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

Abstract

The illustration is an important tool to aid in the description and understanding of anatomy, and penguins (Aves: Sphenisciformes: Spheniscidae) are an important clade in environmental monitoring, paleontology, and other research fields. Traditionally, anatomic illustration has been informed by dissection. More recently, micro-computed tomography (micro-CT) has proven to be a powerful tool for three-dimensional anatomic imaging, although larger specimens are more challenging to image due to increased X-ray attenuation. Here, we used traditional dissection and micro-CT to illustrate the skulls of Aptenodytes patagonicus, Eudyptula minor, and Pygoscelis papua, and the extracranial soft tissue of E. minor. Micro-CT prevented the loss of orientation, disarticulation, and distortion of bones that might result from cleaning and drying skulls, while immobilization was achieved by freezing the specimens before imaging. All bony elements in the head were accurately depicted. Fixing, dehydrating, and diffusion staining with iodine (diceCT) enabled the identification of muscles and other large nonmineralized structures, but specimen preparation precluded the ability to show smaller nerves and vessels. The results presented here provide a guide for anatomic studies of penguins and our summary of sample preparation and imaging techniques are applicable for studies of other similarly sized biological specimens. K E Y W O R D S avian orbit, bird head, bird skull, jaw muscles, penguin 1 | INTRODUCTION Penguins (Aves: Sphenisciformes: Spheniscidae) are an important clade in multiple research disciplines including, for example, animal physiology, coastal ecosystem health, and paleontology. Extant species of Spheniscidae have been the subject of particular anatomic attention, being the extant representatives of an ancient lineage with a fossil record as old as the early Paleocene and distinct from other birds for the entirety of the Cenozoic (Jadwiszczak, 2009; Ksepka & Ando, 2011;Slack et al., 2006;Triche, 2007). Although other avian lineages share some of the features of Spheniscidae, including a long history of flightlessness (e.g., ratites such as the emu, Dromaius novaehollandiae) and diving ability (e.g., the common murre common guillemot, Uria aalge, Piatt & Nettleship, 1985), penguins are the only

An Overview of the Penguin Visual System

“…Conversely, each eye is separated from the other by the thin Septum interorbitale, which is only partially ossified, and the paired Musculi recti mediales. The orbit is most open dorsolaterally, although the presence of Annulus ossicularis sclerae extends the bony coverage of the soft tissue in this location [ 35 , 36 , 39 , 40 ].…”

“…The lacrimal gland (Glandula lacrimalis) occupies the dorsolateral orbit and secretes salty tears, as in mammals. The Haderian gland (Glandula membranae nictitantis) is situated ventrally/ventromedially and makes an oily sebaceous secretion along the edge of Membrana nictitans [ 36 , 41 ].…”

“…Suburo and Scolaro [ 44 ] described the anterior segment of the Magellanic penguin eye, noting that the shape of the anterior portion of the eye resembles a cone truncated by the cornea and is somewhat less spherical than the posterior half of the globe. The ocular dimensions of several different species have been measured [ 34 , 36 , 47 ]. Bliss et al [ 48 ] found a mean axial length of 20.34 ± 0.79 mm (anterior cornea to retina) in the macaroni penguin using a 7 MHz B scan probe.…”

“…An exception is the king penguin pupil which, when constricted, becomes square rather than circular—with an up to 300-fold change in illumination between full miosis and full mydriasis [ 18 ]. As demonstrated in Figure 7 , the little penguin iris also appears somewhat octagonal when mid-dilated [ 36 ]. The iris can also become irregular when diseased.…”

“…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.…”