Thyroxine and vitamin D in the gorgonian Leptogorgia virgulata

The gorgonian coral Leptogorgia virgulata contains thyroxine, or a thyroxine-like substance, referred to here as G-T₄. The G-T₄ levels were significantly higher in colonies collected in the summer vs. winter months. Using immunocytochemical techniques, G-T₄ was localized in the axis, polyp epithelium, and within the electron dense bodies of scleroblasts (spicule-forming cells), as well as on the periphery of spicules. G-T₄ was also localized in the mesoglea between closely adjacent scleroblasts. The effects of exogenous T₄ on the uptake of Ca⁴⁵ was determined in spicule, tissue and axis fractions of L. virgulata. The uptake of Ca⁴⁵ increased in T₄ treated spicules but decreased in the tissue fraction for all time periods tested. The uptake of Ca⁴⁵ into axes was not affected by exogenous T₄ until day 10 of the study. These data suggest that G-T₄ may function in the process of spicule formation. 1,25-dihydroxyvitamin D apparently is synthesized via ultraviolet radiation. Colonies deprived of ultraviolet radiation had significantly more ‘irregular’ spicules than colonies maintained in ultraviolet radiation. Exposure to sunlight therefore may be associated with the process of normal spicule formation.