reproductive cycles in atlantic and mediterranean ...

Journal of Shellfish Research, Vol. 30, No. 3, 813–820, 2011.

REPRODUCTIVE CYCLES IN ATLANTIC AND MEDITERRANEAN POPULATIONS OF VENUS NUX GMELIN, 1791 (BIVALVIA: VENERIDAE), FROM SOUTHERN SPAIN

CRISTINA TIRADO,1 JOSE´ LUIS RUEDA1,2 AND CARMEN SALAS1* 1 Departamento de Biologıá Animal, Facultad de Ciencias, Universidad de Ma´laga, E-29071, Ma´laga, Spain; 2Centro Oceanogra´fico de Ma´laga, Instituto Espanõl de Oceanografıá, E-29640, Fuengirola, Ma´laga, Spain ABSTRACT The recent reduction of catches of common shellfish in southern Spain has pointed out an overexploitation of these resources. For this reason, new shellfish resources have been investigated—among them, the venerid Venus nux Gmelin, 1791. To provide information to managers for a better regulation of this new fishery, we studied the reproductive cycle in the Atlantic (Huelva) and Mediterranean littoral (Malaga) of southern Spain from June 1999 to May 2000 using histology and changes in flesh dry weight. Histological examination of the gonads showed a long and asynchronous reproductive period. Spawning occurred throughout the year in both populations. Two different spawning peaks were observed: April/May for the Mediterranean population and June/July for the Atlantic one. These peaks correspond to the highest percentage of population in spawning stage, together with the greatest loss of tissue weight. During the annual reproductive cycle, new activation of the gonads from postactive stages occurred without passing through a resting period in both populations. The absence of a resting period during the reproductive cycle could be related to mild seawater temperatures (12–16°C) and high levels of chlorophyll a (2.4–4mg/L). Considering these data, we propose that if this fishery continues, and taking into account the low recruitment rate and the monocohort structure of the populations, a closed season from June to July for the Atlantic population and from April to May for the Mediterranean one should be considered for management of this new resource. KEY WORDS: fishery, gametogenic cycle, biomass, histology, Venus nux

INTRODUCTION

In Spain, fishing products have been of relatively great importance to the eating habits of Spaniards during past decades; however, nowadays fishing is a declining activity. Andalusia is a region with a lot of tourist activity, which puts high pressure on coastal fish and shellfish fisheries. Because of this, regional authorities have promoted the search of new shellfish resources as well as aquaculture practices in several coastal areas. The venerid clam Venus nux Gmelin, 1791, is an Atlantic– Mediterranean species, ranging in the Atlantic from southern Portugal to Senegal, and is also present in the Mediterranean (Poppe & Goto 1993). Dense populations seem to be restricted to the Alboran Sea and the Ibero-Moroccan Gulf (southern Spain), usually inhabiting muddy sand bottoms from ;30 to 350 m in depth (Salas 1996). In the past, V. nux was not considered a fishery resource in southern Spain, probably because it generally occurs deeper than other shellfish species. In southern Portugal, V. nux was the most abundant species discarded in the fish trawls, accounting for 42% of the total number of individuals (Malaquias et al. 2006). In a first step for introducing V. nux as a new resource, the regional fishery authorities promoted studies regarding the distribution of these populations and the viability of this fishery in Huelva (Gulf of Cadiz, Atlantic coast) (Mata-Va´zquez et al. 2001) and Malaga (Alboran Sea, Mediterranean coast) (Sańchez-Molina et al. 2001). In these previous studies, the macroscopic aspect of the gonads and the observations of some oocytes and sperm pointed out a long reproductive period; however, few juveniles and monocohort populations were found on either the Atlantic or Mediterranean coasts. To establish a management policy *Corresponding author. E-mail: [email protected] DOI: 10.2983/035.030.0322

that may contribute to a sustainable fishery of this species, a more accurate knowledge of the reproductive cycle of V. nux from the Gulf of Ca´diz and the Alboran Sea populations was proposed using histological techniques. In other shellfish species from the littoral of Andalusia, such as Donax trunculus L. 1758, different spawning periods were found for the Atlantic and Mediterranean populations (Tirado et al. 2011), related to differences in environmental variables. Because of this, our initial hypothesis was that differences in the reproductive cycle of Atlantic and Mediterranean populations of V. nux must be expected. The deeper habitat of V. nux (depth, 30–350 m) in comparison with D. trunculus (depth, 0–3 m), could, however, attenuate these differences. MATERIALS AND METHODS

A total of 7,830 specimens of V. nux were examined for the current study and measured for shell length, of which 6,846 were studied for dry weight variation from the first survey (Table 1). An additional 984 specimens were used for histology. From the second survey in the littoral of Malaga (Table 1), only 91 juveniles were studied histologically to determine age of sexual maturity. The Mediterranean samples were collected off Fuengirola (Malaga; 36°34# N, 4°31# W), at a depth of 30 m in a muddy sand bottom, whereas the Atlantic samples were collected off Punta Umbrı´ a (Huelva; 36°56# N, 6°59# W) at a depth of 80 m also in a muddy sand bottom (Fig. 1). The samples were taken from June 1999 to May 2000, with monthly frequency in autumn/ winter months from October to March and with fortnightly frequency in spring/summer months, when other species, such as Venus verrucosa L. 1758, develop the gonads (Tirado et al. 2003). The specimens were captured using a dredge with a toothed aperture, with a mesh size of ;20 mm, which is usual for use among fishermen in both areas. To capture more

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TABLE 1.

Range of length of the Atlantic and Mediterranean populations during the first (I) and second (II) survey (with the latter survey only on Mediterranean population). Shell length in mm; N: number of specimens. Atlantic Population (Huelva) Months Survey I

Mediterranean Population (Malaga)

Range of Length (mm)

No. of Specimens

Months Survey I


No. of Specimens

Months Survey II


No. of Specimens

June

19–45

196

July 1 July 2

20–44 27–45

185 199

August 1 August 2 September 1 September 2 October November December January February March April

24–45 27–45 18–42 20–44 22–44 21–46 17–44 18–50 17–43 13–47 17–50

199 203 201 195 194 191 199 199 194 199 199

85 202 202 200 199 203 201 200 199 200 200 201 200 200 201 201 200 200 200

14–51 14–51 23–51 12–52 22–50 13–52 12–54 16–57 25–52 31–51 30–53 27–53

443 510 283 369 343 316 339 327 332 400 416 297

199 200

31–50 3–51 30–49 29–54 31–48 29–51 30–50 29–48 32–53 27–49 27–49 31–50 29–50 27–50 30–53 30–49 28–52 30–52 28–49

May June July August September October November December January February March April

18.3 21.5

June 1 June 2 July 1 July 2 July 3 August 1 August 2 September 1 September 2 October November December January February March April 1 April 2 May 1 May 2

May 1 May 2

juveniles for determining the size of sexual maturity, additional samples were taken from May 2001 to April 2002 in the littoral of Malaga, where the populations are closer to the coast (depth, 30–80 m) and the fishery is more sustainable (Table 1, second survey). For collecting small individuals, a dredge with a double net, with a 10- and 20-mm mesh size, was used. To evaluate the influence of environment on the cycle, the seawater temperature at 30 m and 80 m (Malaga and Huelva sampling sites respectively) was measured. Samples of water (2 L) were taken from the bottom with a Niskin water sampler for chlorophyll a determination. Pigment analyses were carried out by filtering the water through Whatman GF/C glass filters (Whatman, Maidstone, Kent, UK). The pigments of the retained cells were then extracted with acetone for 12 h in cool, dark

Figure 1. Sampling sites in the Atlantic and Mediterranean littoral of southern Spain.

conditions, following the recommendations of Lorenzen and Jeffrey (1980). Concentrations of chlorophyll a were calculated using the trichromatic equations of Jeffrey and Humphrey (1975). The flesh dry weight (FDW) of V. nux was obtained from a total of 6,846 specimens—3,152 from Huelva and 3,694 from Malaga (;200 specimens per sample). The length of every specimen was measured, and the soft parts were then pulled out of the shell, placed in the drying stove at 100°C for 24 h, and then weighed to the nearest milligram. Two different condition indices were applied: (1) FDW/L3, where L is the shell length in millimeters, and (2) that proposed by Crosby and Gale (1990) of condition index (CI) FDW 3 1,000/Volume of the internal cavity of the shell (considered in equivalent grams of water). The regression of FDW and shell length was calculated for each monthly sample to estimate the variation in biomass of a standard individual, based on the logarithmic transformation of Ricker’s function, W ¼ aLb (Ricker 1975), where W is the weight, L is the shell length, a is the ordinate at origin, and b is the slope. The size of the standard individual was the nearest to the mean size of the population. The histological study was performed on 984 specimens (usually 30 per sample), consisting of 476 from Huelva and 508 from Malaga, with a shell length ranging from 16–49 mm in the Huelva population and 18–56 mm in the Malaga population. An additional 91 individuals from Malaga, with a range in shell length of 12–34 mm, were studied for determining size of sexual maturity (Table 1). The sex of the specimens of V. nux can be distinguished macroscopically by the color of the gonads during the active period of the reproduction (whitish in females and orange in males). The sex of inactive individuals must be determined microscopically. For the histological processing, specimens were anesthetized with MgCl2, fixed in 10% formaldehyde,

REPRODUCTION OF VENUS NUX FROM SOUTHERN SPAIN embedded in paraffin, sectioned at 10 mm, and stained with hematoxylin of Carazzi and eosin, and a trichrome staining (VOF, according to Gutie´rrez 1967) of hematoxylin of Carazzi, light green, orange G, and acid fuchsine. The stages of gonad development were scored according to the scale proposed by De Villiers (1975) for Donax serra Ro¨ding 1798 in South Africa as follows: Cytolized: The alveoli are very small and wide apart. Some clams can be sexed when a few gametes are present. Preactive: The alveoli have clearly defined alveolar walls. They are intersected by broad, continuous transverse fascicles. Most clams can be sexed. Active: The alveoli are large and usually adjacent. The alveolar walls are always complete. Germ cells in various stages of development fill the alveoli and are both actively increasing and enlarging. The early phase of this stage was named ‘‘early active’’ and was taken into account in the reproductive cycle. Spawning: The alveolar pattern is disturbed and the alveolar walls are often broken. The alveoli are often flattened and show an orientation toward the center of the gonad. Postactive: The amount of germ cells varies, depending on the intensity of spawning and the time that has elapsed since spawning took place. Phagocytic cells are common. The Kolmogorov-Smirnov test was used to check the distribution of the data, and Kendall and Pearson’s rank correlations, included in the software SPSS 14.0 (SPSS, Chicago, IL), were carried out for cross-correlation among the indices of condition, standard individual, and percentage of spawning to clarify the relationship between changes in the FDW and spawning. Crosscorrelation between biological parameters and seawater temperature and chlorophyll a levels were also calculated for assessing the influence of environmental variables on the reproductive cycle. RESULTS Sex Ratio

In the littoral of Huelva (Atlantic population), a total of 476 specimens were considered for the sex ratio—227 (47.69%) males and 249 (52.31%) females. In the littoral of Malaga (Mediterranean population), of 508 specimens considered for the sex ratio, 280 (55.12%) were males and 228 (44.88%) were females. The sex ratio for both populations can be considered as 1:1 (chi-squared ¼ 0313, P < 0.1 for the Atlantic population; chi-squared ¼ 0021, P < 0.1, for the Mediterranean one). The smallest size (shell length) with a developed gonad was 12 mm. The size of sexual maturity (50% of individuals with developed gonads) was calculated for the Mediterranean population of Malaga as 23 mm. Sexual Cycle Biomass Analysis

The variation of the FDW/L3 ratio during the annual cycle is shown in Figure 2A (Atlantic population) and Figure 2B (Mediterranean population). The mean values of both variables—FDW (Fig. 2C, D) and size (L) (Table 2)—were considered. SDs ranged from 15.78–38.74% for the Atlantic population, and from 13.35–23.60% for the Mediterranean one. V. nux from the Atlantic population (Fig. 2A) showed

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a nearly continuous decrease of FDW/L3 from May until December, with small increases in August and September. From January to May, a continuous increase of weight can be observed. The Mediterranean population (Fig. 2B) also showed a general decrease of FDW/L3 from May to November. From November to February, the weight remained more or less stable, but with low values. In March, there was a strong increase of this index, followed by a strong decrease in April. The Crosby and Gale CI showed a similar pattern in the Atlantic population (Fig. 2E), whereas the Mediterranean population (Fig. 2F) showed a slightly different pattern than that of the FDW/L3 index, probably resulting from the less pronounced increase of the values in March. The variation of FDW was estimated for a standard individual of 32 mm and 41 mm in shell length, corresponding to the mean sizes in the Atlantic and Mediterranean populations respectively (Fig. 2G, H). The weight values of the standard individual were estimated using the regression lines between FDW and shell length obtained in every monthly sample (Table 2). In general, decreases in FDW were observed from May to January in the Atlantic population and up until February in the Mediterranean population, with some small increases. From February to April, there were weight increases in the Mediterranean population, and from February to May in the Atlantic population. These patterns are coincident with those observed in the CIs, particularly those of FDW/L3 (Fig. 2). Gametogenic Cycle

The data from the histological study are presented in Figure 3. The two studied populations of V. nux had continuous spawning throughout the year, with percentages of spawning greater than 50% in 10 of the 16 samples examined from the Atlantic population (Fig. 3A) and in 12 of 17 samples from the Mediterranean one (Fig. 3B). The whole of the Atlantic population spawned in summer, from the second half of July to the first half of August, whereas the lowest percentage of spawning (10% of the sample) was registered in January (Fig. 3A). The whole of the Mediterranean population spawned during the first half of May, but from March to August there were more than 80% of the individuals spawning (Fig. 3B). This seems to indicate a certain seasonal trend in the percentages of the individuals spawning in both populations. There is a direct and simultaneous significant correlation between percentage of population spawning and FDW/L3 (tau ¼ 0.7871, P < 0.001; tau ¼ 0.7671, P < 0.001), CI (tau ¼ 0.7355, P < 0.001, tau ¼ 0.7370, P < 0.001), and weight of standard individual (tau ¼ 0.7273, P < 0.001, tau ¼ 0.7671, P < 0.001) for Atlantic and Mediterranean populations, respectively. According to these data, the decreases of weight seem to be related to peaks of spawning. If we considered the absence of any individual in the cytolized or postactive stage, the activation of the gonads for both populations begins in February, and they remain active until the first half of August (Fig. 3). The regression of the gonads in both populations begins in September, with individuals in the postactive stage predominant until January. The gametogenic cycle is also asynchronous within both populations, as is evidenced by the presence of at least two development stages in nearly all the samples and the SDs of the CIs. An asynchrony is also detected in the individuals, resulting from the coexistence of areas with different stages in the same gonad. Several cohorts of oocytes can also be detected throughout

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Figure 2. (A, B) Monthly average flesh dry weight (FDW)/length3 (L3) ratio of V. nux throughout the year of study. (C, D). Monthly average FDW of the specimens of V. nux throughout the year of study. (E, F) Monthly average of the condition index (CI) of Crosby and Gale of V. nux throughout the year of study. (G, H) Monthly variations in FDW in a standard individual with a shell length of 32 mm from the Atlantic population (G) and a standard individual with shell length of 41 mm from the Mediterranean population (H). Bars indicate SD.

the year in both populations, together with the direct step in many individuals from the postactive to preactive or active stage, without a previous cytolized or preactive stage. Environmental Factors

In the Atlantic waters of the littoral of Huelva, the seawater temperature at a depth of 80 m was very constant throughout the year, with a minimum of 12°C in October and a maximum of 16°C for several months (Fig. 4A). In this area, the values of chlorophyll a at 80 m in depth were less than 1 mg/L in all the samples except in November (Fig. 3A). Nevertheless, there were several peaks throughout the year, with a maximum in November, followed by another one in May. In the Mediterranean waters of the littoral of Malaga, the temperature at a depth of 30 m ranged between 13°C in December and 21.8°C at the end of July (Fig. 4B). The decrease during early July is noteworthy and it could be related to the presence of upwelling in this area. Here the values of chlorophyll a at 30 m were higher than those observed in the Atlantic area, reaching levels of 4 mg/L in April, 2.82 mg/L in October, and 2.44 mg/L in August (Fig. 4B). Nevertheless, the chlorophyll a values displayed strong fluctuations throughout the year, with minimum values (near 0) in September and December.

Because of the absence of normal distributions in the temperature and chlorophyll a data (according to the KolmogorovSmirnov test), Kendall’s coefficients of correlation were estimated between these environmental variables and FDW/L3, CI, weight of standard individual, and percentage of population in spawning. The coefficients of correlation were calculated concomitantly and also with a 1-mo and 2-mo delay. In the Mediterranean population (Malaga), only the chlorophyll a was directly correlated, with 2 mo of delay, with CI (tau ¼ 0.000, P < 0.001), whereas in the Atlantic population (Huelva) there were no significant correlations between the environmental variables and the biological parameters considered. DISCUSSION Sex Ratio

The sex ratio obtained for V. nux was 1:1, which is similar to that obtained by Sańchez-Molina et al. (2001) and MataVa´zquez et al. (2001) in different populations of southern Spain. This ratio is also similar to those found in most bivalves, among them other venerids, such as Venus verrucosa

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TABLE 2.

Monthly linear regression for the weight–length relationship, after the logarithmic transformation of Ricker’s function, calculated for the Atlantic and Mediterranean populations. Atlantic Population (Huelva)

Mediterranean Population (Malaga)

Lm

Regression Line

R2

n

W (L $ 32 mm)

June

33.56

y ¼ 2.034x – 0.433

0.833

196

425.03

July 1 July 2

30.51 34.52

y ¼ 3.581x – 2.848 y ¼ 2.951x –1.959

0.877 0.824

185 199

348.51 304.05

August 1 August 2 September 1 September 2 October November December January February March April

33.85 34.87 31.17 31.25 32.16 33.37 29.33 28.99 31.08 27.48 28.97

y ¼ 2.825x – 1.713 y ¼ 2.989x – 2.024 y ¼ 3.469x – 2.726 y ¼ 3.096x – 2.133 y ¼ 3.037x – 2.101 y ¼ 2.894x – 1.903 y ¼ 3.096x – 2.133 y ¼ 3.431x – 2.801 y ¼ 3.124x – 2.271 y ¼ 3.475x – 2.688 y ¼ 3.336x – 2.472

0.785 0.656 0.884 0.898 0.810 0.803 0.898 0.929 0.802 0.936 0.878

199 203 201 195 194 191 199 199 194 199 199

345.63 298.85 312.71 336.31 294.91 283.14 336.31 230.99 269.11 348.44 353.36

May 1 May 2

31.08 32.17

y ¼ 3.767x – 3.139 y ¼ 3.373x – 2.498

0.866 0.818

199 200

339.56 378.79

Month

Month

Lm

Regression Line

R2

n

W (L $ 41 mm)

June 1 June 2 July 1 July 2 July 3 August 1 August 2 September 1 September 2 October November December January February March April 1 April 2 May 1 May 2

40.65 41.95 40.08 39.66 40.27 40.79 40.45 40.66 41.90 41.06 39.65 40.62 39.82 40.66 37.95 36.55 41.21 40.68 41.22

y ¼ 1.803x + 0.081 y ¼ 2.328x – 0.765 y ¼ 2.679x – 1.316 y ¼ 2.420x – 0.950 y ¼ 2.641x – 1.327 y ¼ 2.731x – 1.459 y ¼ 2.699x – 1.417 y ¼ 2.832x – 1.628 y ¼ 2.682x – 1.462 y ¼ 2.707x – 1.477 y ¼ 2.560x – 1.306 y ¼ 3.093x – 2.138 y ¼ 3.059x – 2.071 y ¼ 2.789x – 1.665 y ¼ 2.637x – 1.198 y ¼ 2.722x – 1.285 y ¼ 2.465x – 1.004 y ¼ 2.747x – 1.402 y ¼ 2.785x – 1.473

0.559 0.565 0.650 0.727 0.673 0.598 0.588 0.643 0.562 0.670 0.623 0.750 0.800 0.701 0.673 0.661 0.636 0.784 0.725

85 202 202 200 199 203 201 200 199 200 200 201 200 200 201 201 200 200 200

973.19 975.69 1,012.64 898.46 854.9 882.54 864.59 869.52 730.27 774.94 665.59 708.2 729.5 683.45 1,136.04 1,272.55 937.29 1,066.68 1,161.7

Lm, average shell length (in millimeters); n, number of observations; R2, coefficient of determination; W (L ¼ 32 mm), weight (in milligrams) of a standard individual of 32 mm in shell length, which is near the mean size of the studied Atlantic population; W (L ¼ 41 mm), weight (in milligrams) of a standard individual of 41 mm in shell length, which is near the mean size of the studied Mediterranean population.

in the littoral of Malaga (Tirado et al. 2003) and the Aegean Sea (Galinou-Mitsoudi et al. 1997) or Callista chione (L. 1758) in the littoral of Malaga (Tirado et al. 2002) and the Adriatic Sea (Valli et al. 1994). No hermaphrodites were found in this study. Venus nux shows some sexual dimorphism during the period of intense activation of the gonads because of the existence of different color in the gonads of males and females, which is uncommon in venerids and scarce in most bivalves (Sastry 1979). The color of the gonads of V. nux, orange in males and whitish in females, is contrary to that found in other marine bivalves that exhibit sexual dimorphism, such as mussels (females with orange gonads and males with whitish gonads) and some scallops (with pink female gonads and white male gonads) (Gosling 2003). In Donax semistriatus (Poli, 1795), Donax venustus (Poli, 1795), and Donax vittatus (Da Costa, 1778), the female gonad is reddish and the male one is whitish (Tirado & Salas 1999). In the littoral of Malaga and Huelva, different colors of the male and female gonads have been also reported in D. trunculus, but contrary to V. nux, the gonads were whitish in males and blue in females (Tirado & Salas 1998, Tirado et al. 2011). Reproductive Cycle

Histological study of the gonads showed spawning of V. nux in both populations throughout the year (Fig. 3), as observed in V. verrucosa in the littoral of Malaga (Tirado et al. 2003). Moreover, the presence of female gametes in all stages of development supports that the spawning is continuous or repeatedly partial, but with a certain seasonal pattern. The absence of

a resting period is usually related to mild seawater temperatures together with high amount of phytoplankton (Valli et al. 1994). In both sites there were mild seawater temperatures (Fig. 4). However, in Malaga, where there are several upwellings of intermediate Mediterranean water rich in nutrients (Vargas-Yań˜ez et al. 2002, Vargas-Yaán˜ez et al. 2007), levels of chlorophyll a were higher than in Huelva seawater (Atlantic) for most of the months. Nevertheless, other bivalves from the littoral of Malaga, such as the donacids (D. trunculus; D. semistriatus and D. venustus), are known to have at least 1 mo of a resting period (Tirado & Salas 1998, Tirado & Salas 1999). Temperature is considered to be one of the most influential factors on the reproductive cycle for many bivalves (Sastry 1979), but in the two studied populations of V. nux from southern Spain, no significant correlation between the temperature and the biological parameters analyzed for the reproductive cycle was noted. Therefore, other environmental or biological factors may be influencing the gametogenic cycle of this species. An alternative hypothesis is that phytoplankton induces spawning (Starr et al. 1990). Starr et al. (1990) showed that blooms of phytoplankton should be sufficient to induce spawning in mussels as well as in sea urchins. The chlorophyll a levels in the littoral of Malaga during this study displayed a significant correlation with the CI of Crosby and Gale (1990), but only when estimating the values considering 2 mo of delay. On the other hand, the three peaks of chlorophyll a occurred before or coincident with spawning peaks (Figs. 3B and 4B). The greater depth (80 m) of the Atlantic population compared with the Mediterranean one (30 m) would explain the more

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Figure 3. Monthly cumulative frequencies of different stages of development of the gonads in V. nux. (A) Atlantic population. (B) Mediterranean population. A, active; C, cytolized; EA, early active; Pr, preactive; Ps, postactive; S: spawning. C Pr EA A S Ps

constant temperature and lower chlorophyll a levels in Huelva, whereas the presence of upwelling in the littoral of Malaga would be the origin of the fluctuations of temperature and chlorophyll a in the year studied. The lower food ration available to the Atlantic clams (Fig. 4A) would explain the lower length and tissue dry weights, together with a reduced increase in weight during active gametogenesis and a reduced decrease in weight after spawning (Table 2). Thus, it is fair to conclude that the size and fecundity in the Mediterranean population can be considered greater than that of the Atlantic population. All these data point out a relationship between the amount of available food and the development of ripe gonads and posterior release of gametes, as has been observed in Crassostrea gigas (Thunberg, 1793) in Galicia, northwestern Spain (Ruiz et al. 1992).

Figure 4. Seawater temperatures (T) and concentration of chlorophyll a (Chla) in seawater throughout the studied year. (A) Atlantic littoral of Huelva. (B) Mediterranean littoral of Malaga.

Although spawning occurred throughout the year of the study, the most intense period of spawning in both populations occurred from April to mid-August, indicating a certain seasonal pattern. Nevertheless, if we take into account the data from the Cis (FDW/L3; Fig. 2A–F) and the standard individual (Fig. 2G, H), different peaks of release (greater percentages of population in spawning together with greater decreases in weight) could be observed in both populations. In Malaga (Mediterranean), the peak was from April to May, whereas in Huelva (Atlantic), it was from June to July (Figs. 2 and 3). The existence of a significant correlation between percentage of spawning and FDW/L3, CI, and the standard individual indicates that the decreases in FDW are related to spawning events. In Malaga, the absence of individuals with shells smaller than 25 mm from December to April, and the presence of numerous individuals with a shell length of 12–30 mm in July (which were scarce in other months), point out that April and May can be considered the effective spawning period (Table 1). Differences in spawning peaks were also observed between the populations of D. trunculus from Huelva and Malaga, but in that case it is the Atlantic population that spawns two months earlier and this may be triggered by chlorophyll peaks (Tirado et al. 2011). The direct jump from the postactive stage to the active stage, without a previous cytolized stage, has also been observed in D. trunculus and V. verrucosa from the littoral of Malaga (Tirado & Salas 1998, Tirado et al. 2003). In field conditions, it is difficult to know whether a particular individual has more than two spawning events per reproductive cycle. In the laboratory, Chamelea striatula (da Costa, 1778) spawns repeatedly at intervals throughout the spawning season (Ansell 1961). In the littoral of Malaga, two spawning periods per individual were detected in D. trunculus, whereas in others species, such as D. venustus and D. semistriatus, one spawning per individual and cycle was observed (Tirado & Salas 1998, Tirado & Salas 1999, Tirado et al. 2011). In V. nux, more than one spawning event per individual may occur during the annual reproductive cycle as a result of the presence of several cohorts of oocytes in the same individual throughout the year in both populations. The asynchronous gametogenic cycle in both populations is reflected by the high SDs in FDW (Fig. 1C, D), the existence of several cohorts of oocytes, and the coexistence of several stages of development in the same gonad. The resorption of gametes during the postactive stages could also influence the asynchrony of the gametogenic cycles of the studied populations. The coexistence of different stages has been found in many bivalves from temperate areas, among them, C. striatula (Ansell 1961), D. serra (De Villiers 1975), Tapes rhomboides (Pennant, 1777) (Morvan & Ansell 1988), D. trunculus (Tirado & Salas 1998, Tirado & Salas et al. 2011), D. venustus, and D. semistriatus (Tirado & Salas 1999). According to the data obtained during the studies of viability of these fisheries, the populations were structured by practically one cohort, with few small individuals collected and no older specimens found (Mata-Va´zquez et al. 2001, Sańchez-Molina et al. 2001). This type of population has been related to irregular or fluctuating patterns in recruitment, such as that observed in Ensis (Del Piero & Dacaprile 1998), where after a year of high recruitment, the recruitments remained very low in following years, resulting in populations that are nearly a monocohort. Overexploitation of the species can also result in monocohort populations, such as that seen in R. philippinarum (Adams & Reeve, 1850)

REPRODUCTION OF VENUS NUX FROM SOUTHERN SPAIN in Arcachon Bay (Dang et al. 2010) or Cerastoderma edule in the Wadden Sea (Imeson & van den Bergh 2006). In the case of V. nux, overexploitation may not be the factor to be considered because the species has not been fished. A low rate of recruitment may occur in this species and/or low growth rate, and because of this, we consider that this fishery could be sensitive to environmental fluctuations and overexploitation. Consequently, if the fishery continues in the future, a closed season without perturbation of the environment during the periods of higher emissions would help to ensure higher levels of recruitment. The management of a fishery implies taking into account all the environmental and biological data as well as the sensibility of the fisheries. The fisheries on the Atlantic were initially considered not profitable (Mata-Va´zquez et al. 2001) because of the long distance of the V. nux populations from the shore. However, in Malaga, where the populations are located closer to the shore than in Huelva, these fisheries were considered able to obtain benefits (Sańchez-Molina et al. 2001). In Andalusia, the fishery managers followed the principle of precaution and, since 2005, the policy has been to give only two licenses for the Mediterranean populations with a TAC of 1,000 kg/wk and license (about 200 kg/day and vessel). The authorities have not currently established a closed season because clams are only

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harvested on two Mediterranean populations, with the other ones from the Andalusian littoral considered a refuge for recruitment. However, we consider that, if the populations become overexploited, apart from a reduction of TAC, a closed season must be established during the months with peaks of spawning—April and May for the Mediterranean populations and June and July for the Atlantic ones—to minimize the fishing impact on the recruitment of the populations. ACKNOWLEDGMENTS

We thank David Lo´pez, Daniel Go´mez, and Ma Jose´ Garcı´ aPatinõ for helping in the laboratory process. The manuscript has been improved with the comments of an anonymous referee, to whom we are grateful. This study is part of a project supported by the Junta de Andalucı´ a (Department of Fishery). We thank Ma Dolores Atienza, former general manager of the Department of Fishery, for her trust in our work and permission to publish. We are grateful to Manuel Castanõn (former provincial manager of fishery) for encouraging this research. The project was entrusted to D.A.P. Enterprise, which we thank for their help given to realize this work and publication of the results. We are grateful to I. Ma´rquez, J. Ignacio Lo´pez, and Manuel Aguilar for their help given during the development of this research.

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