and aggressively chase the intruder away if the intruder is of the same sex. However, when the secondary male, or the intruder male approaches, he, instead of the female, exhibits trembling behavior, and also helps to defend the female and primary male's isolated territory. If the primary male does not respond to female's trembling, the female leaves periodically to visit the secondary male and displays trembling behavior which often results in mating. Selection of mates Although the females initiate courtship, female and male cardinalfish are mutually selective. Females have been observed courting larger males more intensively. Even when they are already in the process of courting one male, they often moved on to an intruder male if it is larger than the original. Females have also been observed producing larger eggs for larger, hence more 'attractive,’ males. Females are also able to increase egg size even after the onset of egg maturation if they encounter a new, larger male to brood its eggs. Male Banggai cardinalfish also exhibit preference by size, as larger females tend to lay larger eggs, and egg size positively affects traits such as offspring size, survival, growth, time until maturation, and swimming performance. However, it also takes into account information conveyed by females' courting movement. The 'side by side trembling' described in the previous section can be subcategorized into two different movements: 'rush' and 'twitch'. 'Rush' refers to the first part of the movement when the females approaches the male, folding her pelvic, dorsal, and anal fins together and quickly swimming past the male for a distance of 10–40 cm. Then she performs the 'twitch' by twitching or trembling her body close to the male. The intensity of the 'rush' behavior is important for the male in predicting the female's potential reproductive effort, as the intensity of the behavior correlates positively with clutch weight. However, the number of twitches performed per day strongly associates with how close to spawning the female is. By forming a pair with a female with more mature eggs, or closer to spawning, a male can increase his reproductive rate. Furthermore, he can match his readiness to spawn to females' by observing her 'twitch' behavior. Reproduction Spawning When the male accepts female courting, the female expels eggs from her urogenital papilla. Female Banggai cardinalfish produce a relatively small clutch, consisting of no more than 90 eggs that are 2–3 mm in diameter.