Form, function, and embryonic migration in large gelatinous egg masses of arenicolid worms

Attachment and gas exchange are design problems for the spheroidal egg mass es that annelids and molluscs tether in sediments. At an intertidal site in Bodega Harbor, a high proportion of masses of arenicolid worms became deta ched, but they were not stranded higher than attached masses, and the embry os developed to advanced stages. The large masses, weighing 100-200 g, chal lenge predictions on limiting size for oxygen supply by diffusion. The esti mated concentrations of embryos and thickness of the embryo layer exceed th e limits predicted by a simple model for oxygen supply by diffusion from th e surrounding water, but several features may enhance oxygen supply to embr yos. (1)There is an internal cavity. Distal tears in the gel layer admit pu lses of water into the center, thus adding an inner surface for gas exchang e. (2) The motile embryos aggregate near the outer and sometimes inner surf aces of the gel layer. Embryos migrate in the gel toward regions of greater oxygen concentration. (3) Pennate diatoms colonize the masses, coating the surfaces and penetrating into the gelatinous matrix. Photosynthesis can ex ceed respiration when a mass is in the light. The pH in gel changes diurnal ly from a low of similar to7 to as much as 9.5, indicating an excess of res piration over photosynthesis at night and at least sometimes an excess of p hotosynthesis over respiration by day.