Despite similar appearances and shared status as impressive ocean predators, swordfish and marlins are distinct species. Both are renowned for their powerful builds and elongated, pointed snouts, yet belong to separate biological classifications. This common misconception stems from their shared general shape and formidable presence.
How to Tell Them Apart
Distinguishing swordfish from marlins involves observing several key physical characteristics. The most prominent difference lies in their bill or rostrum. A swordfish has a long, flat, broad bill, resembling a sword in cross-section. Marlins, in contrast, have a more rounded, spear-like bill that is generally more pointed.
Dorsal fins also differ. Swordfish have a tall, rigid, crescent-shaped dorsal fin, typically separated from a smaller second dorsal fin. Marlins have a longer, softer dorsal fin extending further along their back, sometimes resembling a sail, which can often be retracted or folded. Body shape also differs: swordfish are robust and stocky, while marlins are elongated and slender. Swordfish have fixed pectoral fins, unlike marlins whose fins are flexible and can be retracted.
Their Biological Connection
While swordfish and marlins are not the same species, they do share a broader biological classification as “billfish.” This term refers to large predatory marine fish characterized by their prominent, elongated upper jaws. However, this shared trait is where their closest taxonomic relationship ends.
Swordfish are the sole member of the family Xiphiidae, represented by the single species Xiphias gladius. Marlins, along with sailfish and spearfishes, belong to a different family known as Istiophoridae. This means that despite their superficial similarities, they diverged millions of years ago into separate evolutionary paths, leading to their unique physical and behavioral adaptations.
Where They Live and How They Behave
Swordfish and marlins exhibit differences in their preferred habitats and hunting strategies. Swordfish are highly migratory and are found globally in tropical, temperate, and occasionally cold waters of the Atlantic, Pacific, and Indian Oceans. They typically inhabit deeper, cooler waters during the day, often ranging from the surface down to 550 meters, but can dive exceptionally deep, even beyond 2,200 meters. They are primarily nocturnal hunters, using their flat bill to slash at schools of fish and squid, stunning their prey before consuming them.
Marlins prefer warmer, shallower waters, often found near continental shelves, seamounts, and areas with strong ocean currents that attract baitfish. They are also highly migratory, traversing vast distances across oceans. Unlike swordfish, marlins are more active during the day, using their spear-like bill to stun or spear prey, including smaller fish and cephalopods. While swordfish are typically solitary, marlins can sometimes be found in loose aggregations.