineages during the evolutionary elaboration of their olfactory systems. A comparison with insects suggests their olfactory systems ö especially that of the vinegar fly ö to be superb examples for "economy of design". Such a comparison also inspires new thoughts about olfactory coding and the functioning of malacostracan olfactory systems in general.

Brain architecture of the Pacific White Shrimp Penaeus vannamei Boone, 1931 (Malacostraca, Dendrobranchiata): correspondence of brain structure and sensory input?

Penaeus vannamei (Dendrobranchiata, Decapoda) is best known as the "Pacific White Shrimp" and is currently the most important crustacean in commercial aquaculture worldwide. Although the neuroanatomy of crustaceans has been well examined in representatives of reptant decapods ("ground-dwelling decapods"), there are only a few studies focusing on shrimps and prawns. In order to obtain insights into the architecture of the brain of P. vannamei, we use neuroanatomical methods including X-ray micro-computed tomography, 3D reconstruction and immunohistochemical staining combined with confocal laser-scanning microscopy and serial sectioning. The brain of P. vannamei exhibits all the prominent neuropils and tracts that characterize the ground pattern of decapod crustaceans. However, the size proportion of some neuropils is salient. The large lateral protocerebrum that comprises the visual neuropils as well as the hemiellipsoid body and medulla terminalis is remarkable. This observation corresponds with the large size of the compound eyes of these animals. In contrast, the remaining median part of the brain is relatively small. It is dominated by the paired antenna 2 neuropils, while the deutocerebral chemosensory lobes play a minor role. Our findings suggest that visual input from the compound eyes and mechanosensory input from the second pair of antennae are major sensory modalities, which this brain processes.

Dimorphic olfactory lobes in the arthropoda

Specialized olfactory lobe glomeruli relating to sexual or caste differences have been observed in at least five orders of insects, suggesting an early appearance of this trait in insect evolution. Dimorphism is not limited to nocturnal species, but occurs even in insects that are known to use vision for courtship. Other than a single description, there is no evidence for similar structures occurring in the Crustacea, suggesting that the evolution of dimorphic olfactory systems may typify terrestrial arthropods.

Brain architecture in the terrestrial hermit crab Coenobita clypeatus (Anomura, Coenobitidae), a crustacean with a good aerial sense of smell

Background

During the evolutionary radiation of Crustacea, several lineages in this taxon convergently succeeded in meeting the physiological challenges connected to establishing a fully terrestrial life style. These physiological adaptations include the need for sensory organs of terrestrial species to function in air rather than in water. Previous behavioral and neuroethological studies have provided solid evidence that the land hermit crabs (Coenobitidae, Anomura) are a group of crustaceans that have evolved a good sense of aerial olfaction during the conquest of land. We wanted to study the central olfactory processing areas in the brains of these organisms and to that end analyzed the brain of Coenobita clypeatus (Herbst, 1791; Anomura, Coenobitidae), a fully terrestrial tropical hermit crab, by immunohistochemistry against synaptic proteins, serotonin, FMRFamide-related peptides, and glutamine synthetase.

Results

The primary olfactory centers in this species dominate the brain and are composed of many elongate olfactory glomeruli. The secondary olfactory centers that receive an input from olfactory projection neurons are almost equally large as the olfactory lobes and are organized into parallel neuropil lamellae. The architecture of the optic neuropils and those areas associated with antenna two suggest that C. clypeatus has visual and mechanosensory skills that are comparable to those of marine Crustacea.

Conclusion

In parallel to previous behavioral findings of a good sense of aerial olfaction in C. clypeatus, our results indicate that in fact their central olfactory pathway is most prominent, indicating that olfaction is a major sensory modality that these brains process. Interestingly, the secondary olfactory neuropils of insects, the mushroom bodies, also display a layered structure (vertical and medial lobes), superficially similar to the lamellae in the secondary olfactory centers of C. clypeatus. More detailed analyses with additional markers will be necessary to explore the question if these similarities have evolved convergently with the establishment of superb aerial olfactory abilities or if this design goes back to a shared principle in the common ancestor of Crustacea and Hexapoda.

Glomerular organization in developing olfactory and accessory lobes of American lobsters: stabilization of numbers and increase in size after metamorphosis

Olfactory glomeruli are columnar and radially arranged at the periphery of the primary chemosensory areas, the olfactory lobes (OLs), in the American lobster Homarus americanus. The number of olfactory glomeruli reaches nearly 100/lobe in midembryonic life, increases rapidly during larval life, and stabilizes at about 200 in juvenile and adult lobsters. The accessory lobes (ALs), higher order integration areas, are composed of cortical columns and of spherical glomeruli. Two populations of spherical glomeruli are defined, the cortical glomeruli located at the bases of the columns, and the medullary glomeruli scattered throughout the ALs. Both cortical columns and spherical glomeruli are seen for the first time in the second larval stage. There are about 1000 cortical columns and 1700 glomeruli/AL in the postlarva and these numbers remain constant during the life of the lobster. In both OLs and ALs, it is the size of the interglomerular spaces and of the glomeruli themselves that increases. Therefore, the data suggest that in both OLs and ALs the glomeruli were already generated when the lobster metamorphoses (stage III to IV) and switches from a planktonic to a benthic existence, and that the new sensory neurons that are formed at each molt in the antennulae grow into existing olfactory glomeruli. Stability of the glomerular population in the primary olfactory centers, once the full complement of glomeruli is acquired, has also been reported in insects, fish, and mammals.

Neurogenesis in the developing crab brain: postembryonic generation of neurons persists beyond metamorphosis

A considerable amount of information is available about the structure and function of the central nervous system in adult crustaceans. However, little effort has been directed toward understanding embryonic and larval neurogenesis in these animals. In the present study we recorded neurogenesis in the brain of laboratory-reared larvae of the spider crab Hyas araneus. Proliferating cells were detected immunocytochemically after in vivo labeling with 5-bromo-2'-deoxyuridine. This method has already been used to study the proliferation of neuroblasts in the ventral nerve cord of spider crab larvae. In the brain, a set of mitotically highly active neuroblasts was found in newly hatched zoea 1 larvae. These neuroblasts are individually identifiable due to their position and therefore a schematic map of the cerebral neuroblasts could be established. The number of active neuroblasts is high from hatching throughout the molt to the zoea 2. This proliferative action then decreases dramatically and has ceased at the time of first metamorphosis toward the megalopa larva. However, many ganglion mother cells born by unequal division of neuroblasts then go through their final division throughout the subsequent megalopa stage. In the brain, all mitotic activity has ceased at the time of second metamorphosis with the exception of a cluster of labeled nuclei within the olfactory lobe cells. In this cluster, the generation of neurons persists beyond the second metamorphosis into the crab 1 stage. Meanwhile, the neuropil volume of the olfactory lobes increases 10-fold from hatching to the crab 1. These results are discussed with regard to reports on neuronal proliferation during adult life in insects and rodents.

Are the most numerous sensilla of terrestrial isopods hygroreceptors? ultrastructure of the dorsal tricorn sensilla of Porcellio scaber

The ultrastructure of the tricorn sensilla of the woodlouse Porcellio scaber was investigated in cryofixed and freeze-substituted, or chemically fixed specimens. The tricorn sensilla have a foramenized triangular-shaped outer hair and bear a poreless rod-like inner hair. The conical base of the inner hair is connected to the base of the outer hair by a complex cuticular structure. Each sensillum contains three sensory cells. The tip of one of the three dendrites contains a tubular body and is clamped between two bulges of the dendritic sheath. The two other dendrites protrude to the tip of the inner hair, flush against the cuticular wall. The microtubules in the ciliary segments are arranged in nine double tubuli that have neither osmiophilic cores nor arms. The ciliary rootlets are small. The inner segment of the largest dendrite wraps around the two smaller dendrites and one of seven enveloping cells in a mesaxon-like manner. Although this ultrastructure deviates considerably from most crustacean mechanosensitive sensilla, it nevertheless suggests a mechanosensitive function, at least for one of the sensory cells. In many aspects, the tricorn sensilla resemble the thermo-hygrosensilla of insects. However, our results suggest that the structural criteria for thermo-hygro-sensitivity used in insects cannot simply be applied to crustaceans.