Male parental assistance in embryo hatching of barred-chin blenny Rhabdoblennius nitidus

Most teleostean embryos develop and hatch without parental assistance, though some receive parental care. We focused on a paternal brood-care species, the barred-chin blenny (Rhabdoblennius nitidus [Gunther, 1861]). As hatching approached, fanning behavior by the male parent drastically increased and then embryos hatch. In the absence of the male parent, most embryos failed to hatch. However, the hatching rate was greatly assisted by introducing an artificial water current, suggesting that paternal assistance other than for aeration is required for successful embryo hatching. Next, we analyzed genes for the hatching enzyme and egg-envelope protein, which were successfully cloned from barred-chin blenny, and found the expression patterns differed from those of other euteleosts. Generally, high choriolytic enzyme swells the intact egg envelope, and then low choriolytic enzyme solubilizes the swollen envelope. The expression levels of both the enzymes, but especially the latter, were much lower in barred-chin blenny that is known in most other oviparous species. In addition, the main component of the egg envelope was changed into ChgHm and choriogenin L (ChgL) in barred-chin blenny, whereas ChgH and ChgL for other euteleosts. These in barred-chin blenny would result in ineffective egg-envelope digestion because the posthatching egg envelopes were observed to be swollen but not solubilized. Male parental assistance by fanning until hatching may compensate for this insufficiency. Our study illustrates an example of the evolution of parent-embryo interaction built on a novel relationship: Degradation of the hatching enzyme/egg-envelope digestion system, accompanied by male parental hatching assistance.