Things are rather peculiar in the realm of reef fish. Few phenomenons are clear cut in black and white. At a glance it’s as if the myriad of biological processes walk the grey zone, waiting for the deciding waves to push them to either the side. Dig a little deeper and you’ll realise that things aren’t what they seem, and the initially lackadaisical processes are actually well thought out regimes dictated by mother nature herself. We’re talking about sex change today, and many of you may know that certain reef fish can either be a boy or a girl. Or both. Whenever they want? Surely there must be a more complicated and controlled process.
Sequential hermaphroditism is a type of hermaphroditism that occurs in many fish, certain plants as well as gastropods. This occurs when an organism changes its sex at some point of time in their life. The prefix “sequential” suggests that this change goes about in a coordinated fashion with a series of processes in-between. More importantly, it dictates that an organism can change from male to female or female to male.
The evolutionary advantage for hermaphroditism in reef fish is a much debated topic, with numerous hypotheses. One of the earliest evolutionary models for this phenomenon – the ‘size advantage’ hypothesis – today remains as one of the most powerful explanations for sequential hermaphroditism.
As originally phrased by Ghiselin , ‘Suppose that the reproductive functions of one sex were better discharged by a small animal, or those of the other sex by a large one. An animal which, as it grew, assumed the sex advantageous to its current size would thereby increase its reproductive potential.’ Simply put, it would be advantageous for a fish to change sex when the reproductive benefits of the other sex outweighs that of the current one, especially if sexual fecundity is directly correlated to size or age of a fish.
This may seem quite complicated, but it’s just a brief understanding into the world of hermaphroditism. For a more complete look, the paper on “Evolutionary Perspectives on Hermaphroditism in Fishes” by J.C. Avisea and J.E. Mankb is a wonderful place to start, and is referenced heavily here. Back to topic, sequential hermaphroditism occurs in three primary forms. Protogyny, in which a species begins life as a female and is capable of changing into a male at a later stage; Protandry, the opposite of protogyny, in which a species begins life as a male and may later switch to a female; and serial bi-directional sex change, where a species is capable of switching back and forth between male and female.
The first form of hermaphroditism, called protogynous (proto -first, gyno -female) hermaprodites, is the commonest and most prevalent of the three types. It is widely known to occur in Labridae, as well as many other reef fishes including Pomacanthidae and Serranidae. In protogynous hermaphrodites, species are capable of sex change from females to males, and this is predicted to be evolutionary favoured when reproductive outputs of males outweighs that of being a female. Simply put, being a male rocks and he gets to have more sex, and therefore this theory supports the evolution of protogynous hermaphrodites. As such, protogynous hermpahroditism is often seen in fish who display a haremic lifestyle, where most of the breeding is dictated and controlled by a large dominant male.
Using Cirrhilabrus as an example, females and initial phased (IP) males are often smaller, drab and less showy than large terminal phased (TP) males. IP males are primary, or satellite males that are born as males, while TP males are secondary males which arise from sex changed females. In a harem of Cirrhilabrus, the TP males are the brightest, most colourful males that monopolise most of the breeding rights. Take note that while Cirrhilabrus is an example of protogynous hermaphrodites, it is a diandric genus where males are not exclusively derived from females.
In monandric genera such as Centropyge and Genicanthus, males are exclusively derived from females. I.e, there is no such thing as a male born fish. All fishes in monandric genera are born as females, and when the need arises, are capable of changing into males. This is unlike Cirrhilabrus where IP males and TP males are present. Regardless of monandric or diandric protogynous hermaphrodites, the basis of sex change is the same.
With the removal of the terminal and dominant male from the harem, the next most aggressive female in line will sex change to take its place. All female protogynous hermaphrodite species possess germ cells for both sex organs, and when the social situation calls for a change in sex, are capable of suppressing the female gonads in lieu of the development for male ones. Testosterone production suppresses the female gonads and fuels the development of male sex organs and secondary characteristic. In sexually dimorphic and dichromatic species, this change is evident externally, with increased development of secondary male linked characteristics such as colours, finnage and other traits not found in females.
Above is an example of Genicanthus semifasciatus undergoing a sex change. All Genicanthus are sexually dichromatic, and so the process of sex change is easily documented based on physical attributes. The females are mainly unmarked sans a black gill cover, mask and tail lobes. The males are striped vertically halfway down the body, and in exchange for the black facial and tail markings, gains instead a yellow mask that runs along the limits of the vertical stripes. During the changing process, a sequence of simultaneous colour changes occurs whereby the fishes loses female traits and gains male patterns at the same time. Internally, the male sex organs are also developing. In genera like Centropyge where sexual dichromatism is not so obvious, witnessing the transformation and therefore sexing the species is a little bit more tricky.
This change can take as little as 2-3 weeks, and within that time, the changing female continues to express her dominance until the change is ready. By then, the now fully functional male can assume breeding privileges. It is noteworthy that in aquarium contexts, the change is sometimes not complete, or in other cases, terminal males may lose some of their colours and gain a little bit of female traits. One theory to support this, is that in the wild, males maintain either a lek or haremic style mating system with numerous females under their control. In aquarium, the same sexual stimulation is not easy to replicate, and large males often have difficulty maintaining their coloration, and either lose it partially, or get stuck in a male-female transition zone. Large aquariums with multiple females may prove effective in maintaining a fully functional male.
Some popular examples of protogynous hermaphrodites that we as aquarist see on a regular basis include fishes from the genus Thalassoma, Halichoeres, Cirrhilabrus, Paracheilinus, Pseudojuloides, Pseudocoris, Macropharyngodon, Anampses, Genicanthus, Centropyge, Pseudanthias, Gobiosoma, Corphopterus, etc. Notice how majority of them come from the family of wrasses, angelfish and anthias. Invariably there are many more.
The second form of hermaphroditism is called protandrous (proto -first, androus -male) hermaphrodites, and is very much rarer than the preceding. In protandrous hermaphrodites species, females are derived from sex changed males. In reef fishes, the most widely known protandrous hermaphrodites are clownfishes, which includes all species in the only two genus Amphiprion and Premnas.
Like the reverse of protogynous hermpahrodites, protandrous hermpahrodites start out life as males, and where social pressure dictates, are capable of sex changing into fully functional females. In the typical scenario for clownfish, a large female normally lives together with multiple males and juveniles, the reverse of protogynous hermaphrodites. This would make “Finding Nemo” rife with scientific inaccuracies, because instead of taking his son all over the ocean after his wife was eaten by a barracuda, Marlin would have partnered up with another male and then proceeded to change his sex. Perhaps falsifying scientific information for the sake of a controversial sex change topic might not be a bad idea after all, especially with a target group intended for ten year olds.
Serial bidirectional hermaphrodites
This is a unique and uncommon form of hermaphroditism seen in certain reef fish. While many species are capable of sex change, most often change only once in their life time. Serial bidirectional sex changers can switch to and fro, depending on the situation. In the ‘risk-of-movement’ model by proposed by Nakashima et al., 1996 and Munday et al., 1998, predation pressure on reefs makes movement for mate selection highly risky and dangerous, especially for small and sparsely distributed species like gobies. It would be advantageous therefore, for sedentary or ‘stay-at-home’ individuals to switch genders as the need arises, such as when a mate dies or when the sex ratio is skewed strongly toward a specific gender. Gobiodon is a well studied genus for bidirectional sex change hermaphroditism.
Here’s a bonus additional form of hermaphroditism before we end off. Synchronous hermaphrodites occur in relatively few species, and are capable of producing male and female gametes simultaneously. This form of hermpahroditism is seen most often in Serranidae and Gobiidae. Hypoplectrus indigo is one species that displays the standard out crossing style of mating. Prior to fertilisation, two individuals pair up for a process called egg-trading, which involved three steps.
a) both fish package their eggs into parcels
b) courtship is initiated by the individual releasing their parcel first, and
c) partners take turns releasing an egg parcel every few minutes and externally fertilising the mate’s released parcel. This carries on until both individuals have fertilised each other.
This form of mating enables any two individuals to come together to mate and ensure continuation of their lineage. This is especially useful for species that are low in population density.
We hope you have found this article informative, and interesting. Reef fishes have evolved many ways to counteract pressures related to spawning and mate selection, and this is carried forward to those in captivity as well. The next time you’re smitten to buy two male Genicanthus, stop and think. What you really want is more females.