The eyes of a “nobody”,Anoplodactylus petiolatus (Pantopoda; Anoplodactylidae)

Abstract

The fine structure of the four ocelli of Anoplodactflus petiolatus was examined using serial longitudinal and transversal sections of the eye hill. Each pigment cup oceilus is composed of a (planconvex) cuticular lens, lens forming hypodermal cells, inverse retinula cells with latticed rhabdom and surrounding tapetum and pigment layers. Within the retinula cells a distal "vitreous" zone, a nucleus zone and a proximal rhabdomeric zone can be distinguished. Retinula cell axons originate proximally. The tapetum cells contain several layers of reflecting crystals. Distally, they have a common microvillous region. The intraretinal "vitreous" zone contains glycogen-like particles in the centre and rough ER in the periphery. Contrary to other Pantopoda vitreous ceils, a praeretinal membrane and a vertical lens groove have not been observed in Anoplodactylus. While the presence of four (median) ocelli appears to be a primitive characteristic, the inverse orientation of the retinula ceils in combination with a tapetum lucidum represents a highly derived characteristic among arthropod median eyes.

Sense organs in Pycnogonida: A review

“…This general architecture has been confirmed by studies using electron microscopy (Heß et al., ; Jarvis & King, ; Lehmann et al., ) (Figure c–e). However, as the retinula axons leave the eyecup in the form of several axon subbundles proximally rather than distally, Heß et al.…”

“…However, as the retinula axons leave the eyecup in the form of several axon subbundles proximally rather than distally, Heß et al. () applied the morphological term “pseudoinverted” R‐cells, as in genuine inverted eyes the axons leave the eyecup distally in the direction of the incoming light. In old studies, the basal midline of eyes where the axon bundles leave the eyecup has been called “raphe”.…”

“…Heß et al. () measured reflecting crystals in the tapetal cells and described them as biconcave, with 50 nm thickness in their centre and 80 nm at their edge, a length of between 0.5 and 4.5 μm, and a distance of 200–250 nm between the layers revealing a “multilayer reflector”, and hence a highly effective mirror system (Nilsson, ).…”

“…However, do we know this for sure also for Pycnogonida? The eyes of a few sea spiders have indeed been studied in some detail, and from this small body of knowledge, we can assume that the above‐described R‐cell structure and the wiring of their axons probably holds true for Pycnogonida as well (e.g., Heß, Melzer, and Smola (); Jarvis and King (); Morgan (); Sokolow (); Wiren ()). The trajectories of R‐cell axons and the visual neuropils have recently been studied by Lehmann, Heß, and Melzer () and Lehmann, Heß, Wanner, and Melzer ().…”

“…In the present report, we have therefore not only summarized published knowledge, but also present original data from, for example, ultrathin sections we collected in the past two decades as by‐products of our studies on pycnogonid eyes (Heß et al., ; Lehmann et al., ; Lehmann, Heß, Wanner, et al. ) and glands (Melzer, Heβ, Dunkel, Ludwig, & Smola, ).…”