Albinism refers to a group of inherited conditions resulting in little or no pigment (hypopigmentation) in the eyes alone or in the eyes, skin, and hair. In humans, all types of albinism exhibit abnormalities in the optic system, including misrouting of the optic fibers between the retina and the brain, and incomplete development of the fovea, the area of the retina where the sharpest vision is located. Thus, these characteristics can provide useful diagnostic criteria for identifying albinism. Inheritance of an altered copy of a gene that does not function correctly is the cause of most types of albinism. Albinos have white or light skin and hair, and often pink eyes, although the eye color can vary from dull gray to brown. The “pink” eyes are due to the reflection from choroid capillaries behind the retina. Albinism is differentiated from piebaldism (body pigmentation missing in only some areas) and leucism (dark-eyed anomalously white animals). Pigmentation patterns should not be the only criterion used to define albinism, as some mutant phenotypes (pseudo-albinism) may be due to the action of genes at other loci.
Figure 1 Anomalously white humpback whale sighted off Australia.
Mammalian color is almost entirely dependent on presence (or absence) of the pigment melanin in the skin, hair, and eyes. Melanin is produced through a stepwise biochemical pathway in which the amino acid tyrosine is converted to melanin. The enzyme tyrosinase plays a critical role in this pathway, and alterations or mutations in the tyrosinase gene can result in a defective enzyme that is unable to produce melanin, or does so at a reduced rate. Mutations in five other genes have also been identified in different types of albinism in humans.
II. Problems Associated with Albinism
Humans with albinism often are photophobic and have other vision impairments, such as extreme far-sightedness, near-sightedness, and astigmatism. There are unpublished reports of apparent vision problems for albino seals, when they are on shore. Costs of this aberrant pigmentation for marine mammals may include reduced heat absorption in colder waters, increased con-spicuousness to predators, and impaired visual communication.
III. Albinism and Marine Mammals
Anomalously white individuals have been reported for 20 cetacean species (Fertl et al., 1999) (Fig. 1); they have also been reported for pinnipeds (e.g., Rodriguez and Bastida, 1993). No reports are known of anomalously white sea otters (Enhydra lutris) or sirenians. Anomalously white individuals are often presumed to be tme albinos. Some of those individuals match the description of true albinism [e.g., there are well-documented reports of albino sperm whales (Physeter tnacroeephalus) and bottlenose dolphins (Tursiops truneatus)], but many do not. “Chimo,” an anomalously white killer (Orcinus orca) captured for display in Canada, was diagnosed postmortem with Chediak-Higashi Syndrome, (Fig. 2), a type of albinism. This inherited disorder is characterized by diluted pigmentation patterns that appear pale gray, white blood cell abnormalities, and a shortened life span. Whales and dolphins also may appear white if extensively scarred, or covered with a fungus, such as Lobo’s disease.
Figure 2 An albino killer whale (“Chitno”) postmortem diagnosed with Chediak-Higashi syndrome.