At aquariums, as evening approaches and closing time approaches, you'll often see couples swimming close together in front of the tanks. But this time, we're going to talk about what happens inside the tanks. Take a look at the tank representing the seas of Tokyo and the Izu Islands, and try to find a couple of "Spotnape Cardinalfish" (hereinafter referred to as "Black-spotted Cardinalfish"). Did you spot the two fish swimming close together?



Furthermore, take a closer look at the jaw of one of them. Is it bulging? If it is, the male is guarding the eggs. If you stop and observe for a moment, you will occasionally catch a glimpse of a cluster of eggs peeking out from its mouth.

Black-spotted butterflies usually live in large groups, but during the breeding season, they separate from the group and form couples. After courtship rituals, the female lays eggs, which the male then protects in his mouth.



Now, you might think that the female stays close to the male guarding the eggs, and that they happily raise their young together, but it seems that's not always the case. According to field research on the large-striped cardinalfish (hereinafter referred to as the large-striped cardinalfish), a member of the same cardinalfish family as the black-spotted cardinalfish, it is very common for the female to abandon the male guarding the eggs and run off with a new male. Hmm, is that really okay...? However, behind this behavior lies a strategy to leave behind as many offspring as possible during their lifetime.



In the case of the large-striped damselfish, male brooding takes 5 to 17 days, depending on the water temperature. On the other hand, it takes a minimum of 4 days for the female to produce new eggs. In other words, the female should be able to lay more eggs during the breeding season if she lays eggs with another male after 4 days rather than waiting for the male to finish brooding. It is more advantageous for her to abandon her offspring (her eggs) and her husband and mate with a new partner.

However, the males aren't just passive victims. In fact, they've been observed eating the eggs after the females have finished laying them and left! While they seem like a loving couple, both the male and female are actually behaving quite selfishly.



In the case of black-spotted maggots in an aquarium, the way they interact will vary depending on various factors such as the ratio of males to females in the tank. But sometimes, even when all the other maggots are couples, you can see a single male guarding the eggs. "Was he abandoned? Surely he's not planning to eat the eggs?" Am I the only one who gets excited thinking about things like that?

[Tokyo Sea Life Park Research Staff, Michi Amano]



(The photo shows a pair of Spotnape Cardinalfish. The individual in the foreground with the swollen lower jaw is the male incubating the eggs.)