The genus Xenotilapia can be divided into two groups; one group consists of maternal mouthbrooders and the other of species which form pairs during the breeding period and which employ the biparental mouthbrooding technique. However, this subdivision may not always be as clear-cut as is stated here.
X. spiloptera belongs to the group of biparental mouthbrooders. The holotype of this species was collected at Nkumbula Island near Mpulungu in Zambia, but its distribution seems to spread along other parts of Zambia and along the Congonese and Tanzanian shores.
During the breeding period X. siloptera is restricted to the rocky habitat. This has led to the existence of several geographical races. Most races have a colorless dorsal fin with some black markings on its edge. The race at Kigoma has a dorsal fin with tiny colored spots but lacking the blotchy markings found in all other known populations. The races inhabiting the rocky shores near Kipili in Tanzania have an attractive yellow coloration in addition to the black markings on the dorsal fin.
When not breeding, X. spiloptera sometimes lives in large schools over the sand. Here it forages by sifting the substrate for something edible. It may intermingle with other species to form larger schools but most often groups consisting only of X. spiloptera are observed.
With the approach of the breeding season at present it is not known whether there is a regular annual breeding season, or whether breeding is triggered by the effect of some external stimulus on the members of the school - the school moves closer to the rocky habitat and splits up into pairs. Each pair stays in a territory which is defended mainly against conspecifics.
The pair bond is established by repeated courtship displays by the male as well as by the female. In the artificial environment of the aquarium it seems that the bond between the pair is strengthened by the continuous protection of the territory. If only one pair is kept in the tank there is a risk that the male and female will quarrel and that one of them, not always the female, might end up cowering in one of the tank's corners. Even when the pair is not brooding, male and female stay together in a relatively small area.
The four pairs I keep in an 800 litre aquarium are each satisfied with an area about 30 cm in diameter. When one pair decides to spawn, it defends an area about double that size. It is difficult to predict an imminent spawning but mutual courtship increases noticeably, sometimes days before the actual spawning. A slight change in the color pattern occurs as well; female as well as male acquires black markings in the upper and lower part of the iris. These, together with the pigment in the eye, form a black vertical bar across the eyeball. I don't know if this is a sign of readiness to spawn, but it is seen mostly when the pair is engaged in spawning or shortly before the act.
Spawning takes place inside the pair's territory but there is no specific site or nest where the eggs are deposited. In fact, the deposition site may change during spawning. In the maternal mouthbrooder group of Xenotilapia , male and female circle around each other before the eggs are laid, but X. spiloptera starts spawning when the female suddenly deposits some eggs on the substrate. I have never noticed a signal from the male for the female to start. He usually waits behind her, about 3 cm above the substrate, until she clears the site and leaves the eggs to be fertilized by him. While the male positions his vent over the eggs to fertilize them, the female turns around and waits until the male moves away. Then she picks up the eggs. The female will not lay any eggs if the male is not behind her. While he is chasing intruders from the territory she may remain at the site but she may also join the male in the territorial defence. After a short interruption the female again lowers her body to the substrate and waits for the male to take position behind her before she lays a new batch of eggs.
Spawns may be as large as 40 eggs but much seems to depend on the buccal capacity of the female. If the female's mouth is getting full she has to shuffle the eggs around before she can pick up new ones. Usually the male has his head near the eggs when the female collects them and sometimes picks up some eggs himself. Once I noticed that he picked up two eggs, which the female apparently didn't collect quickly enough, and swam away while chewing on them. Then the pair went through another cycle, and when the female wanted to pick up the new batch of eggs the male spat the two eggs in front of the female's mouth. They were immediately picked up by the female. During the next round, the female's mouth was probably too full and it took so long before she had arranged the eggs already inside her mouth that the male again took some eggs, but this time he ate them. The female didn't produce more eggs after this incident.
Shortly after spawning the pair reduces the territory to the usual 30 cm, but this is defended with more energy than usual. The first nine to twelve days the female broods the embryos and refrains from feeding. After this period the larvae are transferred to the male's mouth. I have never witnessed the procedure in its entirety but it is likely that, after a set ritual, the female spits all the larvae in front of the male's mouth. The male retrieves them quickly and continues the brooding for another ten days.
When the brooding is completed the male releases the fry in the territory where they are protected by both parents. For the first few days the fry may take refuge inside the male's mouth hut most of the time they sit on the substrate. They sometimes wander off into other territories where they are protected by the resident pair. The fry measure about 15 mm at the time of release and it takes them about two years before they have reached the adult size of approximately 10 cm.
The biparental mouthbrooding technique is not more advanced than the maternal mouthbrooding procedure. While mouthbrooding pairs have to devote three to four weeks taking care of one brood, males of a maternal mouthbrooder can fertilize eggs of many females in the same period. In theory, this should not make a difference as long as there are as many females as males. The drawback of biparental mouthbrooding is the fact that a territory has to be maintained by the pair. In maternal mouthbrooders only the strongest males occupy a territory, and the mouthbrooding females gather in nursery schools. Where population density is high, as is the case in Lake Tanganyika, this offers a distinct advantage. Instead of having many territories spread over a large area, a species can propagate in a relatively small region of the biotope. Only the males have to worry about a territory and since there are fewer needed a loss during the selection process probably enhances the viability of the species.
The stimulus for the female of the biparental mouthbrooder to transfer the larvae to the male's mouth may be hunger. During the 9 to 12 days incubation she does not eat at all. Females of a maternal mouthbrooder endure a longer period without food but may eat small morsels in the second half of the incubation period. Such mouthbrooding females also expend much less energy than those of a biparental mouthbrooder which also have to defend territory.
In Lake Tanganyika a maternal mouthbrooder is better off in several respects. This could explain the observation that breeding X. spiloptera are found at deeper levels of the biotope than are maternally mouthbrooding Xenotilapia. Breeding pairs are usually found at depths of between 15 and 40 meters.