University of Groningen. Butterflyfishes of the Southern Red Sea Zekeria, Zekeria Abdulkerim

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1 University of Groningen Butterflyfishes of the Southern Red Sea Zekeria, Zekeria Abdulkerim IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2003 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Zekeria, Z. A. (2003). Butterflyfishes of the Southern Red Sea: Ecology and population dynamics Groningen: s.n. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date:

2 Chapter 6 Butterflyfishes of the Southern Red Sea 53 Spawning seasonality in the Brownface Butterflyfish (Chaetodon larvatus) Z. A. Zekeria and J. J. Videler

3 54 Chapter 6. Spawning Seasonality Abstract The spawning behaviour of C. larvatus was investigated in the southern Red Sea. Three methods were employed to study seasonal patterns of spawning: 1. A monthly change in gonadosomatic index was monitored for two years. 2. Changes in histological development of gonads were monitored by analysing monthly samples of gonads. 3. Field observations of spawning behaviour were made and spawning seasonality monitored. Results from the three studies indicate that May and June were the major spawning months. Gonad size was highest during these months; most of the gonads attained maturity when investigated histologically, and highest courting frequency was recorded during these months. The results agree with the recruitment period, which was found to be in June and July.

4 Butterflyfishes of the Southern Red Sea 55 Introduction Reproduction in coral reef fishes has been extensively studied in Pomacentrids (Sale 1980). Five methods have been employed to detect the spawning periods: field observations of spawning events, visual evaluation of maturity of gonads using macroscopic characteristics, the use of gonad indices, the examination of frequency distribution of oocyte diameters, and the observation and analysis of histological sections of gonads. These methods differ in their accuracy and each has its own advantages and disadvantages. Some of these methods have been employed to study spawning seasonality of butterflyfishes. For example, Lobel (1989a) and Yabuta (1997) monitored the spawning behaviour of butterflyfishes in the field. Seasonal change in the gonadosomatic index was investigated by Ralston (1981) and Lobel (1989b). Tricas and Hiramoto (1989) analysed the histology of gonad development in butterflyfishes. Histological investigation provides better information on the spawning seasonality of tropical reef species, which have a protracted spawning season (Tricas & Hiramoto, 1989). Butterflyfishes are among the best-studied families of coral reef fishes (Motta 1989). However, information on reproduction habits exists only for a few species (Ralston 1981, Lobel 1989a, 1989b, Tricas & Hiramoto 1989, Yabuta 1997, Yabuta & Kawashima 1997). Most of these studies were conducted in the tropical east Pacific or in Japan. There are only two reports on the spawning of butterflyfishes from the Red Sea. Spawning in Chaetodon paucifasciatus was reported by Fricke (1986) while Ghraibeh and Hulings (1990) investigated reproduction seasons of three chaetodontids. Both studies were conducted in the Northern Red Sea. There is no information in the literature on the reproduction of chaetodontids in the southern Red Sea. Chaetodon larvatus, the most abundant chaetodontid in the southern Red Sea, is endemic to the Red Sea and the Gulf of Aden (see chapters 2 & 3). The adults are corallivores and live in heterosexual pairs, defending relatively small territories (see chapter 5). Growth and recruitment behaviour of C. larvatus has been investigated (see chapter 7 & 8). The objective of this study is to investigate the reproduction pattern of C. larvatus in the southern Red Sea. Three approaches have been employed to determine the spawning season of this species. First, spawning behaviour was monitored by following focal fishes in the field. Second, fish samples were periodically collected and seasonal changes in the size of gonads monitored. Third, changes in oocyte development were investigated using histological techniques.

5 56 Materials and Methods Chapter 6. Spawning Seasonality Study site Field observations were conducted on Resimedri reef, near Massawa harbor, in the southern Red Sea (Figure 6.1). This fringing reef is reaches a maximum depth of about 10m and it is exposed to moderate waves. Fish specimens were collected from a reef located east of Sheik Said Island; about 1.5 km south of Resimedri reef. Figure 6.1. Study site. FCS = fish collection site; MP = Massawa proper; OS = observation site; RR = Resimedri reef; SSI = Sheik Said (Green) Island; TW= Twalot. Field Observation Spawning in C. larvatus was monitored by snorkeling on the study site in the evening hours. Activity of the fishes and their social behaviour was recorded. Field observations started two hours before sunset and continued until the fishes retired to coral crevices for the night rest, a few minutes after dusk. The observations took place twice a month from January to December Studies on recruitment suggested April and May as spawning period for C. larvatus. Hence, during these months the frequency of observation was increased to twice a week.

6 Butterflyfishes of the Southern Red Sea 57 Gonadosomatic Index Monthly collections of about thirty fishes were made from August 1998 to August Fish collection took place between 9:00 and 13:00 using a barrier net and handnets. Immediately after capture the fish samples were transported on ice to the laboratory where the total and standard lengths (to the nearest mm) and the body mass (to the nearest g) were measured. The fish were then preserved in a deep freezer until dissection. After a couple of days in the freezer, the fishes were dissected; the gonads removed; and the mass and colour of the gonads recorded. Five gonads from each month were selected for further histological analysis. The gonadosomatic index was calculated as the ratio of gonad mass to the body mass expressed as expressed in a percentage. Histological analysis of gonads The gonad samples were fixed in Bouin s solution for at least 24 hours. Gonad tissue was dehydrated and cleared in ethanol and xylene respectively. The tissues were then embedded in paraffin and 7 µm-thick sections cut on a microtome. The sections were mounted on microscope slides and stained using hematoxylin / eosin following the procedure given in Preece (1965). Oocytes were examined and their developmental stages determined using a light microscope. At least 100 oocytes were randomly selected from each oocyte stage. The area of each oocyte was measured using an image analyser and the diameter of the oocytes was calculated from the area assuming a spherical shape of the oocytes. Results Sex ratio and size of maturation A total of 619 fish specimens were collected of which 598 were sexed. The sex ratio for these specimens was 48 males to 52 females. The size of most of the specimens was more than 80mm (Table 6.1). The smallest mature male and the smallest mature female recorded from the sample were 58 mm and 68 mm respectively. Fifty-percent maturity was attained at 66 mm in females and 71 mm in males. The slightly higher abundance of females and their earlier maturity could be due to the fact that females can be distinguished at a smaller size than males. Unlike testis, ovaries are yellowish in colour and lack fatty tissue. Hence, the female gonads can easily be distinguished at a smaller size. Gonad Histology The ovaries in C. larvatus are heavy, rounded, bilobed and yellowish in colour while testes are elongated, slender and white. In gravid females, the ovaries occupy about half of the visceral cavity. Fatty tissues, which surround the tests and other internal organs

7 58 Chapter 6. Spawning Seasonality of ripe males, are not present in females. Since ovaries are larger and easier to stage compared to testes only ovaries were used for histological analysis. A. Oocyte development Oocytes of C. larvatus were classified into five stages following the description given by West (1990). These stages are chromatin nucleolar stage, perinucleolar stage, yolk-vesicle formation, vitellogenic stage, and ripe stage. Table 6.1. Sex ratio by size class expressed in mm total length (TL) for Chaetodon larvatus Size class n Percentage mm (TL) Females Males undifferentiated < Chromatin nucleolar (oogonia) stage At this stage, the ovary consists of numerous ovigerous folds extending from the ovarian wall towards the centre of the ovary. Nests of oogonia arise within the ovarian luminal epithelium during the primary growth. The chromatin nucleolar stage is characterised by small oogonia (7µm) and by the presence of only one nucleolus. Ovaries collected in January and February had oogonia, which were not well preserved during the histological preparations. 2. Perinucleolar stage The oocytes are characterised by their small size (±15 µm) and by the presence of a relatively large central nucleus that occupies more than half of the cell volume (Fig. 6.2A). Concomitant with oocyte growth, the nucleus (germinal vesicle) increases in size.

8 Butterflyfishes of the Southern Red Sea 59 At a later stage of development the nucleus contains many nucleoli around its periphery. Perinucleolar oocytes were present throughout the year in adult females but were dominant in samples collected in January, February and March. A B Y P C D T Z G Figure Stages of oocyte development in Chaetodon larvatus. A-perinucleolar oocytes (p); B-yolk vesicle formation stage (y); C-vitellogenesis stage; D-magnified vitellogenesis stage showing the three membranes (Z = zona radiata, T = Theca, G = Granulosa) 3. Yolk vesicle (cortical alveoli) formation This stage is characterised by the appearance of yolk vesicles in the cytoplasm. The vesicles first appear as vacuoles but later increase in size and number to form several peripheral rows and give rise to cortical alveoli, which surround the central nucleus (Fig 6.2B). A non-cellular membrane, the zona radiata, begins to form between the follicular layer (theca and granulosa) and the developing oocyte. The size of the yolk vesicle stage oocyte is still small (±81 µm). Cortical alviolar stage oocytes were observed in samples collected on February, March and August. 4. Vitellogenesis stage. The vitellogenic (yolk) stage oocyte is characterized by the appearance of yolk vesicles in the cytoplasm. During early vitellogenesis small yolk granules appear in the periphery of the oocyte. Later in the development the yolk, granules migrate towards the center and completely fill the cytoplasm (Fig. 6.2C). The mean size of vitellogenic oocytes is ±185 µm and the size ranges from 97 to 618 µm. As the cell enlarges, the

9 60 Chapter 6. Spawning Seasonality vitelline membrane thickens and develops radial striation to form the zona radiata (Fig. 6.2d). Vitellogenic stage oocytes were found throughout the year, except in samples from January and February. 5. Ripe (mature) stage This is the final stage of oogenesis where oocyte development leads to the release of mature oocytes into the ovary lumen. In many marine teleostes, there is concomitant rapid increase in size due to hydration of the oocytes. The final stage of oocyte maturation is difficult to follow because of shrinkage and distortion of the cells during histological processing. In addition, ovulated oocytes may be lost from the ovarian lumen during the initial tissue processing (West 1990). Tricas and Hiramoto (1989) found late phases of hydrated oocytes only in females collected a few hours before sunset. They argue that oocyte hydration starts a few hours before spawning. None of the ovaries examined during the present study were hydrated and no spent ovaries were observed. B. Gonadosomatic Index. Monthly changes in gonadosomatic index (GSI) for male and female C. larvatus are depicted in figure 6.3. The GSI data for the two sexes show significant variation among months (t-test, P<0.05). Relatively higher GSI values were recorded from March to July and lower values were recorded for January and February. The values remained relatively low during summer and autumn. For example, in 1999/2000 mean GSI for females was 0.8 in January and peaked in March to a mean value of 3.8 (Fig. 6.3B). This was followed by a steady decline from April to August. From September to December the GSI values remained low at 1.9. The trend is apparently similar in the two years studied (Figure 6.3A and 6.3B). The only notable difference between the two years is the very low GSI for the sample collected in April This sample was taken from fishes found aggregated in one spot. Fishes in this sample had small ovaries with an average mass of 0.46 g. The development of testis also showed a similar trend as that of the ovaries. Significant correlation was recorded between males and females in the monthly GSI patterns (r=0.78, p>0.05). However, the GSI curves for males are less clear (Figure 6.3C and 6.3D). For most of the samples, large variation was recorded in the size of testes collected in the same month. The large variation could be due to the error in weighing the testis. Testis is normally found embedded in fatty tissue, which is difficult to clear. As a result testis mass includes the mass of adhered fatty tissue which has a profound effect on the in calculation of the gonadosomatic index.

10 Butterflyfishes of the Southern Red Sea A (Gonad wt / somatic wt) X B C D Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Months Figure 6.2. Monthly gonadosomatic indexes (GSI). a) Females 1998/99 b) Females 1999/2000 c) Males 1998/99 d) Males 1999/2000

11 62 Chapter 6. Spawning Seasonality Histological analysis of gonads also shows seasonal patterns. In January and February, all ovaries were dominated by oocytes in early developmental stages (chromatin nucleolar and perinucleolar stage oocytes). No advanced oocytes (vitellogenic and mature stages) were observed during the two months. On the other hand, from April to December advanced stage ovaries dominated the gonads. During these months, a few gonads with cortical alveoli were recorded and most of the gonads had vitellogenic or mature oocytes. Perinucleolar oocytes where observed throughout the year. The results from the histological analysis show that the fishes do not spawn during the winter when the ovaries are dominated by oocytes in their early development stages. Fast changes in oocyte development occur in February and March. The changes in oocyte development may indicate the approach of the spawning season. C. Field observations. C. larvatus live in heterosexual pairs and defend small territories (see chapter 4). With the exception of occasional antagonistic interactions related to territorial defence, C. larvatus rarely show social interactions with neighbouring conspecifics. The only time neighbours showed positive interactions was when they formed aggregations during evening hours. These aggregations took place mainly in the evening hours of April and May. Each aggregation lasted for only a few minutes and was repeated many times per evening. During aggregation, groups of up to 16 individuals gather from neighbouring territories and swim together for about five minutes a few cm from the bottom. During this time, territorial borders are abolished and the neighbours freely mingle with one another. After swimming for a while the group separates into many pairs. The pairing lasts for about 30 seconds during which the fishes exhibit brief 'courting-like' behaviour. This behaviour takes place among pairs that may or may not be partners from the same territory. One individual of the courting pair (assumed to be a male) courts the other member of the pair (assumed to be a female) by swimming behind her placing his snout against her abdomen. After following the female for a few seconds the male dashes forward and abruptly brakes in front of the female, blocks her way, and waves his caudal fin. The courting lasts for about 30 sec and ends with the female chasing the male. At the end of the courting, the whole group breaks up and the fishes disperse pair-wise to their respective territories. Despite observations of courting behaviour on many occasions, no spawning in C. larvatus was observed. Discussion Spawning in butterflyfishes was observed in the western Atlantic (Colin 1989), in Hawaii (Lobel 1989a & b), in the Red Sea (Fricke 1986) and in Japan (Yabuta 1997,

12 Butterflyfishes of the Southern Red Sea 63 Yabuta & Kawashima 1997b). These studies point out that spawning takes place at dusk following a short period of courting. A male in a heterosexual pair courts the female by swimming closely behind her and by placing his snout close to her abdomen. After a brief courting display, the pair ascends from the bottom and they simultaneously release their gametes on the water column (Colin 1989, Lobel 1989, Yabuta 1997, Yabuta & Kawashima 1997). In some case, non-paired males intrude into a spawning pair and release sperm (Lobel 1989). In most of the observed cases courting and spawning took place between heterosexual pairs, which defend permanent territories. However, Yabuta and Kawashima (1997) reported spawning between one male and many female C. trifascialis. This species lives solitary and individuals defend territories against others of the same sex. Each male territory covers 2 3 female territories and mating occurs between a male and the females living in his territory. Courting and spawning usually takes place within the territories of paired butterflyfishes. In C. trifasciatus, however, pairs were observed to migrate to a temporary spawning territory located a few hundred meters from their feeding territories (Yabuta 1997). In the present study, despite substantial effort to investigate the spawning pattern of C. larvatus no actual spawning was observed. Several pairs displayed courting-like behaviour. During this time, territory borders were crossed but no agonistic interactions were observed. After brief courting, all fishes returned to their respective territories. It is not clear whether the observed aggregation and subsequent courting is a prologue to spawning or if it is a completely different ritual unrelated to reproduction. We are sure that spawning did not take place during our observations. The fishes were followed until after sunset when they retired to their night shelters. Further investigation is required to find out if spawning takes place during other times of the day. The gonadosomatic indices and histological analysis of gonads show that C. larvatus spawns seasonally from April to June. Studies of recruitment patterns of the fish species in the study area revealed that new recruits settle on the reef mainly during June and July (see chapter 7). The pelagic larvae spend about 20 days in the water column before settling on the reef as recruit (Zekeria unpublished data). The timing of spawning and the duration of the larval phase suggest May and June as spawning months, which coincides with the results obtained in the present study. Seasonal reproduction of butterflyfishes was observed in the Western Atlantic (Colin 1989), in the Pacific (Ralston 1981, Walsh 1987, Lobel 1989a & 1989b, Tricas & Hiramoto 1989), in eastern Pacific (Yabuta 1997, Yabuta & Kawashima 1997) and in the Red Sea (Fricke 1986, Gharaibeh & Hulings 1990). In the western Atlantic and the pacific spawning took place in winter and in spring whereas in the Red Sea spawning occurred during summer and autumn (June to December).

13 64 Chapter 6. Spawning Seasonality The seasonal pattern of spawning in coral reef fishes was found to follow annual variations in water temperature and day length (Walsh 1987). Colin (1986) correlated spawning of five Chaetodontids in the western Atlantic with the water temperature. He pointed out that winter, when water temperatures range between 25 C and 28 C, would be the most appropriate season for butterflyfish spawning and ruled out the possibility of spawning when water temperatures exceed 26 C. However, the present study showed that C. larvatus spawn from April to June with water temperatures ranging from 28 C to 33 C. Yabuta (1997) observed C. trifasciatus spawning between 25.6 C and 30.9 C. The observed spawning of C. larvatus at higher temperatures could be due to adaptation to the local conditions. The monthly mean water temperatures in the study area range from 27.7 C in January to 33.6 C in September. Juvenile C. larvatus grow at a fast rate for the first six months of their life (see chapter 8). The fast growth of the young fishes enables the juveniles to quickly pass the risk of a young age when they are vulnerable to heavy predation (Ralston 1981). Growth of young fishes is enhanced at higher temperatures (Sogard & Olla 2001). The delayed spawning of C. larvatus until late spring could be an adaptive strategy for juveniles to be recruited in early summer when the temperature is higher. Seasonality in ocean currents and food availability could also play important role in affecting the seasonality of spawning. However, little work has been done on the oceanographic and planktonic systems of the southern Red Sea. Hence, it is not possible to relate the spawning season with patterns in water movement and food availability.