Ultrastructure of the lateral sense organs on the ocular tubercle ofAnoplodactylus pygmaeus (Pycnogonida)

Abstract

Ultrastructure of the lateral sense organs on the ocular tubercle of Anoplodactylus pygmaeus (Pycnogonida). Former light microscopic studies on the lateral sense organs of sea spiders yielded divergent results. Consequently, different authors ascribed different functions to these organs. The present ultrastructural study shows that each lateral sense organ of A. pygmaeus consists of approximately 15 sensory cells of two different types, approximately 20 sheath cells with numerous long microvilli, and an outer cuticutar covering. Essentially the same elements are characteristic features of arthropod sensilla. There are, however, differences between the sense organs described in this paper and the sense organs of other arthropods. The inner dendritic segments of sensory cells S 1 of the A. pygmaeuslateral sense organs are very short, the sensory cilia are invaginated, and the pericarya of the sensory cells contain electron lucent cytoplasmic regions with large granules (glycogen?). In addition, the lateral sense organs of A. pygmaeuslack a marked receptor lymph cavity and junctions between the ceils. The results of the present ultrastructural study clearly indicate that the lateral sense organs of A. pygmaeus are not glands as was postulated for other sea spider species by earlier authors. Some investigators hypothesized that the lateral sense organs of other sea spider species were auditory organs or rudimentary eyes. The present results do not support such speculations. Some structural details of the sensory cells of A. pygmaeus resemble those found in chemoreceptive or putative chemoreceptive organs of other arthropods. Accordingly, chemoreceptive or thermoreceptive functions should be taken into consideration for the lateral sense organs of A. pygmaeus. EINLEITUNG

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

“…This region is not connected to the cuticle and lacks structures that might indicate a secretory or resorptive function. In the distal region of the lateral sense organ of Anoplodactylus pFgmaeus, microvillous sheet cells are also found (Richter, 1982). As in many other Pantopoda, nuclei of pigment cells have not been observed.…”

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

“…Based on these two lines of evidence, the AMN is here identified as first-order processing center of the sensory information relayed by the LO. Unfortunately, even though fine structural details are available, the sensory modality of the LO is still unclear, with thermo- or chemoreception being considered the most plausible candidates [ 41 , 42 ]. An older interpretation of the LO as a rudimentary eye [ 36 ] lacks not only support from the cellular fine structure but is likewise challenged by the absence of histamine immunolabeling in its constituent cells.…”

“…Further, a so-called lateral sense organ (LO), which typically lies between the eyes on the ocular tubercle, has been identified in early studies on some species [ 36 , 39 , 40 ]. Even though fine structural details are available by now, clarification of the LO’s sensory modality is pending [ 41 , 42 ]. The segmental composition of the pycnogonid brain has been controversially discussed among developmental biologists and neuroanatomists since the late nineteenth century ([ 34 ] vs .…”

Sense organs in Pycnogonida: A review

“…Hence, in the pre-TEM era, it was unclear what kind of organ the lateral sense organs might be. Richter (1982) made the first modern fine structural examination of the lateral sense organs in Anoplodactylus pygmaeus (Hodge, 1864) supporting the idea of a sensory function. Under a roundish, slightly arched and relatively thin cuticle area, Richter observed a complex sense organ made of more than 30 cells including two types of sensory cells and sheath cells.…”

“…A weak point in these studies is that the exact correlation between sheath cells, sensory cells and origin of the microvilli (from sheath cells or both sensory and sheath cells) is as yet not known, and thus, a functional interpretation is still difficult. As Richter (1982) proposed, a chemosensory or thermosensory function is possible, but the old interpretations should be kept in mind. Wiren (1918) described that from the lateral sense organs a fine nerve project to the eye nerves, and form a joint nerve to the visual neuropil.…”