August 25 th , 2001

1. Sexual reproduction in solitary corals of the genus Balanophyllia (Scleractinia, Dendrophylliidae): is there a correlation between sexual condition and mode of environmental colonization? Radetic’ J., Goffredo S., Zaccanti F. Department of Evolutionary and Experimental Biology, University of Bologna, via F. Selmi 3, I-40126 Bologna, Italy Balanophyllia pruvoti is a common azooxanthellate scleractinian coral living in the Mediterranean Sea and along the Atlantic coast from Portugal to southwestern of England. It lives in shaded habitats at depths ranging from shallow water to a depth of more than 100 meters. We are currently studying the annual cycle of the sexual reproduction in this coral in Calafuria (Leghorn, Tuscany, Eastern Ligurian sea), the same locality where we studied the reproductive biology of the Mediterranean congeneric species B. europaea, a zooxanthellate coral living in shallow water in open habitats. B. pruvoti is a gonochoric species with a 1:1 sex ratio and very high population density. On the contrary, B. europaea is a simultaneous hermaphrodite with very low population density. We hypothesize that the presence of opposite sexual conditions in these congeneric species could be related to their different modes of environmental colonization. In the case of B. pruvoti, the adaptive sexual condition could be gonochorism because, in conditions of high population density, it allows for reproductive success with low energy cost for the individual. On the contrary, although energetically more expensive for the individual, in conditions of low population density, the adaptive sexual condition could be hermaphroditism in B. europaea because it maximizes the rate of fertilization. 30 mm 15 mm 2 mm I g sti N IV sni yp cc sdi N ss sdi g pm o A 15 mm 80 mm D A szoi V m mf o III 15 mm g sdi m II D B 60 mm N sni 20 mm yp g g o m szoi E V N 25 mm n B V g 30 mm Adult specimens of Balanophyllia pruvoti photographed at 16 meters depth in Calafuria (Leghorn, NW Mediterranean) C 30 mm cc V Balanophyllia pruvoti oogenesis. (A) Early stage. Small oocyte located in the gastrodermis characterized by high nucleus:cytoplasm ratio. (B-C) Intermediate stages. (B) A vitellogenic oocyte located in the central portion of the mesentery. (C) Vitellogenic oocytes. The cytoplasm is still quite homogeneous. The spherical nucleus is located centrally and contains a single nucleolus. (D-E) Late stages. (D) The nucleus:cytoplasm ratio is low, the ooplasm is full off small yolk plates and the plasma membrane appears thickened. (E) Detail of the nucleus of a late-stage oocyte. The nucleus is now located in the outer portion of the oocyte and is oval-shaped. g: gastrodermis; m: mesoglea; mf: mesenteric filament; o: oocyte; N: nucleus; n: nucleolus; pm: plasma membrane; ss: skeletal septum; yp: yolk plates g sti E III sti sti g C Balanophyllia pruvoti. Stages of spermary maturation. (A) Stage I: the spermary originates from the migration of undifferentiated germ cells that migrate from the gastrodermis (arrow). (B) Stage II: the spermary is differentiated with a spherical shape. It is made up of a group of spermatogonia. (C) Stage III: the spermary is made up of a mass of spermatocytes. (D) Late stages. Stage IV: the spermary is made up of a layer of spermatocytes peripherally and of a group of spermatids centrally. Early stage V: the spermary is made up of a group of sperm centrally but still presents a layer of spermatids at the periphery. (E) Stage V: the spermary is made up of a mass of sperm. Sperm are released from the spermary in the coelenteron (arrow). cc: coelenteric cavity; g: gastrodermis; m: mesoglea; sdi: spermatids; sni: spermatogonia; sti: spermatocytes; szoi: sperm; I, II, III, IV, V: stage of spermary development. Oocytes Spermaries I II III IV V N = 91 July 30th, 2001 no available data N = 30 N = 1004 August 25th, 2001 N = 54 N = 266 September 22th, 2001 N = 164 October 25th, 2001 no available data N = 29 N = 458 January 3rd, 2002 N = 108 N = 33 January 19th, 2002 Diameter (mm) Balanophyllia pruvoti. Preliminary data of the size-frequency distributions of oocyte and of the 5 spermary maturations stages in monthly samples of females and males collected off the coast of Calafuria (Leghorn, Eastern Ligurian Sea) from July 2001 to January 2002. Values reported in the graphs indicate the total number of oocytes or spermaries measured per monthly sample. Acknowledgements: This work is supported by grants from Scuba Schools International Italy, Mobby’s, the Italian Ministry for University and Scientific and Technological Research, and sponsored by Underwater Life Project. We thank Dr Rosanna Falconi for her advice in laboratory techniques, the Bologna Scuba Team for its logistical and practical support for the dives, Gianni Neto for photographic material and the Marine Science Group for organizing and supervising the dives.