REPRODUCTIVE-BIOLOGY OF THE NEOTROPICAL ARMORED CATFISH HOPLOSTERNUM-LITTORALE (SILURIFORMES-CALLICHTHYIDAE) - A SYNTHESIS STRESSING THE ROLE OF THE FLOATING BUBBLE NEST

The most distinctive feature of the reproductive biology of Hoplostern um littorale is the building and guarding of an elaborate floating bub ble nest. Reproduction starts at the age of one year. Males grow to a larger size than females and, during the breeding season, they develop fat deposits in the pectoral fin and an elongated recurved pectoral s pine that often assumes a reddish colour. Analysis of gonadal developm ent and surveys of nesting in swamps show that initiation of reproduct ion is associated with the onset of the rainy season. Males start buil ding the nest at night, and spawning takes place around noon the day a fter. Most nests are built in newly flooded swamps, especially in open water in the peripheral area of the swamp. A minimum distance of 10 m between nests is usually observed. The floating bubble nest consists of a dome of plant material supported by oxygen-rich foam produced by the male. Diameter and height of the nest average 30 acid 6 cm, respec tively. The nest is the centre of a territory that is vigorously defen ded by the male, using its enlarged pectoral spine, against conspecifi c and heterospecific intruders. The number of nests that did not acqui re spawns (48 %) suggests a strong competition among males for females . There are indications that the female drinks the sperm and that fert ilization takes place after the sperm has passed through her digestive tract. Hoplosternum littorale is a multiple spawner. Investment in re production is high in females since they can spawn up to 14 times duri ng a 7-month breeding season and each spawn consists of 6 000 to 9 000 eggs. On average, two to four females spawn simultaneously, resulting in an average number of 20 000 eggs per nest. Incubation of the eggs takes two to three days depending on the temperature in the nest. Desp ite intense predation pressure on eggs and larvae, the guarding behavi our of the male extends for only one or two days after hatching. In th e hypoxic water of tropical swamps, the main function of the floating bubble nest appears to be to provide oxygen to the developing eggs by lifting the eggs above the water surface while protecting them from de siccation. Other functions of the nest may be protection of the brood against predators, temperature regulation, identification of the centr e of the territory and synchronization of reproductive activities. We suggest that H. littorale may be a useful model for the study of the r eproduction of tropical freshwater fish in hypoxic environments. (C) I fremer/Elsevier, Paris.