Description
Arthroleptis bioko is a slim-bodied, squeaker frog described from five male specimens and no female specimens. The adult male snout-vent length range from 26.7 - 33.4 mm. The head is slim and the snout protrudes slightly past the lower jaw with a rounded tip and small, oblong nostrils. The canthus rostralis is small and curved and the loreal region is a little concave. The eyes bulge out from the sides of the head and extend a little out from eyelids with a round pupil. The tympanum is round with no supratympanic fold. The skin is relatively smooth skin over most of the body when in preservative and may be more bumpy in live specimens. They have skin ridges on the sides of their body and tiny, spiny projections on the posterior back and sides of the body. This frog has thin limbs and digits. There are four digits on the front limbs with the third digit being the longest. There are five digits on the hind limbs with the fourth digit being the longest. The tips of the fingers and toes are enlarged and round with no webbing between the digits. All of the digits have one pronounced subarticular tubercle (Blackburn 2010).

DIAGNOSIS:

Arthroleptis bioko is about 1.25 times bigger than other similar species in the Arthroleptis genus. The back legs and toes of this animal tend to be longer than most similar species and they can also be distinguished from other similar species by unique color markings on the ventral side of the body. Lastly, the males have white, spiny projections on the lower legs that are not present in other species (Blackburn 2010).

COLORATION:

In life, the overall background color is dark grayish-brown. The spots on the dorsal side can be light brown or yellow and there may be some dark brown or black patches as well. The spiny projections on the posterior back and sides of the body are white. The spots on the ventral side are off-white. The iris of the eye is yellowish and the pupil is black (Blackburn 2010).

In alcohol, the background color of is medium-brown over the whole body. There are blotchy spots of light-brownish gray on the dorsal side of the frog as well as a few well-defined darker brown spots. The same coloring is seen on the dorsal side of the legs. The ventral side has many small, unevenly shaped, off-white spots with well-defined edges. The same coloring is seen on the ventral side of the legs. The iris and pupil are gray. The tympanum is brown and slightly transparent. The tips of the toes and fingers have no pigment. The underside of the mouth is completely rimmed with white blotchy marks (Blackburn 2010).

VARIATION:

Although no females were collected for the species description, photographs of females exist. From those photographs, males appear to have several characters that are not present in females. Males have a lengthened third finger and spiny projections on the groin area and toes. The skin around their throat is also stretched and darker in color. It was not possible to evaluate size dimorphism from the photographs (Blackburn 2010).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Equatorial Guinea

View distribution map in BerkeleyMapper.

Arthroleptis bioko has only been found on Bioko Island in Equatorial Guinea. It is the only amphibian species known to be endemic to this island, and lives in montane forests on the northeastern and southwestern sides of Pico Basile, a mountain on Bioko Island. Specimens were collected from leaf litter on the forest floor. It is estimated that the total area they occupy is less than 20,000 km^² (Blackburn 2010). They occur at an elevation range of 1,321 to 1,1820 m above sea level (IUCN 2012).

Life History, Abundance, Activity, and Special Behaviors
Arthroleptis bioko is a terrestrial species, but is sometimes be spotted by streams (IUCN 2012).

Information about reproduction is limited, but A. bioko is assumed to be a direct developer, similar to the other species in the same clade (IUCN 2012). Males have been found calling during the day in the rain, but there is no official record of its calling (Blackburn 2010).

Trends and Threats
The Bioko squeaker frog is listed as “Endangered” on the IUCN Red List and has a decreasing trend in its population size. The majority of the area that this species occupies is located within Pico de Basile National Park. However, the building of roads and intrusion of farmland into the habitat of these animals is causing habitat loss and fragmentation, which is threatening the species. It is also suspected that climate change may increase the likelihood of these frogs being infected with a deadly chytrid fungus disease. All of these threats are exacerbated by the fact that this species is only found in a very small range on Bioko Island (IUCN 2012).

Arthroleptis bioko are found in conservation areas that are under legal protection; specifically the forests of Bioko, the Reserve Scientifique de la Caldera de Luba, and the Pico de Basilé National Park. However, new law changes might affect the conservation efforts in place (IUCN 2012).

Possible reasons for amphibian decline

General habitat alteration and loss
Habitat modification from deforestation, or logging related activities
Climate change, increased UVB or increased sensitivity to it, etc.

Comments
Maximum Likelihood, Maximum Parsimony, and Bayesian analyses on 12S, 16S, and tRNA for Valine, as well as morphology first distinguished A. bioko from A. poecilonotus (Blackburn 2008). However, the species was not formally described until 2010, when it was later found to be sister to A. poecilonotus from Cameroon. The next most closely related species is A. brevipes (Blackburn 2010).

The specific epithet, “bioko,” refers to Bioko Island, the location where this species is found (IUCN 2012, Blackburn 2010).

The common name for A. bioko is the “Bioko squeaker frog” (IUCN 2012, Blackburn 2010).

References

Blackburn, D. C. (2008). “Biogeography and evolution of body size and life history of African Frogs: Phylogeny of squeakers (Arthroleptis) and long-fingered frogs (Cardioglossa) estimated from mitochondrial data.” Molecular Phylogenetics and Evolution, 49(3), 806–826. [link]

Blackburn, D. C. (2010). “A new squeaker frog (Arthroleptidae: Arthroleptis) from Bioko island, Equatorial Guinea.” Herpetologica, 66(3), 320-334. [link]

IUCN SSC Amphibian Specialist Group. 2015. Arthroleptis bioko. The IUCN Red List of Threatened Species 2015: e.T16575406A16575409. http://dx.doi.org/10.2305/IUCN.UK.2015- 2.RLTS.T16575406A16575409.en. Accessed 20 February 2022.

Species Account Citation: AmphibiaWeb 2022 Arthroleptis bioko: Bioko Squeaker Frog <https://amphibiaweb.org/species/7529> University of California, Berkeley, CA, USA. Accessed Nov 26, 2024.

Citation: AmphibiaWeb. 2024. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 26 Nov 2024.

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