Skeletal elements of the penguin eye and their functional and phylogenetic implications (Aves: Sphenisciformes: Spheniscidae)

Abstract

Scleral ossicles and other bony elements are present in the eyes of many vertebrates, including birds. In this study, the skeletal elements present in the penguin eye and orbit were imaged using macro photographs and micro‐computed tomography (micro‐CT), to help elucidate their function and significance. A total of 36 scleral rings and three whole skulls were imaged. King (Aptenodytes patagonicus), Fiordland crested (Eudyptes pachyrhynchus), Snares crested (Eudyptes robustus), royal (Eudyptes schlegeli) and yellow‐eyed (Megadyptes antipodes) penguins had between 12 and 14 elements in their scleral ring while the gentoo (Pygoscelis papua) had 14 and 17; little penguins (Eudyptula sp.) consistently had between 10 and 12 elements. All had at least two elements that overlapped, usually totally, each neighbour, and two that were overlapped by each neighbour. The interior structure of all ossicles revealed a lattice‐like arrangement of struts typical of cancellous bone, the whole being surrounded by thick cortical bone. The scleral ring of a 10 week gentoo chick was not completely ossified but rather had multiple small holes within it on micro‐CT. A large os opticus was present in one king penguin but in another bird of the same age and gender there was no such bone. Much smaller accessory bones were found in the posterior pole of one Snares crested and one little penguin. We conclude that the penguin scleral ring not only maintains the shape of the eye but also provides protection and a site of insertion for rectus muscles. However, the extreme variability in the os opticus suggests that it is not essential to normal function.

Micro‐CT guided illustration of the head anatomy of penguins (Aves: Sphenisciformes: Spheniscidae)

“…However, there would likely still be differential staining between tissues closer to external surfaces and those deeper in such large specimens and thus require a compromise, although different, in the choice of scanning parameters. Greater resolution of individual structures would have been possible were we to have dissected out and imaged each element separately, as we have done previously for Anulus ossicularis sclerae (Hadden et al, 2021), although the relationships between structures would be lost. Perfusion‐based contrast enhancement using fresh specimens would be necessary were one to wish to image Systema Cardiovasculare.…”

“…As it provides two‐dimensional transverse, coronal and sagittal planes with accurate dimensions, Data View V 1.5.4.0 was also used to obtain measurements. Three‐dimensional visualization programs such as CTVox are not as accurate in this regard because of distortions induced by perspective, a function of the camera in that program (Hadden et al, 2021).…”

“…The detail visible in K1 is markedly lower than that visible in L1 or G1 due to the lower resolution scanner required to image the larger head. There is also an artificial discontinuity present at the level of the mid orbit of K1, due to transient vibration during the imaging process in a factory environment (the arrowheads mark the medial border of Fossa glandulae nasalis, which should be continuous) program (Hadden et al, 2021).…”

“…In similar future studies, consideration could be given to leaving the specimen in iodine for longer to improve staining and allow Anulus ossicularis sclerae (Hadden et al, 2021), although the relationships between structures would be lost. Perfusion-based contrast enhancement using fresh specimens would be necessary were one to wish to image Systema Cardiovasculare.…”

An Overview of the Penguin Visual System

“…We identified articles relating to penguin head anatomy and vision by internet searches using the keywords: penguin, vision and eye on Google Scholar and the University of Auckland Library catalogue (which includes most major databases), from the reference list of a book on avian sensory ecology [ 2 ] (pp. 259–296), and from the references used by the current authors’ previously published works relating to penguins [ 34 , 35 , 36 , 37 ]. There are few articles that specifically relate to the penguin visual system itself.…”