Experimental Manipulation of Polyandry in a Marine Gastropod Reveals How the Number of Mates Affects Reproductive Output, Offspring Size, and the Distribution of Paternity Within Broods

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Polyandry, where females mate with multiple males, often mediates how ecological and evolutionary forces shape populations, with various explanations for why it occurs. However, these explanations often stem from separate studies on model species, field observations, or lab experiments. Given polyandry's potential context‐dependent effects, it is crucial to design studies that concurrently test multiple hypotheses within wild populations. Therefore, we conducted two experiments over 2 years that experimentally manipulated the number of males a female mates with in the marine gastropod, the Florida crown conch ( Melongena corona ). We tested if experimentally increasing polyandry leads to more offspring, larger offspring at hatching, and broods with greater variation in offspring size and higher genetic diversity. We also investigated paternity skew, the effects of mate order, male size, and copulating time on paternity. We genotyped 3157 offspring from 20 mothers to quantify paternity share at hatching. We found that females mating with more males did not produce more offspring or larger offspring than monandrous females at the embryo or hatching stage. However, multiple mating increased within‐brood variation in offspring size at hatching, possibly as a response to exploitative intracapsular competition for oxygen in mixed broods or sire effects. Paternity share within broods at hatching was skewed, rather than evenly distributed, resulting in a lower effective number of sires compared to the number of mates. Paternity share per brood declined with mate order and increased with copulation duration but was unaffected by male size. Overall, the commonly hypothesized consequences of polyandry were not observed in our experiments. Instead, we hypothesize that multiple mating in this species arises from convenience polyandry or mate encounter rates.