Why are my baby shrimp different sizes?

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Is it unusual to have two different batches of yellow Neocaridina davidi shrimplets which are different sizes? A reader asks...

Q) Is it unusual to have two different batches of yellow Neocaridina davidi shrimplets which are different sizes? The first batch are ‘normal’ sized while another batch from the same female contains shrimplets much smaller in comparison to their siblings. What could have caused this?

VIVIANE REBELO-REIS, VIA EMAIL

A) Neale Monks says: 

Neale says: In theory, each batch of shrimps should be more or less similar to the ones before and after, but you’re right, a certain degree of variation is sometimes seen.

Neocaridina davidi is sexually dimorphic — females are substantially larger and more intensely coloured than the males, and if for some reason one batch of shrimplets were mostly males, and the second batch mostly females, you might well get the impression that one group was a different size to the second group.

Studies have shown that there seems to be a strong relationship between temperature and sex ratio with Neocaridina davidi — when the water is relatively cool, at around 20˚C, about 80% of the offspring are female; this reduces to about 50% at 23˚C, and is just 18% at 26˚C. So, if your tank is unheated and is cooler in winter and warmer in summer, you could very easily have mostly female shrimplets produced in winter, and mostly males in summer.

In a study that reared Neocaridina davidi at a range of stocking densities across 90 days, it was found that at 2.5 shrimps per litre the shrimps were about one-half bigger than the shrimps grown at 10 shrimps per litre, so stocking density also has an impact.

This sort of experimental work is obviously important to commercial shrimp producers who need to balance stocking density against how quickly the shrimp reach a saleable size, but it might also explain what’s happening in your tank. If you started off with a few shrimps, they might have produced offspring that grew up quite quickly, but as the population increased, and presumably there was less food and oxygen available, later batches of juveniles grew more slowly.

Genetics can also play a part. While the shrimps we see for sale are nominally assigned to the species Neocaridina davidi, in truth we really don’t know how much hybridisation is going on with other Neocaridina species. Adult size is almost certainly a polygenic characteristic, meaning that there are multiple genes responsible for the potential size of a given shrimp under ideal circumstances.

If the parents are heterozygous (meaning they have various ‘big’ and ‘small’ genes in their genome) it is perfectly possible for ‘big’ adults to produce some offspring that develop into ‘small’ shrimps. In the absence of a selection pressure, such as predation or the need to compete for females that would favour the larger adults, each subsequent generation might well include a random mixture of bigger and smaller adults. You see this sort of thing with domesticated tropical fish all the time, with mollies and Angelfish being two obvious examples of fish that rarely grow into adults as big as those seen in the wild, partly because they’re genetic ‘mutts’, including genes from large and smaller species, and partly because fish breeders are not selecting for adult size, but rather traits like colouration or fin length.

Quality of diet and the use (or otherwise) of mineral supplements such as iodine and calcium can also affect the growth rate, so while it is possible to hypothesise what’s going on in your tank, it’s hard to be sure exactly why you have this size issue.

(Image credit: Shutterstock)