Control of social monogamy through aggression in a hermaphroditic shrimp.

Janine Wy Wong,Nico K Michiels

doi:10.1186/1742-9994-8-30

Abstract

IntroductionSex allocation theory predicts that in small mating groups simultaneous hermaphroditism is the optimal form of gender expression. Under these conditions, male allocation is predicted to be very low and overall per-capita reproductive output maximal. This is particularly true for individuals that live in pairs, but monogamy is highly susceptible to cheating by both partners. However, certain conditions favour social monogamy in hermaphrodites. This study addresses the influence of group size on group stability and moulting cycles in singles, pairs, triplets and quartets of the socially monogamous shrimp Lysmata amboinensis, a protandric simultaneous hermaphrodite.ResultsThe effect of group size was very strong: Exactly one individual in each triplet and exactly two individuals in each quartet were killed in aggressive interactions, resulting in group sizes of two individuals. All killed individuals had just moulted. No mortality occurred in single and pair treatments. The number of moults in the surviving shrimp increased significantly after changing from triplets and quartets to pairs.ConclusionSocial monogamy in L. amboinensis is reinforced by aggressive expulsion of supernumerous individuals. We suggest that the high risk of mortality in triplets and quartets results in suppression of moulting in groups larger than two individuals and that the feeding ecology of L. amboinensis favours social monogamy.

Highlights

Sex allocation theory predicts that in small mating groups simultaneous hermaphroditism is the optimal form of gender expression
Social monogamy in L. amboinensis is reinforced by aggressive expulsion of supernumerous individuals
We suggest that the high risk of mortality in triplets and quartets results in suppression of moulting in groups larger than two individuals and that the feeding ecology of L. amboinensis favours social monogamy

Summary

Introduction

Sex allocation theory predicts that in small mating groups simultaneous hermaphroditism is the optimal form of gender expression. Under these conditions, male allocation is predicted to be very low and overall per-capita reproductive output maximal. Male allocation is predicted to be very low and overall per-capita reproductive output maximal This is true for individuals that live in pairs, but monogamy is highly susceptible to cheating by both partners. Sex allocation theory predicts optimal investment into male and female function in sexually reproducing organisms [1] and has been the focus of many studies (for example [2,3,4,5,6]). A small mating group will induce reproductive success, because sperm are cheaper to produce than eggs [15]

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