The Banggai cardinalfish is remarkable among marine fishes for the extreme levels of genetic diversity found between its populations. Populations as little as five kilometres apart are as genetically distinct as separate species, a consequence of the Banggai cardinalfish’s highly limited dispersal capability, the strong currents and deep channels between the islands preventing interaction with populations at other sites (4) (5) (6).
Banggai cardinalfish live in small groups of usually between 1 and 6 individuals, though a group of 500 has been recorded (1). Reproduction in the Banggai cardinalfish begins with the female choosing a male; the pair then separate from the group and establish a territory, aggressively defending it from other fish that come too close. The female then initiates an unusual courtship ritual, swimming alongside the male, bodies touching, and making a trembling motion. This behaviour occurs repeatedly on both sides, with the only signs of the male’s receptivity being a darkening of the lower jaw and an occasional opening of the mouth (7). After several hours of this behaviour, the female spawns a mass of up to 75 large eggs (a very small number for a marine fish). These are quickly swallowed by the male, and brooded in a special pouch inside the mouth (2). The eggs take about 20 days to hatch, after which, the newly hatched embryos continue to develop in the male’s mouth pouch. After a further 10 days, when the young have reached around five to six millimetres in length, they are released (1). During the 30-day brooding period the male does not eat, and attends to the brood by frequently turning the eggs and expelling dead eggs and embryos. Once the brood are released, the male does not interact with them anymore (7). The small juveniles are most commonly found sheltering around sea anemones for protection. However, older juveniles and adults are more commonly found around branching corals and sea urchins (2) (8).
The Banggai cardinalfish feeds principally upon tiny planktonic crustaceans, with copepods, in particular, making up about 80 percent of the diet. Nevertheless, this species is an opportunistic feeder, and will take a variety of small organisms from the water column and the seabed, including marine worms, molluscs and fish larvae. In addition, it plays an important role in its environment by preying on the larval stages of coral reef fishes’ parasites (9). The Banggai cardinalfish is preyed upon by various species, such as the crocodile-fish (Cymbacephalus beauforti), various lion-fish species (genus Pterois) and the grouper (Epinephelus merra) (1). This species may live for two years in the wild (2)