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

The sejugal furrow in camel spiders and acariform mites

Camel spiders (Arachnida: Solifugae) are one of the arachnid groups characterised by a prosomal dorsal shield composed of three distinct elements: the pro-, meso- and metapeltidium. These are associated respectively with prosomal appendages one to four, five, and six. What is less well known, although noted in the historical literature, is that the coxae of the 4th and 5th prosomal segments (i.e. walking legs 2 and 3) of camel spiders are also separated ventrally by a distinct membranous region, which is absent between the coxae of the other legs. We suggest that this essentially ventral division of the prosoma specifically between coxae 2 and 3 is homologous with the so-called sejugal furrow (the sejugal interval sensu van der Hammen). This division constitutes a fundamental part of the body plan in acariform mites (Arachnida: Acariformes). If homologous, this sejugal furrow could represent a further potential synapomorphy for (Solifugae + Acariformes); a relationship with increasing morphological and molecular support. Alternatively, outgroup comparison with sea spiders (Pycnogonida) and certain early Palaeozoic fossils could imply that the sejugal furrow defines an older tagma, derived from a more basal grade of organisation. In this scenario the (still) divided prosoma of acariform mites and camel spiders would be plesiomorphic. This interpretation challenges the textbook arachnid character of a peltidium (or ‘carapace’) covering an undivided prosoma

New findings and a new species of the genus Ammothea (Pycnogonida, Ammotheidae), with an updated identification key to all Antarctic and sub-Antarctic species

Specimens of the pycnogonid genus Ammothea collected during the Polarstern cruise XXIII/8 (23 November 2006–30 January 2007) were studied. Nine species were recognized in this collection: Ammothea bentartica, A. bicorniculata, A. carolinensis, A. clausi, A. longispina, A. minor, A. spinosa, A. striata and A. tibialis. Three of them (A. bentartica, A. bicorniculata and A. tibialis) are reported for the second time, enlarging their known geographical and bathymetric range. In the present contribution, the observed morphological variability of all collected Ammothea species is described and discussed. For the identification and description of the material, different museum specimens were consulted. Among them, we have consulted part of the Discovery collection housed at the Natural History Museum in London. That material was initially identified by Isabella Gordon, a reputed author in the field of pycnogonid taxonomy. A new species, based on a museum specimen previously highly confused in the literature, is proposed in the present contribution as Ammothea isabellae n. sp. The new taxon is compared with its closest congeners, especially with A. longispina and A. stylirostris. Finally, we propose an updated dichotomous key to species covering all currently known Antarctic and sub-Antarctic Ammothea specie

A postlarval instar of Phoxichilidium femoratum (Pycnogonida, Phoxichilidiidae) with an exceptional malformation

Individuals of the marine chelicerate lineage Pycnogonida (sea spiders) show considerable regenerative capabilities after appendage injury or loss. In their natural habitats, especially the long legs of sea spiders are commonly lost and regenerated, as is evidenced by the frequent encounter of specimens with missing or miniature legs. In contrast to this, the collection of individuals with abnormally developed appendages or trunk regions is comparably rare. Here, we studied a remarkable malformation in a postlarval instar of the species Phoxichilidium femoratum (Rathke, 1799) and describe the external morphology and internal organization of the specimen using a combination of fluorescent histochemistry and scanning electron microscopy. The individual completely lacks the last trunk segment with leg pair 4 and the normally penultimate trunk segment bears only a single aberrant appendage resembling an extension of the anteroposterior body axis. Externally, the proximal units of the articulated appendage are unpaired, but further distally a bifurcation into two equally developed leg‐like branches is found. Three‐dimensional reconstruction of the musculature reveals components of two regular leg muscle sets in several of the proximal articles. This confirms interpretation of the entire appendage as a malformed leg and reveals an externally hidden paired organization along its entire proximodistal axis. To explain the origin of this unique malformation, early pioneering studies on the regenerative potential of pycnogonids are evaluated and (a) an injury‐induced partial fusion of the developing limb buds of leg pair 3, as well as (b) irregular leg regeneration following near complete loss of trunk segments 3 and 4 are discussed. Which of the two hypotheses is more realistic remains to be tested by dedicated experimental approaches. These will have to rely on pycnogonid species with established laboratory husbandry in order to overcome the limitations of the few short‐term regeneration studies performed to date.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659National Science Foundation http://dx.doi.org/10.13039/100000001Peer Reviewe

Taxonomy and distribution of Italian and Antarctic Pycnogonida, with identification keys to genera and species

openQuesta tesi fornisce una descrizione dei progetti realizzati nel triennio 2018-2021 nell'ambito dello studio dei Picnogonidi. Viene riportata una panoramica generale della classe Pycnogonida (Arthropoda, Chelicerata), evidenziando le principali caratteristiche di questi organismi. Successivamente viene proposta la checklist aggiornata dei Picnogonidi italiani (Colasanto e Galli, 2021) con la distribuzione dettagliata delle specie, generata undici anni dopo quella di Bartolino e Chimenz (2010). La chiave dicotomica proposta per l'identificazione delle specie fa riferimento a questi documenti. La chiave è stata ottenuta attraverso indagini bibliografiche sulle caratteristiche morfologiche della specie, integrando con informazioni tratte da chiavi già presenti in letteratura (in particolare, Munilla e Soler-Membrives, 2014) e analisi di campioni in laboratorio. Parallelamente allo studio dei Picnogonidi italiani, sono stati esaminati gli esemplari conservati al Museo Nazionale dell'Antartide ed è stato effettuato uno studio di DNA barcoding su alcuni di essi. Sono state abbozzate chiavi di identificazione di generi e specie, sulla base delle caratteristiche morfologiche. Conseguentemente, viene riportata anche la checklist aggiornata dei Picnogonidi antartici e sub-antartici. Si riportano infine alcune considerazioni sulla possibilità di coinvolgere i subacquei nel monitoraggio dei Picnogonidi attraverso progetti di citizen science.This thesis provides a description of the projects carried out in the three-year period 2018-2021 as part of the study of Pycnogonida. A general overview of the class Pycnogonida (Arthropoda, Chelicerata) is reported, highlighting the main characteristics of these organisms. Subsequently, the updated checklist of the Italian pycnogonids (Colasanto and Galli, 2021) is proposed with the detailed distribution of the species, generated eleven years after that of Bartolino and Chimenz (2010). The dichotomous key proposed for the species identification refers to these documents. The key was obtained through bibliographic investigations on the morphological characteristics of the species, integrating with information taken from keys already present in the literature (in particular, Munilla and Soler-Membrives, 2014) and laboratory analysis of samples. Parallel to the study of the Italian pycnogonids, specimens preserved at the National Museum of Antarctica were examined and the barcoding DNA of some of them was sequenced. Identification keys to genera and species were sketched, based on morphological characters. Therefore, the updated checklist of Antarctic and sub-Antarctic Pycnogonida is also reported. Finally, some considerations on the possibility to involve Scuba-divers on pycnogonids monitoring through citizen science projects are reported.openXXXIV CICLO - SCIENZE E TECNOLOGIE PER L'AMBIENTE E IL TERRITORIO (STAT) - Scienze del mareColasanto, Elis

Nuevo modo de desarrollo postembrionario en el género Ammothea (Pycnogonida: Ammotheidae) procedente de aguas Antárticas

In this paper the postembryonic development of Ammothea glacialis (family ammotheidae) is described. The studied material was collected during the Italica XIX cruise to Victoria land, ross sea, antarctica. The external morphology of three larval instars is described and illustrated. The development of A. glacialis has the following characteristics: (1) protonymphon hatch from the eggs; (2) the larvae have yolk reserves and relatively large size (0.7 mm in length); (3) the larvae remain on the ovigerous legs of males during several moults; (4) the larvae have reduced larval II-III appendages and the spinning apparatus is absent; (5) the development of walking legs is sequential. This development is compared with those previously known, especially with Propallene longiceps and Nymphon grossipes.Nuevo modo de desarrollo postembrionario en el género AmmotheA (Pycnogonida: Ammotheidae) procedente de aguas Antárticas. – En este trabajo se describe el desarrollo postembrionario de Ammothea glacialis (familia ammotheidae). El material estudiado fue recolectado durante el crucero Itálica XIX a Tierra Victoria, en el mar de ross, antártida. se describe e ilustra la morfología externa de tres estadios larvarios. El desarrollo de A. glacialis se caracteriza por: (1) la larva eclosiona como protonymphon; (2) la larva es de tamaño relativo grande (0.7 mm de longitud) y con reservas de vitelo; (3) la larva permanece en los ovígeros del macho durante varias mudas; (4) la larva presenta los apéndices larvarios II y III reducidos y el “spinning apparatus” está ausente; (5) el desarrollo de las patas es secuencial. Este desarrollo es comparado con otros desarrollos postembrionarios previamente conocidos, especialmente con los de Propallene longiceps and Nymphon grossipes

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