Claims that swordfish court their mates date back to the 19th century, but science had never confirmed the phenomenon. Now, Spanish scientists have documented this reproductive behavior, including the first photographs showing a male circling a female as she prepares to lay millions of eggs for fertilization.

The photos, taken from a boat in incredibly clear water and published in a recent issue of Revista de Biología Marina y Oceanografía (Journal of Marine Biology & Oceanography), are poignant: From 10 meters underwater fishermen hoist a female, whose body cavity is visibly dilated to release eggs, to the surface. As they do, a male, unrelenting in his courtship, accompanies her forced ascent and swims at her side.

The photo confirms stories told by Galician sailors who named an area of intense reproductive activity in the ocean “o mar dos namorados,” or “the sea of the lovers,” in the 1980s. Occasionally, when sailors captured a female on the verge of laying her eggs, one or two males were observed lingering near their “intended,” remaining close to the boat until she had been lifted onboard. The males might even remain at the site for a few minutes afterward, as if unwilling to lose their potential partner.

The new photos are especially valuable because it is incredibly difficult to document this phenomenon in the areas of the ocean where this type of fish spends the majority of its life, says researcher Jaime Mejuto of the Spanish Oceanographic Institute in La Coruña. “The images are amazing,” agrees George Tserpes of the Hellenic Center for Marine Research and chair of the International Commission for the Conservation of Atlantic Tunas. “This is the first time that the courtship is documented during the spawning period in this species, which contributes greatly to the efforts to manage this resource.”

Mejuto says the intimate details of swordfish courtship are still unknown and could include other rituals. Previous data in related species suggest “males could stimulate the laying of eggs and proceed to release sperm once the stimuli are received,” he says. Like most fishes, swordfish fertilize eggs outside their bodies in the water rather than internally.

In new work with colleague Blanca García-Cortés, Mejuto recorded and analyzed 40,000 specimens caught by Spanish boats between 1995 and 2003 to gain more insight into swordfish sexual habits. For example, in areas with a lot of reproductive activity it seems there are between two and four males to every female of any size. “We do not know if there is any preselecting of mates by the females but we should not reject the idea of any type of selection, or competition between males,” Mejuto says.

For other scientists, the latter is the most likely scenario. “In the case of all fish, the female always chooses,” says Matías Pandolfi, a researcher at University of Buenos Aires. He studies the chemical signals of courtship in native tropical and subtropical freshwater fish. “To lay eggs requires a huge amount of energy. If the male does not convince the female he has the best genes, she won’t release her eggs for fertilization.”

In ancient times it was believed that swordfish could pierce the hulls of ships and the skin of whales, but in reality swordfish have a different kind of strength. Their geographic distribution is expansive—they can swim 10,000 kilometers in a year and travel from warm water to cold in a matter of months. A swordfish can spend the night near the ocean surface then dive 900 meters or more during the day. One fish might weigh more than 500 kilograms, and every year fishermen catch more than 100,000 tons of the species. Mejuto’s and García-Cortés’s work, including the first photos of courtship, represents the largest effort to characterize the swordfish’s reproductive process. “To investigate this species is truly fascinating,” Mejuto says, “but also a huge challenge.”

Caption: The female with mature sexual organs displayed and body cavity visibly dilated. In the background, her mate circles.
Credit: Mejuto & García-Cortés 2014