What happens in cases where there is an abundance of female fish and hardly any males, or vice versa? Sometimes, fish change their sex to balance things out! Considered an evolutionary advantage in environments with increasing pressures, the capacity for rapid sex change can be the sole solution to a population’s survival. Sexually dimorphic fish, which have distinct male and female hormones and anatomies, need to have fertile males and females to persist as a species in the wild.
The official term for an anatomical sex change in fish, as a response to external stimuli or age-related hormones, is sequential hermaphroditism. For some groups of freshwater and marine fish, this is a natural process that allows for the fittest specimens to transition to the opposite sex when the time is right. For example, a large female may eventually develop male gonads and reproductive functions in response to the death of a dominant male.
As the female is able to take the place of the lead male, the population’s longevity can be secured. In the same way, species with females as the dominant members of a hierarchy may initially produce all male fish. Over time, the largest males may transition to take the place of the previous matriarchs. Sex change, which can also occur to fix an unbalanced male-to-female ratio, is common for marine species but is very rare in groups of freshwater fish.
1) Blue-banded goby (Lythrypnus dalli)
Popularized by the aquarium trade due to its vivid colors and easily manageable size, the blue-banded goby is a benthic, marine species. It is found in shallow to deep waters throughout the coastal areas of its native range. As it is a relatively small yet eye-catching fish, it often hides in structurally-diverse reefs and along rock faces with many crevices. One of its most notable adaptations is its ability to rapidly change its sex.
The blue-banded goby is considered a bidirectional hermaphrodite because it can quickly change its sex more than once. Essentially, a male can transition into a female when the need arises. Inversely, females can do the same. Once the social hierarchy requires an increased proportion of sexually mature females to males, they can return to their original sex.
In the wild, the sex ratio of this species is usually skewed in favor of females. Blue-banded gobies tend to be grouped in small-sized harems with a single, dominant male. By ensuring that the largest and most dominant fish becomes the sole male, its reproductive success is significantly increased. This way, the most favorable genes are passed down to the next generation.
2) Chinook salmon (Oncorhynchus tshawytscha)
The largest species in its genus, the Chinook salmon is an economically important sport fish and a source of high-quality proteins throughout its native range. This valuable salmonid is anadromous, which means it spends most of its adult life in oceanic waters, only venturing into freshwater systems to spawn. Unfortunately, as it must rely on inland rivers and lakes to sustain its young, some of its populations are now in decline.
One of the possible reasons for its decline has to do with increased rates of sex change. So far, this has been observed in a few studied populations. Sex reversal in this species is quite unusual, and it appears to be a result of environmental estrogens from ecologically harmful sources (e.g. industrial runoff, pesticides). Another possible reason for mass feminization in salmon populations is unstable water temperature.
Researchers who’ve begun to look into patterns of sex reversal in Chinook salmon are worried about its implications for this species’ survival. Unfortunately, with many males transitioning into females, leading to heavily skewed sex ratios, the gender shift could compromise reproduction rates. For now, the consequences of sex change in salmon populations remain unclear.
3) Clark’s anemonefish (Amphiprion clarkii)
Also known as the yellowtail clownfish, Clark’s anemonefish is a small-sized marine species that favors the warm waters of tropical reefs and lagoons. As suggested by its common name, it has a symbiotic relationship with a handful of host anemones. While it protects the anemone from parasites and predators, the anemone provides it with cover and food scraps.
If you’ve ever had the chance to observe this anemonefish before, you may have observed that it is usually found in small groups. The largest and most aggressive member of each group is a fertile female, whereas the smaller members tend to be male. As this species is a sequential hermaphrodite, all individuals start off as male. The largest and fittest fish of each group then transitions into a female fish.
The dominant female spawns with only one male at a time. Apart from size, females are usually distinguished by their white caudal fins; males, in contrast, have yellow caudal fins. Researchers have found that, even before morphological changes begin to occur, the transition from male to female fish begins in the preoptic region of this species’ brain.
4) Black sea bass (Centropristis striata)
A member of the Serranidae family of groupers and sea basses, C. striata is a commercially important perciform. Set apart by its elongated dorsal fin, relatively large mouth, and its blue-black coloration, it grows to a full length of about 26 inches (66 cm). In the wild, this slow-growing fish favors the rocky shores of the east coast of North America. Its aggregations are also found in deep, offshore locations.
Important to many coastal fishers of the US, the black sea bass ensures that its genetic lineage is secured by undergoing a sex change whenever its male-to-female ratio is unfavorable. As protogynous hermaphrodites, their young initially develop as females. Once the numbers of fertile males begin to dwindle, the largest females transition into males.
The stimuli that trigger anatomical changes in black sea bass are not yet fully understood, but biologists have proposed that it simply is a response to the loss of males. As the largest and most physically fit females undergo the transition, their genes have more chances of being passed down to subsequent generations. More research is necessary as knowledge about the consequences of the removal of males (due to fishing pressures) should help determine fishing limits.
5) Ambon damselfish (Pomacentrus amboiensis)
A common sight in the remarkably colorful reefs of the Coral Triangle, P. amboiensis is a small, ray-finned fish. As it grows to a maximum size of just 3.5 inches (9 cm), it is quite popular in the aquarium trade. In the wild, adult damselfish gather in small, distinctly separate groups. The dominant fish of each group is a large male, and all other members are females. The males are responsible for guarding and aerating the eggs.
A protogynous hermaphrodite, the Ambon damselfish starts off as female. Over time, as females become part of sexually mature groups, the largest ones undergo a sex change to take the place of males. The female members of each group follow a hierarchy with linear dominance. The alpha female (the largest one) is able to access the male-guarded nest and is usually permitted to feed in the most productive areas of the group’s territory.
This hierarchical system, which imposes a strict allocation of resources in favor of the dominant male and alpha female, ensures that the growth rate, gamete production, and longevity of the largest fish are prioritized. This leads to higher reproductive success for the largest damselfish specimens, increasing the overall fitness of their subsequent generations.
6) Asian sheepshead wrasse (Semicossyphus reticulatus)
Also known as ‘Kobudai’, the Asian sheepshead wrasse is an enormous marine fish that can weigh as much as 32 pounds (14.5 kg). Its current popularity is largely due to the attention it gained as a sex-changing fish in BBC Earth’s Blue Planet II. A show-stopping inhabitant of temperate rocky reefs, this wrasse is distinguished by the bulbous bump on the head of its fertile males. Females are much smaller and have a less conspicuous hump.
As a protogynous hermaphrodite, the Asian sheepshead wrasse starts off as a female fish. Once it sexually matures and its body size enlarges, the largest females may begin to transform into males. The death of a dominant male is not a necessary precursor to its transformation. Occasionally, large females who have fully transformed may even challenge the dominance of a male they once paired and spawned with.
During their transformation into males, large females gradually develop testes and the hump on their heads becomes significantly larger. The hump continues to enlarge with age. Once the female emerges as a full-fledged male, he begins to attract a harem of younger females. This oviparous species usually mate in distinct pairs.
7) Australian barramundi (Lates calcarifer)
As the Australian barramundi is a catadromous species, it inhabits freshwater systems when it is not preparing to spawn. During the monsoon period, which signals the beginning of its spawning season, it migrates into brackish and coastal areas. Spawning occurs downriver as the eggs and fry require saline water to develop and survive. In areas where freshwater may be scarce or inaccessible, populations may undergo their entire life cycle in marine habitats.
Unlike many reef species that start off as female fish, the Australian barramundi initially matures into a male specimen. It retains its sex for at least one spawning period, after which the largest individuals may begin to transition into females. Generally, females are much larger than males and can have a maximum length of about 4 – 6 feet (1.2 – 1.8 meters)!
In some cases, fry may develop into females that never transition into males. There are likewise males who may never transition into females within their full lifespan. This is especially true for specimens that are reared in captive environments.
8) Potter’s angelfish (Centropyge potteri)
A small, ray-finned, marine species, the potter’s angelfish lives in coral and rubble reefs in relatively shallow waters. As a benthopelagic fish, it may be found searching for detritus on the seafloor or for suspended particles of algae in the water column. Though it is occasionally collected from the wild to meet the aquarium trade’s demand for ornamental fish, its populations have remained stable.
Commonly found in Hawaii’s coastline, this angelfish is a protogynous hermaphrodite. In the absence or decline of dominant males, the largest female may begin to transition. Its social group, usually referred to as a harem, consists of an alpha male and several females. The male pairs up with one of the females during the spawning period, which occurs sometime from December to May.
At the peak of their mating ritual, the red coloration of spawning pairs intensifies. The male performs a courtship dance above its territory’s highest outcrop, encouraging the female to follow and indicate her readiness to spawn. Once the gametes are released, the pair quickly retreats into their territory for cover. Successfully fertilized eggs will eventually hatch into female fry.
9) Ribbon eel (Rhinomuraena quaesita)
Ribbon eels are an absolute joy to observe due to their quirky manner of poking their heads through crevices and hollows. They are normally found in the shallow, warm waters of lagoons and fringing reefs. For a long-lived moray eel, this colorful species is relatively thin and ribbon-like. Most of the time, it keeps its body hidden in the substrate.
Depending on their life stage and sex, ribbon eels display one of three color patterns. Juveniles that have yet to undergo sexual differentiation are usually jet black, save for a fine streak of yellow along their dorsal fin. As this species is a protandric hermaphrodite, the juveniles initially develop into males. Once they are sexually mature, they are set apart by their bright and vivid blue body. Only their elongated snout and their dorsal fin remain yellow.
Once the largest males grow to their full length, they transition into female eels. Again, their color drastically changes as their blue pigments are replaced by a bright shade of yellow. Once the eels have undergone their full transformation into fertile females, they may begin to produce eggs in preparation for the spawning period.
10) Blackhead seabream (Acanthopagrus schlegelii)
The blackhead seabream is a commercially important fish throughout its native range due to its valuable meat and appeal as a sport fish. To meet demand for the high-quality proteins of this marine species, it is occasionally farmed in Japan. In the wild, it is found in brackish bays and coastal waters, where it tends to remain in the shallows of rocky shores. Its diet is largely composed of bristle worms and mollusks.
The blackhead seabream has an interesting life history because it naturally undergoes a sex change at around 3 – 5 years of age. All specimens start off as male, making them protandrous hermaphrodites. Their gonads first differentiate when they are just 4 months old. Though their male gonads are able to function, they gradually develop into bisexual gonads before transitioning fully.
Males of this species usually measure around 11 inches (28 cm) when they undergo the most drastic phases of their sex change. It’s likely that this life history, which ensures that larger and more robust specimens are responsible for producing eggs, aids in improving this species’ resilience when it is faced with heavy fishing pressures.
