What connects pufferfish, triggerfish, sunfish, and spiky porcupinefish? Visually, they seem like a completely random group of utterly bizarre, oddly shaped fish. However, what ties them together is usually not always a visible physical feature.
Making up around five per cent of all tropical fish in the world, tetraodontiform fish (Order: Tetraodontiformes) are a group of tropical marine fish of nearly 350 species grouped under ten families. Common representatives of this group are triggerfish (Balistidae), pufferfish (Tetraodontidae), boxfish (Ostraciidae), and porcupinefish (Diodontidae).
These fish display fascinating diversity in colour, body shape, and special adaptations within and across families. They range in size from two centimetres (the smallest) to the ocean sunfish (Molidae), which can average around 1.8 metres.
Tetraodontiform fish occupy various habitats across the world’s tropical, sub-tropical, and temperate oceans. Most are found in shallower coastal waters, but a few, like spikefish and triplespines, dwell in relatively deeper waters. Some tetraodontiform fish are sought after as culinary delicacies in certain parts of the world, though access may be regulated. Fugu (pufferfish and porcupinefish) in Japan, certain filefish species are a popular Korean snack food, and triggerfish are sought for their light, white meat across several coastal regions.
Tetraodontiform fish are broadly characterised by having spines and a sharp beak made of fused teeth. The word “tetraodontiformes” breaks down into the Greek words tetra, meaning four, odous, meaning teeth, and the Latin word formes, meaning shape. The number of fused teeth defines some of the families under this order, namely Diodontidae (two-toothed), Triodontidae (three-toothed), and Tetraodontidae (four-toothed).
As inflexible swimmers, these fish have evolved defence strategies to compensate for their lack of speed and agility. Some exude toxins through their skin or hold them in their organs, while others inflate in response to a threat. Most have tough, leathery skin with hardened scales. Some exhibit multiple lines of these defences. This group of fish has a fascinatingly wide array of defence mechanisms tucked under their fins and skins.
Tetrodotoxin: While many fish of this order are palatable, some, like pufferfish, have another defence mechanism in their repertoire. For instance, this (1) blue-spotted puffer (Arothron caeruleopunctatus) and (2) white-spotted puffer (Arothron hispidus) contain a very potent and lethal toxin called tetrodotoxin (TTX), which is typically found in their gonads (testes and ovaries), intestines, liver, and sometimes, skin. Known to be around 1,200 times more toxic than cyanide, TTX is a powerful neurotoxin that works by paralysing the unfortunate ingestor. TTX only works if the pufferfish is eaten, highlighting a major flaw in the defences.
Interestingly, though a small dose can kill humans, its effect on dolphins is completely different — for them, TTX acts as a narcotic. Dolphins get “high” on pufferfish or porcupinefish, biting lightly into the fish to “get a hit” before tossing it to the next dolphin. Photos: Dhritiman Mukherjee (1), Phalguni Ranjan (2)
Pahutoxin: Boxfish and cowfish like the (1) long-horn cowfish (Lactoria cornuta) and (2) roughskin trunkfish (Ostracion trachys), when threatened or stressed, secrete a layer of mucous from their skin. This mucous contains a unique ichthyotoxin (a toxin produced by fish and/or kills fish) less potent than tetrodotoxin, but an effective defence mechanism nonetheless. Pahutoxin, previously called ostracitoxin, breaks down red blood cells (haemolytic). It foams on contact with water and is toxic to marine fish.
Unlike tetrodotoxin, which needs to be ingested, pahutoxin just needs to come in contact with a fish to take effect. Interestingly, pahutoxin does not reside in the skin of the boxfish, and is only synthesised when they are stressed. As seen here, these fish are brightly coloured and marked, possibly advertising their toxicity. Boxfish might not be immune to the toxins themselves, but they are not toxic to humans. Photos: asawinimages/Getty Images (1), Francois Libert, CC BY-SA 2.0 (2)
Thick skin and dermal spines: Specialised skin is perhaps the most common defence mechanism seen across the ten fish families in this group. Filefish, triggerfish, and sunfish like the (1) ocean sunfish (Mola mola) have toughened, thick, leathery skin that is hard to break. Filefish like this (2) longnose filefish (Oxymonacanthus longirostris) are called leatherjackets because of this.
In addition to tough skin, certain deep sea fish like spikefish (Triacanthodidae) have small, sharp, spiky spinules on their scales, giving them a sandpaper-like texture. They also have a large, pointed spine accompanying the pelvic fin, and sharp dorsal spines, living up to their “spiky” common name.
Pufferfish also have small spines on some parts of their bodies (dermal spines) – a backup mechanism to tetrodotoxin guaranteed to give a predator a nasty shock in case these fish get swallowed before they can inflate.
The (3) black-blotched porcupinefish (Diodon liturosus) displays how scales in porcupinefish are modified into conspicuous spines all over their body that lay flat when they are not inflated, and stick out when inflated. These spines are non-venomous but sharp. Photos: Adrian Kaye/Shutterstock (1), Paul Starosta/Getty Images (2), Vardhan Patankar (3)
Dorsal spines: Triggerfish and filefish have another line of defence other than their toughened skin — dorsal spines (spines atop the body). Dorsal spines are used to defend and anchor the fish under rocks and refuge.
Filefish, like this Japanese leatherjacket (Paramonacanthus japonicus), have one large dorsal spine that locks into an erect position supported by a smaller one, giving the appearance of a single, long, slightly curved spine. Photo: Dhritiman Mukherjee
Similarly, triggerfish have a sharp dorsal spine, which can be erected and laid flat into a groove, locked into place by another smaller spine. This locking and unlocking motion (or trigger) gives them their name. This titan triggerfish’s (Balistoides viridescens) dorsal spines lie flat in the groove on top.
Deep-sea-dwelling triplespines (not pictured here) have three prominent spines — two dorsal spines fused into one long, sharp spine, and long spines in place of the ventral fins. Photo: Umeed Mistry
Horns and armours: Cowfish like the (1) long-horn cowfish (Lactoria cornuta) typically have two long spines or “horns” above their eyes, much like cows, and may sport more spines on other parts of their bodies.
Boxfish have evolved with a tough, bony, plate-like carapace that provides additional protection against predators. This rigid carapace is made up of hexagonal dermal scutes (that have a collagen base), visible in the patterns on this (2) yellow boxfish adult (Ostracion cubicus). Photos: Raimundo Fernandez Diez/Getty Images (1), Vardhan Patankar (2)
Camouflage and Mimicry: The mottled appearance and cryptic patterns and colouring of some filefish like this pig-faced leatherjacket (Paramonacanthus choirocephalus) provide an added advantage in camouflage.
More specifically, the mimic filefish (Paraluteres prionurus) mimics the toxic black-saddled toby (Canthigaster valentini), a pufferfish, thus avoiding predation by signalling false toxicity. The diamond filefish (Rudarius excelsius) mimics benthic (bottom-dwelling) algae, using its greenish-mottled appearance to camouflage effortlessly. Photo: Umeed Mistry
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Phalguni Ranjan
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