Possesses a solid, suboval to triangular bivalve shell that is markedly inequilateral, featuring an anterior end that is significantly longer and rounded, contrasting with a short, abruptly truncated posterior margin. The maximum length of the shell ranges between 70 and 80 millimeters, though average individuals typically measure around 40 to 50 millimeters. The outer surface is smooth, exhibiting only fine concentric growth lines covered by a thin, bright yellowish, cream, or pale brown periostracum, which occasionally shows subtle purple tints near the umbo. Internally, the shell is pearly white with a hinge characterized by a deep, triangular chondrophore housing the internal ligament or resilium, flanked by elongated, striated lateral teeth that ensure a tight, secure closure of the valves in Amarilladesma mactroides.

Geographical distribution: Occurs exclusively along the southwestern coast of the Atlantic Ocean. Its distribution range extends from the state of Rio de Janeiro in Brazil, spanning the entire maritime coastline of Uruguay, down to the San Matías Gulf in the province of Río Negro, within Argentine Patagonia. This continuous linear coastal distribution spans thousands of miles of warm-temperate and cold-temperate sandy beaches subject to highly dynamic tidal regimes.

Habitat: Inhabits strictly the intertidal and shallow infralittoral zones of high-energy sandy beaches. It exhibits a strong preference for well-sorted fine to medium sand substrates, where the constant action of waves guarantees optimal water oxygenation and a continuous supply of suspended nutrients. The species is vertically distributed from the low-tide mark down to shallow depths of a few meters, successfully adapting to the intense hydrodynamic forces of the surf zone.

Feeding: Functions primarily as a suspension-feeding filter organism. It utilizes specialized siphons, which are extended above the sediment surface, to draw in water laden with suspended particles. Its diet consists mainly of unicellular microalgae, with a dominance of both benthic and planktonic diatoms, alongside particulate organic matter and suspended bacteria, all of which are captured by the ciliated gills and directed to the digestive tract.

Behavior: Demonstrates an extraordinary capacity for rapid burrowing, driven by a highly developed muscular foot that allows the organism to anchor itself securely into the sandy substrate within seconds to avoid being swept away by strong waves or tidal currents. It exhibits an active vertical migration pattern synchronized with the tides, moving up and down within the sand column to maintain an optimal position relative to the water line, thereby minimizing the risks of desiccation and predation by shorebirds, crabs, and demersal fish.

Reproduction: Characterized as a dioecious species with external fertilization, showing no external sexual dimorphism. The reproductive cycle typically features two annual spawning peaks, a primary event during the spring and a secondary, less intense event in late summer or early autumn. The synchronized release of male and female gametes into the water column produces a free-swimming trochophore larva, which subsequently develops into a planktonic veliger larva that, after a brief pelagic phase, undergoes metamorphosis and settles permanently onto the sandy bottom.

Conservation status: Lacks a global assessment by the IUCN, but is regionally categorized as highly vulnerable or threatened due to recurring mass mortality events triggered by sea surface temperature anomalies, harmful algal blooms, and pathogens. Historically, populations in Argentina and Uruguay suffered severe collapses driven by uncontrolled commercial harvesting and tourist pressure, prompting the implementation of strict harvesting bans and local management plans to support its slow biological recovery.