Swallowtail Angelfish & Dimorphism

Swallowtail Angelfishes (genus Genicanthus) are relatively small angelfish ideally suited for the saltwater aquarium. Not only do aquarists like these angelfish because they:

1. are reef-safe
2. possess a peaceful disposition
3. remain fairly small

but they also love this genera’s stunning sexual dimorphism.

Sexual dimorphism is a fancy way of saying that the male and female of a species are observably different. In animals ranging from humans to fish, these differences take the form of size, anatomy, ornamental appendages, or color (dichromatism). While there are many permanently diachronic fish known to the hobbyist (e.g. wrasses), some of the most stunning species have, until recently, not gotten the attention they deserve. This article looks specifically at the angelfishes from the genus Genicanthus—the Swallowtail Angelfishes.

Many aquarists know that swallowtail angelfish have markedly different coloration based on their gender, but how many aquarists have ever wondered why? Perhaps the beauty of a pair of these fish is enough to warrant their relatively high price without necessarily needing to know why the male and female are so different in appearance, but the story behind their sexual dimorphism is equally appealing.

Swallowtail angelfishes, like most fish exhibiting color differences based on gender, do so as both a proverbial red and green light. The coloration of a male is a warning to other males, yet it is also an invitation for females. This is because swallowtail angelfishes in the wild live and travel in small harems or groups composed of one male and several females.

The male of the species G. caudovittatus (the Zebra Angelfish), for example, travels in a harem of one male and up to five females. The male is generally larger with distinctive zebra stripes and black accents along the dorsal fin’s base. The female, on the other hand, has a grey body with a streaked caudal fin. The male’s caudal fin is also often significantly longer than the female’s. Indeed, this sexual dimorphism makes for a stunning display when keeping a pair in an aquarium, but what is the biological rationale for the differences?

In the world of G. caudovittatus, females definitely have a more difficult time of it when it comes to the primary objective—sexual reproduction. Females are only able to mate when they are producing eggs, and they are genetically programmed to be more selective about the males with whom they reproduce. This is perhaps one reason the male Zebra Angelfish is so boldly colored—to attract attention to himself given that the females are more selective.

Another fascinating trait of the swallowtail angelfishes is that they are protogynous hermaphrodites. In other words, all swallowtail angelfish begin their lives as females but have the ability to become males. For example, the popular G. Bellus (the Bellus Angelfish) will remain a functioning female until the absence of a dominant male in the group. At that time, the dominant female in the harem becomes a functioning male. The transformation generally takes about a month and is generally believed to be irreversible (although anecdotally, a Lamark’s Angelfish (G. Lamarck) in captivity once did exactly that).

There are many reasons to own a pair of swallowtail angelfish. Whether it is a pair of G. caudovittatus (Zebra Angelfish), a pair of G. Bellus (Bellus Angelfish) or a pair of any other species from the genus Genicanthus (such as the Wantanabe's Angelfish), the aquarist will be rewarded with a reef-safe angelfish whose sexual dimorphism makes keeping a pair amongst the most attractive displays readily available to the marine aquarist.

Additional Resources

FishBase's List of Species from the Genus

G. caudovittatus

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