Reproductive Ecology of Diaseris Distorta (Michelin) (Fungiidae) in the Galápagos Islands, Ecuador S.B
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Proceedings 9th International Coral Reef Symposium, Bali, Indonesia 23-27 October 2000, Vol. 1 Reproductive ecology of Diaseris distorta (Michelin) (Fungiidae) in the Galápagos Islands, Ecuador S.B. Colley1, J.S. Feingold2, J. Peña1 and P.W. Glynn1 ABSTRACT The sexual reproduction of Diaseris distorta is described from a population at Devil’s Crown, Floreana Island, Galápagos Islands, Ecuador. Gametogenesis is classified in four developmental stages for each gender. Individuals are gonochoric and most likely broadcast spawners. Gametogenesis was active during the warm, wet season from at least February to June, peaking at the end of April. Mature female gametes were abundant around full moon with lesser numbers present around new moon. Synapticular spaces were replete with eggs or spermaries in all stages of development. It is likely that gametes develop continuously during the breeding season. Studies of fecundity of female individuals yielded estimates of 7,894-13,000 mature eggs cm-2 live surface tissue per cycle. Total egg volume was approximately 4.90-8.06 mm3 per cycle with 4-8 spawning cycles yr-1 (19.6-64.5 mm3 eggs yr-1). The sex ratio of the study population was highly skewed toward males, approximately 5:1. Individuals as small as approximately 240 mm2, or approximately 1.75 cm in diameter, were sexually active. Asexual fragmentation is important locally. The potential for sexual reproduction is high, but its effectiveness locally or in establishing new, more distantly located populations is unknown. Keywords Diaseris, Mushroom coral, Coral collections were made, at 13-15 m depth, from 1989 to reproduction, Eastern Pacific. 1998. Three to five polyps were histologically processed for each date. Sample size ranged from 5 to 59 individuals Introduction yr-1 (N=171). Fixation, decalcification, infiltration and Although there have been relatively few studies on embedding were performed according to techniques fungiid coral reproduction, some aspects of their sexual described in Glynn et al. (1994, 1996). Two pieces of and asexual reproductive biology are known (Abe 1937, each polyp were embedded for cross sectional and/or Krupp 1983, Yamashiro and Nishihira 1998). In Palau, longitudinal viewing. One slide of serial sections was Fungia actiniformis [formerly Heliofungia actiniformis] taken from each of three areas: oral, middle, and aboral. was reported to be a hermaphroditic brooder (Abe 1937), Sections were cut and stained following procedures however, Willis et al. (1985) observed this species to be a described in Glynn et al. (1994). gonochoric broadcaster on the Great Barrier Reef. In Development of ova and spermaries was divided into Hawaii, studies by Krupp (1983) demonstrated that 4 stages, which are described below. One section on each Fungia scutaria was a gonochoric broadcast spawner. slide was scored for each developmental stage and data Although some spawning behavior has been reported were plotted with Sigmaplot 5.0 as in Glynn et al. (1996). (Oliver 1985, Willis et al. 1985), patterns of gamete Chi-square analyses of seasonal and lunar trends were development are virtually unrecorded, with the exception performed according to Glynn et al. (1996). of Krupp (1983). Our work is the first to document Egg diameters (Stage IV eggs) were randomly gametogenic cycles in an eastern Pacific fungiid coral and selected (N=10) from one section per sample (N=10 its progression throughout the calendar year. samples). Eggs are located in mesenteries that are not The study population of Diaseris distorta is highly uniform in morphology, i.e. with respect to depth within sexually active, although it is unclear if it is benefiting the polyp. Histological preparations of longitudinal slices significantly from autochthonous sexual recruitment. of polyps showed meandering masses of eggs from which Understanding the sexual reproductive ecology of several finger-like projections extended. These were not Diaseris distorta is obscured by its high rate of consistent in shape or size and were located between specialized fragmentation (Yamashiro and Nishihira spaces created by the dissolution of the synapticulae 1998), which is dominant. Sexual reproduction is during processing. probably its predominant means of geographic dispersal, Due to variations in egg distribution, the number of although our results do not confirm that this seemingly mature eggs cm-2 surface area of polyp was estimated as isolated population at Floreana Island sexually recruits follows. The mean number of mature eggs was calculated successfully. from one delineated cm2 of tissue sliced between the oral and aboral surface for N=4 female samples (slides). Mean Methods numbers of mature eggs also were determined for areas of whole slices cut through both oral and aboral planes Individuals of Diaseris distorta were collected near (N=2). These one dimensional egg counts were adjusted Devil’s Crown (Corona del Diablo), off Floreana Island, volumetrically for a 2.8 mm (N=4) tissue depth. Two Galápagos Islands, Ecuador (Fig. 1). Thirty-two independent estimates resulted from this method (7,894 and 13,000 eggs cm-2 ). 1 Division of Marine Biology and Fisheries, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149 2 National Coral Reef Institute, Nova Southeastern University Oceanographic Center, 8000 North Ocean Drive, Dania Beach, Florida 33004. [email protected] 1 position of the nucleus, which had migrated peripherally, and now appeared flattened against the plasma membrane. Although all four stages of ova were present in most female samples, no spawned ovaries were identified. Fig. 1 Study site showing Devil’s Crown and location of coral community. Inset shows the Galápagos Islands and the relative location of Floreana Island. One size series collection was made during the peak of the yearly breeding season to determine the approximate size of individuals at gametogenesis. Twenty-four individuals were sampled, ranging from 150- 1980 mm2 in planar area. One sexual recruit in the size series data was identified by its completely circular shape and a lack of mature fragmentation grooves or slits. The septal array in this individual was spoke-like, whereas in regenerating polyps, regenerating segments develop Fig. 2 (A) Oocyte Stages I-IV within the mesenterial septae at abrupt angles to the original fragment. In gastrodermis of Diaseris distorta. (B) Male gonad daughter fragments, i.e. asexually generated individuals including spermary Stages II-IV and two spawned (Yamashiro et al. 1998), the initial orientation often spermaries. approximates a teardrop, with irregular tissue portions attached. Spermaries Results Aproximately seven light gray or lavender cells within Gonadal development and gametocyte classification the mesenterial mesoglea comprised Stage I spermaries. Ova They formed a cluster 15-22 μm in diameter. Stage II Most of the eastern Pacific scleractinian coral species spermaries ranged from 24-40 μm in diameter (Fig. 2B). studied in our laboratory display distinct color changes during Cells were generally lightly stained. In rare cases, these egg development. We did not observe this in Diaseris distorta, cells were well defined. Stage III spermaries were 40-82 nor were texture changes obvious. Therefore, egg stages were μm and were darker pink or more purple. Often they generally distinguished by size and nuclear migration (Fig. 2A). contained a lumen and were more organized than at Stage Stage I ova were oval, 12-20 μm in diameter and located in the II. Stage IV spermaries were usually larger than 84 μm. A mesenterial gastrodermis, often resting near the mesoglea. reduction in cell size denoted the beginning of this stage, Usually a thin layer of yellow cytoplasm enveloped a light when primary spermatocytes divided into brownish beige nucleus and dark pink nucleolus. Stage II ova had secondary spermatocytes, starting from the interior and migrated into the mesoglea and ranged from 22-62 μm, progressing peripherally. A second division into smaller but usually were not greater than 53 μm. The cytoplasm purple spermatids also took place. Mature Stage IV was yellow and occasionally light pink. The nucleus was spermaries were shaped as funnels or teardrops and now white and granular with a dark yellow nucleolus. stained from light purple to dark magenta with golden- Stage III ova increased in size and changed slightly in tailed spermatozoa bunched to one side. Since only four color. At 64-85 μm, they were usually darker and were samples were found with spawned spermaries, this often blotchy or mottled. Stage IV ova were 88-140 μm. category was also included in Stage IV scoring. They Although the cytoplasm granularity increased, no color were deflated with a few spermatozoa within the change was noted. They were distinguished by the shrivelled gastrodermis. 2 Reproductive Condition both sexes. Males were highly productive throughout the breeding season, while females were most plentiful and Gamete development was gonochoric, no most often mature during the latter half of April (Fig. 3A hermaphrodites were found. Approximately half (56%) of and B). Spawned spermaries were found in collections the 171 polyps examined harbored gametes, however, made on 8 May 1989, 27 April 1991, 18 March 1992 and males dominated the population 5.4:1 (81 males and 15 25 February 1994. females). Chi-square analysis revealed a significant difference from the expected ratio of 1:1 (p<0.001). All Lunar Periodicity four stages of gametocyte development were usually present in male or female polyps and were intermingled Although the fourth quarter of the lunar cycle (days along each reproductive mesentery. Gametes were 22-28, new moon=day 0) was more sparsely sampled, concentrated in the mid-region of the polyp tissue. male polyps harbored gametes throughout the month. Scleroseptae are bridged by numerous calcareous Gamete development in female individuals was found synapticulae. Consequently, mesenterial tissues must fit only in samples taken around new and full moon phases into these partitioned areas and around the skeletal (Fig. 3C).