While you can observe the behavior of various fish in an aquarium, what's particularly interesting is the behavior of males and females during the breeding season. In this scenario, both males and females behave selfishly in order to leave behind as many offspring as possible, exhibiting a variety of maneuvers and strategies.

This time, let's take a look at the "chaotic" male-female relationships of the "Black-spotted Spotnape Cardinalfish" (hereinafter referred to as "Black-spotted Cardinalfish"). In the large tank of "Nagisa," which recreates the rocky shore at the mouth of Tokyo Bay, there is a group of about 100 small, flesh-colored fish, smaller than the palm of your hand, in the center. These are Black-spotted Cardinalfish. If you look carefully, you can see several pairs swimming close together a little distance away from the group. These are Black-spotted Cardinalfish couples (photo above).

Male and female black-spotted butterflies are almost identical in size and color, but there are ways to distinguish between them when they are in a pair. First, look at their jaws! If the area under their jaws is slightly bulging, it's a male. In black-spotted butterflies, the male protects the egg mass in its mouth until it hatches (see photo below).

However, it's impossible to tell the difference if the male isn't guarding the eggs. But you can easily tell them apart by their behavior. Do you see one female repeatedly circling the other, or bending its body slightly to show its belly? This more assertive one is the female. In black-spotted butterflies, the female attracts the male. It's as if she's saying, "My belly is full of eggs! Please protect them!" The whiteness of the female's belly is also more pronounced and noticeable.

Once you can distinguish between males and females, some interesting things become apparent. When a couple swims close together, other females sometimes approach them. The female in the couple immediately chases the other female away. When she returns to the male, she approaches him while displaying her belly, as if to say, "Don't cheat on me!" However, there are also cases where other females attack repeatedly, and eventually the male ends up with someone else. Couples that are close to the school seem to be interfered with by other individuals more often.

The dynamics within a group are even more complex. Within a group, there are many males with swollen jaws and females with large bellies. When one female starts chasing a male, several other females join in the chase. Sometimes they swim close together like a couple, and other times a female suddenly separates to chase another male. In a short amount of time, you can witness a truly diverse range of male-female relationships.

Observations in the natural ocean have shown that black-spotted dolphins leave their groups during the breeding season, form pairs and establish territories as early as two months before spawning. They spawn several times in a season, but whether they change partners during that time seems to depend on the population density and whether there are suitable places to establish a territory. In areas with high population density, there have been cases of frequent partner changes. This is because both males and females, if they can easily find a partner, will quickly dissolve their pair and team up with a more suitable partner, such as a larger female that will lay more eggs, or a male that seems likely to readily take the eggs.

Perhaps because of the high density of black-spotted clams in the tank, their relationships change quite rapidly, making it fascinating to watch. Why don't you all start by looking for a couple?
[Michi Amano, Research Section Tokyo Zoological Park Society]