Stage-Specific Cannibalism and Spontaneous Cyst Hatching in the Freshwater Fairy Shrimp Streptocephalus Proboscideus Frauenfeld

Freshwater fairy shrimp that live in episodically filled rainpools are obligate bisexual animals that produce drought-resistant, encapsulated embryos called cysts. A fraction of these hatches upon each inundation, a bet-hedging strategy, and the resulting larvae grow in a concerted manner, forming a cohort. Here, we show that in the Sudan fairy shrimp, Streptocephalus proboscideus, about one-third of the cysts hatches shortly after being released from the brood-sac, without intervening period of drought. However, as long as the adult population overshoots a density of about one specimen per litre, the emerging nauplii are cannibalized by their parents. At such a density and above it, fairy shrimp filter the whole pond volume in less than 24h and since it takes about the same amount of time for first-instar nauplii to moult to an immune size, they are all consumed. In addition to being auto-regulatory, this behaviour is adaptive: animals fed their own nauplii on top of an algal diet showed a 30% increase in reproductive output, and an increase in somatic growth rate. This allocation almost quantitatively recouped the loss of inclusive fitness to cannibalism. In contrast, hatching percentages of cysts obtained from animals fed this nauplii-supplement were the same as in algal-fed animals, such that their bet-hedging strategy remains intact and is carried forward to the next generation.