News / Species Spotlight / Barspot Cardinalfish (Apogon retrosella) (09/26/18)

Barspot Cardinalfish (Apogon retrosella)

Barspot Cardinalfish (Apogon retrosella)
Coral reefs are a surprisingly bustling place at night. This is when most corals extend their feeding tentacles, crabs are scuttling about, and certain fishes make themselves known after having remained hidden all day within the shadows of caves and crevices. The most diverse and characteristic of this nocturnal reef fish fauna are the apogonid cardinalfishes.

The family includes roughly 350 species, spread all across the planets tropical, subtropical, and even temperate shallow marine ecosystems. Despite this immense biodiversity, only a handful of species find themselves regularly exported for the aquarium trade, and most of those are diurnal species like the Banggai Cardinalfish and the Pajama Cardinalfish. But for a true cardinalfish experience, aquarists might wish to consider the beautiful red species in the nocturnal genus Apogon.

Cortez Red Barspot Cardinalfish (Apogon retrosella)

There was a time when nearly every cardinalfish found itself lumped into this genus, and youll still often see it treated like this in aquarium references. Presently, there are just over 50 species in Apogon, but this is likely to change further in the coming years. The type species of the genus is the Mediterranean Cardinalfish (A. imberbis), which is in turn closely related to a number of others found in the Atlantic and Eastern Pacific, as well as the handful of fishes classified in Zapogon, Phaeoptyx, Paroncheilus, and Astrapogon. Then theres another group of species found only in the Indo-Pacific which also bears the name Apogon, but these are more distantly related and will presumably be shifted into a new genus (probably Asperapogon) when the matter is studied.

Together, these various cardinalfishes bear some striking similarities in their shape, coloration, and behavior. They are mostly small, elongate, reddish fishes with some amount of black patterning, though Astraopogon is noteworthy for being largely black or bronze in color and for having developed an association with conch shells, either living or dead. Those in Apogon tend to spend the daylight hours huddled together in loose shoals, either in dark recesses of the reef or in close association with the stinging tentacles of anemones (though not in direct contact with these).

At night, they emerge from their sanctuaries to feed upon the zooplankton springing forth into the water column, dispersing across the reef to hunt alone. In captivity, youll see similar behaviors, with specimens grouping together while the lights are on and scurrying about every which way when the lights go off. These fishes benefit highly from a dense rockwork, as they naturally want to hide away from the boisterousness of the reef. In an especially active tank, they can potentially be outcompeted for food, so care should be taken to ensure that they get their share. Feeding at night is a simple way to manage this, though this isnt usually required.

A number of different species are exported from the Western Atlantic and Eastern Pacific, and its not uncommon to see these available either unidentified or misidentified. The differences between some are slight and can be easily confused. For instance, the Eastern Pacific is home to a total of six species. Three are unmarked and rarely seen, but the other three have some combination of a vertical black bar or a spot on the caudal peduncle, and all three are widespread and may be seen.

If your fish has a black bar AND a tail spot, it should be the Blackbar Cardinalfish (A. retrosella). If your fish has just the tail spot, its the Tailspot Cardinalfish (A. dovii). And if theres only a black bar, then youve got yourself the Pink Cardinalfish (A. pacificus). Of course, this only applies to the species in the Eastern Pacific, so knowing where a specimen originates is important when identifying it. Fully grown, these three reach about 3-4 inches, and they may be kept either singly or in small groups.