MARINE ECOLOGY PROGRESS SERIES Vol. 1, 133-144, 1979 - Published September 30 Mar. Ecol. Prog. Ser. The Reproduction of the Red Sea Coral Stylophora pistillata. I. Gonads and Planulae B. Rinkevich and Y.Loya Department of Zoology. The George S. Wise Center for Life Sciences, Tel Aviv University. Tel Aviv. Israel ABSTRACT: The reproduction of Stylophora pistillata, one of the most abundant coral species in the Gulf of Eilat, Red Sea, was studied over more than two years. Gonads were regularly examined using histological sections and the planula-larvae were collected in situ with plankton nets. S. pistillata is an hermaphroditic species. Ovaries and testes are situated in the same polyp, scattered between and beneath the septa and attached to them by stalks. Egg development starts in July preceding the spermaria, which start to develop only in October. A description is given on the male and female gonads, their structure and developmental processes. During oogenesis most of the oocytes are absorbed and usually only one oocyte remains in each gonad. S. pistillata broods its eggs to the planula stage. Planulae are shed after sunset and during the night. After spawning, the planula swims actively and changes its shape frequently. A mature planula larva of S. pistillata has 6 pairs of complete mesenteries (Halcampoides stage). However, a wide variability in developmental stages exists in newly shed planulae. The oral pole of the planula shows green fluorescence. Unique organs ('filaments' and 'nodules') are found on the surface of the planula; these might serve as attachment organs. A hypothesis is offered for further examination suggesting a trend of brooding versus non-brooding species in scleractinian corals: Coral species which develop gonads in their body cavities (usually branching forms or small polyped corals) reduce the number of eggs during oogenesis, have small eggs and brood planula-larvae, while coral species which develop gonads within their mesenteries (usually massive growth forms or large polyped corals) have numerous and large ova per polyp and expel their eggs into the water. INTRODUCTION the lagoon, rear-reef and reef-flats, and abundant in the fore-reef (Loya and Slobodkin, 1971; Loya, 1932). There are many gaps in our knowledge of the bio- This species has a branching growth form. Each branch logy of scleractinian corals, especially in regard to consists of numerous minute polyps, about 0.8 mm in their sexual reproduction. A significant portion of the diameter. Colours of living colonies range from dark- research on corals deals with their community struc- brown, purple, brownish-yellow to pale-pink. This ture, growth rates, calcification processes, nutrition species is characterised by a rapid growth rate (Loya, and their role in reef formation. Most of the studies on 1976a),high recruitment (Loya 1976b) and rapid rege- coral reproduction were performed in the beginning of neration rate (Loya , 1976~).S. pistillata forms a sub- this century and during the Great Barrier Reef Expedi- strate and habitat for many species of crabs (mainly tion (1928-1929). Recently, Stimson (1976, 1978) repor- Xanthidae and Alpheidae), fishes (especially Poma- ted on the mode and timing of reproduction in some centridae and Gobiidae), as well as a rich fauna of common hermatypic corals of Hawaii and Enewetak. cryptic organisms such as sponges, bivalves, polychae- Approximately 100 scleractinian coral species are tes and others. The knowledge of the life history of S. known from the Gulf of Eilat, Red Sea (Loya and pistillata may thus enhance a better understanding of Slobodkin, 1971). Very little is known about their bio- the complex, interspecific relationships of the animal logy and so far almost nothing about their reproduc- community structure accompanying this species. tion. Detailed knowledge of coral reproduction may Marshal1 and Stephenson (1933) claimed that the elucidate some aspects of their life history strategies problem of studying reproduction in corals can be (Loya, 1976a), possible perturbations caused by man approached from two points of view: Since some of the (Rinkevich and Loya, 1977), and a better understand- corals are viviparous one may attempt to discover their ing of the coral reef as a whole. breeding season by finding out at what time they Stylophora pistillata (Esper) is one of the most abun- liberate planulae. On the other hand, one may regu- dant coral species in the Gulf of Eilat. It is dominant in larly examine their gonads in order to determine when O by Inter-Research Mar Ecol. Prog. Ser., 1, 133-144, 1979 they are ripe. Integration of both approaches seems to acid in distilled water (1:l) and the other sodium be most informative. In the present study we chose to citrate in distilled water (20 g 100 cm-3).Before decal- use both histological work combined with field obser- cification the two solutions were mixed (1:l) vations in order to study the different stages in the and poured into vials containing the samples. After de- development of gonads and planulae in Stylophora calcificationwas completed the tissue was rinsed in tap pistilla ta. water and transferred into 70 O/O methyl-alcohol. This method was preferred to the previous one since it was faster, easier to handle and, therefore, it is recommen- MATERIALS AND METHODS ded for coral histological studies. A piece of tissue was taken from each branch and serial histological sections The study area is located five km south of the Coral were prepared, 10 pm thick, stained with Delafield Nature Reserve of Eilat, Red Sea. In this area the hematoxyline and eosin. lagoon is about 20 m wide, dominated by Stylophora Planulae were collected in situ using plankton nets pistillata colonies. The reef-flat and fore-reef zones are (125 jm mesh). Stylophora pistillata colonies in various highly diverse with coral species. The study location size groups were covered by the nets in the late after- was visited approximately every month from March noon and were removed at night (9 to 12 p. m.). This 1974 to September 1975. form of sampling took place every month for a period of Every month 6 to 14 big colonies of Stylophorapistil- approximately two years. After removal, the nets were lata (approximately 10 cm in their larger diameter) tied underwater and transferred to the laboratory in were randomly sampled in order to study the rate of buckets filled with sea water. The captured planulae development of their genital cells. During the study were carefully collected, counted and observed under approximately 350 colonies were sampled and histolo- a disective microscope. Some of the planulae were gically examined. One branch was broken from each fixed in a solution of 4 formalin and 6 jm histologi- colony by a diving knife and put into a marked plastic cal sections were prepared. bag. Since there is synchronization in breeding be- tween different branches within the same colony (Rn- kevich and Loya, 1979), a sampled branch represents RESULTS the rate of reproduction of the whole colony. The height (H), width (W), and length (L) of each colony From the histological study it is concluded that Sty- were measured underwater by a plastic ruler. Since the lophora pistillata is hermaphroditic. Ova and testes are shape of S. pistillata approximates a sphere, the mean situated in the same polyp (Fig. 1: 3, 4) scattered be- geometric radius (4 of each colony was calculated tween and beneath the septa. Only few colonies within using the expression F= (L. W. H.) "3/2 (see Loya, the population, especially young specimens (i less 1976a). In addition, four big colonies were chosen in than 3.5 cm), contain spermaria only (Rinkevich and the beginning of this study, in order to follow the Loya, 1979). However no colonies were found to con- development of gonads within the same colony tain only female gonads. Ova and spermaria develop throughout the year. A small branch was broken from from the mesenterial epithelium, extending into the each of these colonies every month for histological body cavity. The gastric cavities of polyps, which were sectioning. not in reproductive state were found to be empty (Fig. 1: 1). During the breeding season polyps contain The coral samples were fixed in 4 "/" formalin solu- a high density of genital cells, from their bases (Fig. 1: tion in sea water for 24 h, rinsed '/z h under tap water and transferred to 70 O/Omethyl-alcohol for further pro- 3) to their upper parts, such as the pharynx (Fig. 1: 2) cedures. The preservation proved to be excellent for and tentacles (Fig. 2: 4). The gonads are attached to the the purpose of the histological work. In the laboratory, mesenteries by stalks which are built of mesoglea and decalcification was carried out in two different proce- endoderm (Fig. 1: 4). Most of the individual polyps in a dures: (1) Nitric acid treatment: Each branch was breeding colony contain several ova and spermaria transferred into a special glass vial, containing 50 to (usually not more than 5 male and 5 female gonads per 100 cm3 of 20 O/O methyl-alcohol and then, nitric acid polyp). Egg development starts in July preceding the was slowly added until decalcification was completed. spermaria which start to develop only in October. The remaining tissue was rinsed for a few min under tap water and transferred into 70 '/0 alcohol. This Male Gonads method proved to be inefficient for hstological study, since quite often the nitric acid damaged the structure When colonies are in extensive reproduction, sper- of the coral cells. (2) Formic acid and sodium citrate maria fill up all available spaces within the polyps treatment: Two solutions were prepared, one of formic (Fig. 1: 2). Testes appear at the basis of the polyp, in its Rinkevich and Loya: Gonads and planulae of Stylophora pistillata 135 gastric cavity, and penetrate up into the middle of the mesentery into the cavity.