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Classic studies of gill vasculature indicate the gross anatomy of elasmobranch gills is similar in most species, and in general, resembles that of teleosts. Recent studies of fine gill structure of several species of batoid elasmobranchs (Order: Rajiformes), however, have reported previously undescribed fine structural differences including; the presence, in Urolophus jamaicensis, U. mucosus, U. paucimaculatus, Rhinoptera bonasus, Dasyatis americana and D. sabina (Suborder: Myliobatoidei), of; 1) a tip channel, possibly an extension of the afferent filament artery that runs along the distal end of the filament corpus cavernosum; and 2) a vascular arcade, a vessel serially connecting afferent filament arteries of individual hemibranchs near the tip of the gill filaments. Using light and scanning electron microscopy we examined the fine gill structure of Raja erinacea, R. ocellata and R. radiata (Suborder: Rajoidei). Contrary to previous reports which suggested that these structures might be unique to Myliobatids, results of this study indicate the presence of a similar vascular arcade and tip channel in Rajoidei. However, differences were found in the gross structure of the ventral aorta and afferent branchial arteries between the suborders. Additional research is necessary to further elucidate the adaptive significance, if any, of these morphological differences between closely related groups of Rajiform fishes.

The exact role of the immune system in normal spermatogenesis is poorly understood. The attachment, however, of the lymphomyeloid epigonal organ specifically to the testis's mature pole in many shark species is a curious finding. Unlike the histology of the lymphomyeloid tissues of many other elasmobranchs, the epigonal organ leukocytes of wild-caught blue shark (Prionace glauca), besides exhibiting extensive nuclear heterogeneity, contain some of the largest known granules ever seen in vertebrate white blood cells. It was previously shown that the blue shark epigonal organ remains unremarkable and functionally unchanged despite cestode parasites embedded into its surface, suggesting that it might have other functions in addition to microbial defense. We show here that Prionace epigonal leukocytes shed their granule-laden cytoplasm into the cyst resorption zone (RZ) of the testis, i.e. the region separating the spermatogenic tissue from the epigonal organ, as they begin to migrate into the RZ. Using the immunoreactivity of the conserved transcription factor (proliferating cell nuclear antigen) as marker, it is shown that the granule-lacking leukocytes exclusively infiltrated spermatozoal cysts leftover after the wave of wide-spread multinuclear cell death in summer-breeding males in a seasonally dependent manner. By contrast, Prionace caught 2 months later showed fully recovered testes containing numerous completely intact spermatozoal cysts. Conversely, degenerating immature spermatids were gradually phagocytized by their accompanying Sertoli cells, and leukocytes did not infiltrate such cysts. The autoimmune response described here resembles in every aspect the testicular autoimmune response induced experimentally in a teleost fish. These observations suggest functional adaptation of shark leukocytes in response to specific changes in the testicular microenvironment., (© 2014 Anatomical Society.)