TRANSPORT PATHWAYS IN THE NEOTROPICAL SPONGE APLYSINA

Strands of cells distinct from the rest of the tissue were found runni ng lengthwise through the endosome of four species of the sponge Aplys ina. Although the strands were more highly pigmented than the adjacent tissue and could be removed intact with forceps, ultrastructural stud ies revealed no obvious barrier separating the cells in the strands fr om the rest of the tissue. The strands consist of stretches of elongat e cells tightly aligned along densely bundled collagen fibrils, and ar eas of other elongate cells that possess numerous filopodia. When spon ges 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 sit e of feeding. Beads injected into endosomal strands were also transpor ted 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 len gth showed no bulk movement of cells; but individual cells were seen m oving in both directions along the strands at 0.025-0.04 mu m.s(-1). T he endosomal cell strands are suggestive of a primitive nutrient trans port pathway in sponges.