A re‐examination and re‐evaluation of salamander orbital glands

Abstract

The amphibian integument contains numerous multicellular glands. Although two of these, the nasolabial and orbital glands and the associated nasolacrimal duct (NLD), have historically received considerable attention, interpretation of the original observations can be problematic in the context of current literature. Salamanders, in particular, are frequently regarded as at least indicative of aspects of the morphology of the common ancestor to all extant tetrapods; hence, an understanding of these glands in salamanders might prove to be informative about their evolution. For this study, the orbitonasal region of salamanders from three families was histologically examined. Three themes emerged: (1) examination of the effect of phylogeny on the nasolabial gland and NLD revealed a combination of features that may be unique to plethodontid salamanders, and may be correlated to their nose-tapping behavior by which substances are moved into the vomeronasal organ; (2) ecology appears to impact the relative development of the orbital glands, but not necessarily the nasolabial gland, with smaller glands being present in the aquatic species; (3) the nomenclature of the salamander orbital gland remains problematic, especially in light of comparative studies, as several alternate possibilities are viable. From this nomenclatural conundrum, however, it could be concluded that there may be a global pattern in the location of tetrapod orbital gland development. Molecular questions in terms of ontogeny and genetic homology affect the nature of the debate on orbital gland nomenclature. These observations suggest that rather than reflecting an ancestral condition, salamanders may instead represent a case of specialized, convergent evolution.

The Lack of Nasolacrimal Ducts in Plethodontid Salamanders?

“…Wilder's () previous work appears to contradict that conclusion because Desmognathus possesses septomaxillae, albeit, highly reduced (Wake, ). In accordance with the work by Rehorek et al () on plethodontids, many taxa possess septomaxillae but lack nasolacrimal ducts, if their conclusions were correct, as numerous plethodontids possess septomaxillae (Wake, ).…”

“…A recent study concluded that although orbital glands are present in salamanders of the Plethodontidae, plethodontid salamanders (of which there are currently 450+ extant species; AmphibiaWeb, ) lack nasolacrimal ducts, contrary to the findings in other families of salamanders; that is, “In plethodontid salamanders, their nasolabial glands are substantial, the nasolacrimal ducts (NLDs) are absent, and secretions from the orbital glands thus have no direct route to the vomeronasal organ” (Rehorek et al, ). These conclusions were based on examination of Desmognathus , Eurycea , Pseudotriton , and Plethodon , representing three tribes (Desmognathini, Plethodontini, and Spelerpini) within the two subfamilies (Hemidactyliinae and Plethodontinae) of Plethodontidae reconstructed by Shen et al ().…”

Ontogeny of the Orbital Glands and Their Environs in the Pantropical Spotted Dolphin (Stenella attenuata: Delphinidae)

“… Schematic diagrams of the orbital glands in (a) generic nonmammalian tetrapod (adapted from Rehorek et al, ) and (b) generic mammal (adapted from Skalicky, ). This is an oblique view.…”

“…In the case of S. attenuata, the identity of the palpebral glandular region, relative to those of other mammals, is problematic: since there is no clear distinction between the lateral and medial aspects of the palpebral gland mass, at this stage no clear identification can be made. On the other hand, the diffuse distribution of the palpebral glands in embryonic/fetal S. attenuata, especially the portion residing in the lower eyelid, is reminiscent of the condition observed in both salamanders (Rehorek et al, 2013) and alligators (Rehorek et al, 2005, cf., Fig. 1b), in which a widely distributed palpebral glandular mass opens to the conjunctival surface via numerous ducts.…”

“…The very limited literature on orbital gland ontogeny in cetaceans is restricted to a few fetal specimens Rehorek et al, 2013) and (b) generic mammal (adapted from Skalicky, 2016). This is an oblique view.…”