Pterapogon kauderni or commonly known as Banggai cardinalfish, is a species of Perciformes belonging to the Apogonidae family or cardinalfishes. It is a species with a very limited geographical distribution since it is only found in the Indonesian islands of Banggai, where it lives in small groups of specimens in shallow waters (1-5 meters) of coral reefs. Often, especially juveniles of this species, seek refuge among the long, sharp spines of the sea urchin Diadema setosum and sometimes in the vicinity of anemones.
The body of Pterapogon kauderni can reach a maximum total length of 7 to 8 cm. Both its body shape and coloration make this species easy to identify and not easily confused with other species of the same family. The body is laterally compressed and subcircular in shape. The maximum total size of the species is 8 cm, and the maximum weight is 6.5 grams. The head has very large circular eyes, as well as a large upward-facing lower mouth. The operculum has a fleshy posterior margin and ends in the shape of a point.
Specimens of Pterapogon kauderni have two dorsal fins, the first one formed by 7 spines. The second dorsal fin is separate from the first and is located in the posterior third of the specimen's body. It has 1 spine and 13-14 soft rays branched at the ends. The first of the soft rays of the second dorsal fin is the longest of all, even reaching the end of the caudal fin in adult specimens. The anal fin is located at the same vertical height as the second dorsal fin, and has 2 spines and 13 soft rays branched at its ends, the first of the soft rays being the longest of all. The pelvic fins are formed by 1 spine and 5 soft rays. The pectoral fins have between 13 and 14 soft rays, the first and last of which are unbranched, and the remaining ones are branched at the ends. Finally, the caudal fin is very forked, with a total of 17 rays, 9 of them present in the upper lobe and 8 in the lower lobe. As with the pectoral fin rays, the caudal fin rays are branched at the ends except for the outermost rays.
As for the livery of Pterapogon kauderni, the base color of the body is silver, which is interrupted by thick black stripes bordered by a thin whitish line. In total there are 6 black stripes, 4 more or less vertical and 2 horizontal. Starting from the rostrum towards the caudal fin, we find a first more or less vertical stripe that runs along the most anterior part of the rostrum. The second stripe runs vertically through the center of the eyeball the third vertical stripe runs along the posterior edge of the operculum, and includes the anterior margins of the first dorsal fin and the pelvic fins. The fourth and last vertical stripe runs from the second ray of the dorsal fin, crossing the entire body to the first rays of the anal fin. As for the two horizontal stripes, these are born blurred after the fourth vertical stripe, and run along the outermost ends of both lobes of the caudal fin. In these horizontal stripes, the white lines bordering the stripe are transformed at the level of the caudal fin lobes in discrete points. Also prominent on the body of Pterapogon kauderni are a series of bright white dots, whose location and size is unique among specimens. These dots are found in the space between the third and fourth vertical black stripe, on the entire pelvic fins, as well as in the anterior third of the second dorsal and anal fins.
The diet of Pterapogon kauderni is predominantly carnivorous, with small shrimp, crabs and zooplankton among its preferred prey. Unlike the vast majority of species in the Apogonidae family, Pterapogon kauderni is a diurnal species.
Reproduction is a very interesting aspect of this species. Specimens of Pterapogon kauderni have an inverted sexual role compared to what is usual in the animal kingdom. It is the females that have a high reproductive rate, constantly competing and courting the males. On the contrary, males are in charge of the care of the offspring and have longer periods between clutches than females. Females usually reproduce every 25-30 days and there is no specific reproductive period, but they reproduce throughout the year. The reproductive behavior of the species involves the formation of transient pairs that isolate themselves from their groups for a few days. The female, after choosing a potential mate, begins a courtship that involves trembling movements, warping, side-to-side swimming and snouting, which are replicated by the male keeping his mouth open. During this period, the pair aggressively defends the place where they have isolated themselves from the rest of the group. The female will release between 10 and 20 eggs which, in less than 2 seconds, the male will fertilize and place in his mouth to incubate them for the next 25-28 days, during which time the male will not feed. The female, a few hours after laying, will break the isolation to prepare for the next laying.
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