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The Role of Scales on Reptiles and Fish – Why Do They Have Them?

Throughout Earth’s history, one constant challenge for all animals has been attaining protection from the elements, predators, and microorganisms. One protective mechanism that has developed is the integumentary system and its various forms. In ancestral animals, such as fishes and reptiles, a robust and pervasive form of integument–scales–arose and has a variety of functions, including protection and locomotory assistance.

Scales were first developed in fish hundreds of millions of years ago, and since then they have undergone much evolution. There are four types of fish scales: cosmoid, ganoid, placoid, and elasmoid. Elasmoid scales have two subtypes, ctenoid and cycloid scales. Fish scales primarily serve two purposes: protection and locomotion. What distinguishes these various scales from each other is both their composition and how they balance those two functions. The more ancestral a scale, the better its configuration is for protection rather than locomotion (1). The more evolved, or derived, scales have a more balanced functionality between protection and locomotion. Cosmoid and ganoid scales are the most ancestral types of scales while ctenoid and cycloid scales are the most derived types of scales (1).

In a study done on Polypterus senegalus (Senegal Bichir), which possesses ganoid scales, scales were shown to function as preventative structures against punctures, as well as mitigators of damage from force (2). Despite the ganoid scale structure reflecting a more protective function, ganoid scales also assist with locomotion by allowing myomeres, the skeletal muscle tissues, to better pull serial tendons and allow for flexible movement (3). A study done on longnose gar (Lepisosteus osseus), which also possesses ganoid scales, showed that ganoid scales passively stiffen the body and increase tail beat frequency (3). Tail beat frequency controls undulatory propulsion, which influences a fish’s overall locomotory thrust. Taking a look at a more derived scale, the ctenoid scale, shows the same functions that the more ancestral ganoid scale possesses, however, the form of the ctenoid scale places more emphasis on locomotion than that of the ganoid scale (1). In a study done on striped bass (Morone saxatilis), which possesses ctenoid scales, it was found that the form of the ctenoid scales also serves a dual purpose of protection and locomotory assistance (1). Ctenoid scales have two layers to them: the bony outer layer and the collagen-based inner layer (1). The outer layer is characterized by hardness and stiffness, which offers protection against predators and the environment (1). The inner layer is characterized by strength and softness, which offers flexibility and muscular ease of movement (1).

Arkansas darters (Etheostoma cragini), which can be found on Chico Basin Ranch, like many other teleosts (bony fishes) possess ctenoid scales (4). Although there are differences in forms between all fish scale types, they are unified in their functions to protect and assist with locomotion.

The transition of life from aquatic to terrestrial necessitated adaptations in the integumentary system. Dwelling on land meant dealing with more intense ultraviolet light exposure, a higher potential for dehydration, and a much larger instance of abrasion.

We can observe scales in other animals such as the pangolin; these are the result of convergent evolution, where multiple ancestral species adapted similar structures completely genetically separate from each other. While fish and reptiles are closely related evolutionarily, their scales are markedly different. Fish scales are derived from the dermal layer of tissue where reptile scales rise from the epithelial tissue, which also allows them to regenerate faster.

Reptiles’ scales have a variety of specializations. The scales on snakes are smooth to reduce friction from movement, however, the scales on their ventral side are mobile, allowing for increased friction when the snake needs to climb or move up an incline (7). Scales can be specialized for defense as well as camouflage. Reptiles’ scales can be cycloid, granular, a bumpy texture that increases friction, which is seen in iguanas. They can also be keeled, which means they have a central ridge that allows for more rigidity, as well as taking a different form with an ossified (boney) base which are called scutes. This is seen in turtle shells, crocodiles, and stegosaurus plates.

Overall, the development of scales has been incredibly important for fish and reptiles. Despite their differences in composition, they function very similarly. Protection and locomotory support are the central similarities between the two animal groups’ scales. These key functions tie back to the need for animals to have a defense against the environment and predators, as well as the ability to move easily for a variety of different purposes. Scales play a vital role in the survival and success of fish and reptiles.

References:

1. Zhu, D., Ortega, C. F., Motamedi, R., Szewciw, L., Vernerey, F., & Barthelat, F. (2011). Structure and mechanical performance of a ‘‘modern’’ fish scale. Advanced Engineering Materials, 14(4), B185-B192. https://doi.org/10.1002/adem.201180057

2. Bruet, B. J. F., Song, J., Boyce, M. C., & Ortiz, C. (2008). Materials design principles of ancient fish armour. Nature Materials, 7(9), 1-8. https://doi:10.1038/nmat2231

3. Long, J. H., Jr., Hale, M. E., McHenry, M. J., Westneat, & M. W. (1996). Functions of fish skin: flexural stiffness and steady swimming of longnose gar Lepisosteus Osseus. The Journal of Experimental Biology, 199, 2139-2150. https://doi.org/10.1242/jeb.199.10.2139

4. BioLogics, RTEC, Inc. (2002, April). Recovery plan for the Arkansas darter, Etheostoma cragini Gilbert, in Kansas. https://kgi.contentdm.oclc.org/digital/collection/p16884coll14/id/601/

5. Chang, C., Wu, P., Baker, R. E., Maini, P. K., Alibardi, L., & Chuong, C. (2009). Reptile scale paradigm: Evo-Devo, pattern formation, and regeneration. Int J Dev Biol, 53(5-6), 813-826. https://doi.org/10.1387/ijdb.072556cc

6. Dhouailly, D., Godefroit, P., Martin, T., Nonchev, S., Caraguel, F., & Oftedal, O. (2019). Getting to the root of scales, feather, and hair: As deep as odontodes? Exp Dermatol, 28(4), 503-508. https://https://doi.org/10.1111/exd.13391

7. Marvi, H., Gong, C., Gravish, N., Astley, H., Travers, M., Hatton, R. L., Mendelson, J. R., III., Choset, H., Hu, D. L., & Goldman, D. I. (2014). Sidewinding with minimal slip: Snake and robot ascent of sandy slopes. Science, 346(6206), (224-229). https://doi.org/10.1126/science.1255718

By Colorado Parks and Wildlife Southeast Native Aquatic Species Team, Technicians Samantha Duven and Preston Ford.

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