Marine Biology 100, 203-210 (1989) Marine ==BiOlOgy 9 Springer-Verlag 1989 Development of planulae within a mesogleal coat in the soft coral Heteroxenia fuscescens Y. Benayahu 1, T. Berner 2 and Y. Achituv 2 1 Department of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv 69978, Israel 2 Department of Life Sciences, Bar Ilan University, Ramat Gan 52100, Israel Abstract Embryology of the hermaphroditic and dimorphic alcyona- cean Heteroxeniafuscescens has been examined by scanning electron microscopy and light microcopy. Initial embryonic development in this octocoral occurs while the embryos migrate freely inside the anthocodiae and the tentacles. Im- mature planulae are extruded externally into inter- siphonozooid spaces, where they mature. All stages of pla- nular morphogenesis, from egg to planula, occur while the embryo is coated by the original egg mesogleal coat derived from the parent colony. Hatching from this mesogleal coat occurs as late as immediately prior to planulation. H. fus- cescens demonstrates highly specialized brood care involv- ing the retention of embryos in internal polyp cavities as well as in external spaces. This highly specialized brood care, coupled with the embryo coating, may provide better pro- tection for the embryo and greater fecundity for the colony. 1985). Its colonies are easily recognized among other xeniids by their large size and long pulsating polyps. Although Gohar (1940) and Gohar and Roushdy (1961) reported some reproductive features of H. fuscescens, various aspects of embryogenesis and the structural relationships between the developing planulae and the parent colony remain poor- ly known. The objectives of this study are (1) to examine the mode of development of planulae and the brooding procedure in Heteroxenia fuscescens and (2) to determine the structural relationships between a gravid colony and its developing embryos. Using histological sections and scanning electron mi- croscopy (SEM), we examine the morphological features of the eggs and embryos. We describe, for the first time, a peculiar mode of planula development within a coat of pa- rental mesoglea and the consequent hatching of mature lar- va from this coat at planulation. Introduction Several soft corals (Octocorallia:Alcyonacea) have repro- duction that involves planulae shedding (Benayahu and Loya 1984b). Alcyonaceans of the family Xeniidae have been observed to brood their embryos to a planula stage (Gohar 1940, Gohar and Roushdy 1961). In Xenia species, embryogenesis of the fertilized egg is carried out within internal brooding pouches where the planulae mature (Benayahu and Loya 1984b, Benayahu etal. 1987). All Heteroxenia species are characterized by polyp dimorphism, i.e., two kinds of polyps are present in a colony: autozooids and siphonozooids. Some Heteroxenia species are herm- aphroditic and have a rather complex brooding mechanism that involves the retention of the expelled planulae among the siphonozooids (Gohar 1940). Heteroxeniafuscescens (Ehrenberg, 1834) is a common shallow-reef xeniid in the northern Red Sea (Benayahu Material and methods Colonies of Heteroxenia fusceseens were collected during 1986 and 1987 from the coral reef in front of the Marine Biological Laboratory at Eilat. Field collection, underwater observations and photography were conducted while SCUBA diving at depths of 4 to 12 m. Immediately after collection, living colonies were dissected longitudinally and examined under a binocular stereoscope. Gonads and em- bryos from the colonies were collected and fixed (see below). Additional colonies of H. fuscescens were carefully detached from the natural substratum and transferred into aerated aquaria. Released embryos or planulae were pipetted out of these aquaria. Material for light microscopy was fixed in Bouin's solution and then decalcified in a mixture of equal amounts of formic acid (50%) and sodium citrate (15%). Paraffin sections 8/~m thick were cut and stained in Mallory Trichrome or hematoxylin-eosin. Material for scanning elec- tron microscopy (SEM) was fixed in sea-water 2% glutaral-