Lurking under the depths of Hong Kong’s waters, a grouper-like yellow fish is patrolling around the corals and rocks, it is 𝘋𝘪𝘱𝘭𝘰𝘱𝘳𝘪𝘰𝘯 𝘣𝘪𝘧𝘢𝘴𝘤𝘪𝘢𝘵𝘶𝘮 Cuvier, 1828, the barred soapfish🐟.
Adults of this species are brightly yellow coloured 🟡 with two black bars, attaining a maximum length of 25 cm. This fish is laterally compressed and resembles a cute yellow grouper, which it is indeed, as they are members of the grouper family, the Serranidae. Don’t get your attention distracted by the colourful appearance of this species though, as it is a voracious predator, swallowing fishes and crustaceans🦐 whole with their expandable jaws.
Bright colours worn by animals in nature are often associated with a warning signal for predators of a potential danger🚨. This phenomenon is dubbed aposematic colouration. Here, the fish’s colouration presents a high contrast with the surrounding environment, making it easily noticeable and recognisable by predators. Thus predators are reminded of the unpleasant trait the colourful animal has when they are presented with one. So, what exactly is this yellow fish hiding? Let's dive into it now 🔍!
In the ocean every predator might become prey, and barred soapfish are no exception; thus they have equipped themselves with a few “superpowers” 🦸. When young, they are in fact blue and yellow instead of the barred colouration we see in adults. The blue and yellow marks serve a witty purpose, to mimic a venomous blenny (genus Meiacanthus). Blennies that they mimic give predators a painful venomous bite 💉. When predatory fishes see the juvenile soapfish, they associate them with the venomous blenny and avoid eating them. Generally speaking, it is mainly the mimicker who benefits from this behaviour. But here is an example of Müllerian mimicry in which the mimicker soapfish and the blenny, the one being mimicked, are both benefited 🤝.
Indeed, the common name of 𝘋𝘪𝘱𝘭𝘰𝘱𝘳𝘪𝘰𝘯 𝘣𝘪𝘧𝘢𝘴𝘤𝘪𝘢𝘵𝘶𝘮, the “barred soapfish”, refers to the fish behaving like a bar of soap 🧼, and that’s exactly one of its defense mechanisms. This fish species is able to secrete a lot of thick mucus through its dermal skin’s mucus cells, containing a toxin, called Grammistin, which possesses an unpleasant taste to deter predators (and humans) 🤢. Thus, the predator-deterring effect of these fishes are enhanced by both of their “superpowers”, unpleasant taste of the soapfish and an illusory venomous bite of the blenny, making predators further avoid these two fishes. In other words, when predators see a blue and yellow fish, they associate them with both the bad taste of the soapfish and the venomous bite of the blenny.
The toxin picked the interest of scientists for at least two reasons: its toxicity and its taste 😋. Japanese scientists had taste-tested mucus secreted by the fish and discovered that it resulted in an immediate unpleasant bitter taste with a stinging sensation.
Given its unpleasant taste, it is evident that it will also function as a deterrent for hungry predators 😒. Under laboratory conditions, researchers 👨🔬 introduced a juvenile soapfish into a tank containing a ferocious but naive predator — the lionfish. When the soapfish swam to the bottom of the tank, the lionfish slowly swam towards the soapfish, and in a blink of an eye, the soapfish was held by the lionfish 🍽️. Yet, this story ended with a twist, the lionfish immediately spitted the fish out. Then the lionfish continues to do a “spitting” motion 🤮, suggesting that the bitter taste lingered in the predator’s mouth. The lionfish did not do any further attempt to prey on this fish, suggesting that it learnt a lesson through this distasteful event. Furthermore, grammistin not only deters predators, but it also prevents bacterial infections 🧫 by causing bacterial cells to burst. Soapfishes are not affected by their own bitter, toxin-filled mucus, and it is known that soapfish can cannibalize other smaller soapfish. Hence soapfishes are not truly predator-free.
Additionally, grammistin in the mucus is proved to be deadly towards other fishes 😵. When a barred soapfish is irritated in a water tank for 5 minutes then removed, followed by putting in 5 small fishes into the tank, these small fishes show immediate motor excitation and will all die within 15 minutes.
Want to see our bitter-tasting soapy friend? Come join a tour in the Hong Kong Biodiversity Museum, where you can find it lurking near its grouper relatives! And if you enjoy this post, please share it with friends and follow us for more amazing biodiversity stories weekly. 🙌
Text: Hugo
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