Excretory glands of sea spiders (Pycnogonida, Nymphonidae)

All major arthropod taxa possess excretory glands - a type of filtration nephridium considered ancestral for this group. Pycnogonids form a basal branch of the arthropod phylogenetic tree and are ancient aquatic chelicerates, but they were believed to lack specialised excretory organs, except for Nymphopsis spinosissimum (Ammotheidae). Whether this condition is unique or common remained unknown due to lack of anatomical data for many species. Here we examined four nymphonids: Nymphon brevirostre, Nymphon grossipes, Nymphon serratum and Pentanymphon antarcticum using scanning and transmission electron microscopy, as well as light microscopy. In adults of all four species, we found excretory organs and describe ultrastructural details of all their parts: sacculus, reabsorption channel, excretory channel and the pore. In addition to the definitive (adult) excretory organs, we also detected some larval and juvenile transitory ones and were able to trace the origin of the sacculus podocytes from the non-epithelial mesoderm of the horizontal septum. All excretory organs are located in the appendages of the first three postocular segments of the cephalosoma (although not necessarily in all of them at once) because these areas can maintain the high hemolymph pressure necessary for ultrafiltration. The ultrastructure and development of the sacculus point toward the secondary nature of this cavity, although the coelomic status of the sacculi in sea spiders and other arthropods is still unclear.