Calcification in the planula and polyp of the hydroid Hydractinia symbiolongicarpus (Cnidaria, Hydrozoa)

This study examines calcification in planulae and polyps of the hydroid Hyd ractinia symbiolongicarpus. We observed that established colonies produce a crystalline mat on their substratum and that crystals visible by polarized light microscopy occur in the vacuoles of the gastrodermal cells of both p olyps and planulae. The crystalline mat was found by infrared spectroscopy to contain calcium carbonate in the form of aragonite. The composition of t he vacuolar crystals and the cellular mechanisms for manufacturing them wer e explored by alteration of calcium levels in the environment and by the us e of pharmacological agents (acetazolamide, caffeine, DIDS, diltiazem, nife dipine, procaine, Ruthenium Red, ryanodine and verapamil) that affect cellu lar uptake and transport of calcium and bicarbonate. The results indicated that the crystals in the vacuoles contained calcium carbonate. The gastrode rmal cells are hypothesized to serve as a physiological sink for excess cal cium that enters the organism during motility, secretion and metamorphosis of the planula, and to create a crystalline substratum for the colony of po lyps.