Wiring a periscope--ocelli, retinula axons, visual neuropils and the ancestrality of sea spiders.

The Pycnogonida or sea spiders are cryptic, eight-legged arthropods with four median ocelli in a 'periscope' or eye tubercle. In older attempts at reconstructing phylogeny they were Arthropoda incertae sedis, but recent molecular trees placed them as the sister group either to all other euchelicerates or even to all euarthropods. Thus, pycnogonids are among the oldest extant arthropods and hold a key position for the understanding of arthropod evolution. This has stimulated studies of new sets of characters conductive to cladistic analyses, e.g. of the chelifores and of the hox gene expression pattern. In contrast knowledge of the architecture of the visual system is cursory. A few studies have analysed the ocelli and the uncommon "pseudoinverted" retinula cells. Moreover, analyses of visual neuropils are still at the stage of Hanström's early comprehensive works. We have therefore used various techniques to analyse the visual fibre pathways and the structure of their interrelated neuropils in several species. We found that pycnogonid ocelli are innervated to first and second visual neuropils in close vicinity to an unpaired midline neuropil, i.e. possibly the arcuate body, in a way very similar to ancestral euarthropods like Euperipatoides rowelli (Onychophora) and Limulus polyphemus (Xiphosura). This supports the ancestrality of pycnogonids and sheds light on what eyes in the pycnogonid ground plan might have 'looked' like. Recently it was suggested that arthropod eyes originated from simple ocelli similar to larval eyes. Hence, pycnogonid eyes would be one of the early offshoots among the wealth of more sophisticated arthropod eyes

Terenski pokusi na zajednici dekapodnih rakova s vlasuljama, Anemonia viridis (Forsskål, 1775)

We have undertaken for the first time removal and introduction field experiments with both symbiotic (taken from anemones) and anemone-naïve decapods (taken from somewhere else) and tested defence reactions of putative host anemones. Our findings indicate that decapods go through an individual habituation phase before they become symbionts. Once habituated, they are protected from neighbouring anemones as well.Po prvi puta smo proveli terenske pokuse sa simbiotskim dekapodnim rakovima (uzetim s vlasulja) i onima koji nisu u kontaktu s vlasuljama (uzeti s drugih mjesta) te smo testirali obrambene reakcije vlasulja-domaćina. Naši rezultati pokazuju da dekapodni rakovi prolaze individualnu fazu navikavanja prije nego što postanu simbionti. Nakon toga budu zaštićeni i od susjednih vlasulja

Looking like Limulus? - Retinula axons and visual neuropils of the median and lateral eyes of scorpions

Background: Despite ongoing interest in the neurophysiology of visual systems in scorpions, aspects of their neuroanatomy have received little attention. Lately sets of neuroanatomical characters have contributed important arguments to the discussion of arthropod ground patterns and phylogeny. In various attempts to reconstruct phylogeny (from morphological, morphological + molecular, or molecular data) scorpions were placed either as basalmost Arachnida, or within Arachnida with changing sister-group relationships, or grouped with the extinct Eurypterida and Xiphosura inside the Merostomata. Thus, the position of scorpions is a key to understanding chelicerate evolution. To shed more light on this, the present study for the first time combines various techniques (Cobalt fills, DiI / DiO labelling, osmium-ethyl gallate procedure, and AMIRA 3D-reconstruction) to explore central projections and visual neuropils of median and lateral eyes in Euscorpius italicus (Herbst, 1800) and E. hadzii Di Caporiacco, 1950. Results: Scorpion median eye retinula cells are linked to a first and a second visual neuropil, while some fibres additionally connect the median eyes with the arcuate body. The lateral eye retinula cells are linked to a first and a second visual neuropil as well, with the second neuropil being partly shared by projections from both eyes. Conclusions: Comparing these results to previous studies on the visual systems of scorpions and other chelicerates, we found striking similarities to the innervation pattern in Limulus polyphemus for both median and lateral eyes. This supports from a visual system point of view at least a phylogenetically basal position of Scorpiones in Arachnida, or even a close relationship to Xiphosura. In addition, we propose a ground pattern for the central projections of chelicerate median eyes

Mechanisms of eye development and evolution of the arthropod visual system: The lateral eyes of myriapoda are not modified insect ommatidia

AbstractThe lateral eyes of Crustacea and Insecta consist of many single optical units, the ommatidia, that are composed of a small, strictly determined and evolutionarily conserved set of cells. In contrast, the eyes of Myriapoda (millipedes and centipedes) are fields of optical units, the lateral ocelli, each of which is composed of up to several hundreds of cells. For many years these striking differences between the lateral eyes of Crustacea/Insecta versus Myriapoda have puzzled evolutionary biologists, as the Myriapoda are traditionally considered to be closely related to the Insecta. The prevailing hypothesis to explain this paradox has been that the myriapod fields of lateral ocelli derive from insect compound eyes by disintegration of the latter into single ommatidia and subsequent fusion of several ommatidia to form multicellular ocelli. To provide a fresh view on this problem, we counted and mapped the arrangement of ocelli during postembryonic development of a diplopod. Furthermore, the arrangement of proliferating cells in the eyes of another diplopod and two chilopods was monitored by labelling with the mitosis marker bromodeoxyuridine. Our results confirm that during eye growth in Myriapoda new elements are added to the side of the eye field, which extend the rows of earlier-generated optical units. This pattern closely resembles that in horseshoe crabs (Chelicerata) and Trilobita. We conclude that the trilobite, xiphosuran, diplopod and chilopod mechanism of eye growth represents the ancestral euarthropod mode of visual-system formation, which raises the possibility that the eyes of Diplopoda and Chilopoda may not be secondarily reconstructed insect eyes

Morfología de la primera zoea de Portunus acuminatus (Stimpson, 1871) obtenida en el laboratorio

Larvae of Portunus acuminatus (Stimpson, 1871) from one female, collected by trawling at a depth of 12 m in the Gulf of Nicoya, Pacific Costa Rica, Central America (090°48.899’N, 084°40.498’W) were hatched in the laboratory. The morphology of zoea I is described and illustrated for the first time and compared with known zoeae of other portunid species belonging to the subfamily Portuninae. We present a combination of three features which allows zoea I larvae of P. acuminatus to be distinguished from other described larvae of the genus. Descriptions are based on dissected larvae analysed by SEM and light microscopy.Se describe el primer estadio larvario del cangrejo Portunus acuminatus. Las larvas se obtuvieron en el laboratorio a partir de una hembra ovígera capturada en el Golfo de Nicoya (090°48.899’N, 084°40.498’W), Pacífico de Costa Rica. La descripción se ha realizado con la ayuda del microscopio electrónico de barrido y el microscopio óptico. Los caracteres morfológicos son comparados con los de otras especies de la subfamilia Portuninae. Presentamos una combinación de tres caracteres que permiten distinguir la primera zoea de P. acuminatus de otras larvas del género

Dissecting a neuron network: FIB-SEM-based 3D-reconstruction of the visual neuropils in the sea spider Achelia langi (Dohrn, 1881) (Pycnogonida)

Background: The research field of connectomics arose just recently with the development of new three-dimensional- electron microscopy (EM) techniques and increasing computing power. So far, only a few model species (for example, mouse, the nematode Caenorhabditis elegans, and the fruit fly Drosophila melanogaster) have been studied using this approach. Here, we present a first attempt to expand this circle to include pycnogonids, which hold a key position for the understanding of arthropod evolution. The visual neuropils in Achelia langi are studied using a focused ion beam-scanning electron microscope (FIB-SEM) crossbeam-workstation, and a three-dimensional serial reconstruction of the connectome is presented. Results: The two eyes of each hemisphere of the sea spider's eye tubercle are connected to a first and a second visual neuropil. The first visual neuropil is subdivided in two hemineuropils, each responsible for one eye and stratified into three layers. Six different neuron types postsynaptic to the retinula (R-cells) axons are characterized by their morphology: five types of descending unipolar neurons and one type of ascending neurons. These cell types are also identified by Golgi impregnations. Mapping of all identifiable chemical synapses indicates that the descending unipolar neurons are postsynaptic to the R-cells and, hence, are second-order neurons. The ascending neurons are predominantly presynaptic and sometimes postsynaptic to the R-cells and may play a feedback role. Conclusions: Comparing these results with the compound eye visual system of crustaceans and insects - the only arthropod visual system studied so far in such detail - we found striking similarities in the morphology and synaptic organization of the different neuron types. Hence, the visual system of pycnogonids shows features of both chelicerate median and mandibulate lateral eyes

The visual system of harvestmen (Opiliones, Arachnida, Chelicerata) - a re-examination

Background: The visual systems in chelicerates are poorly understood, even though they show strong variation in eye and visual neuropil architecture, thus may provide valuable insights for the understanding of chelicerate phylogeny and eye evolution. Comparable morphological characters are desperately sought for reconstructions of the phylogeny of Chelicerata, especially with respect to Arachnida. So far, reliable data exist only for Pycnogonida, Xiphosura, Scorpiones, and Araneae. The few earlier studies of the organisation of the visual system in harvestmen are contradictory concerning the number, morphology, and position of the visual neuropils. Results: We undertook a descriptive and comparative analysis of the neuroanatomy of the visual system in several phalangid harvestmen species. Various traditional and modern methods were used that allow comparisons with previous results (cobalt fills, Dil/DiO labelling, osmium ethyl gallate procedure, and TEM). The R-cells (photoreceptor and arhabdomeric cells) in the eyes of Opiliones are linked to a first and a second visual neuropil. The first visual neuropil receives input from all R-cell axons, in the second only few R-cells terminate in the distal part. Hence, the second visual neuropil is subdivided in a part with direct R-cell input and a part without. The arcuate body is located in a subsequent position with direct contact to the second visual neuropil. Conclusions: This re-examination comes to conclusions different from those of all previous studies. The visual system of phalangid Opiliones occupies an intermediate position between Pycnogonida, Xiphosura, and Scorpiones on the one side, and Araneae on the other side. The projection of the R-cells is similar to that in the former grouping, the general neuropil arrangement to that in the latter taxon. However, more research on the visual systems in other chelicerate orders is needed in order to draw inferences on phylogeny or eye evolution

A new species of Austrodecus Hodgson, 1907 (Pycnogonida, Austrodecidae) from the Chilean fjords

A new species of the "glaciale" group of Austrodecus Hodgson, 1907, Austrodecus nausinoos sp. n., is described from specimens collected by "Huinay Fjordos" expeditions to the southern Chilean fjords in 2005, 2011 and 2013. Specific to the new species is the combination of the following morphological characters: six-articled ovigers, auxiliary claws, two spines on the first coxa of leg 1, and a spur at the tip of the long abdomen. In addition to the species description we give an update of Child's ( 1995) key to the glaciale group of Austrodecus including the new species