Biomechanics and behaviour in the sea urchin Paracentrotus lividus (Lamarck, 1816) when facing gradually increasing water flows

Water motion, because of its potential to dislodge intertidal organisms, plays a crucial role in shaping marine communities as it creates available spaces suitable for interactions, settlement and colonization. To understand how water flow influences the behavioural ecology of benthic species such as echinoids, we investigated how the sea urchin Paracentrotus lividus (Lamark, 1816) can cope with increasing flow velocities through a biomechanical and behavioural approach. Whereas sea urchins maintained in static conditions for several weeks showed weak adhesive properties, they immediately reacted to increasing hydrodynamic forces. Sea urchins were dislodged at a flow velocity of about 70 cm s⁻¹, with a few individuals enduring up to 90 cm s⁻¹. This response was modulated by two behavioural strategies: (1) at low flow velocities, an escaping strategy characterized by fast upstream locomotion relying on tube feet; and (2) above a threshold flow, a streamlining strategy characterized by changes in overall shape and spines orientation, accompanied by a fast decrease of the animal's movement until complete immobility. Although the threshold at which the switch in behaviour occurs and the detachment velocities were both probably underestimated because individuals were aquarium-acclimatized, the behavioural sequence reported reveals how P. lividus can avoid or withstand rough weather conditions. From an ecological perspective, the occurrence of a range of water velocities too slow to dislodge sea urchins but sufficient to inhibit their active movement is relevant to understand how echinoids can structure benthic communities by controlling macroalgae development through their grazing activity.