Immune cell profiling in the sand dollar Mellita quinquiesperforata (Leske, 1778) (Echinodermata: Echinoidea): morphology, phagocytosis, and ABC transporters activity

The immune system cells of Echinodermata, known as coelomocytes, have been extensively studied in groups such as regular sea urchins and sea cucumbers. In contrast, investigations addressing coelomocytes in Clypeasteroida remain limited. The present study aimed to investigate the immune system cellular profile of the sand dollar Mellita quinquiesperforata, focusing on the morphological characteristics of coelomocytes and two physiological parameters: phagocytic activity and ABC transporters activity. Coelomocyte morphology and physiology were assessed using light, confocal, and fluorescence microscopy, along with flow cytometry. Four coelomocyte types were identified: phagocytes, large spherical corpuscles, spherulocytes (colorless, yellowish-brown and red), and fusiform cells. Phagocytes were the most abundant cell type (81.12% ± 6.16%). The average total concentration of coelomocytes was 1.19 x 10⁷ ± 5.01 x 10⁶ cells mL^-1. Phagocytes were the only cell type capable of internalizing latex beads, with a mean phagocytic capacity of 20.44 ± 9.70%. Coelomocytes exhibited ABCC transporter activity, but no ABCB1 transporter activity was detected. The results presented here contribute to the understanding of the echinoderm immune system diversity and open new possibilities for using the immunological profile of the widespread M. quinquiesperforata in environmental monitoring studies.