. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 282 M. AND Y. LOYA. Figure I. Cliulinulu sp. larval and post-larval fusion, (a) A pair of fused larvae, 2 h after initial contact, (h) A pair of fused larvae. 24 h after fusion, start to fuse with a third larva, (c) Fusion of two post-larvae. Fusion followed their settlement in proximity, (d) A new larva starts to settle on and fuse with a two-day old post-larva. In all the light micrographs, arrows indicate the contact zone (scale bar = 200 ^m). specific reaggregation of dissociated sponge cells (reviewed by Muller, 1
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 282 M. AND Y. LOYA. Figure I. Cliulinulu sp. larval and post-larval fusion, (a) A pair of fused larvae, 2 h after initial contact, (h) A pair of fused larvae. 24 h after fusion, start to fuse with a third larva, (c) Fusion of two post-larvae. Fusion followed their settlement in proximity, (d) A new larva starts to settle on and fuse with a two-day old post-larva. In all the light micrographs, arrows indicate the contact zone (scale bar = 200 ^m). specific reaggregation of dissociated sponge cells (reviewed by Muller, 1982; Coombe and Parish, 1988). The fusion of larvae derived from the same parent may have occurred for several reasons: (1) the larvae may lack a capacity for self/nonself discrimination; (2) they may possess such discrimination, but may also express an in- hibition of the rejection mechanism; and (3) the larvae may have been genetically identical (products of parthe- nogenetic reproduction), thus resulting in the fusion of grafts that were actually isografts and not allografts. The last cause of larval fusion is less likely, because the larvae were taken from sponges 10 to 300 m apart from each other in their natural habitat; therefore they were probably genetically different. Thus, the larvae that fused in the experiments were probably genetically different, even if asexual development of larvae occurs in Chalinula sp., which is unlikely (Ilan and Loya, 1990). These results differ from the situation reported by Van de Vyver and Willenz (1975), who studied the freshwater sponge Ephy- datia fluviatilis and described larval fusion as occurring only between larvae belonging to the same strain. If indeed larvae were incapable of self/nonself discrim- ination, the results with adult grafting indicate acquisition of this capability during ontogenesis. Juvenile immuno- logical incompetence is well known among vertebrates (Cooper, 1976) and has been suggested also for corals (, Du
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