In at least 3 bays of south Brittany, in the Concarneau bay, offshore Le Croisic archipelago and in the external part of the Loire river estuary, dense settlements of several thousands individuals per square meters, of tubes dwelling amphipods, Haploops antennata, have been reported as being perfectly superimposed to fields of pockmarks (crater-like figures visible in soft sediment seafloor that mark fluids expulsions). This ecosystem's engineer species plays a sedimentological key role in the environmental processes and increases local biodiversity. 

This work presents results of the investigation of these 3 pockmarks-Haploopsfields, where each area is characterized by different hydrodynamics and rivers inputs conditions, allowing us to strictly link the presence and development of Haploopsto the venting activity of the pockmarks fields. Indeed, whatever the external inputs of organic and inorganic particles, the Haploopsstrictly developed inside the area of the pockmarks field. 

Focusing on the settlement offshore Le Croisic, a two year space-time evolution monitoring (between 2017 and 2018) of the Haploops settlement, pockmarks field and gas movements into the sedimentary column have been conducted. Boundaries limits of the Haploops settlement have been defined and compared over these two years and a large expansion have been determined in the southwestern part of the settlement. There, around 1.7 square kilometer have been colonized by the amphipods. Manual count of the pockmarks have been carried out on representative areas and positive correlation were obtained between expansion zones and pockmarks apparitions. Moreover, pockmarks apparitions and activities may be linked with the depth of acoustic mask into the sedimentary column. The areas where theHaploopsare expending corresponds to the areas where numerous pockmarks already exist or appear, and these pockmarks are located were the enhanced gas reflector (traducing the gas stopped under an impermeable layer) is shallower in the sedimentary column. 

AsHaploops seem to not contain the necessary bacteria to directly consume methane, indirect explanation should be proposed. Pockmarks should thus either (i) directly expulse some nutrients necessary for Haploopsdevelopment, or (ii) the upcoming methane induce chemical reactions at seafloor surface, which may be used to locally increase the primary production of phytoplankton.