Maternal Control of Calcium Concentration in the Larval Nursery of the Bromeliad Crab, Metopaulias depressus (Grapsidae)

It has been proposed that parental care in invertebrates controls for physicochemical conditions. This suggests that the parent detects deviations from optimal conditions and responds by correcting the deviation for the benefit of the offspring. In the bromeliad crab, Metopaulias depressus, mothers accumulate snail shells (CaCO₃) in the leaf axil containing the larvae, which results in an increase in Ca²⁺ and pH, both beneficial to the young. I tested the hypotheses that mothers: (i) control for calcium levels and only collect shells when calcium is too low; (ii) always collect shells, independent of the calcium content of the nursery; (iii) collect shells if encountered, but increase their activity if the calcium content becomes critical. In a field experiment, I reduced the initial calcium concentration in the nurseries of one group and increased it in another group. In both groups mothers collected shells. However, they collected significantly more shell mass per day in the group with reduced levels than in the group with high levels. The results support hypothesis (iii) and unequivocally demonstrate maternal control of physicochemical conditions in the nursery. This is the first proof that crustacean mothers actively regulate abiotic conditions for their offspring.