Sense organs in Pycnogonida: A review

Abstract

The Pycnogonida or sea spiders are exclusively marine invertebrates, numbering about 1,300 described species worldwide. Given their remarkable position in phylogeny as basal chelicerates or even basal euarthropods, the structure of their sense organs can reveal important characters, which—in a comparative framework—provide arguments to phylogenetic discussions and help to develop scenarios of evolutionary transformations. This review summarizes current knowledge and presents new original data on the sense organs in pycnogonids, that is, the eyes, the lateral sense organs and the ciliary or sensillar sense organs. Except for the eyes, there are not many detailed studies available. The ultrastructure of the R‐cells of the four eyes located on the ocular tubercle is described as “pseudoinverted”. The eyes are innervated to two visual neuropils located in the protocerebrum. The features of the lateral sense organ, also located on the ocular tubercle, are hitherto not conclusively resolved, a chemo‐ or thermoreceptive function is suggested. Finally, an overview of the various ciliary or sensillar sense organs distributed all over the body is given and the fine structure of branched setae is shown for the first time. The morphology of the sense organs of pycnogonids is compared with that of other arthropod taxa and assessed against the background of current theories of arthropod evolution.

The (not very) typical protonymphons of Pycnogonum litorale

“…V‐shaped bristles have the same design in larvae of N. brevirostre (Alexeeva et al, ) and P. litorale, and in adults of Anolodactylus spp. (Lehmann, Heß, & Melzer, ), while only in P. litorale did we find concealed, embedded receptors, working without the cuticular sheath. We interpret them as mechanoreceptors (more specifically proprioreceptors) and hypothesize that in chelae and larval legs, the entire appendage serves as an outer sheath: deformation of its surface generates the signal.…”

Anatomical changes in postembryonic development of Pycnogonum litorale

“…Palps (homologous to pedipalps in Arachnida and walking legs 1 of Xiphsura, Vilpoux & Waloszek, 2003) are commonly used for chemical sensing (as in some arachnids) and occasionally for grooming (e.g., by sea spider Austrodecus ; Arnaud & Bamber, 1987), but never for locomotion. Reasons for the disappearance of these appendages are unclear, but the chemical sensing role may be taken over by chemoreceptors (e.g., pore plates) on the proboscis (Alexeeva et al, 2017, 2018) and on the other appendages (Fornshell & Harlow, 2018; Lehmann et al, 2018). Ovigers (homologous to the walking legs 1 of Arachnida and walking legs 2 in Xiphosura, Vilpoux & Waloszek, 2003) take part in grooming in several but not all pycnogonid groups.…”

The origin and evolution of the euarthropod labrum

“…Intriguing potential remnants of the protocerebral complex associated with the anterior sclerite may remain in modern arthropods, in a variety of ventral sense organs, notably the "ventral organ" of Limulus (Fig 8; e.g. Battelle 2016); the ventral eyes/organs of Bellonci of crustaceans, and perhaps and (even) more speculatively, the still mysterious organs of Tömösvary (Müller et al 2011), or even the lateral sense organs of pycnogonids (for example, see Paulus 1979;Elofsson 2006;Lehmann et al 2018 for discussion, of varying degrees of scepticism, about possible homologies). All of these structures are innervated from the protocerebrum If so, then the arrangement in Limulus of ventral organ, epistome and labrum (Fig.…”