In the Rio Uatsi (Rio Sixaola drainage, Costa Rica, 2002), we could observe several Archocentrus myrnae females with mixed broods. Withing the group of A. myrnae fry there were some heterospecific fry of about the same size. All the heterospecific juveniles we could catch turned out to be Amphilophus bussingi, a species that occurs in sympatry in reasonable densities. Brood caring A. myrnae and A. bussingi could be observed right next to each other.
How these mixed broods accomplish is so far unclear. Probably, females are not able to distinguish heterospecific fry. If lost junveniles just join the next group of fish passing by, this could lead to mixed broods. However, the formation of mixed broods could also be an adaptive strategy of females to reduce predation of the own fry. The acceptance or even active kidnapping of fry from heterospecifics or the same species results in a dilution effect that already reduces the predation risk. The acceptance of heterospecific and thus differently looking fry is even more effective, since aberrant looking fish appear much more conspicuous in an otherwise homogenous group.
The phenomenon of brood adoption in cichlids is well known (Wiesenden, 1999). Intraspecific brood adoption is also known from Archocentrus nigrofasciatus (Wisenden & Keenleyside). Interspecific brood adoption was observed in mouth brooding cichlids from Lake Malawi in Africa (Ribbink et al. 1980, Szymanski 1996, Spreinat 1996).
I'd be very interested in further observations of brood adoption. Has anybody observed a similar behavior in captivity?