Head to head
Swordfish vs Blue Marlin
Swordfish and marlin are both large, fast, open-ocean predators that extend the upper jaw into a weapon, and anglers and diners often lump them together as billfish. But they belong to different families and are built for different depths. The swordfish is a deep-diving loner; the blue marlin is a warm surface hunter and the classic big-game fish.
| Swordfish | Blue Marlin | |
|---|---|---|
| Family | Xiphiidae (swordfish alone) | Istiophoridae (marlins, sailfish, spearfish) |
| Bill | Long, flat, smooth, blade-like | Shorter, rounded, spear-like |
| Pelvic fins | Absent | Present, long and narrow |
| Scales as an adult | None | Small scales embedded in the skin |
| First dorsal fin | Low and stiff, well forward | Tall and pointed |
| Where it hunts | Deep and cold by day, shallower at night; eyes and brain heated | Warm surface waters |
| Size | To about 4.5 m and 650 kg | Females to 5 m and over 800 kg; males much smaller |
| Behaviour | Solitary | Solitary to loosely aggregated; highly migratory |
Swordfish vs Marlin: common questions
What is the quickest way to tell a swordfish from a marlin?
Look at the bill and the belly. A swordfish's bill is long, flat, and smooth, like a sword or a broad knife, and can be a third of the fish's length. A marlin's bill is shorter and round in cross section, more like a spear or a pool cue. The other giveaway is the pelvic fins on the underside behind the head: a marlin has them, a swordfish has none at all. A marlin also carries a tall, sail-like front dorsal fin, while a swordfish's dorsal fin is a low, rigid crescent set near the head.
How can a swordfish survive in deep, freezing water?
It heats the parts that matter most. Swordfish have a specialised organ derived from one of the eye muscles that acts as a heat exchanger, keeping the eyes and brain up to 10 to 15 degrees Celsius warmer than the surrounding water. Cold slows down an animal's vision and reaction time, so by keeping its eyes and brain warm the swordfish can still see and respond quickly while hunting squid hundreds of metres down, where the water may be only a few degrees above freezing. The rest of the body cools to near the water temperature, and the fish returns to warmer surface layers to recover.
Do swordfish and marlin actually spear their prey?
They use the bill, but usually to slash rather than to skewer. Both fish charge into a school of squid or baitfish and swipe the bill rapidly from side to side, stunning, cutting, or killing several prey at once, then turn back to eat the disabled ones. Prey is occasionally found impaled, and there are well-documented cases of both species driving the bill into boat hulls, whales, and even submersibles, but that is not the normal hunting method. The sideways slash is faster and lets the fish disable many targets in a fast-moving school.