The visual pathway in sea spiders (Pycnogonida) displays a simple serial layout with similarities to the median eye pathway in horseshoe crabs

Abstract

Background Phylogenomic studies over the past two decades have consolidated the major branches of the arthropod tree of life. However, especially within the Chelicerata (spiders, scorpions, and kin), interrelationships of the constituent taxa remain controversial. While sea spiders (Pycnogonida) are firmly established as sister group of all other extant representatives (Euchelicerata), euchelicerate phylogeny itself is still contested. One key issue concerns the marine horseshoe crabs (Xiphosura), which recent studies recover either as sister group of terrestrial Arachnida or nested within the latter, with significant impact on postulated terrestrialization scenarios and long-standing paradigms of ancestral chelicerate traits. In potential support of a nested placement, previous neuroanatomical studies highlighted similarities in the visual pathway of xiphosurans and some arachnopulmonates (scorpions, whip scorpions, whip spiders). However, contradictory descriptions of the pycnogonid visual system hamper outgroup comparison and thus character polarization. Results To advance the understanding of the pycnogonid brain and its sense organs with the aim of elucidating chelicerate visual system evolution, a wide range of families were studied using a combination of micro-computed X-ray tomography, histology, dye tracing, and immunolabeling of tubulin, the neuropil marker synapsin, and several neuroactive substances (including histamine, serotonin, tyrosine hydroxylase, and orcokinin). Contrary to previous descriptions, the visual system displays a serial layout with only one first-order visual neuropil connected to a bilayered arcuate body by catecholaminergic interneurons. Fluorescent dye tracing reveals a previously reported second visual neuropil as the target of axons from the lateral sense organ instead of the eyes. Conclusions Ground pattern reconstruction reveals remarkable neuroanatomical stasis in the pycnogonid visual system since the Ordovician or even earlier. Its conserved layout exhibits similarities to the median eye pathway in euchelicerates, especially in xiphosurans, with which pycnogonids share two median eye pairs that differentiate consecutively during development and target one visual neuropil upstream of the arcuate body. Given multiple losses of median and/or lateral eyes in chelicerates, and the tightly linked reduction of visual processing centers, interconnections between median and lateral visual neuropils in xiphosurans and arachnopulmonates are critically discussed, representing a plausible ancestral condition of taxa that have retained both eye types.

Cambrian origin of the arachnid brain reveals early divergence of Chelicerata

“…The reflected configuration of the arachnopulmonate cerebrum here ascribed to mollisoniids requires a divergence from the conventional view that Limulus is included within Arachnida. Pycnogonida, recognized as the sister group of Euchelicerata, also possess linear brains: although in many pycnogonid species the neuraxis is tilted upwards, their chelicerae are innervated by a deutocerebrum situated posterior to the protocerebrum (32)(33)(34), as in Xiphosura (23,33). The character state 'linear brain' thus supports the proposition that megacheirans with linear cerebra comparable to the developing limuliid brain (17) are best viewed as upper stem euarthropods most closely related to a xiphosuran/pycnogonid grouping with the exclusion of Arachnida (Fig.…”

Single-cell transcriptomics reveals the brain evolution of web-building spiders

“…After washing, samples were blocked by 5% goat serum in 0.5% PBST at room temperature on a shaker for 30 min. The primary antibody and its dilution ratio were adopted from previous publications 112,113 . Mouse anti-SYNORF1 and anti-Fasciclin II antibody was purchased from Developmental Studies Hybridoma Bank and diluted to 1:1,000 in the blocking buffer.…”

Sea spiders (Arthropoda: Pycnogonida) collected during the Madibenthos Expedition from Martinique shallow waters

“…No dorsomedian ornamentation. Ocular tubercle long, about 5.5 times as long as base width, distal part broad and ovoid carrying pigmented eyes, pointy tip and 2 small pointy lateral sense organs (see Lehmann et al 2017;Brenneis 2022). Lateral processes about twice as long as wide, well separated by about half of their own diameter, unornamented.…”