Macropodus fry

The first batch of fry are doing well, but the size disparity is getting larger. Most of the fry are "average" size - around 6 mm long. Several are smaller than average - around 4 mm long. And some are much larger than usual - over 10 mm long. [Update: The large ones are over 14 mm long]

It's the giants that are really interesting. Not only are they about twice the length of average fry, they are also 4 or 5 times the overall size of the average fry. At least one of them appears to be an air-breather (the labyrinth organ doesn't develop until the fry reach a certain size).

I'm interested in what the evolutionary strategy is here - or whether there is one. Floyd asked the question of whether there is some sort of advantage to having some fry remain stunted - perhaps as a food source for their larger siblings. While that might explain the smallest ones, it really doesn't explain the very large ones. There's a continuum of body sizes, but the very large fry seem to be outliers.

I have no idea whether this is typical or not. It's possible that these are truly unusual individuals, but it's also possible that this is a normal strategy for Macropodus. If so, it makes me wonder whether this could be viewed from the perspective of asynchronous germination in seeds. The normal way to interpret this would be as some sort of scramble competition - that the most successful strategy is to get big as quickly as possible, because faster growth allows you to outcompete your siblings and escape many predators (including your parents).

The problem with this interpretation is that getting big probably isn't a winning strategy for a territorial species living in a limited environment. Chances are, not only is reproduction only an issue for a fish that can claim a territory, it's also likely that reaching adult size is only an option if a territory becomes open (much like a tree colonising a gap). If that is the case, then growing large quickly doesn't guarantee success - it only represents one out of a range of strategies. It may be that some fish get big quickly to occupy an immediately available territory (if one exists) while other mature more slowly, with the "objective" of staying to occupy a territory that becomes available later on.