Armed with spears, they rush through the water: How do striped marlins avoid hurting each other during their rapid group hunts on schools of fish? A study now documents: Shortly before the start of a lightning attack, each group member increases the contrast of their body markings. The scientists explain that this apparently serves as an optical signal to coordinate the group behavior of the marlin fish and thus make it safer.
They are among the fastest swimmers in the oceans and pointed snouts and bands are their trademarks: The striped marlins (Kajikia audax) belong to the group of marlins (Istiophoridae) and are widespread in the Indo-Pacific. The sea predators, up to four meters long, hunt sardines etc. They often attack the swarms of prey fish in groups: one of the animals takes turns racing into the swarm to grab a victim and tear the formation apart. One may wonder how the robbers coordinate the rapid group behavior. This seems to be important because a collision between the sharply armed giants could result in life-threatening injuries.
Contrasts in sight when hunting marlins
The research team led by Alicia Burns from the Humboldt University in Berlin has now followed up on a lead: Are the predatory fish perhaps giving each other optical signals? It is known that striped marlins can change the contrast of their body markings very quickly. For this purpose, so-called chromatophores are used in the skin. These are pigment-filled cell structures whose visibility can be changed through contractions: the animals can make the whitish bands on the side of their body appear more or less bright. Until now, however, it was unclear for what purposes the fish use the optical signal.
To clarify to what extent changes in appearance play a role in group hunting, the researchers analyzed drone footage of striped marlins in the Indo-Pacific. The predatory fish were close to the surface hunting together for schools of sardines. The marine biologists recorded the behavior of each animal and the changes in the contrast of their body markings.
Lightning attacks are preceded by a warning signal
As the team reports, it turned out that when the fish are not attacking, their stripes look comparatively dull. “The attacking marlin then appears much brighter than its group mates,” says Burns. The researchers found in detail: About ten seconds before the fish launches one of the lightning attacks on the prey, it “switches” to a high contrast stripe pattern. The predatory fish then strikes in such a visually striking way, capturing a school of fish or driving the formations apart. Immediately after completing this maneuver, the animal then adjusts the contrast of its markings back to the dull initial level, the researchers report. “This is the first time we have documented a rapid contrast change in a group-hunting predator,” says Burns.
As the team explains, it is therefore obvious that the predatory fish use the optical signal to coordinate group behavior when hunting. Accordingly, by increasing the contrast, a member of the community announces that he is about to launch an attack. This visual stimulus presumably prevents the other group members from also throwing themselves spear-first into the school of fish – which could lead to life-threatening collisions between the predators. “Although it is known that marlins can change color, this is the first time this behavior has been linked to hunting or other social behavior,” says Burns. The discovery suggests that these predatory fish have a surprisingly complex communication system.
However, according to the researchers, it would also be possible for the optical signal to serve an additional purpose. The increase in contrast, combined with the very fast movement, could confuse the nimble marsupials, making it easier for the marlins to catch them. The researchers now want to investigate the extent to which this is true and further questions through additional studies. For example, it has so far remained unclear whether predatory fish also show the contrast change when they are hunting alone. The researchers also want to find out to what extent other fish species use a similar concept. “We already have footage of the hunting behavior of sailfish and mahi mahi, where we have seen even more pronounced and variable color changes than marlin,” says Burns.
Source: Cell Press, specialist article: Current Biology, doi: 10.1016/j.cub.2023.12.040