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Sea urchins are key members of hydrodynamically intense habitats, such as open coast intertidal and shallow reefs. Secure attachment to the substrata, by means of tube feet and spines, is essential for survival. Previous studies suggest that environmental variables (i.e. hydrodynamics, substrate) can influence the morphology and adhesive performance of tube feet. Catastrophic failure of tube feet occurs often due to strong hydrodynamic forces. The time and ability of amputated tube feet to return to pre-amputation adhesive performance, and the interaction with environmental factors are unknown. To assess the dynamics of adhesive performance and regeneration of tube feet, we 1) evaluated phenotypic plasticity of disc surface area of two urchin populations inhabiting sites with different lithologies (field - in-situ conditions) and, 2) assessed regeneration of their functionality (tube foot length, time of disc first appearance, disc surface area, and tenacity) after amputation (laboratory - ex-situ conditions). Results showed that in the field, tube feet are plastic in their phenotype: sea urchins from the two sites differed in their disc surface area. Plasticity was also observed in the laboratory, where urchins regenerate tube feet that are shorter in length and have smaller discs compared to pre-amputation measurements. Thus, sea urchin tube feet recovering from amputation events in the intertidal may differ in their initial morphology and in the recovery period after amputation. Moreover, tube feet that were never amputated also decreased their disc area, suggesting that plasticity occurs on tube feet of urchins maintained in laboratory conditions. In shallow temperate reef systems, urchins are ecosystem engineers that affect the abundance and distribution of many other organisms. The plastic nature of morphological and adhesive abilities of sea urchin tube feet influences their ability to survive and dominate wave-battered habitats. • Sea urchins are highly plastic in their tube feet morphology • Substrate lithology appears to influence the plasticity of tube feet • Regenerating tube feet do not recover pre-amputation attributes when kept ex situ • Disc area decrease in ex-situ conditions, but tenacity and length remain constant [ABSTRACT FROM AUTHOR]

Purple sea urchins (Strongylocentrotus purpuratus) profoundly impact nearshore rocky coasts through their feeding habits. Their intense grazing sculpts substrates through bioerosion using their teeth and spines and controls the alternative stable state dynamic between kelp bed and urchin barrens. These states have contrasting food availability for sea urchins, with abundant food in kelp beds and scarce food in barren grounds. However, the relationship between food availability and bioerosion is unknown. We predicted that when kelp is available, it would ameliorate the action of teeth on the substrate. Our 11-week long, 2 × 2 factorial experiment, crossed community state (kelp present vs absent) and rock type (sandstone vs mudstone). We also quantified the contribution of spine abrasion to bioerosion on the two rock types. The bioerosion rates did not differ between treatments with and without kelp. Although there was no significant difference in net bioerosion between the rock types, there was a large difference between the proportion of bioerosion from teeth vs spine abrasion. Approximately a third of the sandstone bioerosion was from spines whereas less than 2% of mudstone bioerosion could be attributed to spines. As anticipated, growth of sea urchins fed kelp ad-libitum was higher than food-limited sea urchins. Surprisingly, sea urchins on mudstone (which has a higher organic component) grew faster than sea urchins on sandstone. Although bioerosion rates may not differ on a per-urchin basis between community states, the sea urchin population densities between kelp beds and urchin barrens likely causes a difference in net bioerosion between these communities. Our results point to the importance of lithology on the mechanics of sea urchin bioerosion. Differences in texture, grain size, and hardness of rock substrates undoubtedly contribute to bioerosion rates and dynamics., (© The Author(s) 2024. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology.)

Climate change will increase the frequency and intensity of low-salinity (hyposalinity) events in coastal marine habitats. Sea urchins are dominant herbivores in these habitats and are generally intolerant of salinity fluctuations. Their adhesive tube feet are essential for survival, effecting secure attachment and locomotion in high wave energy habitats, yet little is known about how hyposalinity impacts their function. We exposed green sea urchins (Strongylocentrotus droebachiensis) to salinities ranging from ambient (32‰) to severe (14‰) and assessed tube feet coordination (righting response, locomotion) and adhesion [disc tenacity (force per unit area)]. Righting response, locomotion and disc tenacity decreased in response to hyposalinity. Severe reductions in coordinated tube foot activities occurred at higher salinities than those that affected adhesion. The results of this study suggest moderate hyposalinities (24-28‰) have little effect on S. droebachiensis dislodgement risk and survival post-dislodgment, while severe hyposalinity (below 24‰) likely reduces movement and prevents recovery from dislodgment., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2023. Published by The Company of Biologists Ltd.)