Vertical distribution of brittle star larvae in two contrasting coastal embayments: implications for larval transport

Morgane Guillam,Claire Bessin,Éric Thiébaut,Philippe Cugier,Aline Blanchet-Aurigny,Amandine Nicolle,Thierry Comtet

doi:10.1038/s41598-020-68750-4

Abstract

The ability of marine invertebrate larvae to control their vertical position shapes their dispersal pattern. In species characterized by large variations in population density, like many echinoderm species, larval dispersal may contribute to outbreak and die-off phenomena. A proliferation of the ophiuroid Ophiocomina nigra was observed for several years in western Brittany (France), inducing drastic changes on the benthic communities. We here studied the larval vertical distribution in this species and two co-occurring ophiuroid species, Ophiothrix fragilis and Amphiura filiformis, in two contrasting hydrodynamic environments: stratified in the bay of Douarnenez and well-mixed in the bay of Brest. Larvae were collected at 3 depths during 25 h within each bay. In the bay of Brest, all larvae were evenly distributed in the water column due to the intense vertical mixing. Conversely, in the bay of Douarnenez, a diel vertical migration was observed for O. nigra, with a night ascent of young larvae, and ontogenetic differences. These different patterns in the two bays mediate the effects of tidal currents on larval fluxes. O. fragilis larvae were mainly distributed above the thermocline which may favour larval retention within the bay, while A. filiformis larvae, mostly concentrated near the bottom, were preferentially exported. This study highlighted the complex interactions between coastal hydrodynamics and specific larval traits, e.g. larval morphology, in the control of larval vertical distribution and larval dispersal.

Highlights

The ability of marine invertebrate larvae to control their vertical position shapes their dispersal pattern
Ophiocomina nigra larvae were characterized by an active Diel Vertical Migration (DVM), with young larvae rising towards the surface during the night, and by an ontogenetic vertical migration (OVM)[31], which indicated that larvae could be able to actively control their vertical position
Thermoclines may act as a barrier to vertical migration, this would differ between species and developmental stages[32,33,34]

Summary

Introduction

The ability of marine invertebrate larvae to control their vertical position shapes their dispersal pattern. Echinoderms play key roles in the structure and the functioning of many marine ecosystems, and are known for large variations in population density, alternating periods of outbreaks and periods of die-offs[1] These rapid and drastic changes in their abundances can have important ecological consequences on the diversity, resilience and functioning of these e cosystems[1]. The causes of such extreme phenomena are still poorly known, but may be of anthropogenic origin, such as the degradation of water quality, the overexploitation of marine resources or even the introduction of invasive s pecies[1,2] Besides these primary causes, particular life-history traits of echinoderm species (e.g. external fertilization, planktotrophic larval development) may modulate, either amplifying or reducing, the variations of abundances of adult p.

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Conclusion