Attachment capacity of the sea urchin Paracentrotus lividus in a changing ocean

Chair: Sue-Ann Watson

Mishal Cohen-Rengifo (1,2)*, Antonio Agüera (2), Tjeerd Bouma (3), Alexandre Campo (4), Philippe Dubois (2), Patrick Flammang (1)

1 Université de Mons, Institut des Biosciences, Laboratoire de Biologie des Organismes Marins et Biomimétisme, Mons, 7000 Belgium
2 Université Libre de Bruxelles, Laboratoire de Biologie Marine, Brussels, 1050, Belgium
3 Royal Netherlands Institute for Sea Research, Yerseke, 4401, The Netherlands
4 Université Libre de Bruxelles, Laboratoire d’Ecologie Sociale, Brussels, 1050, Belgium

Background
Intertidal rocky shores are extremely stressful environments wherein benthic invertebrates are subjected to strong wave-induced forces that are likely to cause dislodgement from the substrate. Hydrodynamics is therefore one of the most important factors determining survival and distribution of species. To counteract hydrodynamic forces, echinoids have developed adhesive appendages, the tube feet, made up of a distal adhesive disc and a proximal load-bearing stem. In a context of global climate change, oceans are becoming more acidic and warmer, but extreme hydrodynamic events are also occurring more frequently. The aim of this study was to determine whether sea water temperature and pH can induce variations in attachment capacity of the sea urchin Paracentrotus lividus.

Methods
A fully crossed experimental design was set up (temperature: 17°C, 21°C X pH: 8.0, 7.7, 7.4) during 12 weeks. Sea urchin morphology and physiology as well as tube foot mechanics were studied. Flume tank experiments were also run in order to evaluate behaviour and detachment of sea urchins subjected to an increasing flow regime.

Findings
Sea urchin size and growth were not affected by any of the experimental treatments. Physiological parameters showed that P. lividus is able to compensate internal pH when seawater pH decreased due to the high buffer capacity of its coelomic fluid. Exposure to the lowest pH resulted in significantly weakened tube foot stems both in terms of extensibility and toughness. Adhesive properties of the disc, global tenacity and detachment velocity were not affected. However becoming streamlined probably counteracts negative impact of pH on tube feet.

Conclusions
Climate change conditions do not appear as a chronic factor of stress for this robust species. Although sea urchins presented weakened mechanical properties of the stems, their overall attachment capacity was not affected, presumably because behaviour can play and important buffering role.