Understanding the sex of fish isn’t easy for everyone. Some species show sexual dimorphism, while others require gonadal analysis. The color and shape of the gonads provide clues to the fish’s sex. Male gonads (testes) are typically whitish and elongated, whereas female gonads (ovaries) are reddish. Fish like groupers and sea bream undergo sex changes during their life due to social or genetic factors. For instance, the dominant female clownfish can switch to male in the absence of a dominant male.
Recognizing fish gender through gonadal observation: Testes are generally whitish and elongated, especially visible during reproductive periods when filled with seminal fluid. On the other hand, ovaries are reddish.
Gonochoristic species, like many teleosts and cartilaginous fish (e.g., sharks and rays), maintain separate sexes throughout life. However, some teleosts, such as sea bream and groupers, exhibit hermaphroditism, where individuals may possess both male and female gonads or switch sexes at different life stages.
Synchronous and Sequential Hermaphrodites Synchronous hermaphrodites, like certain serranids, possess both gonads simultaneously, allowing them to be both male and female at the same time. Examples include species like Serranus cabrilla, Serranus scriba, and Serranus hepatus.
Sequential Hermaphroditism In contrast, species like sea bream exhibit sequential hermaphroditism. Males develop first, later transitioning to females—a condition known as protandry. Conversely, in protogyny, fish begin life as females and later transition to males, as seen in species like the Xyrichthys novacula.
Sex Change and Morphological Changes During sex inversion, fish may experience external morphological changes. Males typically exhibit more vibrant colors, while females retain more subdued tones. Occasionally, self-fertilization occurs in species with limited social interactions.
Factors Influencing Sex Change In some species, genetic factors program sex inversion, triggered by age or size. For example, brown groupers become sexually mature as females around age 5, and most undergo sex inversion by age 12, although some transition earlier or later. In Epinephelus merra, a smaller grouper species, sex change is triggered at a size of about 20 cm, independent of environmental factors.
Social Factors and Sex Change Physical environmental factors rarely influence fish sex changes, even in species without programmed inversion. Social factors, however, are critical. In bisexual fish species, social structures often determine sex. For example, in Labroides dimidiatus (cleaner wrasse), the dominant male controls a harem of females. The male’s presence prevents sex changes, but if he disappears, the dominant female undergoes sex inversion and assumes control of the harem. Similarly, in large harems, some females may change sex and form new harems when the dominant male cannot exert control.
Clownfish and Gobies In protandric clownfish, females are larger and dominant within social groups. Upon the dominant female’s disappearance, the male quickly grows in size and reverses sex. Interestingly, in groups lacking sexually mature individuals, some juveniles may directly become females.
In gobies (Trimma okinawae), the dominant female in a social group changes sex to replace a lost male. However, if a larger male later appears, the new male may revert to being female.
These social interactions trigger internal endocrine responses, leading to sex changes. While data is fragmented, the most widely accepted hypothesis is that social signals influence the hypothalamic-pituitary-gonadal axis, activating cerebral GnRH, which triggers gonadotropin release, stimulating steroid hormone production in the gonads.
