The Blue marlin's body is blue-black above, with a silvery white underside. It has two dorsal fins and two anal fins. Its first anal fin, along with its pectoral and caudal fins, can be folded down into grooves for better streamlining. It has a long, stout bill. Males rarely exceed 160 kg (353 lb) in weight, and females will commonly weigh over 540 kg. (1190 lb).
The Atlantic Blue Marlin is known to migrate towards the equator in winter and away again in the summer. Ichtyologists believe that some of these migrations may encompass the entire Atlantic Ocean. With a top recorded speed of over 50 MPH (80 Kmph), that is plausible.
Atlantic blue marlins primarily feed on a wide range of fish (such as Bigeye tuna and Chub mackerel) and cephalopods (such as Southern shortfin squid). They tend to attack schools of fish or invertebrates near the surface of the water, although they will often feed near the bottom. They will swim through the schools at high speeds, slashing at fish with their bills, and then come back to eat the dead or stunned ones.
The sexual maturity of Atlantic blue marlins is reached at two to four years. The breeding season in the Atlantic Ocean is in part of summer and fall. In the Pacific the breeding season is in December and January. Females may spawn as many as four times in one season. Atlantic blue marlins have planktonic young. Females may release well over seven million eggs at once, each being around 1 mm in diameter. The young may grow as much as 16 mm in a day. Males may live for 18 years, and females as many as 27.
The status of the Indo-Pacific blue marlin (Makaira mazara) as a separate species is subject to some debate, although recent genetic data supports the existence of a single species, albeit one with "striking phylogeographic partitioning" of mitochondrial and microsatellite loci.