Fish
Asian Sheepshead Wrasse
Semicossyphus reticulatus · also called asian sheepshead wrasse, kobudai
The Asian sheepshead wrasse, or Kobudai, is a metre-long fish from the cool reefs of Japan and Korea, and it is best known for two things: a bizarre, lumpy face — a bulging forehead over a jutting chin — and the fact that every one of them is born female. The biggest fish in a group can switch sex, grow that face, and take over as the resident male.
Overview
The Asian sheepshead wrasse lives on rocky reefs in cool temperate water around Japan, Korea, and nearby China. It reaches about 1 m and 15 kg, reddish-grey with a pale lower face, and in large adults a swollen, dome-like forehead and an enlarged lower jaw. It crushes crabs, shellfish, and sea urchins with strong jaws and grinding throat teeth.
Female first
The species is a protogynous hermaphrodite: all individuals mature as females, and only some later change to male. The switch is social — when a territory loses its male, the largest, most dominant female responds by changing sex over weeks to months, developing male anatomy and behaviour and taking over the group.
The forehead
The bulging forehead and heavy chin develop mainly in males and the largest fish, and are used in display and head-to-head shoving contests over territory and mates. A big head signals an old, dominant fish.
A television star
Footage of a female Kobudai transforming into a male and then displacing the resident male, filmed off Japan for Blue Planet II, brought the species to a global audience.
Status
The Asian sheepshead wrasse is listed as Data Deficient. It is naturally uncommon, slow-growing and long-lived, and caught by hook and spear — a life history that makes removing the biggest fish especially risky.
Asian Sheepshead Wrasse: common questions
Do Asian sheepshead wrasse really change sex?
Yes. The species is a protogynous hermaphrodite, which means individuals function first as females and some later become males. This is widespread in the wrasse family, but the Asian sheepshead wrasse is a striking example because it is so large and the change is so visually obvious. Sex change is triggered socially. In a group holding a territory, the male suppresses the females behaviourally, and if he dies or is removed, the largest and most dominant female responds by changing sex. Over the following weeks to months her ovaries are replaced by testes, her behaviour shifts to male patterns, and she develops the male's exaggerated head and jaw.
What is the big lump on its head for?
The domed forehead, along with the heavy protruding chin, develops mainly in males and in the biggest individuals, and it appears to be a signal and a weapon in male competition. A larger head makes a male look more imposing in the frontal displays that rivals use to settle disputes, and when displays escalate the fish push and butt each other head to head, so a bigger, denser forehead is an advantage. Because both size and maleness are tied to social dominance, a fish with a large head is advertising that it is old, big, and in charge.
Where does the name Kobudai come from?
Kobudai is the Japanese name for the fish, and it refers to the lump on its head, kobu meaning a bump or protuberance and dai relating to the sea bream and similar fish. In English the species is called the Asian sheepshead wrasse by analogy with the unrelated sheepshead fish and sheepshead wrasse of the Americas, which also have blunt heads and strong shell-crushing teeth. The species became widely known internationally under the name Kobudai after it was featured in a nature documentary.
Is the Asian sheepshead wrasse endangered?
Its status is officially listed as Data Deficient, meaning there is not enough information to judge its population trend confidently. However, the species has the profile of a fish that can be depleted easily. It grows slowly, lives a long time, matures as a female before any individuals become male, and is targeted by hook-and-line and spearfishing, especially for large fish, which are the males and the potential future males. Removing the biggest individuals from a population that depends on social rank for its sex ratio can have outsized effects, so the fish is generally regarded as one to watch even though it lacks a formal threatened listing.