Co-evolution of the Shrimp Hippolyte inermis and the Diatoms Cocconeis spp. in Posidonia oceanica: Sexual Adaptations Explained by Ecological Fitting

Abstract

Microalgae influence the life of grazers in such stable ecosystems as Posidonia oceanica meadows. Competition and co-existence require adaptations for both organisms: algae produce metabolites able to reduce the grazing activity, and invertebrate react to the chemical weapons of algae, for feeding on their thalli. Several diatoms produce wound-activated compounds and some of them have been demonstrated to trigger apoptosis and teratogenic effects in planktonic copepods. The case of Hippolyte inermis and its diatom food is different and peculiar because the shrimp transformed the effects of apoptogenic compounds produced by Cocconeis into a spring signal to obtain a higher abundance of females, so stabilizing its natural populations. As in crustacean decapods, the sex is determined by the presence/absence of a single gland (the Androgenic Gland; A.G.), in H. inermis the apoptogenic effect of secondary metabolites is limited to the destruction of the A.G. in spring, when various species of Cocconeis dominate the epiphytic layer of Posidonia leaves. This relationship, evidently co-evolved through a competitive relationship, allows the shrimp to produce a secondary reproduction burst in fall, when Cocconeis spp. are less abundant on the leaves of the plant. Co-evolutionary relationships are often viewed in light of mutual cooperation between two species. However, the peculiar case of H. inermis indicates the need to widen the concept, integrating various adaptations that may lead to different degrees of advantages for two co-evolving organisms. Shrimp’s populations are stabilized in P. oceanica meadows thanks to this very specific relationship, and they can survive a high predation pressure by fish and other invertebrates because the secondary reproductive burst in fall produces sufficient specimens for the next spring.