Swordfish and marlin, both formidable predators of the open ocean characterized by their elongated, pointed snouts, are frequently mistaken for one another. Their shared trait of possessing a “bill” and inhabiting similar marine environments contributes to this common confusion. Despite these superficial resemblances, swordfish and marlin are distinct species with unique biological classifications and physical attributes.
The Core Difference
Swordfish and marlin are not the same; they belong to different taxonomic families. Swordfish are the sole member of the family Xiphiidae, making them unique within the billfish group. In contrast, marlin belong to the family Istiophoridae, a diverse group that includes multiple species like blue marlin, black marlin, striped marlin, and white marlin.
The most immediate visual distinction lies in their bills and dorsal fins. Swordfish possess a broad, flattened bill, resembling a sword, while marlin have a rounder, more spear-like bill. Additionally, their primary dorsal fins exhibit notable differences, with swordfish having a rigid, crescent-shaped fin, and marlin featuring a more elongated, sail-like structure.
Distinguishing Physical Traits
The bill serves as a primary identifier, with swordfish exhibiting a distinctively flattened, sword-like rostrum. Conversely, marlin possess a rounder, more conical, spear-like bill.
Adult swordfish have a tall, rigid, and crescent-shaped first dorsal fin that is separate from the tail, and they uniquely lack pelvic fins. Marlin, however, feature a long, sail-like dorsal fin that extends almost the entire length of their back. They also possess distinct pelvic fins, which are slender and can often be depressed into grooves along their body to enhance streamlining.
Swordfish have an elongated, stout, and cylindrical body that tapers towards the rear. Their skin is smooth and scaleless in adulthood, typically appearing blackish-brown or purplish to bluish on top, fading to a lighter underside. Swordfish can reach maximum reported lengths of up to 4.55 meters (14 feet 11 inches) and weights of 650 kilograms (1,430 pounds). Marlin generally have elongated, slender bodies, often with a deep blue or blue-black coloration on their back and a silvery-white underside, and some species display pale vertical stripes. Blue marlin, for instance, can grow to 5 meters (16 feet) in length and weigh over 800 kilograms (1,800 pounds), with females typically being much larger than males.
Beyond Appearance: Biological and Behavioral Variations
While both are highly migratory oceanic predators, swordfish and marlin have distinct preferred habitats and hunting strategies.
Swordfish are found globally in tropical and temperate waters. They are primarily midwater fish, typically found at depths of 200-600 meters (650-1970 feet), but can tolerate a wide temperature range, from 5°C to 27°C (41-80°F). This wide tolerance is partly due to a specialized organ near their eyes that warms their brain and eyes, improving vision and cognitive function in cold, deep water. Swordfish are opportunistic feeders, preying on various fish and cephalopods. They use their flattened bill to slash and stun their prey before consuming it.
Marlin also inhabit tropical and subtropical waters across the Atlantic, Pacific, and Indian Oceans. They typically prefer warmer surface waters, often staying above the thermocline, and are considered the most tropical of billfishes. Marlin are known for their speed and acrobatic leaps. They primarily feed on schooling fish and squid near the surface, using their spear-like bill to stun or injure prey by knifing through schools. Some marlin species, like the striped marlin, can change color rapidly to coordinate hunting efforts with other fish, signaling their intentions to encircle and capture prey.