The photoreceptors of the West Indian manatee

Morphological evaluation of the orbit, eye tunics, eyelids, and orbital glands in young and adult aardvarks Orycteropus afer, Pallas, 1766 (Tubulidentata: Orycteropodidae) - similarities and differences with representatives of the Afrotheria clade

The Afrotheria clade includes a large group of extant mammals, and the aardvark (Orycteropus afer) is the only representative of the order Tubulidentata in it. Here, we studied the morphological nature of the orbital region, eye tunics, upper and lower eyelids, superficial gland of the third eyelid, the third eyelid, deep gland of the third eyelid, and lacrimal gland in post-mortem specimens obtained from three captive aardvarks, two young and one adult. The obtained samples were analyzed using macroscopic, histological, and histochemical methods. We observed choroidal tapetum lucidum fibrosum in all specimens, which was typical for aardvarks. The superficial gland of the third eyelid was a compound multilobar tubular branched gland of a mucous nature. The deep gland of the third eyelid produced a serous secretion. The seromucous secretion was typical for the lacrimal gland. We compared the morphological data of the O. afer skull with that from other endemic African mammals in the Afrotheria clade. We found that other authors provided different anatomical names for some bones and foramina located within the orbit. The types and function of eyelid glands, as well as eyeball glands of aardvarks, can primarily be connected with their habitat. Our study may constitute an introduction to the ontogenesis of individual eyeball glands in aardvarks. This article is protected by copyright. All rights reserved.

Review of sensory modalities of sirenians and the other extant Paenungulata clade

Extant members of Paenungulata (sirenians, proboscideans, and hyracoideans) form a monophyletic clade which originated in Africa. While paenungulates are all herbivorous, they differ greatly in size, life history, and habitat. Therefore, we would expect both phylogenetically related similarities and ecologically driven differences in their use and specializations of sensory systems, especially in adaptations in sirenians related to their fully aquatic habitat. Here we review what is known about the sensory modalities of this clade in an attempt to better elucidate their sensory adaptations. Manatees have a higher frequency range for hearing than elephants, who have the best low-frequency hearing range known to mammals, while the hearing range of hyraxes is unknown. All paenungulates have vibrissae assisting in tactile abilities such as feeding and navigating the environment and share relatively small eyes and dichromatic vision. Taste buds are present in varying quantities in all three orders. While the olfactory abilities of manatees and hyraxes are unknown, elephants have an excellent sense of smell which is reflected by having the relatively largest cranial nerve related to olfaction among the three lineages. Manatees have the relatively largest trigeminal nerve-the nerve responsible for, among other things, mystacial vibrissae-while hyraxes have the relatively largest optic nerve (and therefore, presumably, the best vision) among the Paenungulata. All three orders have diverged significantly; however, they still retain some anatomical and physiological adaptations in common with regard to sensory abilities.

Adaptations for amphibious vision in sea otters (Enhydra lutris): structural and functional observations

Sea otters (Enhydra lutris) are amphibious mammals that maintain equal in-air and underwater visual acuity. However, their lens-based underwater accommodative mechanism presumably requires a small pupil that may limit sensitivity across light levels. In this study, we consider adaptations for amphibious living by assessing the tapetum lucidum, retina, and pupil dynamics in sea otters. The sea otter tapetum lucidum resembles that of terrestrial carnivores in thickness and fundic coverage. A heavily rod-dominated retina appears qualitatively similar to the ferret and domestic cat, and a thick outer nuclear layer relative to a thinner inner nuclear layer is consistent with nocturnal vertebrates and other amphibious carnivores. Pupil size range in two living sea otters is smaller relative to other amphibious marine carnivores (pinnipeds) when accounting for test conditions. The pupillary light response seems slower than other aquatic and terrestrial species tested in comparable brightness, although direct comparisons require further assessment. Our results suggest that sea otters have retained features for low-light vision but rapid adjustments and acute underwater vision may be constrained across varying light levels by a combination of pupil shape, absolute eye size, and the presumed coupling between anterior lens curvature and pupil size during accommodation.

The Retina of Asian and African Elephants: Comparison of Newborn and Adult

Elephants are precocial mammals that are relatively mature as newborns and mobile shortly after birth. To determine whether the retina of newborn elephants is capable of supporting the mobility of elephant calves, we compared the retinal structures of 2 newborn elephants (1 African and 1 Asian) and 2 adult animals of both species by immunohistochemical and morphometric methods. For the first time, we present here a comprehensive qualitative and quantitative characterization of the cellular composition of the newborn and the adult retinas of 2 elephant species. We found that the retina of elephants is relatively mature at birth. All retinal layers were well discernible, and various retinal cell types were detected in the newborns, including Müller glial cells (expressing glutamine synthetase and cellular retinal binding protein; CRALBP), cone photoreceptors (expressing S-opsin or M/L-opsin), protein kinase Cα-expressing bipolar cells, tyrosine hydroxylase-, choline acetyltransferase (ChAT)-, calbindin-, and calretinin-expressing amacrine cells, and calbindin-expressing horizontal cells. The retina of newborn elephants contains discrete horizontal cells which coexpress ChAT, calbindin, and calretinin. While the overall structure of the retina is very similar between newborn and adult elephants, various parameters change after birth. The postnatal thickening of the retinal ganglion cell axons and the increase in ganglion cell soma size are explained by the increase in body size after birth, and the decreases in the densities of neuronal and glial cells are explained by the postnatal expansion of the retinal surface area. The expression of glutamine synthetase and CRALBP in the Müller cells of newborn elephants suggests that the cells are already capable of supporting the activities of photoreceptors and neurons. As a peculiarity, the elephant retina contains both normally located and displaced giant ganglion cells, with single cells reaching a diameter of more than 50 µm in adults and therefore being almost in the range of giant retinal ganglion cells found in aquatic mammals. Some of these ganglion cells are displaced into the inner nuclear layer, a unique feature of terrestrial mammals. For the first time, we describe here the occurrence of many bistratified rod bipolar cells in the elephant retina. These bistratified bipolar cells may improve nocturnal contrast perception in elephants given their arrhythmic lifestyle.

Evolution and spectral tuning of visual pigments in birds and mammals

Variation in the types and spectral characteristics of visual pigments is a common mechanism for the adaptation of the vertebrate visual system to prevailing light conditions. The extent of this diversity in mammals and birds is discussed in detail in this review, alongside an in-depth consideration of the molecular changes involved. In mammals, a nocturnal stage in early evolution is thought to underlie the reduction in the number of classes of cone visual pigment genes from four to only two, with the secondary loss of one of these genes in many monochromatic nocturnal and marine species. The trichromacy seen in many primates arises from either a polymorphism or duplication of one of these genes. In contrast, birds have retained the four ancestral cone visual pigment genes, with a generally conserved expression in either single or double cone classes. The loss of sensitivity to ultraviolet (UV) irradiation is a feature of both mammalian and avian visual evolution, with UV sensitivity retained among mammals by only a subset of rodents and marsupials. Where it is found in birds, it is not ancestral but newly acquired.

Adaptive features of aquatic mammals' eye

The eye of aquatic mammals demonstrates several adaptations to both underwater and aerial vision. This study offers a review of eye anatomy in four groups of aquatic animals: cetaceans (toothed and baleen whales), pinnipeds (seals, sea lions, and walruses), sirenians (manatees and dugongs), and sea otters. Eye anatomy and optics, retinal laminar morphology, and topography of ganglion cell distribution are discussed with particular reference to aquatic specializations for underwater versus aerial vision. Aquatic mammals display emmetropia (i.e., refraction of light to focus on the retina) while submerged, and most have mechanisms to achieve emmetropia above water to counter the resulting aerial myopia. As underwater vision necessitates adjusting to wide variations in luminosity, iris muscle contractions create species-specific pupil shapes that regulate the amount of light entering the pupil and, in pinnipeds, work in conjunction with a reflective optic tapetum. The retina of aquatic mammals is similar to that of nocturnal terrestrial mammals in containing mainly rod photoreceptors and a minor number of cones (however, residual color vision may take place). A characteristic feature of the cetacean and pinniped retina is the large size of ganglion cells separated by wide intercellular spaces. Studies of topographic distribution of ganglion cells in the retina of cetaceans revealed two areas of ganglion cell concentration (the best-vision areas) located in the temporal and nasal quadrants; pinnipeds, sirenians, and sea otters have only one such area. In general, the visual system of marine mammals demonstrates a high degree of development and several specific features associated with adaptation for vision in both the aquatic and aerial environments.

The visual pigments of the West Indian manatee (Trichechus manatus)

Manatees are unique among the fully aquatic marine mammals in that they are herbivorous creatures, with hunting strategies restricted to grazing on sea-grasses. Since the other groups of (carnivorous) marine mammals have been found to possess various visual system adaptations to their unique visual environments, it was of interest to investigate the visual capability of the manatee. Previous work, both behavioral (Griebel & Schmid, 1996), and ultrastructural (Cohen, Tucker, & Odell, 1982; unpublished work cited by Griebel & Peichl, 2003), has suggested that manatees have the dichromatic color vision typical of diurnal mammals. This study uses molecular techniques to investigate the cone visual pigments of the manatee. The aim was to clone and sequence cone opsins from the retina, and, if possible, express and reconstitute functional visual pigments to perform spectral analysis. Both LWS and SWS cone opsins were cloned and sequenced from manatee retinae, which, upon expression and spectral analysis, had lambda(max) values of 555 and 410 nm, respectively. The expression of both the LWS and SWS cone opsin in the manatee retina is unique as both pinnipeds and cetaceans only express a cone LWS opsin.

Structure and presumptive function of the iridocorneal angle of the West Indian manatee (Trichechus manatus), short-finned pilot whale (Globicephala macrorhynchus), hippopotamus (Hippopotamus amphibius), and African elephant (Loxodonta africana)

The iridocorneal angles of prepared eyes from the West Indian manatee, short-finned pilot whale, hippopotamus and African elephant were examined and compared using light microscopy. The manatee and pilot whale demonstrated capacity for a large amount of aqueous outflow, probably as part of a system compensating for lack of ciliary musculature, and possibly also related to environmental changes associated with life at varying depths. The elephant angle displayed many characteristics of large herbivores, but was found to have relatively low capacity for aqueous outflow via both primary and secondary routes. The hippopotamus shared characteristics with both land- and water-dwelling mammals; uveoscleral aqueous outflow may be substantial as in the marine mammals, but the angular aqueous plexus was less extensive and a robust pectinate ligament was present. The angles varied greatly in size and composition among the four species, and most structures were found to be uniquely suited to the habitat of each animal.

The mammalian photoreceptor mosaic-adaptive design

Unlike in birds and cold-blooded vertebrates' retinas, the photoreceptors of mammalian retinas were long supposed to be morphologically uniform and difficult to distinguish into subtypes. A number of new techniques have now begun to overcome the previous limitations. A hitherto unexpected variability of spectral and morphological subtypes and topographic patterns of distribution in the various retinas are being revealed. We begin to understand the design of the photoreceptor mosaics, the constraints of evolutionary history and the ecological specialization of these mosaics in all the mammalian subgroups. The review discusses current cytological identification of mammalian photoreceptor types and speculates on the likely "bottleneck-scenario" for the origin of the basic design of the mammalian retina. It then provides a brief synopsis of current data on the photoreceptors in the various mammalian orders and derives some trends for phenomena such as rod/cone dualism, spectral range, preservation or loss of double cones and oil droplets, photopigment co-expression and mono- and tri-chromacy. Finally, we attempt to demonstrate that, building on the limits of an ancient rod dominant (probably dichromatic) model, mammalian retinas have developed considerable radiation. Comparing the nonprimate models with the intensively studied primate model should provide us with a deeper understanding of the basic design of the mammalian retina.

Color vision in the manatee (Trichechus manatus)

Four manatees were trained to discriminate between a colored stimulus and a shade of gray in a two-fold simultaneous choice situation. The colors blue, green, red and blue-green were tested against shades of gray varying from low to high relative brightness. The animals distinguished both blue and green from a series of grays but failed to discriminate red and blue-green from certain steps of grays. The manatees could not discriminate between a UV-reflecting white target and an UV-absorbing white target. The results indicate that manatees possess color vision which is most likely dichromatic.