Impact of Evolution on Mexicanus astyanax and Peracarida crustaceans

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Evolution, a phenomenon that has swayed many biological populations of successive generations, has been known to assist organisms in adapting to their surroundings. There have been several examples of this, such as the peppered moth and the cane toad. Investigating these species, I have discovered that the peppered moth had a light coloring that was darkened following the Industrial Revolution, because of the pollution of the time. Light colored moths were seen by birds more readily, which caused the mutation to take place as it was needed for survival in order to reproduce. The cane toads, an invasive species in Australia, developed longer longs and were bigger/more active once studied closely, so that they would be able to adjust to their territory in a better way.

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Species of crustaceans as well as certain fish have fallen prey to the process of evolution as well. These crustaceans consist of the cave dwelling species of the superorder Peracarida, a large group of Malacostracan crustaceans. Malacostraca is the largest of six classes of crustaceans, consisting of approximately 40,000 living species, typically demonstrating a vast array of body forms not limited to crabs, lobsters, crayfish, shrimp, krill, woodlice, and amphipods. Usually found in marine habitats, these animals contain 20 body segments, with the main divisions being at the head, thorax, and abdomen. It has been discovered that certain species of these crustaceans have lost their eyesight (possibly due to evolution), such as the shrimp. There may have been several reasons that contributed to this change, including the environment, the energy cost of vision, as well as the genetics of these species. Not only this, but there is also evidence of a Mexican cavefish species known as Astyanax mexicanus going eyeless as well. Delving into these two species, there could be a possible linkage between the causes of their eye loss, leading into an exploration regarding convergent evolution. Convergent evolution is “the process whereby organisms not closely related (not monophyletic), independently evolve similar traits as a result of having to adapt to similar environments or ecological niches.” This brought me to my research question, which asks, “To what extent are the factors of environment, energetic cost of vision, and genetics in blind crustaceans 3 orders of Peracarida (Mictaceans, Spelaeogriphaceans, and Thermosbaenaceans) similar to those in blind Mexican cavefish (Asytanax mexicanus)? How are they related convergently?” 

The environment of a species is often crucial and in many cases, influences their behavior as well as ways in which they adapt. The orders of crustaceans from the superorder of Peracarida that have lost their eyesight were cave dwelling, which means that they were exposed to the dark. The ancestors of these crustaceans had been evolving in the dark for millions of years, which caused them to abandon parts of the brain that corresponded to their vision. A study investigating how these crustaceans were living without light was conducted by Dr. Martin Stegner, who studied 3 orders of Peracarida. These orders, named Mictaceans (3 mm long), Spelaeogriphaceans (7 mm long), and Thermosbaenaceans (3 mm long), were eyeless both as larvae and adults. While Mictaceans and Spelaeogriphaceans did have stubby eyestalks, there were no eyes within them. The study soon discovered that all of these orders were located in difficult to find, perpetually dark underwater caverns. Mictaceans, also known as Mictoris halope, were only found in limestone caves of Bermuda containing underwater connections to the sea. Spelaeogriphaceans, also known as Spelaeogriphus lepidops, were found in dark limestone/sandstone caves situated on southern continents, with brackish water surroundings. Thermosbaenaceans, also known as Tethysbaena argentarii, were found mostly in thermal springs plus limestone caves. It is evident that limestone caves were one of the most common habitats for these crustaceans. [1]

According to Stegner’s team, observing the way these animals interacted had many practical obstacles, therefore making it quite hard to understand their biology. As the team could not study the living specimens, they compared the brains of the crustaceans with the brains of other 10 legged crusta such as crabs, lobsters, and shrimp in order to interpret the various functions of the brains of the crustaceans. Upon examination, they came to the conclusion that like the brains of the other crustaceans, the Peracarida crustaceans’ brains had 3 parts—the protocerebrum, deutocerebrum, and tritocerebrum. Usually, the optic lobe of sighted crustaceans (located in the eyestalks) has 4 clusters of cells, called neuro pupils. However, this was not the case for the Peracarida crustaceans. Researchers learned that Spelaeogriphaceans only have one neuro pupil located in their eyeless eyestalks, as quoted by Dr. Martin Stegner: “In a striking contrast [to other crustaceans], the optic center in the protocerebrum of S. lepidops is constituted by only a single neuropupil, whose orientation toward the tip of the eye stalk is the only hint at a former visual-coordinative function. Optic nerves are absent.” In the Mictaceans and Thermosbaenaceans, the obvious deficit of eyesight is much more advanced. Mictaceans possess a single neuro pupil that could correspond to the optic lobe, yet it is not inside the eyestalk or even familiarized with it. Thermosbaenaceans do not have any eyestalks, which means that there is no existing point of reference that would assist in determining if there is a neuro pupil. Stegner’s crew believes that Spelaeogriphaceans do not have a functioning neuro pupil, due to where it is placed. But, in the other 2 species, the function of their single neuro pupils remain a mystery as their single neuro pupils may have no correlation to the optic lobe, vestigial or not. Stegner himself made the point that the apparent brain loss within these 3 blind cave dwellers is “a vivid evolutionary example of how changing ecological environments—total darkness—have affected neuroanatomy. Researchers soon began to conclude that perhaps in order to make up for the visual inadequacy that the Peracarida crustaceans were experiencing, perhaps non visual senses might be more efficient than normal. Therefore, they began to investigate other sensory portions within the crustaceans’ brains. Most of the information that they found confirmed that the animals are anatomically compensated in other areas of their brain because of their lack of eyesight. In Mictaceans and Spilaeogriphaceans, their olfactory lobe (used for the purpose of smelling) was extremely large, however, this was not true for Thermosbaenaceans. Yet, all the species had nerve connections to their prominent antennae. It was clear that the crustaceans had received other benefits that would assist them to adapt without their eyesight. [2]

Stegner and his colleagues postulate that these crustaceans had been evolving for a long period of time (almost 200 million years) in order to reach where they are now. They think that the southern continental domains of the Mictaceans and Spilaeogriphaceans make up part of what onced to be a southern supercontinent called Gondwana. It is said that the region in which Thermosbaenaceans are found was surrounded by the Tethys Sea which covered Eurasia during the period of the Paleozoic era. Evolutionary scientists feel as though the predecessors of these species were trapped in underwater caves as a result of the breakup of Gondwana and the regression of the Tethys Sea. Thus, these 3 orders represent remains from a time in which the Earth’s geological movements detached them from the rays of light, and further attention was not brought to these species until the modern man came along for exploration.

Due to all of these aspects as well as research uncovered by Stegner and his team, it is safe to say that the environment of these Peracarida crustaceans did indeed play a large role in their loss of eyesight. Due to the amount of time they spent in the darkness, they were able to reduce usage of brain parts that had become unnecessary. This may also apply to the Mexican cavefish Astyanax mexicanus, who also live in dark environments. Asyntanax mexicanus, who are notable for having no eyes or pigment in their cave fish forms (they are of pinkish-white color making them look somewhat like an albino), reside in the Nearctic ecozone, as their native range includes the lower Rio Grande plus the Neuces and Pecos Rivers in Texas, as well as the central and eastern parts of Mexico. [3] Typically inhabiting freshwater habitats, these cavefish are found in 2 forms: cave and surface fish. Both fish are omnivores and approximately the same size (3 inches long), but the cavefish share a few different characteristics, such as physical appearance. The cave forms can either undergo degenerative sight or complete eye loss, which is reliant upon the exact population. For example, cave fish from the Pachon caves have lost their eyesight completely while cave fish from the Micos cave have limited eyesight. [4]

It is evident that the process of evolution is relevant to these cavefish through their dark environments. Charles Darwin, an English biologist best known for his contributions to evolution, essentially hypothesised that eyes could be lost by disuse over time, stating this: “By the time that an animal had reached, after numberless generations, the deepest recesses, disuse on this view will have more or less perfectly obliterated its eyes, and natural selection will often have affected other changes, such as an increase in the length of antennae or palpi, as compensation for blindness.”[5] This statement is certainly valid, and implications of it can definitely be seen with the cavefish to some extent.The phenotype of the cavefish (which consisted of many biological functions that were dependent on the presence of light) was, in fact, considered prone to natural selection and genetic drift when their surface dwelling predecessors entered the subterranean environment.[6] [7] Taking this into account, it can be seen that because of the dark environment of the cavefish, eyesight is neither disadvantageous nor advantageous, therefore mutations may occur in order to assist the cave fish to efficiently adapt to their environments without any negative consequences. Not only this, but there advantages to losing eyesight for the cavefish, just like with the Peracarida crustaceans. For instance, cavefish lacking their vision have given up their circadian rhythms (a process that influences behavior based on the time of day), which has made them much more efficient than fish that can see, as seeing fish have a metabolism that is still slave to the light-dark circle.[8] Also, as compared to fish that had the ability to see, the blind cavefish suffer a lower chance of infection and accidental damage, since they possess more amounts of protective skin that seals where their eyes would have been. Other advantages that the cavefish get to exclusively enjoy include having a better olfactory sense, which is also demonstrated by the Peracarida crustaceans.[9] There are a few arguments from the perspectives of creationists who claim that the cavefish are an example of devolution, as it showcases an evolutionary pattern of decreasing complexity.[10] However, this is not necessarily true as evolution is not a directional process; despite the fact that evolution usually results in increasing complexities, there should not be a reason for doubt just because the process shifts to a more simplistic approach— as long as it still serves the purpose of allowing organisms to become better accustomed to their environments.

In addition to the apparent loss of eyesight, (also known as a regressive trait due to the fact that the surface fish that had initially colonized the caves were able to see), cave forms evolved constructive traits as well. These traits differ from regressive ones because their presence/purpose is seen as a benefit, and is widely accepted. Much research has been conducted in order to find the mechanisms that drive the evolution of regressive traits, such as in the case of the eyesight loss in Astyanax mexicanus. Evidence has been collected from recent studies that further the claim that the mechanism is indicative of direct selection[11], or indirect selection by means of antagonistic pleiotropy[12], which regards the genes of the species.

The energetic cost of a species is often significant to how members of the species will develop. The high energetic cost of maintaining neural tissue is said to have severe pressure on the evolution of brain size, including aspects of the brain that contribute to sight, smell, touch, etc. Species will typically demand accompanying increases in nutrient intakes throughout the increase of neural tissue mass during evolution. Once food availability becomes low, the opposite occurs, and neural tissue mass decreases over time in an attempt to minimize the amount of whole body energy being spent. The impacts of nutrient limitation on neural mass during evolution are depicted clearly in animals that have evolved under ground or on nutrient poor islands, and it is common for the sensory modality of vision to be reduced under these circumstances. Neural tissue mass, energy demand, and adaptation are all concepts that are connected to one another. As there are innate obstacles in investigating regressive traits and selective forces, not many studies have been able to prove an adaptive reduction in neural tissue mass.[13]

This concept of energy cost has not been found to affect the orders of Peracarida that lost their eyesight to a large extent, but is heavily relevant to Astyanax mexicanus. As the evolutionary processes that resulted in the loss of eyesight for these fish are not known for certain yet, there is much speculation regarding them. One proposition suggests that active selection for regression typically saves energy rather than expending it when located in an environment that most likely has limited access to food and is void of primary production. Mexican tetra are especially applicable to the “expensive tissue hypothesis” due to how “the extant surface ecotype can be used as a proxy for the ancestral surface ecotype that diversified into caves (and continues to do so).”  A team under the lead of scientist Damian Moran decided to investigate further into the matter and embarked to quantify the energy that was saved by the Mexican tetra consisting of a regressed visual system. This was done by the comparison of organ sizes and brain/eye metabolic costs in dissimilar phenotypes of the species. The phenotypes mainly consisted of the surface ecotype, the Pachon cave ecotype, and two morphs that were of immediate eye size. Among the cave ecotypes, the Pachon population is considered to be the most disparate and have lost their eyesight because of the (almost) breakdown and resorption of the embryonic eye. Fish from the Micos cave, a phylogenetically young population with great amounts of genetic and phenotype variability, were one of the more intermediate phenotypes that were tested. Another phenotype tested that was intermediate was a Pachon/surface F2 hybrid. Aside from evident dissimilarities in pigmentation and their ability to process visual detail, the monophyletic group of Mexican tetra seemed to be more or less invariant in traits usually related to energy saving adaptations to environments that are food limited, for instance decrease in body mass or metabolic suppression. Sizes and growth rates regarding surface and cave ecotypes were similar, but there was not much evidence available that supported certain physiological adaptations to starvation resistance beyond differences in metabolic rate. It was also found out that the cave form was at least as engaging as the surface form and that the surface form displayed more cannibalistic behaviors. There is a possibility that  the mass of bodily functions such as the heart, gills, gonads, and digestive systems will alter among phenotypes if selective pressure to change the function or size of the organs has occurred. Differences in the relative mass of these organs are crucial to energy saving during diversification underground as well—but this has not been confirmed or closely looked into. The data that was collected by this study led by Damian Moran focusing on organ size, organ costs, and energy budgets were utilized to make accurate conjectures about the evolutionary results of changes in brain and eye size throughout the diversification of Mexican tetra underground. During the study, organ mass measurements were done in order to analyze whether various morphs of Mexican tetra (for example Micos cave, Pachon cave, surface, and surface/F2 hybrid) had different relative organ size. The organs measured comprised of the gills, heart, digestive system, gonads, brain, and eyes; body mass scaling relationships among ecotypes were assessed by analysis of covariance. (ANCOVA) The absence of differentiation amid ecotypes in the mass scaling of the heart, digestive system, and gonads, propose that these organs did not undergo strong selective pressure to alter during the underground diversification of the Mexican tetra. The gills were notably bigger in the Pachon and Pachon/surface F2 hybrid than the surface and Micos fish, indicating that the Pachon ecotype may have been under selective pressure to develop larger respiratory exchange surfaces to deal with hypoxic periods that might come about in caves. It was anticipated that there would be a perceived difference in relative eye and whole-brain size, and the brain mass discrepancies provided by the study matched results that were produced by another study that utilized high resolution microcomputed tomography scanning to calculate volumetric brain dissimilarities in surface and Pachon ecotypes. “Adult surface ecotypes were reported to have 22% larger brains by volume compared to Pachon ecotypes when the fish were 1 year old, and the present study observed a 30% difference in the relative brain mass of adult fish of varying sizes.” The strong correlation  concerning eye and brain mass noticed with surface and Micos ecotypes demonstrates the near coupling of retinal size and visual processing requirements. This theory is backed up by a study done in the past collating brain region volumes in the two phenotypes, due to the fact that the midbrain (which is given input from the optic nerve) in Micos fish is 50% smaller than that in surface fish, although the forebrain and hindbrain are practically the same in volume. Unalike to surface and Micos ecotypes, the whole-brain mass of Pachon/surface F2 hybrids was quite invariant even with a 10 fold difference in eye mass. A foregoing study of the brain dimensions and eye dimensions of this hybrid disclosed an identical trend, showing a drastically changed eye and brain advancing pathway compared to differing reduced eye phenotypes that gained regressed vision through a gradual evolutionary procedure. The considerable energetic cost of eyesight in Mexican tetra that had intact vision discovered in this study represents the worth and significance of this sense in an environment consisting of light. “The cost of the visual system as a fraction of minimal metabolism in juvenile surface fish (15%) approaches the cost of the human brain (about 20% to 25%)”. Consequently, it is very plausible that vision is under strong selective pressure to regress in caves, where light is not present and food is of short supply. The pressure for Mexican tetra to store energy and to remove the visual system into while diversification is occurring into caves is estimated to have been at its peak during the early life phase when the eyes and brains are commensurably very large, energy reserves are short,  and nutriment is needed to sustain the high growth rates usual of larval and juvenile fish.[14]

Based upon these factors, the two species of Mexicanus astyanax and Peracarida crustaceans can be believed to have gone through convergent evolution. Despite not being distinctively closely related, the two organisms have independently evolved similar traits as a result of having to adapt to factors such as environment or energy costs. Using the information regarding these two species, prognoses for the future regarding other species that may go blind in the future can be made. I personally believe that bats and moles may end up losing their vision. This is due mainly because of their environments as well as genetic modifications. The implications of studying the organisms of the Mexican astyanax and Peracarida crustacean clade are significant as they provide scope to research, comprehend, and predict species in the future that may eliminate their visual system. Not only this, but the Mexican astyanax and Peracarida crustaceans are quite understudied, leaving lots of potential for expansion upon the information that has already been found regarding them. Information regarding their genetics is limited, which is important to examine on deep levels as genetics account for much of the evolutionary process. Further investigation into their genome sequences can provide a vast amount of insight into how they have evolved from the past and why. Evolutionary change is often based upon the accumulation of many mutations, and in order for these mutations to be examined thoroughly, genetic sequences are greatly needed. Personally. I mainly decided to base my paper off these organisms because evolution has always been a concept that I have always been passionate about. I believe that evolution can truly demonstrate the entire history of a species and will potentially assist in the curing of many diseases, which is why it is so significant. The research I was able to do paired with careful examination of the background/history of the 2 clades will be extremely valuable to the endeavors that I take on in the future.

Worked Citations in MLA

  1.  Stegner, Martin, et al. “The Brain in Three Crustaceans from Cavernous Darkness.” BMC Neuroscience, BioMed Central, 7 Apr. 2015,

  1. Moran, Damian, et al. “The Energetic Cost of Vision and the Evolution of Eyeless Mexican Cavefish.” Science Advances, American Association for the Advancement of Science, 1 Sept. 2015,
  2. “Astyanax Mexicanus.” ITIS Standard Report Page: Mayetiola Destructor,
  3. Dowling, et al. “Evidence for Multiple Genetic Forms with Similar Eyeless Phenotypes in the Blind Cavefish, Astyanax Mexicanus.” OUP Academic, Oxford University Press, 1 Apr. 2002,
  4. Moran, Damian, et al. “Eyeless Mexican Cavefish Save Energy by Eliminating the Circadian Rhythm in Metabolism.” PLOS ONE, Public Library of Science, 24 Sept. 2014,
  6. Mitchell, Elizabeth. “Certain Blind Crustaceans Said to Show Evolution in Action.” Answers in Genesis, 7 May 2015,
  7. Reyes, Rodolfo B., and Rainer Froese. “Astyanax Mexicanus Summary Page.” FishBase,
  8. Soares, Daphne, and Matthewe L. Niemiller. “Sensory Adaptations of Fishes to Subterranean Environments.” OUP Academic, Oxford University Press, 1 Apr. 2013,
  9. Wilkens, H. “Genes, Modules and the Evolution of Cave Fish.” Nature News, Nature Publishing Group, 13 Jan. 2010,
  10. Protas, Meredith, et al. “Multi‐Trait Evolution in a Cave Fish, Astyanax Mexicanus.” The Canadian Journal of Chemical Engineering, Wiley-Blackwell, 3 Mar. 2008,

Brains/optic lobes of the 3 eyeless crustanceans

Mictoris halope (Mictacean)                                                Mexicanus astyanax fish



Adaptations of the optic center and olfactory lobes of the Peracarida crustaceans


[2] Mitchell, Elizabeth. “Certain Blind Crustaceans Said to Show Evolution in Action.” Answers in Genesis, 7 May 2015,

[3] Reyes, Rodolfo B., and Rainer Froese. “Astyanax Mexicanus Summary Page.” FishBase,

[4] Moran, Damian, et al. “The Energetic Cost of Vision and the Evolution of Eyeless Mexican Cavefish.” Science Advances, American Association for the Advancement of Science, 1 Sept. 2015,

[5] Charles Darwin, Origin of Species (1859)

[6] Soares, Daphne, and Matthewe L. Niemiller. “Sensory Adaptations of Fishes to Subterranean Environments.” OUP Academic, Oxford University Press, 1 Apr. 2013,

[7] Wilkens, H. “Genes, Modules and the Evolution of Cave Fish.” Nature News, Nature Publishing Group, 13 Jan. 2010,

[8] Moran, Damian, et al. “Eyeless Mexican Cavefish Save Energy by Eliminating the Circadian Rhythm in Metabolism.” PLOS ONE, Public Library of Science, 24 Sept. 2014,

[9] Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 315, 1997, ISBN 0-86542-256-7

[10] Dawkins, R.: Climbing Mount Improbable, W. W. Norton & Co, 1997, ISBN 0-393-31682-3

[11] Protas, Meredith, et al. “Multi‐Trait Evolution in a Cave Fish, Astyanax Mexicanus.” The Canadian Journal of Chemical Engineering, Wiley-Blackwell, 3 Mar. 2008,


[13] Moran, Damian, et al. “The Energetic Cost of Vision and the Evolution of Eyeless Mexican Cavefish.” Science Advances, American Association for the Advancement of Science, 1 Sept. 2015,

[14] Moran, Damian, et al. “The Energetic Cost of Vision and the Evolution of Eyeless Mexican Cavefish.” Science Advances, American Association for the Advancement of Science, 1 Sept. 2015,

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