Morphological and Morphometric Characterisation of Selected Camarodont Sea Urchin (Echinoidea: Camarodonta) Pedicellariae (Paracentrotus lividus, Psammechinus miliaris and Echinus esculentus)

Pedicellariae are small appendages on the surface of the echinoid test, involved in defence, cleaning, and locomotion. Pedicellariae in three species of camarodont sea urchins common in UK waters (Paracentrotus lividus [Lamarck, 1816], Psammechinus miliaris [P.L.S Müller, 1771] and Echinus esculentus [Linnaeus, 1758]) are the focus of this study, with new characterisation and comparisons by light and scanning electron microscopy. Four pedicellariae types are described and measured: globiferous, ophicephalous, tridentate and the smallest trifoliate. Different distributions were estimated on the aboral surface, with significantly higher densities of ophicephalous in P. miliaris (0.79/mm² ± 0.09, mean ± SEM, p < 0.05, Kruskall-Wallis), and lower densities of globiferous in E. esculentus (0.08/mm², ±0.02, mean ± SEM, p < 0.05, ANOVA). Scanning electron microscopy revealed that the globiferous head is highly specialised, aiding a complex venom delivery system, while the ophicephalous and tridentate appendages have part flexible and part calcified stalks and serrated features for grabbing, respectively. Trifoliate pedicellariae were too small to reliably characterise and measure under live light microscopic imaging. P. miliaris, which occurs at the highest intertidal zone, had the shortest pedicellaria, and the deeper subtidal E. esculentus had the longest. The results demonstrate that the morphology of pedicellariae shows similarities among the three species, but the sizes and distribution of their pedicellariae differ, reflecting differences in their ecology, behaviour and evolutionary history. The functional and evolutionary significance of pedicellariae in sea urchins is still insufficiently understood in general and further research into the comparative venom profiles of the globiferous pedicellaria, the continued action after detachment, and the smallest trifoliate pedicellaria could provide valuable insight into novel biological compounds and ecological implications of morphological and chemical defence systems.