Minyobates steyermarki is a small, poisonous dart frog classified in the family Dendrobatidae. Snout-vent-length ranges between12 - 19.5 mm, but most are less than 17 mm. The skin is smooth on both the dorsal and ventral sides except on the throat and sides of the abdomen where skin has fine granules. The head is as wide as it is long. An angular ridge connecting the eye and the nose called the canthus rostralis is present. This frog has a vertical loreal region. The snout is relatively short and not fully truncated at the point. Diameter of the eye is slightly larger than the distance between the nose and the eye, but is much shorter than the snout. A tympanic membrane can be seen from the anterior margin and is about one half the diameter of the eye. Interorbital space is wider than the superior eyelids. Arms of M. steyermarki are long and thin. An external, round tubercle is almost centered on the metacarpal region of the hand and an internal, elongated tubercle is also found on the palm. Subarticular tubercles of the hands are large and easily identifiable. The first finger is longer than the second. Fingers have discs. The first and second fingers have discs wider than the fingers but smaller than the discs of the third and fourth fingers. The larger discs on the third and fourth finger are smaller than the tympanic membrane. Minyobates steyermarki has a defined tarsal fold. A small oval tubercle covers about 2/3 the distance between the base of the foot and ankle. Subarticular tubercles on the feet are much smaller and less prominent than those found on the hands. Two metatarsal tubercles, an external oval tubercle and an internal oval tubercle, are easily identifiable and are located at the base of the first toe. Feet are free of webbing. Discs on feet are much smaller than discs on the hands. When limbs are pressed against the body, the ankle of the hind limb extends up to the middle of the eye (Rivero 1971; Myers 1987).
Several characteristics separate Minyobates steyermarki from the other genera within the family Dendrobatidae. Color of skin is bright and recognizable. Neither an oblique lateral stripe, a ventrolateral stripe, nor a dorsolateral stripe is present along the body. Texture of dorsal skin is smooth, toes on feet lack webbing, and a dark throat collar is absent. The third finger of adult males is not enlarged as in other species of different genera (Grant et al. 2006). The first finger is just barely shorter than the second and finger discs are large. Lastly, M. steyermarki is a very small miniature poison frog that has unique skin peptides that differentiate it from other species (see comments section below) (Myers 1987). When comparing specifically to other species, certain characteristics distinguish M. steyermarki. Dendrobates leucomelas and Dendrobates pictus are both larger in size and have a skin color distinctly different than M. steyermarki. Dendrobates braccatus, D. trivittatus, D. machadoi, and D. paraensis are also larger in size than M. steyermarki and have recognizably different skin coloration. Minyobates steyermarki differs from Dendrobates tinctorius because D. tinctorius has a blue skin color and a different skin pattern than M. steyermarki (Rivero 1971).
In life, the dorsal side of Minyobates steyermarki is described as red-brown, dark red, or scarlet in color. The dorsum is also covered in speckles of dark black spots all over. Limbs, particularly more proximal segments, are either reddish or a salmon color. The limbs have fewer spots and are more solid in color than the dorsal skin. The ventral side is the same color as the dorsal side but dark spots are more extensive, except near the throat. The throat has fewer dark spots than the abdomen and feet. The extensive amount of spots on the ventral side sometimes creates a marbling look to the skin (Rivero 1971).
The species authority is Rivero 1971.
According to current classification, Minyobates steyermarki is the only species classified in the genus Minyobates. Minyobates steyermarki was originally classified in the genus Dendrobates until 1987 when Charles Myers improved classification based on data collected for skin alkaloids, vocalizations, and behavior (Myers 1987). Most recently, classification has been improved based on DNA sequencing. DNA sequences for five mitochondrial loci and six nuclear loci were used. A total of 174 phenotypic characters were identified and defined for adult and larval morphology, alkaloid secretions, and behavior. All species sampled were classified into two clades: family Aromobatidae and family Dendrobatidae. The two clades were classified based on toxicity. Species in Aromobatidae are nontoxic and species in Dendrobatidae are toxic. Further classification based on DNA sequences and defined phenotypic characteristics has divided the family Dendrobatidae into three subfamilies: Colostethinae, Hyloxalinae, and Dendrobatinae. Minyobates steyermarki is classified in the third subfamily, Dendrobatinae. All species in this clade have a smooth texture on the dorsal skin, absence of a pale oblique lateral stripe, an iris that lacks both a pupil ring and metallic pigmentation, lay larvae in plants that hold water (phylotelmata), and secrete lipophilic alkaloids. The subfamily Dendrobatinae contains the most recognized species of poison dart frogs. All are extremely toxic. Dendrobatinae contains five genuses: Adelphobates, Dendrobates, Minyobates, Oophaga, Phyllobates, and Ranitomeya. Minyobates steyermarki is the only species in the genus. Classification for M. steyermarki is weak mostly because there is a lack in DNA sequence data (Grant et al. 2006).
Minyobates steyermarki was first named Dendrobates steyermarki in 1971 when the species was first described. In 1987, the species was reassigned to the newly classified genus Minyobates and has since been named Minyobates steyermarki. Minyobates comes from the Greek word minys (litte, small) and bates (a walker). This name is given for characteristic miniature size of the species in this genus (Myers 1987). The specific epithet, steyermarki, was named in honor of Dr. Julián A. Steyermark for his discovery of the species in Cerro Yapacana, Venezuela and for his contribution to herpetology (Rivero 1971).
Classification for Minyobates steyermarki has been difficult since the species was first written about in 1971. During the time, there were three genera in the family Dendrobatidae: Dendrobates, Phyllobates, and Colostethus. Each genus was identifiable by certain characteristics. At the time of 1971, M. steyermarki belonged to the genus Dendrobates, which had large discs on the second and third finger that were generally the size of the finger. Skin colors of species in this genus were usually black, orange, yellow, red, blue, or green. However, Minyobates steyermarki did not exactly fit this genus because the size of the first finger discs was much wider than the digits, hence reclassification to Minyobates years later. Minyobates steyermarki remains part of the family, Dendrobatidae, consisting of diurnal Neotropical frogs that secrete lipophilic alkaloids for defensive purposes. All species in this family are toxic and secrete lipophilic alkaloids. Certain secretions have been known to be used by Indians for poison in blowgun darts. This is how the family got the name poison dart frogs (Myers 1987). Dendrobatid frogs have several distinct skin toxins. All of the toxins are organic, nitrogenous ring compounds, and are soluble in alcohol. Three major classes of the alkaloids are batrachotoxins, pumiliotoxins, and histrionicotoxins (Daly and Myers 1976). Minyobates steyermarki contains piperidine-based skin alkaloids, for the most part from the pumiliotoxin-A and decahdroquinoline classes, and lacks histrionicotoxins or 3,5-disubstituted indolizidines alkaloids. The different chemicals in skin alkaloids separate M. steyermarki from other genera such as the genus Phyllobates, which do contain 3,5-disubstituted indolizidine alkaloids (Myers 1987).
Featured in Amazing Amphibians on 22 April 2013