Everything you want to know about the glass frog.
The family Centrolenidae refers to a group of small nocturnal and arboreal frogs that live along streams and moving water in the humid forests of the tropical Americas. There are currently more than a hundred species, and they are distributed from southern Mexico, through Central America, to South America, as far south as parts of northern Argentina, Paraguay and Bolivia.
Photo credit: Bill Love
Glass frog
With the recent increase of exploration in the forests of neotropical America, many species have only been described in the last 40 years. Glass frogs are difficult to find and collect, due not only to their small size and color, but also because of the extreme areas they sometimes inhabit. Glass frogs are often found along streams that are nearly impossible to walk along, let alone collect specimens. With new areas constantly being explored in tropical America, more species are sure to be discovered and described.
Glass frogs get their name from the semitransparent nature of their skin, and, in some species, the transparent ventral surface. Most of the specimens from this family are light green with small spots ranging from yellow, white, black and blue. The frogs typically range in size from 20 to 30 millimeters, but some species, like Centrolene geckoideum from the Pacific Andean slopes of Colombia and Ecuador, reach larger sizes.
Centrolenidae Family and Genera
The Centrolenidae family of glass frogs was once composed of just two genera: Centrolenella (Noble, 1920) and Centrolene (Jimenez de la Espada, 1872). All of the smaller species were placed in the genus Centrolenella, and the one large species, geckoideum (Jimenez de la Espada, 1872), was placed in the genus Centrolene. Due to the recent work of Ruiz-Carranza and Lynch in 1991, the family has since been reclassified with respects to the genera. There are now three separate genera recognized in the family: Hyalinobatrachium, Cochranella and Centrolene.
The genus Hyalinobatrachium (Ruiz-Carrana and Lynch, 1991) contains the species that have white bones and a transparent parietal peritoneum (ventral surface) in live specimens. The transparent ventral surface allows one to view the viscera (internal organs), such as the heart, intestines and liver. Most of the species in this genus have a white visceral lining covering most of the digestive organs. This is in addition to the presence of a small bulb-shaped liver, which is different than the three-lobed liver that is present in many species of the other two genera. The name Hyalinobatrachium comes from Greek and means "glassy little frog."
The genus Cochranella (Ruiz-Carrana and Lynch, 1991) contains the species that, in life, have visible green bones, which are most noticeable in such areas as the arms and legs. Upon introduction to preservative, the green coloration in their bones typically fades after only a short time. The specimens from this genus also have a partially white-covered parietal peritoneum, which only allows for the viewing of the viscera in the lower abdominal area. The partial white parietal peritoneum looks like a white vest covering the upper area of the ventral surface. The specimens from this group have a colorless viscera and colorless walls of most the digestive organs.
The genus Centrolene (Jimenez de la Espada, 1872) refers to animals similar to those species from the genus Cochranella. Both genera have green bones in live specimens and a partial white parietal peritoneum, but the specimens from the genus Centrolene are set apart due to the presence of a humeral hook along the anterior surface of the humeral bone on the upper arm. This humeral hook is present on all male specimens, and a few females also have this structure. It is believed that this hook serves a purpose in breeding activities or in combat between territorial males.
Glass Frog Habitat and Behavior
Glass frogs are nocturnal, arboreal and riparian (living along streams). Centrolenid specimens can most commonly be found, while active, in the night among the vegetation of many forest streams and moving water bodies in neotropical America. Glass frogs occupy a large range of altitudes, from sea level to near 3,800 meters (McDiarmid, 1983). During daytime, the species are typically sleeping in the vegetation and terrestrial substrate near streams, and are difficult to locate.
One species can be found guarding its egg masses even in the daylight hours. The reticulated glass frog, Hyalinobatrachium valerioi (Dunn, 1931), from central Costa Rica to Ecuador, is unusual because the male specimens will guard their eggs in the daytime. Hyalinobatrachium valerioi males will guard egg masses all day, apparently protecting the eggs from predatory insects and dehydration. The males of H. valerioi will often be found sleeping next to, or actually touching, the egg masses on the underside of the leaves during daylight hours (pers. obs. 1998, 1999). Other species of glass frogs show egg-attendance behaviors, but usually only for a couple hours following oviposition. The adults retreat to their sleeping sites during the light of day and only sometimes return to attend their eggs the following evening. In a study done in 1982 by Susan K. Jacobson, it was noted that males of Hyalinobatrachium fleischmanni (Boettger, 1893) often attend eggs on subsequent nights later in the evening hours, 71 percent of the observed instances happening after midnight (Jacobson, 1985). The probable advantage in waiting until later in the evening to attend eggs, when there are less females ready to breed, is that it decreases lost breeding opportunities for these males.
In 1998, while lowering a leaf to make a closer observation of male H. valerioi specimens on the underside of a banana leaf, I witnessed one male take a position that spread himself directly over one of his egg masses, apparently in a defensive manner. Male glass frogs can be found calling from leaves overhanging, or near, the stream's surfaces on most humid nights during the rainy season. One cloud forest species of glass frog, Centrolene buckleyi (Boulenger, 1882), is reported to use bromeliads to hide its eggs (Cannatella, 1997). Glass frogs have also been witnessed calling from vegetation above, or near, dried-up stream beds, apparently waiting for approaching rains to bring the water that is needed for the survival and development of the larvae.
Little is known about the females' non-breeding activities, but they are presumably present among the vegetation bordering the forest's streams during the night hours. In the absence of amplexus, the females have proven much harder to locate than the calling males. One method of identifying females in many species is the presence of eggs that are visible through the transparent dermal layer of the lower abdomen. Another female identification method in most specimens of the genus Centrolene is the absence of the humeral hook.
The feeding behavior of glass frogs typically takes place while they are active during the evening hours, but glass frogs may also be active and feeding during rain showers in the daylight hours as well. One phenomenon I have noticed in my field work with glass frogs is the tendency for individuals to be found further up the banks of the streams, in the lower vegetation, from the late hours of dusk to the early evening (l730 hours to 19OO hours). I feel this may be a "feeding period" for these frogs, during which they acquire nutritional support for their ritual evening breeding activities. In this lower thick vegetation, there are typically abundant supplies of small arthropods, and I have witnessed glass frog individuals feeding upon these abundant arthropod populations (pers. obs., 1998, 1999). More investigation of the frogs in regards to this idea of a feeding period need to be done in order to generate some stable data. Glass frogs typically feed on a variety of smaller arthropods that are present among the vegetation along the stream's banks.
Glass Frog Reproductive Behavior
The reproductive behavior of glass frog species is greatly dependent on the rainy season in most areas, except those that lack a defined dry season, and the species typically reproduce throughout the year. On most humid nights, especially after a rainstorm or even during a light shower, the male centrolenids can be heard making their mating calls along the stream's vegetation. One study done in 1982 by Mark Hayes, found that specimens of H. fleischmanni show a much higher activity and calling tendency in relative-humidity levels above 93 percent. The mating calls of male glass frogs are variable between species, but they can typically be described as single, rising whistles that have a duration of less than a second, high-pitched trills or a series of short, high-pitched peeps. Males will call from chosen sites along the stream that exhibit preferable characteristics for egg deposition directly above the stream water.
Male centrolenids are fairly territorial and typically do not tolerate the presence of other males in close vicinity to their particular chosen calling sites. It is not common to witness physical combat between two males over a calling site, but physical confrontation does occur. It usually takes place when an intruding male enters the vicinity (typically within 10 centimeters or the same leaf [pers. obs. with H. fleischmanni in 1998 and 1999]) of another calling male. When the original male observes an intruding male, he will then change his normal advertising call to a lower soft-pitched "preep," and orient himself toward the intruding male. Next, the defending original male will slowly approach the intruder and often jump on his back in a behavior similar to amplexus, but usually within a second or so, the intruding male will wiggle free from the original male's grasp and retreat to another site. The victorious male, if the original, will then resume the typical advertisement call within a minute or two after the confrontation. I have witnessed groups of several male H. valerioi sharing the same banana leaf overhanging a small stream. Evidence of obvious breeding and calling activity from all of these males comes from the presence of several egg masses distributed among the population of resident egg-guarding males.
The reproductive strategies of glass frogs differ among various species, but, in general, the males call from chosen sites, and when an interested female comes close, usually within several centimeters, the male jumps onto her back and initiates amplexus. In some species, such as H. fleischmanni, the female often nudges the side of the calling male and crawls underneath him. The male next grasps the female and commences with amplexus behavior (Jacobson, 1985). Amplexus typically lasts for a couple hours or more, after which, if the pair are successful, they deposit typically between 20 to 30 eggs on an appropriate surface overhanging, or close to, stream water. The eggs typically take about two weeks to develop and hatch, often falling into the water below on rainy nights. One suggestion for larvae hatching on rainy nights, is the possible higher turbidity in the streams, due to the rain washing sediments from the land. In this turbid water, the chance for larvae survival from predatory fish is much higher due to reduced visibility (McDiarmid, 1983).
Little is known or reported about centrolenids natural larvae stages, but it is believed they burrow into the detritus that builds up in slower sections of streams and undergo development there.
Glass Frog Captive Care
Appropriate housing for glass frogs is very important for the long-term survival and health of these anurans. Much care and planning is necessary for both the external and internal structures. Because most glass frog species are arboreal in nature, it is best to house them in an enclosure that provides plenty of vertical space for plants and other structures that the frogs will use in captivity. I like to use enclosures that are a least 150 percent higher than they are wide. For example, if the enclosure is 50 centimeters wide, the preferred height is 75 centimeters. You must consider the number of specimens and species you wish to house. In my experience, it is best to house no more than two males and three females in an area 75 centimeters tall by 30 centimeters lateral depth by 35 centimeters wide. These dimensions equate to a 20-gallon area, so, considering the number of specimens for this allotted space, it works out to one frog per four gallons of leaf space.
Glass frogs also benefit from the available light in their ecosystems. Make sure to provide a source of light with the appropriate spectrum and intensity for all the plant and animal specimens. Ventilation allows for the exchange of fresh air into the enclosure. Fresh, clean air is vital to combat problems with pathogens and fungus. Humid, stagnant air is a potential problem for both glass frogs and the plants they live on. Provide enough ventilation to allow fresh air to constantly enter, but be careful not to overdry the enclosure. An ecosystem that is too arid can be almost as harmful as one with stagnant humid air. The vivarium should always smell humid and fresh.
Glass Frog Captive Diet
Glass frogs are excellent little predators, possessing great vision in their little eyes, which are oriented at 45 degrees forward to view the distance of their small prey. They feed by leaping at their prey with an open mouth, and even at a distance of several centimeters, they are dead-on accurate. Glass frogs in the wild feed on a wide variety of live insects and other arthropods, and in captivity the same menu is best. I feed my specimens a diet of drosophilid flies. Glass frogs have incredible appetites, and there is a vital need to have a constant supply of small insects available for them when they are active at night. Other possible food items are any small, soft-bodied arthropods, but use care that they are not poisonous or otherwise dangerous food items.
Glass Frog Captive Breeding
The captive breeding of glass frogs depends on the conditions of the enclosure and the specimens. Healthy specimens in an appropriate enclosure with lots of plants and a moving water source can be encouraged to breed with higher humidity levels. These levels can be attained with heavy spraying or an automated rain-chamber device. An automated rain-chamber device simply refers to any automated spray system that simulates rain showers. The specimens will be in breeding readiness, which is most often demonstrated by calling males in conditions resembling the rainy season. The females must also be given plenty of food to promote the development of healthy eggs, which may be laid once they have successfully gone into amplexus with chosen males.
Egg masses in the wild are typically deposited on the surfaces of leaves overhanging streams below, but in captivity they will deposit on glass and other surfaces in the enclosure. The number of eggs in an egg mass depends both on the health of the specimens and the species, but typically 20 to 30 eggs can be expected.
It is important to thoroughly check the enclosure each morning to locate any possible egg masses that may have been deposited the prior night. When and if eggs are located, you have basically two choices. One choice is to leave the eggs in the enclosure to undergo development there. The other choice is to remove the eggs and place them into a separate incubation container. I have always removed the eggs and placed them into a special incubation container. An incubation container consists of a small, clean, plastic box where you place the eggs, and any substrate to which they may be attached. I provide a small cup with clean water to increase humidity and spray the eggs lightly every couple of days as well. To finish the container, I place a thin plastic film with a few small holes over the top to keep in the humidity.
It is important to prevent eggs from drying out. Watch the eggs closely to make sure no fungal infections take place. If an egg or two goes bad, simply remove the bad egg(s).
The time of egg development varies with temperature and species, but it typically lasts seven to 15 days. After the larvae hatch from their eggs it is time to introduce them into appropriate rearing containers. Rearing centrolenid larvae is tricky, and is typically best done by housing each larva individually. Glass frogs have a long larval stage, sometimes taking a year to completely undergo metamorphosis, but typically the larval developmental stage lasts seven to 10 months. The type of rearing container I have used is one of smaller volume, typically between 250 and 500 milliliters. It is best to use a clean water source, free of possible pesticides, pollutants and other harmful chemicals. I typically use clean stream water or rainwater, but I have also used water produced from a reverse-osmosis (R/O) DI unit. The main problem with R/O DI units is that they sometimes clean the water too well and vitally needed chemical compounds are often absent. I combat this by adding mixed terrestrial substrate rich in mineral and chemical compounds.
Wild glass frog larvae typically undergo development in the detritus of a stream bed, and you should replicate this in the confines of a captive-rearing container. I often collect forest floor substrate and place it into the rearing container. Moss, dead leaves, small twigs, dirt, mud and decaying plant matter are what typically compose the forest floor substrate. Once again, be careful to not include any toxic items or pollutants into this substrate. I next place this substrate into the rearing container and place these containers in an area receiving decent to strong lighting, but be careful not to exceed a water temperature of 27 degree Celsius (80 degrees Fahrenheit) with containers in strong light areas. The water in the container will soon turn a greenish color with the presence of micro algae, and this marks the readiness of the container for the placement of the larvae. Keep a close eye on the developing specimens, and keep track by documenting observations in their development.
The method just described has worked for me, but by no means is it the final word on rearing glass frog larvae. In my opinion, anyone working with anurans in such a manner is, in fact, doing scientific work, and the need for experimentation is vital. Please experiment and share any results with friends and colleagues.
One larvae-rearing method I have been planning is to set up a river-type aquatic ecosystem, with stream substrate and an undergravel filter fitted with a smaller powerhead to provide current. However, I have not experimented with this ecosystem yet and, therefore, have no results to provide. We all need to work together to enhance the captive-breeding efforts among the various captive-bred anurans with which we work.
Once the larvae have reached the stage of metamorphosis, which is typically defined by the penetration of the front limbs, it is time to place them into an intermediate container. An intermediate container should consist of 50-percent aquatic and 50-percent terrestrial environments. Make the container with a gradual slope, which will allow the specimens to easily exit their aquatic environment when they are ready. The glass frogs will be in a transition stage of absorbing their tail remnants for several days and typically will not consume any food items. Once they have fully absorbed their tails, it is especially important to provide a steady supply of small food items for the developing juveniles.
Glass frogs are usually less than a centimeter long when they leave the water, so remember to provide them with tiny food items appropriate to their small size. Watch the juvenile specimens closely to make sure they are actually feeding on the small insects provided to them. This stage after metamorphosis is the most challenging and crucial.
The tiny froglets can be introduced to a rearing container together with their siblings. The rearing container should only provide minimal plant cover. I advise not placing specimens in enclosures more than 50 centimeters by 25 centimeters by 30 centimeters. After a few months of development, when the captive juveniles have reached a length of about 1.5 centimeters, introduce them into larger, permanent setups that have the characteristics of the adults' ecosystems.
Conclusion
Glass frogs are beautiful and rewarding to keep if given the chance. However, they share the fate of many other anurans in the tropics, that of habitat destruction and pollution.
If you are fortunate enough to encounter these rare little rain forest gems, do your part to provide them with the best care possible. Glass frogs are too beautiful to suffer the grim fate of dying in inadequate captive environments. With our combined captive-care efforts, glass frogs, along with many other species of amphibians, may have a second chance in an otherwise bleak future.
