Does Pontoporeia affinis (Amphipoda) Optimize Age at Reproduction in the Gulf of Bothnia?

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Abstract
Current life-history theory was applied to explain the variation in life-cycle length of the amphipod Pontoporeia affinis Lindstrom in the Gulf of Bothnia. The life-cycles of P. affinis at 9 localities (depths) were evaluated on the basis of a maximization of the per capita growth rate. The per capita growth rates of the observed life-cycles were compared with those of mutants with alternative life-cycles. The observed life-cycle lengths were 2 years at 5 and 210 m depth, 3 years between 50 and 174 m, and both 3 and 4 years at 84 m. All the observed 2 year life-cycles were optimal, and at 5 of the 9 localities the observed life-cycles coincided with the optimal on one or several occasions. About 40% of the observed life-cycles coincided with the calculated optima. Two alternative proximate mechanisms regulating the age at reproduction were evaluated with respect to their ability to contribute to an optimization of the life-cycle length. The life-cycle length was suggested to be either directly, or indirectly determined by the growth rate. The latter mechanism is linked to size rather than growth rate, i.e. such that the life-cycle length is correlated to the time needed to reach a certain threshold size. Both mechanisms were shown to have the possibility to account for the optimal solution. We argue in favour of the latter mechanism.