The spiny chromis (Acanthochromis polyacanthus) is a species of damselfish from the western Pacific. It is the only member of the genus Acanthochromis.
Spiny chromis individuals can grow up to a maximum size of 14 centimetres (5.5 in) as adults. Its fins have 17 dorsal spines, 14 to 16 dorsal rays, 2 anal spines, and 14 to 16 anal rays. This fish is gray on the front and white on the back. The ratio of grey to white varies by specific reef, as there is little interbreeding between reefs due to lack of pelagic larval period. Grey color can range from a blue-grey to brown as the fish matures. The top of its dorsal fin and the bottom of its anal fin is black. Some individuals are gray with a yellow horizontal line in the middle.
The spiny chromis is found in the western Pacific Ocean. It is found in western and central Indonesia, Papua New Guinea, northern Australia, and all the islands in the Philippines except Luzon. They are also found in Melanesia. This fish usually lives in coral reefs. This fish is found at a depth range of 1 to 65 metres (3.3 to 213.3 ft) but is usually found at a depth range of 4 to 20 metres (13 to 66 ft).
This fish is planktonivorous and algivorous, and steals algae from algae-farming species of damselfish. Younger individuals have been known to eat mucus off the adults.
The spiny chromis, unlike many other coral reef fishes, has direct development of their larvae, which means that the parents protect their brood from the egg stage (which are laid on the reef), through hatching and onto the fully developed juvenile stage. There is no pelagic larval stage unlike the vast majority of coral reef fishes. This form of direct development means that the offspring often take up residence on the reef not far from where they were hatched. Consequently, high levels of local adaptation to their local environment has been recorded in this species.
