The practice of “blackwater diving“, literally “diving into black water”, involves, as the name suggests, diving in the middle of the night, when the ocean is completely dark and the only way to see something is to shine with an underwater torch hoping to “peck” a fish or other living creature.
In this way, it may happen that we witness unusual sights, which push us to investigate the situation more systematically: this is what happened to marine biologist Jeff Milisen, who in 2015 spotted a fish swimming with a ball between its fins. Milisen showed photos of the event to his colleague Dave Johnson (1931-2024), who signed, together with colleagues from the Virginia Institute of Marine Science, a study published in Journal of Fish Biology which reveals the identity of this and other “balls”, used (it is their hypothesis) by fish as shields.
Unidentified ball. Let’s start with the identification of the ball spotted in 2015: impossible to do based only on the 2015 photo, and in fact the team made several other night dives between 2018 and 2023, managing to photograph several fish swimming with something between their fins. These are often small species, or very young specimens: all of them held another creature between their fins, and in all cases it was some type of anthozoan.
Survival strategy. I may not tell you much about the name, but the Anthozoa class includes six thousand species spread throughout the world, the most famous of which are corals. Sea anemones are also anthozoans, and these were precisely the animals that were transported around the ocean by the fish observed by the scholars, all active at depths between 8 and 15 meters. Photographed off the coasts of Florida and Tahiti, these fish confirmed to the team that the 2015 photo was not a coincidence, but part of a strategy.
Underwater shield. Yes, but what strategy? The simplest answer, the one also put forward by the authors of the study, is that these fish use anemones as shields: this is demonstrated among other things by the fact that, once spotted by a diverthe animals positioned themselves so as to put the anemone between them and the human. Adult anemones are poisonous, and many larvae are also toxic: fish know this, and use them to keep potential predators at bay.
Free shipping. It is not the first time that we have observed a link of this type between fish and other poisonous marine creatures: jellyfish in particular are one of the favorite “protections”; However, it is the first time that we have discovered that anemones can also act as a shield.
The team also hypothesized that anemones benefit from this association, because fish transport them around and thus help them spread around the ocean. A theory that still needs to be confirmed, however, given that the fish in question swim little and slowly. However, we certainly know that, for them, walking around with an anemone between their fins has its benefits: now it’s a matter of investigating further and understanding which ones.
