Gastrovascular Circulation in an Octocoral: Evidence of Significant Transport of Coral and Symbiont Cells.

The gastrovascular system in the Red Sea soft coral Parerythropodium fulvum fulvum comprises two interconnected networks of canals filled with fluid and circulating cells. The first network is composed of narrow canals (50-80 {mu}m in width) located below the upper ectodermal layer; the second network includes larger canals (300-500 {mu}m in width) that are located deeper in the coral tissue. Particle movement in the second network is faster than in the superficial network, but in both, coral cells with and without healthy zooxanthellae circulate freely. To investigate the movement of metabolites and cellular components within the colony, coral fragments were exposed to 14C-labeled seawater for 24 h in the laboratory and in situ under saturating photosynthetic photon flux and then grafted back to their original colonies. Grafts fused after 24 h. In the laboratory experiment, up to 45% of the fixed 14C was translocated to the unlabeled colony within 48 h after fusion. In the in situ experiment, significant translocation of labeled materials occurred at the furthest parts of the colonies, 390 mm away from the fusion line, in 24 h. Even though the amount of labeling varied between colonies, labeled material spread throughout all the unlabeled parts. It thus appears that the gastrovascular system in Parerythropodium fulvum fulvum functions as an effective circulatory apparatus for fast translocation of organic compounds and cellular components within the colony.