Tube fo o t mechanical properties in the sea urchin
P a ra cen trotus livid u s from habitats presenting a range o f seawater
velocities
Cohen-Rengifo M ishal12, Patrick Flam m ang1 and Philippe D ubois2
1 Laboratoire de Biologie des Organism es m arins et Biom im étism e, In s titu t des Biosciences,
Université de Mons, 23 Place du Parc, 7000 Mons, Belgium
E-mail: m ishal.cohen.r@ qm ail.com
2 Laboratoire de Biologie Marine, Université Libre de Bruxelles, CP160/15
50 Av. F.D. Roosevelt, 1050, Brussels, Belgium
Intertidal rocky shores are considered as extrem ely stressful environm ents wherein benthic marine
invertebrates are subjected to strong hydrodynam ic forces due to wave action and likely to detach
them from the substrate. Hydrodynam ics is therefore one o f the m ost im p o rta n t factors
determ ining the survival and d istrib u tio n o f species as well as shaping benthic com m unities. To
counteract these forces, many marine organism s have developed specialized structures, including
adhesive organs. For instance, echinoids e x h ib it appendages known as tube feet or podia. Tube feet
consist o f a distal fla t deform able disc th a t adheres to the substrate and th a t is connected to the
skeleton th ro u g h a cylindrical shaft called the stem. The aim o f th is study was to evaluate tu be fo o t
adhesive and mechanical properties in the sea urchin Paracentrotus lividus from three
subpopulations exposed to d iffe re n t seawater velocities. In May 2013, a du lt sea urchins were
collected fro m tide pools at 3 localities around the Crozon peninsula in Britany (France). These
localities, llien, M orgat and Cap de la Chèvre, d iffe r according to th e ir hydrodynam ic conditions,
w ith mean w ater velocities o f 9.1 ± 4 . 8cm.s ', 16.4 ± 5 .3 cm .s' and 44.9 ± 13.5 cm .s', respectively.
Specimens were b ro ug ht alive to the laboratory where m orphom etric as well as tenacity
m easurem ents were taken. A safety fa c to r predicting the flo w ve lo city at which sea urchins are
prone to detach from the substrate was then calculated. In a dd ition, tra ctio n tests were perform ed
on tube feet in ord er to determ ine th e ir m echanical properties, i.e. e xten sibility, strength, stiffness
and toughness. Sea urchin size decreased w ith increasing w ater velocity and th is trend was
sta tistically significant. A ttachm ent force and both whole individual and single tube fo o t tenacity
increased w ith increasing flo w velocity. A ll mechanical properties were also higher in the
subpopulation where the highest flo w velocity was recorded, but these differences were statistically
sign ifican t o nly fo r e xte n s ib ility and toughness. Safety fa cto r analysis indicated that, if subjected to
increasing hydrodynam ic forces, sea urchins fro m Cap de la Chèvre w ould be the last to detach
compared to individuals from the o the r tw o subpopulations. Thus the tube feet o f Paracentrotus
lividus show an intraspecific plasticity, th e ir mechanical properties varying according to local
seawater velocities which lead to a better attachm ent capacity in the m ost exposed habitat.
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