Introduction

Snappers (Lutjanidae) are one of the important fisheries resources in tropical and subtropical waters such as the Caribbean and Indo-Pacific waters (Allen 1985). Several previous studies have clarified their spawning, settlement, feeding habits, age and growth (e.g., Newman et al. 2000a, b; Denit and Sponaugle 2004; Kritzer 2004; Heyman et al. 2005; Shimose and Tachihara 2005; Amezcua et al. 2006; Nanami and Yamada 2009). The spawning season in lutjanid species has been reported in several study sites (Grimes 1987; Kaunda-Arara and Ntiba 1997; Shimose and Tachihara 2005; Fry et al. 2009). Another aspect of fish reproduction is their lunar-synchronized gonadal development, which is common in coral reef fishes (Johannes 1978). Lunar-synchronized spawning has been reported for sciaenids (Aalbers 2008), serranids (Lee et al. 2002), siganids (Hoque et al. 1999; Harahap et al. 2001) and sparids (Saavedra and Pousao-Ferreira 2006). For lutjanid species, Heyman et al. (2005) demonstrated a lunar-synchronized spawning aggregation for Lutjanus cyanopterus on the Belize Barrier Reef. However, no studies have demonstrated lunar-synchronized reproductive activities for lutjanid species in Okinawan coral reefs.

The checkered snapper, Lutjanus decussatus, is one of the lutjanid species that is widely distributed in the western Pacific and eastern Indian Ocean (Allen 1985). The species is an important fisheries target in the Okinawan region (Masuda et al. 1984). Although some previous studies clarified their home range size and feeding behavior (Nanami and Yamada 2008a, b), little is known regarding the reproduction. The purpose of this study is to describe the seasonal and lunar-related changes in oocyte development for L. decussatus off Ishigaki Island, Okinawa.

Materials and methods

Biological data were collected from specimens purchased from commercial catches made off the coast of the Ishigaki Island, Okinawa, Japan (24°27′N, 124°13′E) between April 2007 and April 2008. Samples were obtained every week in order to be consistent with lunar cycles (new moon, first quarter moon, full moon and last quarter moon) except in bad weather conditions. A total of 172 female specimens were measured for fork length (FL, nearest 0.05 cm), whole body weight (g) and gonad (ovary) weight (nearest 0.01 g). Ovaries were removed from the specimens and preserved in 20% buffered formalin.

The gonadosomatic index (GSI) for females was calculated using the formula: gonad weight (g)/[whole weight (g) − gonad weight (g)] × 100. In order to clarify the temporal changes of oocyte development, small pieces of the ovaries were placed in 20% buffered formalin for 48 h and then kept in 70% ethanol. After dehydration using a series of ethanol, they were embedded in paraffin (m.p. 56–58°C, Histologie; Merck, Darmstadt, Germany). Embedded tissues were serially sectioned at 5–10 μm and stained with Mayer’s hematoxylin-eosin for microscopic observations. Oocyte development was classified as peri-nucleolus stage, oil-droplet stage, primary yolk stage, secondary yolk stage, tertiary yolk stage and maturation stage in accordance with Harahap et al. (2001) and Shimose and Tachihara (2006). Postovulatory follicles were also recorded. For analysis of oocyte composition, the most developed stage in the oocytes was regarded as the representation of the developmental stage of the ovary.

The spawning frequency was estimated in accordance with Ashida et al. (2008). In this estimation, the samples were obtained at the last quarter moon in June and August since the samples could be regarded as matured individuals with postovulatory follicles (see “Results”). The fecundity was estimated by using ten females with matured oocytes (see “Results”). For each ovary, five small pieces of the ovary (about 30 mg) were extracted. The oocytes of ≥0.4 mm in diameter were regarded as matured oocytes and counted under a Nikon profile projector at 10× magnification. Then, the fecundity was estimated in accordance with Murua et al. (2003).

Results

Figure 1a indicates the monthly changes in female GSI. The GSI was generally higher between April and October (GSI > 1.0) and lower between November and March (GSI < 1.0). Monthly changes in oocyte development and histological sections of ovaries are shown in Figs. 1b and 2, respectively. The tertiary yolk stage was observed during April and October. Since oocytes at the maturation stage were observed in June, September and October, and the postovulatory follicles were observed between June and August (Fig. 1b), the main spawning season was estimated to be between June and October. In contrast, only the peri-nucleolus stage and the oil-droplet stage were found between November and March.

Fig. 1
figure 1

Monthly changes in gonadosomatic index (GSI) ± SE (a) and oocyte composition (b) of female Lutjanus decussatus. Numbers above plots represent sample sizes. Since biological data were collected every week, monthly data were obtained from average values of weekly data. PNS peri-nucleolus stage, ODS oil-droplet stage, PYS primary yolk stage, SYS secondary yolk stage, TYS tertiary yolk stage, MA maturation stage. Numbers in parentheses indicate numbers of ovaries with the postovulatory follicles

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Fig. 2
figure 2

Cross-section of ovaries in Lutjanus decussatus. Three stages in ovaries (peri-nucleolus stage, oil-droplet stage and primary yolk stage) (a); two stages in ovaries (secondary yolk stage and tertiary yolk stage) (b); maturation stage (c); postovulatory follicles (d). PNS peri-nucleolus stage, ODS oil-droplet stage, PYS primary yolk stage, SYS secondary yolk stage, TYS tertiary yolk stage, MA maturation stage, POF postovulatory follicles. Bar 100 μm

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Clear lunar-synchronized GSI fluctuations of females were found during the main spawning season (June–October) (Fig. 3a), showing the highest GSI values around the last quarter moon of each month between June and September. In contrast, the highest GSI value was found around the new moon in October (Fig. 3a). Observations of oocyte development also support that the lunar-synchronized GSI fluctuation is related with spawning (Fig. 3b), since oocytes at the maturation stage were observed at the highest GSI values in June, September and October (Fig. 3b). Although oocytes at the maturation stage were not observed at the highest GSI values in July and August, the postovulatory follicles were observed in these 2 months (Fig. 3b).

Fig. 3
figure 3

Weekly changes in gonadosomatic index (GSI) ± SE (a) and oocyte composition (b) of female Lutjanus decussatus. Numbers above plots represent sample sizes. PNS peri-nucleolus stage, ODS oil-droplet stage, PYS primary yolk stage, SYS secondary yolk stage, TYS tertiary yolk stage, MA maturation stage. In a, lunar phase (last quarter moon) is indicated. Dotted line represents no samples as the typhoon approached Ishigaki Island. Numbers in parentheses indicate numbers of ovaries with the postovulatory follicles. In b, “P” indicates the existence of the postovulatory follicles

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Spawning frequency ranged from 0.14 (August) to 0.22 (June). Fecundity ranged from 105,169 to 541,159 oocytes (average = 301,736 oocytes ± 42,234 SE, n = 10) in fish of 25.5 to 29.4 cm FL.

Discussion

It is suggested that the main spawning season of Lutjanus decussatus is between June and October (during 5 months). Several previous studies have demonstrated that gonadal development and/or spawning for lutjanid species occurs over a period of several months (Heyman et al. 2005; Shimose and Tachihara 2005). Grimes (1987) showed that the seasonality of reproduction for lutjanid species has two patterns: (1) a restricted season centered around summer and (2) more or less continuous year-round spawning with peaks of reproductive activity in the spring and fall. The results of the present study are consistent with the former case.

Lunar-synchronized spawning of L. decussatus was also suggested since the GSI values of females were highest around the last quarter moon between June and September. In contrast, the spawning of four lutjanid species (Lutjanus argentimaculatus, Lutjanus cyanopterus, Lutjanus fulvus and Lutjanus griseus) occurred around the full moon in some tropical regions (Johannes 1978; Heyman et al. 2005), suggesting that the type of lunar periodicity of spawning is species-specific for lutjanid species. Some other coral reef fish species, such as siganids and serranids, also show lunar-synchronized spawning (Johannes 1978; Hoque et al. 1999; Harahap et al. 2001; Lee et al. 2002; Park et al. 2006). Johannes (1978) showed that the majority of tropical fishes that have lunar spawning rhythms spawn on or around the new or full moon. Grimes (1987) also showed that spawning of lutjanid species occurs around the new or full moon. However, some coral reef fishes have spawning rhythms around the first quarter moon (Hara et al. 1986) or last quarter moon (Johannes 1978). Although it is unclear what types of environmental cues act as stimuli for L. decussatus, water temperature, photoperiod and lunar cycle are reported to be cues of ovarian development for lutjanids (Grimes 1987). The highest GSI in October was found around the new moon; however, this might have been caused by the effects of the typhoon delaying the ovarian development.

Several lutjanid species show spawning aggregations at specific sites and/or lunar phases (Heyman et al. 2005). Such spawning aggregations are often targeted by commercial and recreational fisheries (Colin et al. 2003). Although it is not clear whether L. decussatus aggregates at specific sites for spawning, the species might form spawning aggregations since a lunar-synchronized ovarian development was found. If so, protection of spawning aggregations and/or conservation of spawning sites would also be useful for effective fishery management strategies for this species.