Why can the golden box puffer fish swim with a square, box-shaped body?

The yellow fish adorned with distinctive square black spots you behold bears the scientific moniker Ostracion cubicus, commonly referred to as the round-backed pufferfish or yellow pufferfish. These aquatic creatures predominantly call the coral reefs of the Pacific, Indian Ocean, and southeast Atlantic Ocean home, where they sustain themselves primarily on a diet of small prey, which includes tiny worms, crustaceans, mollusks, and minuscule fish, as well as fragments and organic detritus strewn across the reef.

Their vibrant yellow hue, embellished with scattered round black spots, imparts a striking appearance, rendering the entire fish akin to a black-spotted golden box. This distinctive coloration serves as a visual warning, as these creatures prefer a solitary existence and are exceedingly timid, using their brilliant skin to ward off potential predators.

In times of peril, the skin of the golden box pufferfish acts as a potent defense mechanism by releasing a neurotoxin known as Ostracitoxin. Attempting to house these remarkable creatures has posed a unique challenge, as keeping them in relatively confined aquatic spaces, such as aquariums, can lead to the toxins wreaking havoc on other fish residing in the same enclosure.

Most fish species boast streamlined, elongated bodies designed for optimal aquatic locomotion, affording them the ability to traverse water with ease. However, the golden box pufferfish boasts a rigid, box-shaped skeletal structure that deviates drastically from the typical fish anatomy we are accustomed to. This striking dissimilarity prompts questions about their evolutionary path and the mechanics enabling them to navigate their watery realm. How, one might wonder, do these fish, with their boxy, square bodies, manage to swim?

Astonishingly, despite their seemingly cumbersome appearance, the golden box pufferfish is an exemplar of aquatic agility, ranking among the most dexterous inhabitants of the coral reef. In 2002, a team of researchers employed a 3D-printed model of the box pufferfish to gain insights into the fluid dynamics governing their movements. Their observations revealed that when water interacts with the pufferfish, it generates small vortices near their bony shells, serving to stabilize their movements with surprising grace.

This remarkable adaptation plays a pivotal role in minimizing disturbances in turbulent waters and maintaining equilibrium amid strong currents. Through rigorous scientific inquiry, a deeper understanding has emerged, shedding light on the evolution that led to the distinctive form of the pufferfish, both in general and the golden pufferfish in particular.

Nevertheless, a subtle detail may have eluded the researchers' scrutiny: while the vortices surrounding the golden box pufferfish's skeleton enhance their directional stability, it also suggests a reliance on these vortices when navigating sudden threats posed by predators. Furthermore, close observations reveal the remarkable flexibility of these fish. When confronted with danger, they execute swift maneuvers, darting toward the shelter of the coral reef.

A 2015 study by S. Van Wassenbergh and colleagues posed a challenge to the prior vortex theory, contending that the influence of these eddies is counteracted by the torque generated in front of the pufferfish's boxy skeletal structure, rendering them unable to maintain a stable swimming position. In fact, they may be susceptible to being "overturned."

The enigma of the golden box pufferfish's agility lies in its ability to employ dorsal, pectoral, and tail fins for quick flips, allowing them to maintain stability in flowing waters, much like a helicopter relies on rotor blades and a vertical tail rotor to remain steady in the air. When swift changes in direction are required, these ingenious creatures need only manipulate their tail fin, enabling them to execute swift turns within confined spaces.

In essence, the golden box pufferfish's agility arises from the intricate interplay between its aerodynamically unstable body structure and the finesse with which their fins are employed. Furthermore, these creatures need not undertake long journeys, and their armory of venom and "armor" more than compensates for their reduced speed. This unique adaptation underscores the long-term evolution that has sculpted their singular niche in the underwater realm.

One might ponder: How does this fish, with its boxy anatomy, typically navigate its watery world? Observing other fish in motion reveals a common pattern: the coordinated movements of the tail, body, and fins, driven by elongated, streamlined forms suited to aquatic life.

The pectoral and pelvic fins are crucial for maintaining balance in the water, with the pectoral fins, unlike other fins, adopting a horizontal orientation. Their motion generates a characteristic up-and-down splashing pattern, facilitating the fish's vertical control. The dorsal and caudal fins, akin to the keel of a sailboat, provide the necessary stability to keep the fish upright in the water.

These fins play a vital role in both providing forward propulsion and altering the fish's course. When the tail fin swings left, the fish veers left; conversely, a rightward swing propels the fish to the right.

Nonetheless, due to variations in habitat and behavior, the function and configuration of these fins have undergone corresponding changes throughout the lengthy course of evolution.

Source: The Paper