Paternal egg guarding and mouthbrooding in the bubble nest in the swamp-eel, Monopterus albus

Abstract

Paternal egg guarding and mouthbrooding of larvae and juveniles were observed in the swamp-eel, Monopterus albus. In aquaria, the male guarded and cared for eggs in the bubble nest floating inside a plastic tube (5 cm in dia-meter, 50 cm in length). It was suggested that spawning and fertilization occurred outside the nest tube, and that the male carried the fertilized eggs (ca. 4 mm in diameter) in his mouth and inserted them into the bubble mass. Until the hatching of larvae (7-8 days after spawning), the male frequently added fresh bubbles into the bubble mass. As soon as the larvae (18-21 mm in TL) hatched and emerged from the bubble nest, the male sucked them into his mouth. Fifty juveniles (32-37 mm in TL) and two yolk-sac larvae (22 mm in TL) were released from the mouth of a male collected from a natural habitat. Those juveniles were retrieved by the male, some of them voluntarily returning to the male's mouth. The mouthbrooding male frequently performed pumping behavior (i.e., inflating and deflating the buccopharyngeal cavity), thereby acquiring to take fresh air. Eggs removed from the bubble nest successfully hatched only when directly exposed to aeration. In addition, only about 40% of the hatched larvae survived more than 10 days when they were kept in well-aerated water without the male parent. These suggest that both the bubble nest and mouthbrooding are indispensable for successful development and survival of eggs and larvae in this species, which inhabits swamps and paddy fields. © The Ichthyological Society of Japan 1997.