Swordfish and marlin are powerful, fast-swimming predators inhabiting the world’s oceans. Both are recognized by their distinctive elongated bills, which can lead to confusion regarding their identity. Despite their shared characteristic of a bill, these two types of fish belong to different biological families and possess unique physical attributes, habitats, and behaviors. Understanding these distinctions clarifies how they interact with their marine environments.
Distinguishing Physical Features
A primary difference between swordfish and marlin lies in their bill shape. Swordfish possess a long, flat, broad bill, resembling a sword, from which it derives its name. This bill is typically smooth and can account for about one-third of its total body length. In contrast, marlin have a rounded, spear-like bill that tapers to a point, generally shorter relative to their body size.
Their dorsal fins also offer a clear way to differentiate them. Swordfish have a tall, rigid, and crescent-shaped first dorsal fin that stands prominently, similar to a shark’s fin. Their second dorsal fin is much smaller. Marlins, conversely, feature a longer, lower dorsal fin that extends along a significant portion of their back, often appearing more streamlined.
Differences extend to their pectoral fins and body coverings. Swordfish have rigid pectoral fins that cannot be folded flat against their body and often extend noticeably below the body. Marlin, with the exception of the black marlin, possess flexible pectoral fins that can be folded back. Adult swordfish are characterized by a round, stout body that lacks scales and teeth. Marlins, however, have a more elongated, streamlined body and retain rough, bony scales.
Their coloration also varies. Swordfish generally display a dark brown to black coloration on their upper body, fading to a lighter, silvery underside. Marlin species exhibit more vibrant hues, often with a distinct cobalt blue or dark blue back and a silvery-white belly, frequently adorned with pale blue vertical stripes.
Habitat, Diet, and Size Differences
Swordfish and marlin occupy distinct niches within the ocean. Swordfish are found globally in tropical, temperate, and even cold waters, showcasing a wide temperature tolerance. They are oceanic, mesopelagic species, often found at depths of 200 to 600 meters during the day and migrating to shallower, near-surface waters at night to feed. Marlins typically prefer warmer, tropical and subtropical surface waters, frequently staying within the upper 100 meters of the water column. They are also highly migratory, following seasonal temperature changes across vast ocean basins.
Their specialized bills dictate their hunting strategies and diet. Swordfish use their broad, flat bill to slash and stun prey, consuming a variety of pelagic fish, as well as squid and crustaceans. They are opportunistic feeders, capable of targeting prey from the surface to deep waters. Marlin use their spear-like bill to impale or stun individual prey, which primarily consists of smaller fish and squid.
Both species can reach impressive sizes, though marlin generally attain greater maximum weights. Swordfish commonly reach lengths of 3 meters (10 feet) and weights of 650 kilograms (1,430 pounds). Females are typically larger than males. Blue marlin, one of the largest marlin species, can exceed 5 meters (16 feet) in length and weigh over 820 kilograms (1,800 pounds). Black marlin can also reach significant sizes, up to 700 kilograms (1,480 pounds).
Classification and Unique Behaviors
Beyond physical and ecological differences, swordfish and marlin belong to distinct taxonomic families. The swordfish (Xiphias gladius) is the sole living member of the family Xiphiidae. In contrast, marlin are part of the family Istiophoridae, which also includes sailfish and spearfish. This family encompasses several species, such as the blue, black, white, and striped marlin.
Behavioral nuances further distinguish these powerful ocean inhabitants. Swordfish possess a specialized organ near their eyes that heats their eyes and brain, enhancing their vision for hunting in the deep, cold, and dark waters they frequent during the day. They are primarily nocturnal hunters, rising to shallower depths at night. Marlin are known for their spectacular jumping displays when hooked and their ability to rapidly change depths, aided by an efficient swim bladder.