The living coelacanth is a livebearer. Yolk seems to be the main source of nutrients and of oxygen to the embryo (fetus). Long before birth, young may also possibly feed orally on histotrophe secretion and egg debris. This type of reproduction evolved, as in most other fishes, from oviparity. The Carboniferous coelacanth Rhabdoderma exiguum had eggs of much lesser yolk volume and may represent an earlier form of oviparity with hiding, guarding or brooding type of parental care. The Jurassic coelacanth Holophagus (Undina) and the Cretaceous Axelrodichthys appear to have already evolved the internal-bearing style. Much of this evolutionary sequence is similar to that in cichlids. Ancestral cichlids are substrate tenders and nesters, with small eggs, little yolk and a feeding larva with indirect development. Mouthbrooding cichlids evolved a few, large eggs with denser yolk, direct development and, ultimately, orally feeding embryos while yolk is still in ample supply. Mixed feeding from yolk and orally ingested food in cichlids and in coelacanths is shown to be an enhanced mode of food delivery to the embryos over that from each source separately, in order to produce directly a better developed or larger young at the time of release, i.e. independence. Increase in egg size is regarded as an environmentally induced, altered pattern of yolk synthesis and an initial component of the epigenetic mechanism leading towards greater specialization. Carotenoids are incorporated within the yolk to assist the oxidative metabolism of the developing embryo.
