My Sea Monkey Diary, Day 2: I added the eggs/salt mixture to the water about 80 minutes ago. Sure enough, I was able to ID a few hatchlings within a few minutes. Every critter I saw was swimming towards the surface, where they apparently like to hover. Why do they do this? (Probably for the oxygen content, but possibly also to escape from predators that tend to lurk elsewhere, but this is just a WAG on my part.)
In any event, I have a theory as to why so many people have observed their hatchling population decline so steeply over the first few days. I suspect that many of the hatchlings are dying of dehydration on the edge of the tank just above the waterline! This can happen, I think, through two distinct mechanisms: 1) the surface tension at the top of the tank, which may attract and then slowly strand them on the surface of the tank, and 2) the crude mechanical turbulence of the water’s surface (from stirring the water or moving the tank) in which the water laps above the normal meniscus and strands the creatures on the tank surface in either a fine, rapidly evaporating sheen or, more likely, in small droplets that they are helpless to escape. On close (but unaided by magnification) examination of the surface area, I see what looks like at least one hatchling already dessicated on the tank wall, although I can’t be sure.
I very much doubt that cannibalism is taking a toll at this early stage in their development, although others have documented that occuring amongst mature (and quite possibly underfed) populations. The tank water is very finely but thoroughly silted with the micronutrients.
I hope that as they grow (they should double in size over the next 24 hours) they will become less vulnerable to the hazards of surface tension – no small threat to critters so miniscule, weak, and, while suspended in water, virtually weightless (relative to the water, that is).