Originating from the lush, productive waters of East Glicker Tropical Coast, the arthrothere evolved in an environment rife with competition. Due to intense competition with the thornant, the arthrothere's options for food were limited. As a result, it has become incredibly aggressive in order to satiate its hunger. Its adaptations are a telltale sign of its new behavior. Its radula has been fully encrusted in chitin, and is now a powerful jaw used in killing prey and attacking competitors. Most of the radula-jaw's bite force originates from a triangle-shaped projection in the exoskeleton that lies just behind the eyes, serving as an important muscle attachment point that allows the jaws to close. With the thornant primarily consuming plant matter, it was only inevitable for the arthrothere to have a greater emphasis on carnivory. While it still consumes any plant small enough to fit in its powerful jaws, its options are limited, as it can only consume species that can fit in its mouth. As a result, it has progressed towards carnivory at a faster rate than its competitors. It primarily feeds on smaller prey items between 0.5 cm and 2 cm in length. However, it can consume much larger, unarmored organisms like the carpolanta's descendants with ease. Its hunting methods vary from snatching planktonic and in rare cases, nektonic prey out of the water column with its biramous, clawed antennae to scooping up small binucleid worms with its jaws before crushing them and consuming the resulting fragments.
However, this mostly carnivorous diet is oftentimes not enough to sustain the arthrothere due to competition with its kin. As a result, it has taken to kleptoparasitism and scavenging. If it finds a thornant that has downed a relatively large prey item, the arthrothere will attempt to scare it off. While the two are the same size, the arthrothere possesses more venues of attack with its claws and jaws. In addition to this, the arthrothere's body is sturdier than that of its competitor. Its six standard limbs have all become biramous to a certain capacity. The fin-limbs' second growth is used as an extra walking leg, giving it greater stability while walking. Meanwhile, the walking limbs have developed a new, biramous segment that serve as toes. These form claws that increase traction while walking. However, these have only recently evolved, and have not fully developed, and only serve to keep the claws from impeding its movement at the time being. Together, its limbs combined with its stockier body have made it more stable than its competitors, making it more difficult to knock down. However, this came at the sacrifice of much of its swimming ability, as its fins are now too small to effectively propel it through the water. However, it has the ability to fold its walking appendage against its fin, giving it a wider surface area. This prevented it from fully losing its ability to swim. What little swimming power it has left is used to charge at the competitor, and combined with its increased weight and sharp claws and jaws, it is often able to scare away competitors. If the fight becomes more drawn-out, the arthrothere's sturdy construction will allow it to survive attacks from its opponent. Underneath its relatively thick exoskeleton are a series of rib-like projections extending from its dorsal and ventral sides. These form a protective cage around the internal organs while simultaneously serving as attachment points for its muscles. In order to further protect the organs, the dorsal and ventral sides of the segments' exoskeletons possess a thickened ridge of chitin, which protrudes into the body cavity and reinforces the dorsal and ventral sides of the exoskeleton. If it scares off the competitor, it will then consume the competitor's prey.
The Arthrothere's internal anatomy has been altered drastically from their ancestor. In order to better maintain their larger size, their tracheal system now leads to a large, gill-filled chamber located within the ventral side of the abdomen. These the spiracles are connected to the gills by a series of ciliated trachea that draw water into the gills. After passing through the gill chamber, the water is sent through another ciliated tube that leads to the base of the gonopodium. With most of the organs being situated on the ventral side of the body cavity, the arthrothere's open circulatory system has given way to a closed circulatory system that uses hemoglobin-based blood, with the most veins and arteries being centered around the ventral side of the body and the muscles. The arthrothere has developed a simple, heart-like organ located in the third segment. It sits atop the liver, which in turn sits atop the intestines and kidneys. These more advanced circulatory and respiratory systems allow the organism to deliver more oxygen to the organs and muscles.
Due to their lack of predators and a change in mate preferences, the arthrothere has lost its crystal butt. Mates are now selected on the basis of the roundness of the final segment. This has ultimately resulted in the shortening of the gonopodium and the rounding of the segment. Atavistic genes for spikes have been reactivated, resulting in the return of spikes on this final segment. However, this gene is not necessarily selected for, and instead has simply become common within the arthrothere genome by chance. The male's gonopodium is smaller than that of the females, and is inserted into the larger female gonopodium when mating. The outflow tube from the respiratory system aids in pushing the gametes into the gonopodium, where the gametes fuse. After about thirty seconds of being coupled, they decouple and release their eggs.