The oddly shaped boxfish.

By Lydia A. Harris ~ Edited by C.E. O’Brien, Ph.D., The School for Field Studies, Center for Marine Resource Studies, South Caicos

Ask someone to draw a fish, and you can probably guess the basic shape they will draw:  a horizontal oval with a triangle on one end for a tail. Easy enough, and this shape would be accurate for many of the fish out there. Another key feature of this classically shaped fish is that it is laterally compressed, meaning that it is not very wide from side to side. From the front, its profile is a thin line, while the side profile is where we see that elongated oval shape. This body plan seems to work really well in the water as many species of fish across many families have evolved this shape and are very fast and maneuverable. It’s no wonder that this is the body plan we tend to think of when we imagine a fish.

But some fish are built differently. There are many oddly shaped fish in the sea, and many examples from the Turks & Caicos. Think of flat flounders, lumpy frogfish, winged batfish, and even noodle-like eels. Another oddly shaped group is known as Tetraodontiformes, which includes fish such as pufferfishes, boxfishes, filefishes, triggerfishes, and more. These fish families have all sorts of unique shapes. Pufferfish, porcupinefish, and burrfish have large, round bodies, often covered in spines. Filefish and triggerfish have trapezoid-shaped bodies and usually have one large spine poking up from the top of their heads (this is the “trigger” of the triggerfish). But one of the most interesting of these is the family Ostraciidae, or the boxfishes.

A honeycomb cowfish swims over the reef. Its skin is intricately patterned with bright yellow coloration. These fish can also shift the brightness of the patterns on their skin, helping them to blend in with different environments.

The boxfishes are accurately named. Instead of that classic, skinny oval shape, their bodies are distinctly box-like. Rather than laterally compressed with a skinny profile, their bodies form a triangular or hexagonal shape with a wide, flat bottom and rigid sides that slant up to a ridge along their dorsal (top) side. Along with this fascinating body shape, the boxfishes are often brightly colored and intricately patterned, making them an exciting fish to find on a snorkel or dive.

The “box” that gives the boxfishes their distinctive shape is actually made of bones. Each side of the box, or “carapace,” is composed of multiple plates of thick bone that become firmly attached to each other as the boxfish grows. These plates are called “scutes” and their edges have suture-like teeth that help connect the scutes to each other. Also helping to bridge the gaps between scutes is a network of fibers made of collagen, a protein that is a part of connective tissues like tendons and ligaments. The body of the boxfish is almost completely enclosed inside this carapace, but there are openings for its eyes, mouth, gills, fins, and tail. The result is a unique-looking fish with a built-in suit of armor under its skin. Research shows that this armored carapace is very strong and able to withstand both penetrating attacks (with teeth) and crushing attacks from predators.

Is this the whole story behind the boxfish’s box? Other theories suggest that the bony carapace could also provide mineral storage, help protect males from each other as they compete for females, or even increase maneuverability. The edges of the carapace might also act like the keel of a boat and help stabilize the boxfish in turbulent water. However, many studies have had different results, so it’s unclear whether the carapace has a stabilizing or destabilizing effect. More research is definitely needed to assess how this bony carapace impacts the performance of the boxfishes, but it’s possible that these fascinating fish have evolved a way to both protect themselves and maintain their swimming ability.

A spotted trunkfish sits by a rocky wall. This is an excellent view of the different fins: pectoral fin in the middle of its side, dorsal fin on the top of its back, anal fin underneath and before the tail, and the broom-like tail (or caudal) fin.

In addition to their bony armor, some species also have bony spines and some produce toxic substances in their skin. When a boxfish feels stressed or threatened, it releases slimy mucus from its skin that goes directly into the water around it or right into the mouth of the predator that is trying to eat the boxfish. The toxin they secrete is known as ostracitoxin or pahutoxin, and it is highly poisonous to anything in the nearby area. This toxin is generally not very harmful to humans, but very poisonous to other fish. This is why boxfishes are notoriously difficult to keep in aquariums—a stressed-out boxfish can accidentally wipe out all of its tankmates in such a small, enclosed area. 

The bright colors of many boxfish species are a warning sign to predators and other fish that they are poisonous. Using bright colors as a toxicity warning is known as aposematism and is found throughout the animal kingdom, for example in snakes, frogs, and butterflies. 

Given their cumbersome shape and small fins, intuition would suggest that boxfish are slow, clunky swimmers. But some research indicates that they are actually strong, powerful swimmers comparable to more “classic” fish body types. Boxfishes use different fins to swim with than most other fish: they mostly use the fins on either side (pectoral fins) and the fins on the top and bottom (dorsal and anal fins respectively), while other fish will use their tail fin to propel their body. Even so, the boxfish body shape is rather hydrodynamic and capable of speed and maneuverability. They also have a powerful tail that enables them to move very quickly if they need to, but with prey that doesn’t move and multiple defenses against predators, it doesn’t seem that they need to very often. Instead, they are usually found swimming slowly around the ocean floor looking for food in a variety of habitats, including rocky areas and coral reefs, open sandy plains, and seagrass beds.

Boxfishes eat many different kinds of marine invertebrates, such as shrimp, sponges, worms, tunicates, and more. Much of their diet includes organisms that are non-mobile, or sessile, meaning they are attached to a hard, unmoving surface like rocks or a coral reef. Boxfishes have small mouths with protruding lips and pointy teeth. To eat, they turn their mouths downward and graze on the sessile organisms. Some species have also been reported using their mouths to blow jets of water into the sand in order to uncover and find prey.

Boxfishes are found all over the world, and there are five species that you can spot in the Turks & Caicos Islands. There are two kinds of boxfishes: cowfishes and trunkfishes. The cowfishes are distinguished from the trunkfishes by two spines, one over each eye, that look like little cow horns.

A scrawled cowfish hovers above a sandy plain. This is a good example of a common habitat where boxfishes will forage for food.

There are two Caribbean cowfish, the scrawled cowfish (Acanthostracion quadricornis) and the honeycomb cowfish (Acanthostracion polygonius). The scrawled cowfish is yellow with blue markings in a random “scrawled” pattern, and the honeycomb cowfish is a yellowish-brownish color with a distinctive honeycomb pattern. Each of these cowfish are usually about a foot (0.3 meters) in length and found at depths of up to 80 feet (24 meters). While not especially common, they are found throughout the Caribbean, and their range includes as far north as Massachusetts and as far south as Brazil.

The Caribbean is also home to three species of trunkfish: the smooth trunkfish (Lactophrys triqueter), the spotted trunkfish (Lactophrys bicaudalis), and the buffalo trunkfish (Lactophrys trigonis). The smooth trunkfish is identified by its dark body with white spots and a patch of honeycomb pattern on the middle of each side. This trunkfish is called “smooth” because of its lack of spines, while both the spotted and buffalo trunkfish have a spine on either side of the tail (but not above the eyes, these are only found on the cowfishes). The spotted trunkfish has the reverse color pattern of the smooth: a white body covered with dark spots. The buffalo trunkfish is usually a mottled gray or brown color, with white spots on its back.

A spotted trunkfish hangs out in a rocky crevice. Note its small, protruding mouth and the distinctive ridge on its back.

The buffalo trunkfish is the largest of the trunkfish, reaching over a foot (0.3 meters) long with a maximum length of 19 inches (0.5 meters), while other trunkfish are usually around a foot (0.3 meters) long or smaller. Like the cowfishes, the trunkfishes can be found at depths up to 80 feet (24 meters), although the buffalo trunkfish is generally found at shallower depths. Out of these three species, the smooth trunkfish is the most common in the Caribbean, while the spotted trunkfish is found occasionally and the buffalo trunkfish is uncommon. Like the cowfishes, each trunkfish species ranges from Massachusetts to Brazil.

In the Caribbean, the best way to enjoy these eccentric fish is by saying hello if you see them out on the reef. Some boxfish species are kept in hobby aquariums, but these are usually from the Pacific since the Caribbean species are protected from the aquarium trade. Boxfish are also rather difficult to keep in aquariums because of the toxins they can produce from their skin when they get stressed. While they are generally regarded as “excellent eating,” there isn’t a prominent boxfish fishery in the Caribbean, although they are occasionally fished. 

While it’s not the most common fish to see on the reef, the good news is that the populations of all five Caribbean boxfish species are considered to be at healthy levels. Each species is determined to be of “least concern” on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. However, there are many general threats affecting their habitats. Global climate change, pollution, and overfishing are contributing to the degradation of coral reefs, a vital ecosystem that supports around 25% of all marine life, despite only taking up around 1% of the ocean floor.

Coral reefs are considered to have the highest biodiversity of any ecosystem on Earth and provide several ecosystem services including protecting coastlines and providing food and recreational opportunities. It’s extremely important to protect coral reefs and other marine habitats from human-based threats, otherwise we risk losing an ecosystem that is vital to the well-being of the ocean and the organisms that depend on it, including ourselves. 

There is still a lot to learn about the boxfish, not in the least determining the purpose of its bony carapace. More research is being conducted to determine how body shape affects its swimming performance, and the built-in suit of armor and toxic skin of the boxfish provide a great opportunity for research on defenses against predation.

These fascinating, mysterious fishes are an intriguing piece of the coral reef ecosystem and an undeniable treat to spot underwater. While their box-like shape is out of the ordinary, these fishes are definitely thinking inside of the box.

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