I’m no expert on vertebrates but I do remember somewhere in my undergraduate learning that a distinguishing feature of mammals was the mammary glands. Those o’ so important glands that provide nourishment to offspring.
But what does an animal do if it doesn’t have access to milk producing glands. In snails, which I know better, young larvae often just thrown out into the ocean and told “Find your own damn food!”. In the really lucky species, the larvae are hatched with a yolk sac. In the really unlucky species, like those in the genus Neptunea, an entire stalk of eggs are laid by the female. The first few juveniles to hatch go through and consume their unborn siblings.
Discus fish, obviously lacking mammary glands, produce a breast milk surrogate. No, not that kind of surrogate. ”Hey Ms. Manatee*, would you mind coming over here and let my young suckle on your teat?” Discus fish parents secrete a highly nutritious mucus all over their bodies that the young feed on until old enough to feed themselves. If mom is dried up, there is always dad! “Then they left [hatching area] en masse and began feeding on their parents’ mucus, feeding for up to 10 min by biting at the parent’s side until the parent expertly ‘flicked’ the shoal over to its partner to continue feeding.” The mucus, although always secreted, becomes more protein rich and full of antibodies during the juvenile feeding phase.
Buckley, J., Maunder, R. J., Foey, A., Pearce, J., Val, A. L. and Sloman, K. A. (2010). Biparental mucus feeding: a unique example of parental care in an Amazonian cichlid. J. Exp. Biol. 213, 3787-3795.
*yes Discus fish are freshwater and this is ocean blog, but dammit I like the study and its my blog and I’ll do want I want to