Among vertebrates, ray-finned fishes (Actinopterygii) display the highest diversity
in parental care, and their diversification has been hypothesized to be related to
phylogenetic changes in fertilization modes. Using the most comprehensive, sex-specific
data from 7600 species of 62 extant orders of ray-finned fishes, we inferred ancestral
states and transitions among care types and caring episodes (i.e. the stage of offspring
development). Our work has uncovered three novel findings. First, transitions among
different care types (i.e. male-only care, female-only care, biparental care and no
care) are common, and the frequencies of these transitions show unusually diverse
patterns concerning fertilization modes (external, or internal via oviduct, mouth
or brood pouch). Second, both oviduct and mouth fertilization select for female-biased
care, whereas fertilization in a brood pouch selects for male-biased care. Importantly,
internal fertilization without parental care is extremely unstable phylogenetically.
Third, we show that egg care in both sexes is associated with nest building (which
is male-biased) and fry care (which is female-biased). Taken together, the aquatic
environment, which supports considerable flexibility in care, facilitated the diversification
of parenting behavior, creating the evolutionary bases for more comprehensive parenting
to protect offspring in semiterrestrial or terrestrial environments.
