Living In The Dark – Why Some Animals Lack Eyes: The answer lies in the intriguing relationship between natural selection, environmental challenges, and the cost-benefit balance of evolution. Eyes are retained only when they offer a clear advantage for survival or reproduction; otherwise, nature discovers alternative, often clever solutions.
In this article, we explore various animals that either never developed eyesight or have lost it through natural selection. As we examine each species, think about the common factors that may have driven their evolution.
Mexican Tetra (Astyanax mexicanus)
The Mexican Tetra (Astyanax mexicanus) is a noteworthy species of freshwater fish indigenous to North and Central America. It is particularly distinguished by its two distinct forms: a surface-dwelling morph characterized by functional eyes and pigmentation, and the one we are looking at here, a cave-dwelling morph that exhibits the absence of both eyes and pigmentation.
This has established the Mexican Tetra as a significant model organism in the field of evolutionary biology, particularly in the investigation of regressive evolution, sensory compensation, and adaptive mechanisms.
Surface populations of cavefish are located in rivers and streams throughout northeastern Mexico and southern Texas. In contrast, cave populations, also known as troglobitic morphs, are confined to limestone cave systems situated in the Sierra de El Abra region of northeastern Mexico. These caves represent dark, nutrient-scarce environments.
Notably, despite the absence of functional eyes, cavefish develop rudimentary optic primordia that ultimately degenerate. This phenomenon provides valuable insights into the mechanisms by which evolution may suppress certain traits.
Did You Know? Cavefish display insulin resistance, making them a valuable model for studying type 2 diabetes and metabolic adaptations.
Olm (Proteus anguinus)
The Olm (Proteus anguinus) represents one of the most remarkable amphibians known to science. This blind, cave-dwelling salamander has evolved specific adaptations that allow it to survive in the constant darkness of subterranean aquatic environments found in southeastern Europe. Characterized by its pale skin, rudimentary eyes, and elongated, serpentine body, the olm serves as a compelling example of troglomorphic evolution, which encompasses the various adaptations observed in species inhabiting cave ecosystems such as:
- Loss of pigmentation and visual acuity.
- Enhancement of non-visual sensory modalities.
The Olm is the largest known cave-dwelling amphibian. They eat aquatic invertebrates as well as small fish. They have specialised electroreceptors and chemoreceptors, and are sensitive to vibrations, enabling them to catch their prey.
They also have a pretty easygoing lifestyle, with a typical lifespan of between 50 and 70 years.
Giant Tube Worms (Riftia pachyptila)
Riftia pachyptila, also known as the giant tube worm, is an extraordinary organism found in deep-sea hydrothermal vent ecosystems located along mid-ocean ridges, at depths of thousands of meters below the ocean surface. Giant Tube Worms are intriguing due to their unique anatomical features. As well as being blind, they possess neither a mouth, nor a stomach, nor a digestive system. Instead, Riftia pachyptila derives its sustenance through a symbiotic relationship with chemosynthetic bacteria.
They are found exclusively at deep-sea hydrothermal vents, primarily along the East Pacific Rise and the Galápagos Rift, as well as other mid-ocean ridges situated at depths of 2,000 to 3,000 meters. These creatures thrive in clusters around black smokers, which are superheated, mineral-rich vent chimneys that provide a distinct habitat for this extraordinary life form.
The Giant Tube worm is interesting from a scientific viewpoint, as it challenges the conventional idea that all ecosystems depend on sunlight, opening the possibility that life could thrive in environments previously considered inhospitable.
It supports the hypothesis that extraterrestrial life might exist in these dark, icy oceans by illustrating the adaptability of life forms. Furthermore, it presents an extreme example of symbiogenesis, where an animal’s survival hinges entirely on the presence of specific microbes, underscoring the intricate relationships between life forms and the diverse ways in which life can persist and evolve.
Brahminy Blind Snake (Indotyphlops braminus)
Blind snakes are a classification of small, worm-like, burrowing reptiles that have adapted to an underground habitat, categorized under the infraorder Scolecophidia, which represents one of the most primitive and basal lineages of extant snakes. These snakes exhibit distinct characteristics, including diminutive vestigial eyes, smooth, cylindrical bodies, and highly specialized adaptations that facilitate a fossorial lifestyle.
Pictured here is the Brahminy Blind Snake (scientific name Indotyphlops braminus). While they may initially resemble earthworms, blind snakes are indeed true reptiles and fulfill significant ecological functions as subterranean predators.
Scolecophidians, which are believed to be among the most ancient snake lineages, diverged over 100 million years ago. They have their vestigial eyes, covered by skin or scales, and are only used to detect light and dark. These snakes primarily feed on small invertebrates, with a particular preference for ants, termites, and their larvae, pupae, and eggs. Scolecophidians are fossorial creatures, spending nearly their entire lives underground and emerging only during heavy rains, when the soil is disturbed, or at night in warm, humid conditions. Despite their widespread presence in many areas, they are secretive and rarely seen.
Hydra (Hydra vulgaris)
Hydra vulgaris is a small freshwater organism classified within the phylum Cnidaria, which includes jellyfish, corals, and sea anemones. Despite its simple anatomical structure, the hydra is an intriguing organism, predominantly recognized for its exceptional regenerative capabilities, asexual reproduction, and biological immortality under controlled laboratory conditions.
This organism has emerged as a critical model in the fields of developmental and regenerative biology, providing valuable insights into stem cell dynamics, the aging process, and mechanisms of tissue regeneration.
Hydra vulgaris feeds on tiny aquatic invertebrates. They utilize nematocysts, or stinging cells, located on their tentacles to capture and paralyze their prey. Lab studies have shown that hydra do not exhibit senescence, meaning they can live indefinitely under stable conditions. If a hydra is cut in half, each piece can regenerate into a whole new hydra. Although Hydra vulgaris lacks traditional eyes, they are not entirely blind. The animal is equipped with photosensitive cells, especially on its tentacles, which enable it to detect light and react accordingly.
The Cost of Eyes
Eyes are complex organs that require a significant amount of energy and neural resources to maintain and process visual information. A common factor among all the animals we have studied is their environment. In places where there is no light and vision does not provide any advantages, it is more beneficial for energy to be allocated to other areas, such as reproduction, enhancing other senses, or ensuring basic survival.
As a result, natural selection may favor the loss of eyes when they become unnecessary.
There are many diverse species that either never developed functioning eyes or whose ancestors had functioning eyesight, but the capability has subsequently devolved.
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REFERENCES
- Protas, M. et al. (2007) ‘Regressive Evolution in the Mexican Cave Tetra, Astyanax mexicanus’, Current biology, 17(5), pp. 452–454. [Accessed 19/07/2025]
- Balázs, G., Lewarne, B. and Herczeg, G. (2020) ‘Extreme site fidelity of the olm (Proteus anguinus) revealed by a long‐term capture–mark–recapture study’, Journal of zoology (1987), 311(2), pp. 99–105. [Accessed 19/07/2025]
- Robidart, J.C. et al. (2008) ‘Metabolic versatility of the Riftia pachyptila endosymbiont revealed through metagenomics’, Environmental microbiology, 10(3), pp. 727–737. [Accessed 19/07/2025]
- Macias-Muñoz, A., Murad, R. and Mortazavi, A. (2019) ‘Molecular evolution and expression of opsin genes in Hydra vulgaris’, BMC genomics, 20(1). [Accessed 19/07/2025]
