Written By: Arman Momeni

Introduction:

The Glass Frog is one of the most fascinating species on planet earth. With Exactly as the name describes, the glass frog has a glass-like translucent underside, which puts its internal organs on display. As see in Figure 1, when viewing the glass frog from underneath, one can see its heart, gall bladder, intestines, veins, and even some of its bones. There are over 150 different species of the glass frog worldwide, however, only recently have scientists found research on the characteristics and interesting aspects of the Glass Frog (Gallagher, 2022). This article will explore everything from the evolution of the glass frog, all the way to its fascinating respiration techniques.

Due to the tiny size of the Glass frog and its ability to camouflage, experts believe that there are several different species of the glass frog that have not yet been discovered. Glass Frogs are only found in South America and Central America. Their habitats strictly include humid montane rainforests and tropical lowland to mid-elevation mountain forests. Most commonly, glass frogs are found in countries such as Mexico, Belize, Costa Rica, Panama, Columbia, Guatemala, and Honduras (Gallagher, 2022).

Glass frogs have this incredible ability to jump long distances, which is useful in the presence of predators. Fascinatingly, glass frogs can jump more than 10 feet. Glass frogs are also great climbers and use their climbing abilities to move further into trees and avoid any threats. Glass frogs have tiny yellow suction cups on their toes, which allows them to grip onto humid branches and leaves, greatly aiding their climbing ability (Alfred, 2022).

Despite the name, only the underside of the glass frog is translucent, whereas the skin on their backs is typically a bright lime green. Glass frogs are nocturnal animals and like to stay hidden or sleep under tree leaves during the day. Once the sun goes down, glass frogs leave their trees to hunt for food and seek potential partners (Gallagher 2022)

Due to their translucent skin, which gives them the ability to camouflage, glass frogs are less likely to be seen by predator. Additionally, as shown in figure 4, their green skin allows them to effortlessly blend into tree leaves. Their camouflaging abilities make them more likely to survive in the wild than their opaque frog counterparts (Allred, 2022).

Even though the glass frogs have a lack of teeth and have very short tongues, they are actually carnivores. The diet of a glass frog mostly consists of small insects such as ants, tiny spiders, flies, and crickets (AZ Animals, 2023). In certain conditions, glass frogs may even prey on other smaller grogs if they are given the opportunity (Gallagher 2022). Most glass frogs grow to only be an inch long, but due to the wide range of the species, glass frogs fluctuate greatly in size.

Taxonomic Classification:

The taxonomic classification of the glass frog has been highly debated by scientists and biologists ever since the discovery of the Glass Frog in the late 19th century. Glass frogs occur in 12 genera of frogs, however most of the glass frogs belong to the Hyalinobatrachium, Cochranella, or Centrolene genera of frogs (Allred, 2022). Nonetheless, listed below is the most up-to-date and confirmed Taxonomic Classification of the Glass Frog.

Domain: The domain of a species is the largest group of classification. There are currently only 3 agreed groups of domains: the Archaea domain, Bacteria domain, and Eukarya domain (BD Editors, 2019). The domain of a species is even more general than plants and animals and relate more closely to cell anatomy (“Taxonomic Classification: From Domain”, 2022). The Glass Frog falls under the Eukarya domain, which includes everything that has a nucleus and organelles that are membrane-bound (BD Editors, 2019). See Figure 6 for an example of a Eukaryote cell.

Kingdom: Biological kingdoms allow for classification that is based on the ancestry and evolution of a species. Additionally, species in the same kingdom share very similar characteristics when it comes to their functionality and their growth/development. There are 6 kingdoms in biology: Eubacteria, Archaebacteria, Protista, Plantae, Fungi, and Animalia (BD Editors, 2019). The Glass Frog falls under the kingdom Animalia, which includes all animals. Animalia is a kingdom of multi-celled organisms who do not produce their own food (AZ Animals, 2023).

Phylum: After the animal kingdom, there are generally 7 different phylum in which a species would fall under. The Glass Frog belongs to the Chordata phylum, which contains vertebrates that develop a notochord. A notochord is a cartilaginous skeletal rod that is used to support the body when it is in the embryo and can often become a spine (AZ Animals, 2023).

Class: Phylum groups are then split up even further, into their classes. The Chordata phylum splits into 7 different classes, which contain more specific shared characteristics between species. For example, whether an animal is two-limbed or four-limbed would differentiate their class. The Glass Frog belongs to the Amphibia class, which contains four-limbed ectothermic vertebrates (AZ Animals, 2023). Ectothermic animals are cold blooded and cannot regulate their own body temperature (Kennedy, 2019).

Order: Each class is separated even further by establishing a species’ order. Organisms in the same order are generally comprised of families, which share very similar behaviours and nature. The Glass Frog belongs to the order Anura. The order Anura includes frogs and toads that are grouped into approximately thirty families (Helmer and Whiteside, 2005).

Family: In each order, there are several different animal families. At the family level, the species have very similar features. For example, all cats and all bears fall under the same family. The Glass Frog falls under the family Centrolenidae, which contains all species of the Glass Frog (AZ Animals, 2023).

Reproduction Process:

Many species of glass frogs are nocturnal and spend their days hidden under leaves and in branches. At night, the frogs go off to hunt and search for a potential partner. Male glass frogs emit a distinctive mating call to obtain the attention of females. As glass frogs get older, the volume of their mating call becomes louder (Rainforest Alliance, 2012). After male frogs call upon female frogs using their distinctive mating call, they sit on leaves that are over streams or lake edges, waiting for a female to hear the call and arrive to reproduce (Zugg, 2022). Female glass frogs are attracted to the male glass frogs based on the depth and resonance of their croaks. Therefore, the more powerful a frog’s mating call is, the greater chance he has at finding a partner (Allred, 2022)

The frogs reproduce and mate on the leaf in a physical position known as amplexus. As shown in Figure 8, in amplexus, the male clasps onto the back of the female frog with his arms wrapped around her waist. The female frog then deposits her eggs on the leaf and leaves, leaving the male in charge. The male frog’s job is to protect the eggs (see Figure 10) from predators; however, the male frog continues to call upon additional females, hoping to father several batches of eggs. By indulging in reproduction with several other females, the male frog must be aware of the different phases that the eggs are in and must ensure to keep an eye on all the eggs (Zugg, 2022).

One of the main predators of glass frog eggs is the fruit fly. The fruit fly lays its eggs on the batches of the frog’s eggs as a clever mechanism to attack the eggs. The fly’s eggs hatch much faster than the eggs of the glass frog, thus, the maggots are able to feed on the frog embryos. The ability to destroy such a vast number of eggs makes the fruit fly one of the greatest threats to the growing glass frog population (Zugg, 2022).

The glass frog is an arboreal frog, which means that it lives almost exclusively within the trees. However, when it is time to reproduce and lay eggs, the frogs can be found on low hanging branches that are near or around running water. The glass frog prefers to lay its eggs, normally in batches of 18-30, on the underside of the leaf. The reason that glass frogs like to lay their eggs on branches that are near running water is so that once they hatch, the tadpoles drop directly into a body of water (Zugg, 2022).

Respiration

The glass frog is extremely unique when it comes to respiration, as it contains three completely separate respiratory surfaces (see Figure 11). Their three sources of respiration include: obtaining oxygen through their skin when they are submerged in water, using the breathing membrane on the membrane of their mouth, practicing the traditional mouth-lung breathing method. The frogs’ ability to extract oxygen from the surface of their skin has allowed certain frog species – such as the Bornean flat-headed frog – to evolve to not have lungs all together, obtaining their complete oxygen reservoir through their skin (“Meet these incredible animal”). Let’s take a more in-depth look at the Glass Frog’s three respiratory surfaces.

Skin:

A key thing to note when discussing respiration in glass frogs is their ability to perform cutaneous respiration, which is a common trait in amphibians. Cutaneous respiration is when animals have the ability to breath through their skin (Loy, 2021). Despite having lungs, a majority of the frog’s respiration occurs through their skin. A frog’s moist skin is thin and marbled with blood vessels and contains capillaries that are close to the surface (Miller 2020). The thin membranous skin allows respiratory gasses to readily diffuse, moving them along their concentration gradients between blood vessels and the surrounding environment (Brown). Although moisture plays a key role in the skin respiration, frogs can still perform cutaneous respiration when they are not in the water. Frogs have glands in their skin, which produces mucus that keeps the skin moist and allows for successful respiration even when the frogs are on dry land (Miller, 2020).

Lung:

Lungs are only found in adult frogs and are poorly developed, making them one of the less efficient means of gas exchange (Brown). All frogs go through a process called metamorphosis, where they mature from their tadpole stage into mature adult frogs. One of the main transitioning points in metamorphosis, is the frog’s transition from having gills to having fully functioning lungs. A newly hatched tadpole’s gills are external and are able to take in oxygen when water passes over them. As a tadpole matures the gills are absorbed by the body and become an instrumental part of the frog’s internal structure (Miller, 2020).

In mature frogs, they rely on their lungs to breathe when they are in a state where they need more respiration than their skin alone can provide (Miller 2020). The breathing process in frogs contains large similarities to the breathing process in humans. For instance, the frog inhales through their nostrils and takes that air down into their lungs. Nonetheless, the process is not all the same. Frogs lack a rib and a diagram, which means they do not have the same anatomical resources as humans that aid in the expansion of the chest and the decrease of pressure inside of the lungs. To successfully inhale air, the frog lowers the floor of its mouth, enlarging its throat. Next, it opens its nostrils, which allows for air to enter into the open and expanded mouth. The nostrils are then closed and the floor of the mouth contracts, forcing the air down into the lungs. To get rid of carbon dioxide, the frog moves the floor of its mouth down, allowing it to vacuum the air out of the lungs and into the mouth; then it opens its nostrils, takes in the floor of its mouth, and forces the air out of the nostrils (Brown).

Mouth:

Finally, the frogs also have a moist lining on their mouth, which acts as another additional surface for respiration. The respiratory lining on the mouth brings oxygen into the bloodstream from the surrounding air and is also able to diffuse excess carbon dioxide back into the environment. When the frog is at rest, mouth respiration becomes its most prominent means of breathing (Miller, 2020).

Circulation:

The glass frog shares an almost identical circulatory system to all other frogs in the Anura order. As they are amphibians, glass frogs have a closed circulatory system and have only one heart to circulate blood throughout their bodies (BD Editors, 2019). Amazingly, due to its translucent skin, the heart and heartbeat are visible through the underside of the frog (see Figure 12).

In addition to having a visible heart, the glass frog has an astonishing ability to make its arteries and veins disappear as a camouflage tactic (see Figure 13). As they are nocturnal, while the glass frogs sleep during the day, they are vulnerable to predators. Thus, the glass frogs must implement an effective camouflage technique in order to hide from potential predators, such as wasps or even snakes. In order, to hide their veins and arteries, the glass frog hides its red blood cells into its liver. Since they are at rest, their blood cells transport a very low amount of oxygen and scientists have theorized that the frogs have a specific process targeted at keeping their cells alive during the camouflage process. Astonishingly, the frogs can pack up to 90% of their blood cells into their liver – an area of extremely small volume – and still not suffer from blood clotting (Daniel, 2022). Not only is the glass frog’s ability to camouflage and hide its blood cells impressive, but its ability to avoid blood clots poses a potential gateway and background for humans to develop anti-clotting mechanisms.

The glass frog has a three-chambered heart, containing two atria and one ventricle. The right atrium acquires deoxygenated blood from the veins and the left atrium acquires oxygenated blood from both the lungs and the skin of the frog (BD Editors, 2019). Some blood mixes within the heart’s ventricle, which in turn, reduces how efficiently oxygenation happens. Nonetheless, the advantage to this set up is that there is a high pressure build up in the vessels that is able to push blood to the lungs and to the body (“Overview of the Circulatory System”). In humans, there are two circulatory circuits or pathways. The pulmonary circuit and the systemic circuit (National Cancer Institute). However, frogs actually have three circuits for their circulation. Like humans, they have a systemic circuit, which pumps oxygenated blood around the body, and a pulmonary circuit, which transports blood to the lungs to oxygenate it. Additionally, frogs also contain a pulmocutaneous circuit, which is found in most amphibians (BD Editors, 2019).

Pulmocutaneous circulation is a necessary part of the amphibian circulatory system, and its main responsibility is to direct blood to the skin and lung capillaries of the animal (see Figure 14). As stated previously, the blood mixes within the ventricle; however, this is alleviated by a ridge that diverts oxygen-rich blood through the systemic circuit and deoxygenated blood to the pulmocutaneous circuit, which allows for effective and efficient gas exchange through the lungs and the frogs moist skin (Campbell, 1981).

Digestion:

The digestive system in frogs is quite similar to humans, as it is built up of a digestive tract. The frog’s digestive tract is built up of its mouth, pharynx, esophagus, stomach, small intestine, large intestine, and cloaca. Additionally, just like humans, the frog uses accessory organs such as its tongue, teeth, salivary glands, gastric glands, pancreas, liver, and gallbladder to aid in certain aspects of digestion (“Digestive System of a frog”). See Figure 15 for a breakdown on the digestive system of the frog.

Mouth:

Just like humans, the mouth is the starting point of the digestive tract. The frog’s mouth is a very wide and extends entirely from one side of the frog’s face to the other. Frogs have very bony jaws, which bound the mouth. The upper jaw is fixed in place; however, the lower jaw moves up and down. Unlike humans, frogs normally swallow their food whole (Karki, 2020). The frogs’ tongue mixes the food with saliva. Saliva is crucial because it is able to convert starch to sugar.

Pharynx and Esophagus:

After the mouth, the food is moved to the pharynx, and then to the esophagus. Like humans, the esophagus of a frog acts as a path for the food to efficiently reach the stomach. In the stomach the true breakdown of food can begin (“Digestive System of a Frog”).

Stomach:

The stomach of the frog lies on the left side of its body, where it is attached to the dorsal body. The stomach of a frog is very similar to that of a human. The digestion occurs with the help from digestive enzymes that are secreted by digestive gland. The frog’s stomach is also able to expand through the many folds that the inner lining of its stomach contain (“Anatomy of the frog”).

Small Intestine:

Arguably the most important part of the digestive process, the absorption of nutrients takes place in the small intestine of the frog. The small intestine is held in place by membranous tissue known as mesentery. This tissue prevents the small intestine from moving in the abdominal cavity. Food that is partially digested moves through the small intestine. The small intestine is then divided into the duodenum and ileum, both present in the human digestive tract as well. The process of digestion in the small intestine is mainly caused by bile and pancreatic juice. Bile is produced by the liver and pancreatic juice is secreted by the pancreas (“Digestive System of a Frog”).

Large Intestine and Cloaca:

In frogs, the large intestine is quite short, ranging normally around couple centimeters. It opens directly into the cloaca through the anus. The opening is protected by an anal sphincter. Similar to the folds in the stomach, the inner lining of the large intestine also forms several longitudinal folds. The large intestine’s main purpose is dedicated to the re-absorption of water as well as preparing and storing excrement. The cloaca, also known as the terminal, is the final part of the digestive system. The cloaca opens to the outside through a vent, or a cloacal aperture and it lies at the very end of the frog’s body (Karki, 2020).

Evolution & Adaptation:

Evolution of Frogs as a Whole:

Overall, the Anura order is one of the most diverse orders on the planet, with over 5000 known species of frogs discovered by biologists (“Frog Fact Sheet”). See Figure 16, which displays some of the different species of the Anura order that have been discovered.

Even though frogs are one of the most diverse groups of vertebrates, for a long period of time there was a lack of genetic data, which hindered researchers’ abilities to trace the evolution and diversification frog species. Fortunately, a study from a US-Chinese team of researchers in the PNAS journal found that frog evolution and the reason for their immense diversification is primarily due to the asteroid strike that killed the dinosaurs. The asteroid released over a billion times mor energy than an atomic bomb and wiped out three-quarters of all life on earth (see figure 17). However, the asteroid happened to create the perfect conditions for the onset and rise of the frog species. Frogs were able to thrive so well in this environment because they were best able to adapt and evolve to the new habitats that were being rebuilt after the asteroid (“Frog evolution linked to dinosaur asteroid strike”, 2017). Just as Charles Darwin’s theory of evolution states: those best suited for the environment at that time, are most likely to survive, reproduce, and pass those traits on to the next generation. At the time of the asteroid strike, frogs happened to be best suited for the new habitats, which allowed for such diverse evolution and adaptation.

Evolution of the Glass Frog:

Glass frogs are from the family Centrolenidae and is part of the order Anura, which includes all frogs and toads. The glass frogs share similarities to many other frogs; however, the are some key differences that distinguish the Centrolenidae family. Clearly, translucent skin is found only in glass frogs, but another unique trait in glass frogs is their reproductive behaviors. As explained above, male glass frogs lie on their eggs to protect them, and they lay their eggs on leaves instead of inside of the water like most frogs (Zugg, 2022).

The evolution of the glass frog from earlier Anura ancestors is still actively researched, as a conclusive answer is yet to be found on the exact phylogeny of the glass frog. Nonetheless, a molecular phylogenetic study was able to find more conclusive information on the evolution of the glass frog. The molecular phylogenetic study found that the Centrolenidae family is very ancient and diverged from other frog species around 100 million years ago, predating the asteroid strike 66 million years ago. Scientists have been able to find fossils that date back 70 million years, to the Late Cretaceous period (see Figure 18), which show similar molecular homology to present glass frogs. However, the ancient counterparts which were found lacked the translucent skin of the modern glass frog (Juan M Guayasamin et. al, 2008). Nonetheless, researchers are continuing to use molecular phylogeny and other methods to trace the exact evolutionary history of the glass frog and understand when and how it developed its infamous see-through skin.

Why the Glass Frog is Translucent – Strategic Adaptation:

The glass frog has its signature translucent skin, which puts their bones intestines, and beating hearts on display (Bitell, 2022). The glass frogs translucent skin makes it difficult to find in the wild, which is why their skin has not been able to be investigate thoroughly. Nonetheless, new research from the Proceedings of the National Academics of Sciences found that the translucency of the frogs skin is a defense mechanism, which the frog has evolved over time. The glass frogs have green backs and as they spend most of their time in trees, the glass frogs stomach is usually pressed against a leaf, rendering its translucent skin useless (Spivack, 2022). However, researchers discovered that the glass frog had this fascinating ability to change the color of its skin. The frogs mostly looked green, but they had the ability to be brightened or darkened depending on their background. The reason for the frog’s green skin is not because of its natural pigmentation, but because the glass frog is constantly changing its skin color to match the green of the leaf that they are placed on. Overall, research states that the glass frogs translucent skin is a trait that has been adapted over time to protect the glass frog from dangerous predators within their habitat (Fox, 2020).

The Glass Frog’s Adaptation in Changing Environments:

Most frogs live in aquatic and swampy habitat, as their skin requires fresh water for survival. However, glass frogs primarily live within trees, which has opened them up to new prey, and habitats to adapt to. Within the molecular phylogenetic study (mentioned in the “Evolution of the Glass Frog” section), researchers were able to find several key adaptations and differences, which separate the glass frog from other frogs. Glass frogs had reduced webbing on their toes to help them climb trees, as they are arboreal amphibians. They also found that the glass frog had a longer and more pointed snout when compared to other frog species. They concluded that the change in their facial structure helps them catch their smaller sized prey such as insects, which make up the majority of their diet. Finally, the glass frogs adapted adhesive pads on their fingertips, which help them effectively grasp onto smooth surfaces such as leaves where they will mate and lay eggs (Guayasamin et al., 2008).

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An Invisible Animal: The Glass Frog

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