The blacksmith fish (Chromis punctipinnis) is one of the most common and abundant fish species you will see in the waters above rocky reefs and kelp forests off the southern California coast. Adults tend to be a uniform blueish-black, while juveniles, like those in today’s photo are blueish above with more of a silvery gold underside, and black spots on their scales.
Regardless of their being juveniles, the fish in this photo are displaying typical blacksmith behavior by grouping together in an unstructured aggregation. During daylight hours large aggregations of blacksmith can be found hovering over rocky reefs, generally facing into any current that may be running. This behavior results from the fact that blacksmith specialize in eating small plankton organisms that drift over reefs and through kelp forests with the currents. Their ability to eat these tiny organisms, which are often invisible to the unaided human eye, is aided by two adaptations.
The first adaptation is a modification of their jaws, such that when their mouths are opened, the jaws form a tube-like structure. While many fish use a suction system to bring prey into their mouth, the blacksmith extends it mouth out around the prey. They may also use suction, but the tube-like mouth is clearly a special feature of these fish.
The second adaptation, which has not been completely confirmed by scientific research, has to do with the structure of their eyes. Like many animals, blacksmith can see some wavelengths of ultraviolet (UV) light. The exact purpose of this ability in most animals is open to speculation, but it is thought that for the blacksmith, it allows them to see the tiny organisms they eat. As most planktonic organisms are transparent to some degree, visible light is not terribly helpful in being able to see them. But it is possible that UV light refracts or reflects through the transparent bodies of plankton in a way that makes them much more visible in a literal ocean of blue.
Blacksmith are also interesting from an ecological perspective because of they feed on plankton above the reef all day, and then at night they migrate down into crevices in the reef for protection. While there, they empty their guts of the food consumed during the day, providing a source of organic material to organisms on the reef. I spent my formative years as a marine ecologist as a part of a study of this behavior by blacksmith. Much humor was generated by the fact that we were studying fish poop.
This photo was taken with a Canon EF17-40 mm f/4L USM lens zoomed to 26 mm on a Canon EOS 5D Mk III in an Ikelite underwater housing. Lighting was provided by two Ikelite DS161 strobes set to eTTL exposure mode. The exposure was set to 1/45 sec at f/9.5 and ISO 400.