The sexes have similar coloration, and the coloration is usually uniform golden yellow with one to several large black dorsal markings (Savage 1972). Normally the abdomen is also yellow, but when carrying eggs, the female's ventral surface will have a lighter-colored abdomen (Poole 2006).
At stage 36, the tadpole body length is 5.8 mm, with a body width of 4.3 mm, and total length of 12.2 mm. The larval body is ovoid, with the greatest width at the body midpoint. The body is also depressed, and flattened ventrally, being about 3/5 as high as wide. Nares (nostrils) are small and located closer to the eyes than the tip of the snout. Eyes are dorsal and directed anterodorsolaterally; they are moderately large, nonbulging, and separated by a distance 1.5 times the diameter of the eye. Atelopus zeteki tadpoles are gastromyzophorous, possessing a large sucker that covers the anterior part of the belly and numerous tooth rows. The tadpole mouth is large, ventral, and surrounded by labia forming an unbroken oral disc 3.6 mm wide. The posterior lip has no papillae, while other lips bear a single row of small blunt papillae. The rows are complete and roughly equal in length. Beaks are thin and smooth. A large ventral suctorial disc extends and broadens from the posterior labium. The suctorial disc lacks papillae. On the venter, the spiracle is sinistral, medial, elongate, and directed posteriorly. The vent tube is moderate in length and medial. Caudal musculature is well defined and deep on the anterior half of the tail, narrowing abruptly at midlength and extending nearly to the tip. The tip of the tail is moderately rounded (Lindquist and Hetherington 1998b).
Tadpoles (at least in captivity) are completely white for the first few days post-hatching, developing pigmentation after a few days (Poole 2006). At that point the larval coloration begins to change to dark brown to black dorsally with metallic gold flecks. The venter is pale gray to translucent. Metamorphosing tadpoles (stage 45) have lost the gold flecking and gained small dark green markings against the dark brown dorsum, and are slightly darker than fully metamorphic juveniles (Lindquist and Hetherington 1998b).
Newly metamorphosed froglet size is 6 mm in SVL, in captive-bred frogs (Poole 2006). Recent metamorphs are vivid green with dark black or brown markings, matching the color of mosses growing on stones in or around the stream habitat (Lindquist and Hetherington 1998b). Dorsal markings on the metamorphs always include the following: brick-red warts inside the dorsal markings; a rostral spot or band running transversely, an interorbital "X" from the crest of the eyes to the suprascapular area, a single lumbar chevron with the point towards the anterior of the frog and ending at the flanks, a transverse post-sacral band, and 1-3 lateral bands encircling arms and legs. In addition,the digits sometimes have markings. Palmar and plantar surfaces are also yellow (Lindquist and Hetherington 1998b). The juvenile venter may be either yellow or white, without markings or spotted. If the venter is white, there is a yellow anal patch. Juveniles from El Cope, Panama, had differences in ventral coloration (yellow or white), and it is not known whether this might be sexually dimorphic. In contrast, juveniles at Campana Heights, Panama, had only yellow venters, but were also thought to be older than those observed at El Cope (Lindquist and Hetherington 1998). It is not until the frogs reach older juvenile to subadult age that they undergo ontogenetic color change and acquire the typical yellow or goldenrod overall ground color. In addition, adults that have black patterning have considerably narrower markings than do metamorphic or subadult frogs (Lindquist and Hetherington 1998b), implying that the extent of black marking also changes with age.
Atelopus zeteki is the most toxic species of Atelopus, with the skin of a single individual containing enough toxins to kill 1,200 20g mice (Savage 2002). Zetekitoxin AB, the major alkaloid in Atelopus zeteki's skin extracts, is an analog of saxitoxin and an extremely potent blocker of voltage-dependent sodium channels (Yotsu-Yamashita et al. 2004). Atelopus zeteki is currently the only known amphibian with saxitoxin-analog activity, producing zetekitoxin C as in addition to zekekitoxin AB (Yotsu-Yamashita et al. 2004). It has been hypothesized that zetekitoxin production requires the presence of symbiotic bacteria, as various forms of bacteria (marine, anaerobic, and cyanobacteria) are known to produce saxitoxin (Daly 2004; Yotsu-Yamashita et al. 2004).
This species appears to undergo an ontogenetic change in color, from cryptic green and black coloration in new and recently metamorphosed frogs and juveniles, to aposematic bright yellow, or yellow and black, in subadults and adults. Lindquist and Hetherington (1998b) hypothesized that this may parallel the acquisition of skin toxicity. They reported that metamorphs and young juveniles were secretive, unlike the adults which moved about openly (Lindquist and Hetherington 1998b).
The genus Atelopus, with 113 described and putative species, appears to be the most threatened clade of amphibians. At least 30 species appear to be extinct, having been missing from all known localities for at least 8 years. Only 52 of the surviving species have sufficient data with which to evaluate population trends; of these, 81% (42 of 52) have population sizes that have been reduced by at least half. Higher-elevation species (those living at least 1000 m asl) have been hit the worst, with 75% (21 of 28) having disappeared entirely. Chytridiomycosis is thought to be a primary factor in the decline and disappearance of species in this genus. Habitat loss has occurred within the ranges of many Atelopus species, but does not appear to be a major factor in the decline of most Atelopus species; 22 species declined despite occurring in protected areas. Many Atelopus species are local endemics, putting them at particular risk of extinction, with at least 26 species known only from a single population inhabiting a narrow altitudinal range (La Marca et al. 2005).
Atelopus zeteki is possibly extinct in the wild, with frogs having been removed to captive breeding facilities in 2006, to protect them against possible death from infection with Batrachochytrium dendrobatidis (Bd). The BBC filmed these frogs courting, wrestling, and semaphoring in the wild shortly before removal. The amphibian chytrid fungus (Batrachochytrium dendrobatidis, or Bd) is spreading through Central America, decimating frog populations (Lips et al. 2006), and overtook the study location soon after the filming.
Richards-Zawacki (2010) has shown that behavioral thermoregulation may be an important mechanism by which this species (and possibly other amphibians as well) can combat Bd infection. Body temperatures were collected before and during a Bd epidemic in Panama. Average body temperature was higher during the epidemic, regardless of air temperature, and this influenced the prevalence of chytrid fungal infection (Richards-Zawacki 2010).