Sexing Reef Fishes
Maroon clownfish can change their gender. Photo credit: Nick Hobgood/Wikipedia
Determining the gender of reef fish is often easier said than done. Fortunately, some species do exhibit sexual dimorphism. Some of the best known examples are the sharks and rays (subclass Elasmobranchii). In all known elasmobranchs, the male has an extension off each pelvic fin, which are known as claspers, used to inseminate females. The claspers are visible even in neonate sharks and rays.
Other reef fishes that show sexual dimorphism in finnage include the following animals:
- The Cocos Keeling angelfish (Centropyge joculator). Males have longer, more pointed dorsal and anal fin margins.
- The common hogfish (Lachnolaimus maximus). Males have long dorsal filaments.
- The sailfin blennies (Emblemaria spp.). Males have larger, sail-like dorsal fins.
- The yellowsnout shrimpgoby (Stonogobiops xanthorhinica). Males have slightly longer dorsal spines.
- Many pygmy gobies (Eviota spp.). Males have longer dorsal fin rays.
- Many fairy wrasses (Cirrhilabrus spp.). Males have longer pelvic fins.
Some goby species have urogenital papilla (a small projection that occurs immediately behind the anus and right before the anal fin and is used to deposit gametes) that differ between the sexes. Males tend to have long, tapered papillae, while the females’ papillae are short and thick (the differences may not be as apparent if you do not see an example of each side by side). Male seahorses are unique, in that they have a brood pouch where the female’s eggs remain until hatching. Jawfish also exhibit interesting sexual dimorphism related to their unusual reproductive mode. Male jawfishes incubate the fertilized eggs in their mouth until they hatch. As a result, the males of some species have larger mouths than females.
The Banggai cardinalfishes (Pterapogon kauderni) not only hold the eggs in their mouth until they hatch, they also brood the young fish in the oral cavity for several weeks. Photo credit: Jens Petersen/Wikipedia
The most common sexual dimorphism observed in reef fishes is related to size. In most species, mature females tend to have a broader girth, especially during the reproductive season. Just prior to spawning, the eggs become hydrated, which causes the abdomen to swell. For example, female frogfishes swell like a balloon the day before spawning occurs as a result of egg hydration. Some species exhibit more permanent dimorphism when it comes to overall size.
In fish that change from males to females (protandric hermaphrodites), such as the anemonefishes, females are usually larger than males. In some cases, the size difference can be highly exaggerated. Such is the case with the spinecheek or maroon anemonefish (Premnas biaculeatus). In this species, the female may be three times larger than the male. Some fish (protogynous hermaphrodites) start life as females, with some individuals turning into males; in these fish, it is typically the terminal phase male that is larger than female (and initial phase male) conspecifics (see the information on diandry for more information; all of this will be discussed in detail later).
Some species exhibit sexual dichromatism (that is, color differences between the sexes) — though it is not as common as potential marine fish breeders would wish. Sexual dichromatism can be permanent or temporary. In those that display temporary color differences between the sexes, the chromatic attire typically changes during courtship and spawning. Examples of temporary dichromatism are seen in the sergeant major damsel (Abudefduf saxatilis), which takes on a dark blue color during the reproductive period, and the pygmy angelfishes (Centropyge spp.), in which the females of some species have color that blanches during courtship. Of course, these temporary color differences are often not that helpful to the aquarist until the fish are engaging in courtship.
Permanent color differences are much more helpful. In some cases, these chromatic disparities between the sexes are quite subtle. For example, in the king angelfish (Holacanthus passer), the male has white pelvic fins while those of the female are yellow. In the flame angelfish (Centropyge loriculus), the male has more blue on the posterior margin of the dorsal fin than the female. In other species, the differences are extreme — so much so that many of these chromatic forms were recognized as two (or more) distinct species based on the male and female color patterns. Species that display these more radical differences include some anthias (Pseudanthias spp.), swallowtail angelfishes (Genicanthus spp.), parrotfishes, wrasses, trunkfishes and some triggerfishes. As is the case with most terrestrial animals, the male typically displays the most garish and gaudy chromatic attire. Males have to attract females into their territories, and the females usually select their mate based, in part, on his flashiness. Although no one can say for sure why, it may be that it is because in a species that pair for life or for a long time, they do not need to have to rapidly access the sex of another fish during a brief reproductive period and, therefore, dichromatism is not as important.
Sex Allocation and Sexing of Reef Fish
Hermaphrodites either possess both male and female sex organs, or have the ability to change sex. The former are known as simultaneous hermaphrodites, while the latter are classified as sequential hermaphrodites. Mature simultaneous hermaphrodites possess the functional reproductive organs of both sexes and can reproduce as a male or a female during a single spawning bout. One benefit to this is that an individual can mate with any other conspecific it happens to encounter. These fish have not been reported to fertilize their own eggs in the wild. In fact, to prevent self-fertilization from accidentally occurring, the eggs and sperm are delivered through different ducts in front of the anus.
Diandry is the rarer form of protogyny, but it’s the most common in wrasses and parrotfishes. In diandric species, the individual that starts life as a male is known as an initial or primary phase male, while the individuals that change sex are known as terminal or secondary phase males. The latter usually differ radically in coloration and size from the initial phase individuals, which are typically similar in appearance to the females. Terminal phase males are often referred to as “super males” because they tend to be larger and more colorful than initial phase males.
- Male sharks, which have two reproductive organs called claspers, bite females (sometimes leaving massive wounds) during courtship and mating.
- During the mating period, unreceptive female round stingrays (Urobatis halleri) may use their tail spines to jab males that attempt to mate with them.
- Male seahorses (Hippocampus spp.) carry the fertilized eggs in a brood pouch on their abdomens until the eggs hatch at which time they expel the baby seahorses.
- The female ghost pipefish (Solenostomus spp.) forms a pouch with her modified pelvic fins in order to carry her eggs. The eggs are connected to branching cotylephores, which serve as a primitive umbilical cord between mother and eggs.
- Many female frogfishes (Antennariidae) produce egg ribbons that the male ejects and fertilizes during spawning. The ribbon matrix can hold as many as 200,000 eggs and breaks apart, releasing the larvae, after several days.
- The male jawfish (Opistognathus spp.) takes the fertilized eggs into his mouth and holds them their until they hatch — as a result, males of many jawfish species have a larger mouth than the females.
- The Banggai cardinalfishes (Pterapogon kauderni) not only hold the eggs in their mouth until they hatch, they also brood the young fish in the oral cavity for several weeks.
- Anemonefishes (Amphiprion and Premnas spp.) are protandric hermaphrodites — that is, females result from male sex change.
- The eggs of some of the toxic tobies (Canthigaster spp., a.k.a. sharpnose pufferfishes) are also poisonous and thus are not eaten by egg predators.
- Male porcupinefish (Diodon holocanthus) nuzzle the female to induce her to spawn.
- Some reef fish exhibit sexual dichromatism. In the case of the ribbon eel (Rhinomuraena quaesita), the juvenile is black, the male is blue and the female is yellow.
Protandric hermaphrodites transform from males to females. This type of hermaphroditism is relatively rare in coral reef fish. The most popular examples are the anemonefishes (Amphiprion spp. and Premnas biaculeatus). These fish live in groups in which the most dominant, and in some species the largest, individual is a female. The individual just beneath her in the pecking order is a male, and all the rest of the group members are asexual (that is, they are neither sex). If the female should die, the male will change into a female, and the most dominant asexual individual will take his place, transforming into a functional male. The ribbon eel (Rhinomuraena quaesita) is also a protandric hermaphrodite. In this species, juveniles are black, males are blue, and females are yellow. There are also some dottybacks that are protandric (e.g., Cypho spp.).
True Gender Benders
If you would like to know more about reef fish sexuality, I suggest you read Matthew Wittenrich’s book The Complete Illustrated Breeder’s Guide to Marine Aquarium Fishes (TFH Publications/Microcosm Ltd., 2007), which will aid you in not only sexing some species, but it also includes tips on how to induce spawning as well as raising the spawn. Until next time, happy fishkeeping!