Reproductive cycle of the Pearl oyster Pteria sterna (Pteriidae) in the Ojo de Liebre lagoon, B.C.S., Mexico.

ABSTRACT The reproductive cycle of a wild population of the oyster pearl Pteria sterna living in the Ojo de Liebre lagoon was analyzed from February 2001 to February 2002. The gonadic development was studied both qualitatively (through histological analysis) and quantitatively (through an analysis of the percent follicular area). P. sterna's gonadic development consists of five stages (undifferentiated, developing, ripe, spawning, and spent). The percent follicular area turned out to be an adequate quantitative indicator of reproductive activity, with significantly higher values in the ripe months (August to September) and with a decrease directly related to spawning. The reproductive cycle of P. sterna is synchronic and is influenced by temperature and food availability. The reproductive season can be regarded to range from October to April, starting when temperature drops. The size at first maturity was determined at 117.1 mm SH and 106.6 mm SH for females and males, respectively. However, the smallest ripe female and male measured 73 mm SH and 26 mm SH, respectively, and both were undergoing the spawning phase. Differences in the reproductive cycle of P. sterna were found between the one reported here and reports for other Gulf of California's localities.

KEY WORDS: first maturity, gonadic cycle, follicular area, histology, oyster, Pteria

INTRODUCTION

The pearl oyster Pteria sterna (Gould 1851) locally named "concha nacar" produces high-quality pearls, which are highly appreciated in the international market because of their shape and color (Monteforte 1990, Monteforte 1996). Additionally, its adductor muscle has been used for human consumption, and the nacre produced by its shell is used as one of the main ingredients to elaborate pharmaceutical products, including cicatrize-promoting creams.

This species has suffered an intensive and uncontrolled exploitation in the Gulf of California, causing its natural stocks to be virtually depleted, because more than 6 decades ago, mainly at Bahia de La Paz, B.C.S., Mexico (Sevilla 1969, Shirai & Sano 1979, Monteforte 1990, Monteforte & Carino 1992, Carino & Monteforte 1995). The decree of 1939 forbidding the fishery of the pearl-producing oysters P. sterna and Pinctada mazatlanica is still in force (Diario Oficial de la Federacion 1939), and both species are regarded as "under special protection" (Diario Oficial de la Federacion 2004).

Large natural P. sterna banks have recently been located in the Baja California Sur western coast, mainly in the Ojo de Liebre lagoon, B.C.S., Mexico (pers. comm. with local fishermen). This finding has raised interest from the fishing and pearl-producing sectors to start its exploitation. However, and especially in this case, the establishment of the biological framework to carry out a sustainable exploitation of this species is key and hence to avoid the depletion of these banks. In this respect, the Mexican fishing regulations establish that fishing-permit approvals require the previous evaluation of any benthic resource. Several aspects about P. sterna have been investigated in the Gulf of California, but cultured organisms have been analyzed in all cases (Diaz & Buckle-Ramirez 1996, Saucedo et al., 2002a, Saucedo et al., 2002b). Although its reproductive cycle has been established under those circumstances in the Gulf of California (Diaz & Buckle-Ramirez 1996, Arizmendi-Castillo 1996, Saucedo & Monteforte 1997), this topic has not been sufficiently studied in wild populations living in the Mexican Pacific. For this reason, this study aims to establish the reproductive cycle of P. sterna in the Ojo de Liebre lagoon, B.C.S., Mexico on the basis that this may vary according to the geographic location (Hesselman et al., 1989, Baqueiro & Aranda 2000) and in an effort to establish its relationship with temperature and food availability. Furthermore, the size at first maturity was determined to be a criteria that may be used to set minimum catch sizes as a fishing-regulation measure.

MATERIALS AND METHODS

From February 2001 to February 2002, 30 Pteria sterna adult specimens were collected each month by scuba diving, from a bank known as "El Borbollon" at a mean depth of 6 m, in the Ojo de Liebre lagoon, B.C.S. Mexico (113[degrees]50' to 114[degrees]20'W, and 27[degrees]35' to 28[degrees]00'N) (Fig. 1). Organisms were fixed in 10% formalin in sea water. At the same time, the bottom temperature was registered with a U-10 HORIBA water-quality analyzer. Additionally, a water sample was collected near the bottom using a Niskin bottle, these water was filtered in the first place with a 200-[micro]m mesh to eliminate large particles. Then 3 samples, of 1-L each one, were filtered through 47 mm-diameter Whatman GF/F filters and were kept in the dark under refrigeration. From these samples, and as a measure of food availability, the concentration of chlorophyll-a in filters was determined. Chlorophyll-a was extracted with acetone and its concentration was determined using the fluorometric method modified by Parsons et al. (1984).

Shell height (SH) and fresh weight (without shell and byssus) were registered for each organism. Afterwards, visceral mass sections (including the gonad) were obtained. These were processed histologically, embedding them in paraplast and 7-[micro]m thick sections were stained with hematoxylin-eosin (Humason 1979). The gonad structure was analyzed under the microscope, and organisms were sexed based on the presence of oocytes or sperm in follicles. The sex ratio was determined dividing the total number of males by the total number of females. Undifferentiated organisms were not considered.

[FIGURE 1 OMITTED]

To analyze the reproductive process, gonads were classified based on the 5 gonad-development phases (undifferentiated, developing, ripe, spawning, and spent) established for the pearl oyster Pinctada mazatlanica (Garcia-Dominguez et al. 1996). The reproductive season was established as the period when spawning organisms occurred.

The percent follicular area calculated as the ratio between the follicular area (relative area occupied by follicles) and the total gonad area (maximum area available for gonad development) present in the histological section was computed as a quantitative measure of the degree of gonad development, following the criteria of Baron and Ciocco (2001). Measurements were obtained using the SCAN PRO software (V. 5.0) from gonad-section digitalized images. Areas were traced individually (by hand), and measures were automatically calculated by the software. Only developing, ripe and spawning gonads were considered, according to the criteria of Grant and Tyler (1983a, Grant and Tyler 1983b).

To establish the size at first maturity, the cumulative relative frequencies of all mature pearl oysters were calculated for each 5-mm height size (excluding undifferentiated specimens). These data were adjusted to a logistic model, which was used to calculate the height corresponding to 50%, regarded as the population's size at first maturity (the height at which the pearl oyster is mature for the first time) (Somerton 1980).

Statistical Analysis

To check whether the sex ratio can be regarded as 1:1, a hypothesis test was carried out ([chi square], [varies] = 0.05) (Sokal & Rohlf 1979). To analyze the percent follicular area seasonality, a single-factor analysis of variance was applied, followed by a Tukey means- comparison test. To detect whether any relation exists between reproductive activity and environmental variables, Spearman's correlation analyses were conducted between mean monthly percent follicular area and temperature or chlorophyll-a concentration. Variables measured as percentage were arc-sin transformed (arcsin [square root p]) before carrying out the statistical analyses, which were conducted using the Statistica software (V. 6.0).

RESULTS

A total of 368 oysters was collected, including 123 females, 146 males, and 99 undifferentiated specimens. The size interval ranged between 26 and 145 mm SH (mean 112.34 [+ or -] 17.3 SD). The per-sex size distribution evidenced that small sizes (26-70 mm SH) included only males, and from 71 mm SH females were also present (Table 1). However, the sex ratio per size classes did not differ significantly ([chi square]; P > 0.05) from the expected 1M:1F ratio, except in the size classes 56-70 mm SH and 86-100 mm SH ([chi square]; P < 0.05). The overall sex ratio (0.83F:1M) did not differ significantly ([chi square]: P > 0.05) from the expected 1F:1M ratio.

The Pteria sterna's reproductive cycle is depicted in Figure 2. The relative frequencies of the gonad-development phases throughout the study period were obtained by pooling together males and females, because no differences between sexes were detected in the pattern's temporality. From March to August 2001 and in February 2002 undifferentiated organisms occurred, with peak frequencies in May and February 2002 (73.3% and 83.3%, respectively), and the minimum frequency in August (3.5%). Developing organisms appeared in February and from June to September 2001, with a maximum proportion in August (89.7%). Ripe stage occurred from August to October, with the peak frequency in September (55.2%). Spawning took place from February to April 2001 and from October to January 2002, with the highest frequency in November (100%). Last, spent organisms appeared from February to July 2001 and from December to February 2002, with the peak frequency in January 2002 (80%).

The temporal variation in the percent follicular area during the study period is shown in Figure 3a. Mean percent follicular area values showed significant differences between months (one-way ANOVA, P < 0.01) with a clear seasonality (post hoc Tukey test). Percent follicular area values were low from February to June 2001 (31.7% to 40%), because of a small proportion of spawning and developing organisms. From July, these values rose steadily to reach a peak in September (92.6%), coinciding with the highest proportion of ripe organisms. From October, percent follicular area started declining until it reached a minimum value in January 2002 (32.4%), coinciding with a high proportion of spawning organisms. In May 2001 and February 2002 all organisms were either undifferentiated or spent, so that percent follicular area was not estimated.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The temperature variation along the study period was clearly seasonal (Fig. 3b). The lowest value occurred in February 2001 (16.7[degrees]C). Temperature steadily rose from March to reach a peak in September (24.6[degrees]C). In October, temperature started decreasing until it attained its lowest value in December 2001 to January 2002 (17[degrees]C).

Chlorophyll-a concentration showed three important peaks during the study period (Fig. 3c): one in March (0.95 [micro]g/L), another in June to July (0.94 and 0.9 [micro]g/L, respectively) and the most important one, with the highest values, from September to November 2001 (0.97 [micro]g/L, 1.02 [micro]g/L, and 0.95 [micro]g/L, respectively). The lowest values occurred in December 2001 and January 2002 (0.52 [micro]g/L).

Percent follicular area significantly correlated with temperature (R = 0.81; P = 0.004) but not with chlorophyll-a concentration (R = 0.36, P = 0.3).

The first maturity size interval ranged from 115-120 mm SH for females and from 105-110 mm SH for males (Fig. 4). The logistic model revealed first maturity sizes of 117.1 and 106.6 mm SH for females and males, respectively. However, the smallest mature female and male measured 73 and 26 mm SH, respectively, and both were in the spawning phase.

DISCUSSION

In general, the sex ratio of the Pteria sterna population living in the Ojo de Liebre lagoon was found at 1:1 (P > 0.05), and similar results have been reported for this same species in Gulf of California populations (Arizmendi-Castillo 1996, Diaz & Buckle-Ramirez 1996). However, the sex-ratio analyses showed that small sizes (26-70 mm SH) corresponded only to males and these occurred in larger numbers up to 115 mm SH, with extreme values ranging between 86 and 100 mm SH, where a significant proportion of males was found (0.21 F: 1M). For their part, females were more abundant in the largest sizes (116-145mm SH), although no significant deviation from the 1:1 ratio was detected. This population structure of P. sterna might be related to a protrandric condition, but in the population analyzed no histological evidence of sex reversion was observed, which might support such condition. The same was reported in the work by Diaz & Buckle-Ramirez (1996). However, some studies carried out in the Gulf of California reported histological evidence of this phenomenon (hermaphrodite organisms), although in low proportions (0.6-4.17) (Arizmendi-Castillo, 1996; Saucedo & Monteforte, 1997). Assuming that P. sterna displays a certain degree of protandry, a likely explanation of the lack of histological evidence is that sex reversion takes place after reaching a given size (70 mm SH) and during the undifferentiated phase. However, it is important to analyze the fact that under culture conditions a higher proportion of males is found compared with females (i.e., 0.45F:1M, or 0.38F:1M) (Arizmendi-Castillo 1996, Saucedo & Monteforte 1997). Then, it is reasonable to assume that sex reversion in P. sterna may be influenced by environmental conditions, mainly food, because cultures analyzed by Arizmendi-Castillo (1996) and Saucedo and Monteforte (1997) were grown in baskets, usually covered by fouling, which limits the sufficient entrance of food. Consequently, the ovary development (a process which demands a high amount of energy) is not favored. The earlier mentioned would indicate that this species might instead display a transitional hermaphroditism, as is the case of other Pteriidae genera, like Pinctada margaritifera (Tranter 1958d) and P. maxima (Rose et al., 1990), which is regulated by environmental factors. Nonetheless, further studies are required to clarify this topic.

[FIGURE 4 OMITTED]

Given that P. sterna is one of the smallest species of the Pteriidae family, organisms of this species are believed to ripen at small sizes (Saucedo & Monteforte 1997). In this sense, Arizmendi-Castillo (1996) observed spawning organisms measuring only 8-11 mm SH whereas these were manipulated, although it is suggested that gametes were unviable. In the present work, the smallest ripe female and male measured 73 mm SH and 26 mm SH, respectively, and both were in the spawning phase. The size at first maturity reported here and in other studies on P. sterna differ from one another (Table 2), probably because of the size of the organisms analyzed. When the sizes reported here are compared with those reported by Hernandez (1993) and Diaz & Buckle-Ramirez (1996), it is evident that the last ones differ noticeably, not only for being considerably smaller but also for presenting a larger size at first maturity in males compared with females.

Pteria sterna displayed a synchronic gonad ripening in the Ojo de Liebre lagoon, with an annual reproductive pattern. After spawning, a rapid proliferation of phagocytes and granulocytes was observed, which surround and reabsorb any residual gametes (Gaspar-Soria et al. 2002). This is a nutrient-recycling strategy (yolk proteins and lipids), which represents an important contribution to energy recovery after the reproductive season (Narvarte & Kroeck 2002).

The percent follicular area turned out to be a good quantitative indicator of reproductive activity, with significantly higher values during the peak reproductive activity months (August to September), and with a decline associated with spawning. Additionally, the presence of a single peak in the percent follicular area, along with a significant correlation between the percent follicular area and temperature (R = 0.81; P = 0.004) indicates that the P. sterna population in the Ojo de Liebre lagoon has a highly marked annual reproductive pattern influenced by water temperature. The onset of gametogenic development occurs in June (20.5[degrees]C) and goes on until September, when water temperature reaches a peak (24.6[degrees]C).

The geographic location of the Baja California peninsula causes different environmental conditions: subtropical to temperate at the Mexican Pacific side and tropical to subtropical in the Gulf of California, so that the peninsula leads to isolation, acting as a barrier for natural populations living at both sides, so that a genetic difference between them could be expected (Cruz et al., 2000). As a result, they may also be expected to respond differently, according to the environmental conditions in each locality. In general, a single synchronized spawning is known to occur per year in the summer in cold climates (high latitudes) (Giese & Pearse 1974), whereas two peaks may occur in temperate climates, and a continuous spawning takes place throughout the year in tropical climates, although not with the same intensity (Sastry 1979, Mackie 1984).

Thus, differences in the reproductive cycle of P. sterna can be detected between organisms living in the Gulf of California and those studied here (living in the peninsula's western coast). The Gulf of California populations display an asynchronous reproductive cycle, with a continuous gametogenesis throughout the year (Hernandez 1993, Arizmendi-Castillo 1996, Diaz & Buckle-Ramirez 1996; Saucedo & Monteforte 1997); whereas in the Ojo de Liebre lagoon, a synchronic reproductive cycle was observed, with gametogenesis being restricted to a specific period of the year, triggered by a rise in water temperature. Similarly, the reproductive period in the genus Pinctada has been observed to be more restricted at high latitudes, being shorter and characterized by a single reproductive period in the summer, which results in a high spawning synchronization, whereas at lower latitudes the reproductive period is longer-lasting, characterized by two or more reproduction peaks, and not confined to a specific season of the year (Tranter 1958a, Tranter 1958b, Tranter 1958c, Rose et al. 1990).

On the other hand, the factor regarded as the most important one for inducing spawning in bivalves is temperature. In many species the onset of spawning is triggered when temperature reaches a threshold level, characteristic of each species (Sastry 1979, Mackie 1984, Arsenault & Himmelman 1998). In this sense, it has been reported that P. sterna has a temperate affinity and that its optimum spawning temperature lies between 24[degrees]C and 28[degrees]C (JICA 1987). However, various temperature intervals at which P. sterna spawns have been reported, depending on the locality: Bahia de Los Angeles, 14[degrees]C to 20[degrees]C (Diaz & Buckle-Ramirez 1996); Guaymas, Sonora, 16.6[degrees]C to 28[degrees]C (Arizmendi-Castillo 1996); Bahia de La Paz, 22[degrees]C to 23[degrees]C (Saucedo & Monteforte 1997); Ojo de Liebre lagoon, 17[degrees]C to 24.6[degrees]C (present work). On the other hand, the onset of spawning observed in this work occurred in October, when temperature decreased (1.8[degrees]C) after the maximum value was recorded. This leads to assume that the drop in temperature triggers spawning, as reported for other bivalve species (Villalejo-Fuerte et al. 1995). Accordingly, the influence of temperature on spawning has been confirmed in laboratory experiments, observing that thermal stimuli trigger spawning in ripe pearl oysters (P. sterna and P. mazatlanica) (Rangel-Davalos et al. 2000, Hernandez-Olalde 2001).

Besides temperature, another factor that may also influence gamete production in bivalves (from a seasonal perspective) is food availability (Bayne 1976, MacDonald & Thompson 1985, Jaramillo et al. 1993). It has also been suggested that a phytoplanktonic bloom may also be a spawning-triggering factor (Jaramillo et al. 1993, Arsenault & Himmelman 1998, Luna-Gonzalez et al. 2000, Tirado et al. 2002, Arellano-Martinez et al. 2004) because environmental food availability provides favorable nutritional conditions for larvae during their planktonic life (Brockington & Clarke 2001).

In this investigation, phytoplankton abundance (estimated through chlorophyll-a concentration) displayed seasonal variations characterized by three blooms (spring, summer, and autumn). Although chlorophyll-a levels showed a nonsignificant correlation with the percent follicular area, the coincidence between the onset of spawning in October with the peak chlorophyll-a concentration (1.02 [micro]g/L) suggests that the latter might be triggering spawning in P. sterna, as reported by Arellano-Martinez et al. (2004) for Nodipecten subnodosus in the same locality.

ACKNOWLEDGMENTS

The authors thank the Secretaria de Investigacion y Posgrado at the Instituto Politenico Nacional for funding this study, as well as to Empresa Exportadora de Sal, S.A. de C.V., to the Fishing Production Cooperative Society Pescadores Unidos de Guerrero Negro, S.C.L., and the El Vizcaino Biosphere Reserve for the logistics support providing during sampling field work. The authors also thank Maria Elena Sanchez Salazar for her valuable help in editing the English manuscript. B. P. Ceballos-Vazquez, F. Garcia-Dominguez and M. Arellano-Martinez thank the Instituto Politenico Nacional by the grants (EDI and COFAA) and the CONACYT (SNI) and L. Hernandez-Olalde is grateful for the economic support by PIFI and CONACYT.

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LILIANA HERNANDEZ-OLALDE, * FEDERICO GARCIA-DOMINGUEZ, MARCIAL ARELLANO-MARTINEZ AND BERTHA PATRICIA CEBALLOS-VAZQUEZ

Centro Interdisciplinario de Ciencias Marinas, Instituto Politecnico Nacional, La Paz, Baja California Sur 23000, Mexico

* Corresponding author. E-mail: [e-mail]

TABLE 1.

Female and male frequencies, Chi-square ([chi square]) values and sex
ratios of Pteria sterna by height class. Numbers in bold indicate a
statistically significant difference from a 1:1
ratio (P [less than or equal to] 0.05).

 Height
Class (mm)        Female         Male        Total

26-40                               1           1
41-55                               3           3
56-70                               6           6
71-85                  3            4           7
86-100                 6           28          34
101-115               45           51          96
116-130               58           45         103
131-145               11            8          19
TOTAL                123          146         269

  Height                     Sex Ratio
Class (mm)   [chi square]   Female:Male

26-40               1.00         0:1
41-55               3.00         0:1
56-70               6.00         0:1
71-85               0.14      0.75:1
86-100             14.24      0.21:1
101-115             0.38      0.88:1
116-130             1.64      1.29:1
131-145             0.47      1.38:1
TOTAL               1.00      0.90:1

TABLE 2.

Different sizes at first maturity reported for Pteria sterna.

Female      Male           Source

34.9 mmSH   51.8 mmSH      Hernandez 1993,
                           Diaz & Mickle-Ramirez 1996
      70-80 mmSH           Rangel & Chavez 1994
      Since 8 mmSH         Arizmendi-Castillo 1996
      40-45 mmSH           Saucedo & Monteforte 1997
117.1 mmSH  106.6 mmSH     This work