Transport Pathways in the Neotropical Sponge Aplysina

Strands of cells distinct from the rest of the tissue were found running lengthwise through the endosome of four species of the sponge Aplysina. Although the strands were more highly pigmented than the adjacent tissue and could be removed intact with forceps, ultra-structural studies revealed no obvious barrier separating the cells in the strands from the rest of the tissue. The strands consist of stretches of elongate cells tightly aligned along densely bundled collagen fibrils, and areas of other elongate cells that possess numerous filopodia. When sponges were fed fluorescent latex beads in situ, beads were taken up and transported specifically into the strands; eventually they were found at the tip of the sponge and further down the stalk, away from the site of feeding. Beads injected into endosomal strands were also transported upwards in the strands to the tip of the sponge. Video microscopy of cells in strands that had been exposed along a portion of their length showed no bulk movement of cells; but individual cells were seen moving in both directions along the strands at 0.025-0.04 {mu}m.s-1. The endosomal cell strands are suggestive of a primitive nutrient transport pathway in sponges.