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Reproduction of Venus verrucosa L., 1758 (Bivalvia: Veneridae) in the littoral of Málaga (southern Spain)
Fisheries Research 63 (2003) 437–445
Short communication
Reproduction of Venus verrucosa L., 1758 (Bivalvia: Veneridae) in the littoral of Málaga (southern Spain) C. Tirado a , C. Salas a,∗ , I. Márquez b a
Dept. Biolog´ıa Animal, Facultad de Ciencias, Universidad de Málaga, E-29071-Málaga, Spain b Consejer´ıa de Agricultura y Pesca de la Junta de Andaluc´ıa, E-41071-Sevilla, Spain
Received 27 March 2002; received in revised form 14 February 2003; accepted 27 February 2003
Abstract The reproductive cycle of Venus verrucosa Linnaeus, 1758 was studied using histology and changes in flesh dry weight, in the littoral of Málaga (southern Spain), from June 1999 to May 2000. Histological study of the gonads showed spawning throughout the year, with two peaks. The first lasts from March to April, and is accompanied by the highest decrease of weight; the second is from May to August with the highest percentage of population spawning. There is new activation of the gonads from postactive stages, without passing through a resting period (with a high percentage of the population in the cytolized stage). The absence of a resting period in the reproductive cycle could be explained by the mild seawater temperatures and high levels of chlorophyll a in the littoral of Málaga. In the light of these data, we propose a closed season from March to April, months during which there was the most intense release of gametes in the population. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Venus verrucosa; Histology; Condition index; Spawning season; Fishery management
1. Introduction Venus verrucosa Linnaeus, 1758, the “warty Venus”, is an Atlantic–Mediterranean species distributed in the Atlantic from Norway to South Africa (Durban), and also in the Mediterranean (Poppe and Goto, 1993). It lives in poorly sorted sand; sometime with coralline rhodoliths and in channels between beds of Posidonia oceanica down to a depth of about 30 m (Arneri et al., 1998). This species is particularly appreciated in France, where it is known as “praire”. Catches of 3500 t per year are registered from the coast of Normandy and Brittany (Berthou and Le Gall, 1980; Berthou et al., ∗ Corresponding author. E-mail address: [email protected] (C. Salas).
1980). In the European Mediterranean countries the fisheries are local, reaching about 500 t per year in the southern Adriatic (Italy) and Greece (Arneri et al., 1998). Most of the studies on V. verrucosa in the Mediterranean Sea have been carried out in Italy, particularly on the Adriatic populations. The reproduction has been analyzed by Valli and Cester (1980) and Marano et al. (1980, 1982), and the fishery by Bello (1985, 1986) and Del Piero (1994). Rossi et al. (1994) have attempted to reproduce the species artificially in a hatchery at Ferrara (Italy), but in this initial set of experiments the survival did not go beyond the 25th week. In Greece, the reproduction has been studied by Galinou-Mitsoudi et al. (1997) in populations from the Gulfs of Thessaloniki and Termaikos.
0165-7836/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0165-7836(03)00106-1
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C. Tirado et al. / Fisheries Research 63 (2003) 437–445
In southern Spain, V. verrucosa, locally known as “bolo”, is a much-appreciated shellfish, with an illegal market of the same order of magnitude as the official catch, not taken into account by statistics. These latter show an annual average catch of 8.5 t, with a collapse from ca. 41.7 t in 1987 to 0.67 t in 1995 that points to an overexploitation of this resource. This fishery is located in the littoral of Málaga, being very important for the local economy. The lack of a closed season since 1990 in Málaga province is a consequence of the absence of biological studies on the reproductive cycle of the species in the area. The overexploitation of this resource and the other shellfish from southern Spain, has lead the regional fishery authorities (Consejer´ıa de Agricultura y Pesca) to promote a research project on the reproductive cycles of the most important shellfish of Andaluc´ıa. The main objective was to establish a closed season for each species and a better management of these fisheries (Tirado and Rodriguez de la Rúa, 2000). The aim of this paper, as part of the above project, is to provide information on the reproductive cycle of
V. verrucosa in the littoral of Málaga and to suggest a closed season for a better management of this fishery.
2. Materials and methods A total of 3409 specimens of V. verrucosa was examined, from which 2933 specimens were used for the analysis of flesh dry weight variation (ca. 185 specimens/sample) and 476 specimens for histological study (usually 30 per sample). The samples were collected between Fuengirola and Marbella (36◦ 29.5 N–4◦ 48 W) (Fig. 1), at 10 m depth, on a sandy bottom, from June 1999 to May 2000, with monthly frequency from October to March and with fortnightly frequency in the other spring and summer months. The specimens were captured using a dredge with a toothed aperture, with teeth 6 cm long. The mesh was 7.5 cm, usual among the fishermen of the area. The length (L) of every specimen was measured to the nearest millimeter, and the soft parts were then
Fig. 1. Sampling area. The sampling site is indicated with
.
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Table 1 Different stages of development of the gonad, according with the scales of De Villiers (1975), Seed (1969) and Boyden (1971) (G: gonad) Authors
Stages of gonad (G)
De Villiers (1975) Seed (1969) Boyden (1971)
Cytolized Resting G (0) Indeterminated G (I)
Preactive Developing G (1, 2, 3) Developing G (II)
extracted and dried in an oven at 100 ◦ C for 24 h, and weighed to the nearest milligram (flesh dry weight, FDW). Two different indices of condition were applied, FDW/L3 variation, and that proposed for Crosby and Gale (1990) as FDW × 1000/volume of the internal cavity of the shell (considered the milliliters of water as milligrams) which is referred to as CI. The changes in weight could be related to somatic growth and/or variations in the size of the specimens between successive samples. Because of that, the variation of flesh dry weight was estimated for a standard individual of 52 mm long, size close to the mean size of the population, for which the changes in weight must be related with the development of the gonads. For the estimation of the monthly weights of the standard individual, we calculated the monthly regression lines for the weight–length relationship, after a logarithmic transformation of Ricker’s function W = aLb (Ricker, 1975), where W is the weight, L the length, a the ordinate at origin and b the slope. For histological processing, specimens were anaesthetized with MgCl2 , fixed in 10% formaldehyde, embedded in paraffin, sectioned at 10 mm and stained with haematoxylin of Carazzi and eosin, and a trichromic staining (VOF according to Gutiérrez (1967)) of haematoxylin 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) 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 either actively increasing and enlarging.
Active Ripe G (5) Ripe G (III)
Spawning Spawning G (4, 3) Spawning (IV)
Postactive Spawning G (2, 1) Resting G (V)
Spawning. The alveolar pattern is disturbed and the alveolar walls are often broken. The alveoli are often flattened and show an orientation towards 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 equivalent stages from Seed (1969) and Boyden (1971) are given in Table 1. To evaluate the possible influence of environmental factors on the cycle, the temperature of the seawater at 10 m depth was measured simultaneously with the collection of the individuals. Samples of water (2 l) were taken from the bottom for determination of chlorophyll a. Pigment analyses were carried out by filtering the water through Whatman GF/C glass filters. The pigments of the retained cells were then extracted with acetone for 12 h in cool, dark conditions, following the recommendations of Lorenzen and Jeffrey (1980). Concentrations of chlorophyll a were calculated using the trichromatic equations of Jeffrey and Humphrey (1975).
3. Results 3.1. Size composition and sex-ratio The shell length of V. verrucosa sampled ranged from 23 to 76 mm. The sex in V. verrucosa cannot be distinguished macroscopically by the color of the gonads. Therefore, sex determination must be made microscopically and this was done on 476 specimens. It was, however, impossible to determine the sex of four individuals from two monthly samples, so the 56 individuals of these 2 months were not considered for the sex-ratio estimation. From the remaining 420 individuals, 228 (54.29%) were males and 192
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Fig. 2. Monthly average flesh dry weight (FDW)/length (L3 ) ratio of V. verrucosa throughout the year of study (June 1999–May 2000). Bars show standard deviation.
(45.71%) females. The sex-ratio obtained was 1:1.19. According to the chi-square test (χ2 = 3084; d.f. = 1; P < 0.05) the different proportion of sex can be considered as not significant. 3.2. Sexual cycle 3.2.1. Flesh dry weight analysis The variation of flesh dry weight–length ratio [FDW/L3 ] during the annual cycle is shown in Fig. 2. The mean monthly values of both variables, flesh dry weight (FDW) and size (L3 ), were considered. The
standard deviations range between 14.94 and 25.15%, being broader than the standard deviations observed in the monthly average size (between 8 and 11%) (Fig. 3). The monthly average weights show strong standard deviations that range between 20 and 36% (Fig. 4). V. verrucosa shows from September to April three decreases of the above ratio, followed by an increase in the summer months (Fig. 2). The other index of condition (CI) shows a similar pattern (Fig. 5). According to the monthly regression lines for weight–length relationship (Table 2), the monthly
Fig. 3. Monthly average length (L) of the specimens of V. verrucosa throughout the year of study (June 1999–May 2000). Bars show standard deviation.
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Fig. 4. Monthly average flesh dry weight (FDW) of the specimens of V. verrucosa throughout the year of study (June 1999–May 2000). Bars show standard deviation.
variation of flesh dry weight for a standard individual of 52 mm long (mean size of the studied population), were estimated (Fig. 6). There are three decreases from September to April that are coincident but more pronounced than those in the indices of condition (Figs. 2 and 5). The most continuous increment of the flesh dry weight of the standard individual was registered between June and September. 3.2.2. Gametogenic cycle The data from the histological study are presented in Fig. 7. The gametogenic cycle is asynchronous in
the population, such shown by the presence of at least two development stages in nearly all the samples. An asynchrony is also detected in the individuals, due to the coexistence of areas with different stages in the same gonad. The population of V. verrucosa showed continuous spawning throughout the year. The whole population was spawning in August, while the lowest percentage of spawning (20% of the sample) was in December. Activation of the gonads begins in February, with an increment of active individuals, while their regression begins in September, with the occurrence of
Fig. 5. Monthly average CI values (flesh dry weight × 1000/volume of the internal cavity of the shell considered in equivalent grams of water) of V. verrucosa throughout the year of study (June 1999–May 2000).
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Table 2 Monthly linear regression for weight–length relationship, after the logarithmic transformation of Ricker’s function W = aLb (Ricker, 1975)a Month
Lm
Regression lines
1 June 2 June 1 July 2 July April 1 September 2 September October November December January February March 2 April 1 May 2 May
57.37 51.27 50.10 51.11 50.57 57.09 51.59 51.16 52.18 52.08 51.37 49.97 57.23 49.94 50.99 51.89
y y y y y y y y y y y y y y y y
= 1.8409x + 0.1266 = 2.8107x − 1.547 = 3.1404x − 2.0832 = 2.8773x − 1.624 = 3.015x − 1.8493 = 2.8498x − 1.5393 = 3.4475x − 2.6058 = 3.4580x − 2.6630 = 3.6641x − 3.0302 = 3.2511x − 2.2795 = 3.2637x − 2.3323 = 3.3491x − 2.4944 = 2.9081x − 1.6514 = 3.4451x − 2.6648 = 3.4345x − 2.598 = 3.251x − 2.2847
R2
R
N
W (L = 52 mm)
0.588 0.766 0.725 0.705 0.667 0.541 0.763 0.805 0.829 0.799 0.717 0.747 0.696 0.842 0.808 0.813
0.767 0.875 0.851 0.840 0.817 0.736 0.873 0.897 0.910 0.894 0.847 0.864 0.834 0.917 0.899 0.902
61 178 198 199 200 201 191 200 197 200 194 194 195 190 135 200
1930.12 1888.69 2021.74 2058.00 2110.77 2243.66 2042.26 1866.07 1808.82 2243.56 1854.42 1789.19 2182.22 1766.00 1975.16 1967.96
a Lm: average length; R2 : coefficient of determination; R: coefficient of correlation; N: number of observations; W (L = 52 mm): weight of a standard individual of 52 mm long (=the mean size of the studied population).
individuals in cytolized and postactive stages. There is not, however, a true resting (or inactive) period due to the existence of spawning in the population at any time of the year and the absence of high percentage of individuals in cytolized stage. Several cohorts of ovocytes can be detected throughout the year, together with the direct step from postactive to active stage, without the previous cytolized and preactive phases in many individuals.
3.3. Environmental factors The maximum seawater temperature at 10 m depth (Fig. 8) was registered in August (20 ◦ C) and the minimum (14.8–14.9 ◦ C) in April and January, respectively. The maximum of chlorophyll a (Fig. 8) levels occurs in the first half of August 2000 and the second peak in the first half of May 1999. Between these extremes, we can observe several minor peaks, at the
Fig. 6. Monthly variations in flesh dry weight in a standard animal 52 mm long (the mean size of the studied population from M´alaga) of V. verrucosa.
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Fig. 7. Monthly cumulative frequencies of different stages of development of the gonads in V. verrucosa. C: cytolized; Pr: preactive; EA: early active; A: active; S: spawning; Ps: postactive.
Fig. 8. Seawater temperatures and changes in concentration of chlorophyll a in seawater throughout the year of study.
end of summer (September), in autumn (October) and winter (March). Because of problems during sampling procedures, the data series on temperature and chlorophyll a levels in seawater are incomplete, so the correlation analyses have been omitted. 4. Discussion 4.1. Sex-ratio V. verrucosa does not show sexual dimorphism, and has a sex-ratio of 1:1. Our data on the sex-ratio are
similar to those of Galinou-Mitsoudi et al. (1997) from the Aegean Sea. We did not find any hermaphrodites. 4.2. Reproductive cycle According to the histological data, V. verrucosa in Málaga spawned throughout the year (Fig. 7), whereas in the Gulf of Trieste (northern Adriatic) the activity of the gonads lasts from December to September (Valli et al., 1988). In the lower Adriatic the spawning period extends from June to October (Marano et al., 1980, 1982), whereas in the Aegean Sea it lasts from May to November (Galinou-Mitsoudi et al., 1997; Arneri
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et al., 1998). The lower winter seawater temperatures in the Adriatic and Aegean Sea (between 7 and 8 ◦ C in December to 12–15 ◦ C in April) could explain these differences in the spawning period, since the milder seawater temperature in the littoral of Málaga (Fig. 8) would favor a long reproductive cycle. The absence of a resting (or inactive) period in the littoral of Málaga, as in the Adriatic populations, could be related to the high amount of phytoplankton in both areas (Valli et al., 1994), while low amount of phytoplankton and low seawater temperatures during the winter months in the Gulfs of Thessaloniki and Termaikos, could explain the existence of a resting period (cytolized stage) in these populations. Although spawning occurs throughout the year, two peaks of high percentages of spawning were observed from the histological study, one from March to April and another from June to August (Fig. 7). Of the three decreases in weight, reflected by the condition indices, FDW/L3 (Fig. 2) and CI (Fig. 5), and the standard individual (Fig. 6), only one (from March to April) coincides with a high percentage of population in spawning (Fig. 7). The two remaining decreases seem to be related with regression of the gonads, with a high percentage of individuals in postactive and cytolized stages (Fig. 7). August, with 100% of the histological sample in spawning (Fig. 7) did not show any loss of weight, which seems to indicate very poor release of gametes (partial emission). The above data show that the most important spawning period (maximum amount of released gametes) was March–April. Another species with a similar spawning period in the littoral of Málaga is Callista chione, for which the most important release of gametes appears between February and March (Tirado and Salas, 2002). It is interesting to observe that the peak of spawning of V. verrucosa occurs when the seawater temperature is relatively low (14.8 ◦ C in April), but there is, at the same time, a continuous increase of chlorophyll a in the seawater (Fig. 8). This could indicate some relationship between levels of phytoplankton and spawning, as found in Galicia with Crassostrea gigas (Ruiz et al., 1992). Galinou-Mitsoudi et al. (1997) also pointed that the transparency of the water and the decrease of dissolved oxygen affect the spawning time in the Aegean populations.
In the population of V. verrucosa from Málaga, there are new activations of the gonads, with repeated small partial releases, during the spring and summer months that did not coincide significant loss of flesh dry weight (Figs. 2, 6 and 7). Valli and Cester (1980) also observed the existence of several periods of emission during the reproductive cycle in the Gulf of Trieste. The asynchronic gametogenic cycle in the population is reflected by the high standard deviations of the flesh dry weights (Figs. 2 and 4), the existence of several cohorts of ovocytes and the coexistence of several stages of development in the same gonad. The coexistence of different stages has been found in many bivalves from temperate areas, among them, Chamelea striatula (Ansell, 1961), D. serra (De Villiers, 1975), Tapes rhomboides (Morvan and Ansell, 1988), D. trunculus (Tirado and Salas, 1998), D. venustus and D. semistriatus (Tirado and Salas, 1999). Although the total of captures of V. verrucosa is relatively low in Andaluc´ıa according to official statistical data, they are obtained in totality from the littoral of Málaga, and therefore this fishery is important in the local economy. Because there are no previous studies on the biology of this species in southern Spain we do not know the evolution of the average size in the Málaga population. However, the reduction of the volume of catches suggests an overexploitation of this resource, and indicates the need for a closed season. According to the data of this study the closed season should be established from March to April, when there is the most intense release of gametes (high loss of weight together with high percentage of the population in spawning).
Acknowledgements The project has been supported by the Consejer´ıa de Agricultura y Pesca (Department of Fishery) and entrusted to D.A.P. Enterprise (Tirado and Rodriguez de la Rúa, 2000). The authors wish to thank David López, Daniel Gómez and Ma José Garc´ıa-Patiño for helping in the laboratory work. This study is part of a project supported by the Junta de Andaluc´ıa (Department of Fishery). We thank Ma Dolores Atienza, General Manager of the Department of Fishery for her support and permission to publish. We are grateful to Manuel Castañon (Provincial Manager of Fishery) for
C. Tirado et al. / Fisheries Research 63 (2003) 437–445
encouraging this research. The project was entrusted to D.A.P. Enterprise, which is thanked for the facilities given for the conduct of the work and its publication. We are grateful to J. Ignacio López and Manuel Aguilar for their management and the facilities provided. Eva Garc´ıa is thanked for helping with the bibliography.
References Ansell, A.D., 1961. Reproduction, growth and mortality of Venus striatula (Da Costa) in Kame Bay. Millport. J. Mar. Biol. Assoc. UK 41, 191–215. Arneri, E., Giannetti, G., Antolini, B., 1998. Age determination and growth of Venus verrucosa L. (Bivalvia: Veneridae) in the southern Adriatic and the Aegean Sea. Fish. Res. 38, 193– 198. Bello, G., 1985. Analisi biometrica di Venus verrucosa L. (Bivalvia: Veneridae) dell’Adriatico meridionale, e considerazioni sulla sua pesca. Quad. Ist. Ric. Pesca Maritt., Ancona 4 (2), 173–182. Bello, G., 1986. Porcentajes teóricos de retención de Venus verrucosa L. (Bivalvia: Veneridae) con cedazos de varillas. Invest. Pesq. 50 (2), 167–177. Berthou, P., Le Gall, Y., 1980. Biologie et pˆeche de la praire Venus verrucosa en Manche Occidentale (Bretagne et Normandie). 4. Dynamique des populations, evaluation et gestion rationelle du stock. ICES, CM, 1980/K7, 49 pp. Berthou, P., Le Gall, Y., Djabali, F., Yahiaoui, M., 1980. Biologie et pˆeche de la praire Venus verrucosa en Manche Occidentale (Bretagne et Normandie). 2. Reproduction et fecondité. ICES, CM, 1980/K5, 18 pp. Boyden, C.R., 1971. A comparative study of the reproductive cycles of the cockles Cerastoderma edule and C. glaucum. J. Mar. Biol. Assoc. UK 51, 605–622. Crosby, M.P., Gale, L., 1990. A review and evaluation of bivalve condition index methodologies with a suggested standard method. J. Shellfish Res. 9 (1), 233–237. De Villiers, G., 1975. Reproduction of the sand mussel Donax serra Röding. Invest. Rep. Sea Fish. Brch. Repub. S. Afr. 109, 1–31. Del Piero, D., 1994. The clam fishery in the Gulf of Trieste. In: Proceedings of the Conference, Coastal Zone’94 on Cooperation in the Coastal Zone, vol. 4, pp. 1645–1660. Galinou-Mitsoudi, S., Sinis, A.I., Petridis, D., 1997. Reproduction of Venus verrucosa in the Thessaloniki and Thermaikos Gulfs. In: Proceedings of the Fifth Hellenic Symposium on Oceanography and Fishery, vol. 2, Kavala, Greece, pp. 107–110 (in Greek). Gutiérrez, M., 1967. Coloración histológica para el ovario de peces, crustáceos y moluscos. Invest. Pesq. 31 (2), 265–271. Jeffrey, S.W., Humphrey, G.T., 1975. New spectrophotometric equation for determining chlorophylls a, b, c1 and c2 in higher
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plants, algae and natural phytoplankton. Biochem. Physiol. Pflanz. 167, 191–194. Lorenzen, C.J., Jeffrey, S.W., 1980. Determination of chlorophyll in seawater. UNESCO Tech. Pap. Mar. Sci. 35, 1–20. Marano, G., Casavola, N., Saracino, C., 1980. Indagine comparativa sulla riproduzione di Chamelea gallina (L.), Venus verrucosa (L.), Rudicardium tuberculatum (L.) nel basso Adriatico. Mem. Biol. Mar. Oceanogr. 10 (Suppl. 6), 229–234. Marano, G., Casavola, N., Saracino, C., Rizzi, E., 1982. Riproduzione e crescita di Chamelea gallina (L.) e Venus verrucosa (L.) (Bivalvia: Veneridae) nel basso Adriatico. Mem. Biol. Mar. Oceanogr. 12 (2), 97–114. Morvan, C., Ansell, A.D., 1988. Stereological methods applied to reproductive cycle of Tapes rhomboides. Mar. Biol. 97, 355– 364. Poppe, G.T., Goto, Y., 1993. European Seashells, vol. 2. Scaphopoda, Bivalvia, Cephalopoda. Ed. Christa Hemmen, Wiesbaden, 221 pp. Ricker, W., 1975. Computation and interpretation of biological statistics of fish population. Bull. Fish. Res. Board Can. 191, 1–382. Rossi, R., Campioni, D., Conte, A.G., Paesanti, F., Turolla, E., 1994. Artificial reproduction of warty venus (Venus verrucosa): preliminary results. Riv. Ital. Acquacolt. 29 (2), 53–62.4. Ruiz, C., Abad, M., Sedano, F., Garc´ıa-Mart´ın, L.O., Sánchez López, J.L., 1992. Influence of seasonal environmental changes on the gamete production and biochemical composition of Crassostrea gigas (Thunberg) in suspended culture in El Grove, Galicia, Spain. J. Exp. Mar. Biol. Ecol. 155, 249–262. Seed, R., 1969. The ecology of Mytilus edulis L. (Lamellibranchiata) on exposed rocky shores. I. Breeding and settlement. Oecologia 3, 277–316. Tirado, C., Rodriguez de la Rúa, A., 2000. Estudio del ciclo reproductor de los moluscos bivalvos y gasterópodos del litoral andaluz. Informe Técnico. Consejer´ıa de Agricultura y Pesca, Junta de Andaluc´ıa, 357 pp. Tirado, C., Salas, C., 1998. Reproduction and fecundity of Donax trunculus L., 1758 (Bivalvia: Donacidae) in the littoral of Málaga (southern Spain). J. Shellfish Res. 17 (1), 169–176. Tirado, C., Salas, C., 1999. Reproduction of Donax venustus Poli 1795, Donax semistriatus Poli 1795 and intermediate morphotypes (Bivalvia: Donacidae) in the littoral of Málaga (southern Spain). P.S.Z.N. Mar. Ecol. 20 (2), 111–130. Tirado, C., Salas, C., 2002. Reproduction of Callista chione (L., 1758) (Bivalvia: Veneridae) in the littoral of Málaga (southern Spain). J. Shellfish Res. 21 (2), 643–648. Valli, G., Cester, P., 1980. Riproduzione e biometria di Venus verrucosa L. Nota preliminare. Nova Thalassia 4, 193–194. Valli, G., Nodari, P., Castenetto, D., 1988. Reproduction, biometrie et indices de condition chez Venus verrucosa (Mollusca: Bivalvia) du Golfe du Trieste. Rapp. Comm. Int. Mer Médit. 31 (2), 15. Valli, G., Marsich, N., Marsich, M.G., 1994. Riproduzione, biometria e contenuto di metalli in Callista chione (L.) (Mollusca, Bivalvia) del Golfo di Trieste nel corso di un ciclo annuale. Boll. Soc. Adriat. Sci. 75 (2), 441–464.
Short communication
Reproduction of Venus verrucosa L., 1758 (Bivalvia: Veneridae) in the littoral of Málaga (southern Spain) C. Tirado a , C. Salas a,∗ , I. Márquez b a
Dept. Biolog´ıa Animal, Facultad de Ciencias, Universidad de Málaga, E-29071-Málaga, Spain b Consejer´ıa de Agricultura y Pesca de la Junta de Andaluc´ıa, E-41071-Sevilla, Spain
Received 27 March 2002; received in revised form 14 February 2003; accepted 27 February 2003
Abstract The reproductive cycle of Venus verrucosa Linnaeus, 1758 was studied using histology and changes in flesh dry weight, in the littoral of Málaga (southern Spain), from June 1999 to May 2000. Histological study of the gonads showed spawning throughout the year, with two peaks. The first lasts from March to April, and is accompanied by the highest decrease of weight; the second is from May to August with the highest percentage of population spawning. There is new activation of the gonads from postactive stages, without passing through a resting period (with a high percentage of the population in the cytolized stage). The absence of a resting period in the reproductive cycle could be explained by the mild seawater temperatures and high levels of chlorophyll a in the littoral of Málaga. In the light of these data, we propose a closed season from March to April, months during which there was the most intense release of gametes in the population. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Venus verrucosa; Histology; Condition index; Spawning season; Fishery management
1. Introduction Venus verrucosa Linnaeus, 1758, the “warty Venus”, is an Atlantic–Mediterranean species distributed in the Atlantic from Norway to South Africa (Durban), and also in the Mediterranean (Poppe and Goto, 1993). It lives in poorly sorted sand; sometime with coralline rhodoliths and in channels between beds of Posidonia oceanica down to a depth of about 30 m (Arneri et al., 1998). This species is particularly appreciated in France, where it is known as “praire”. Catches of 3500 t per year are registered from the coast of Normandy and Brittany (Berthou and Le Gall, 1980; Berthou et al., ∗ Corresponding author. E-mail address: [email protected] (C. Salas).
1980). In the European Mediterranean countries the fisheries are local, reaching about 500 t per year in the southern Adriatic (Italy) and Greece (Arneri et al., 1998). Most of the studies on V. verrucosa in the Mediterranean Sea have been carried out in Italy, particularly on the Adriatic populations. The reproduction has been analyzed by Valli and Cester (1980) and Marano et al. (1980, 1982), and the fishery by Bello (1985, 1986) and Del Piero (1994). Rossi et al. (1994) have attempted to reproduce the species artificially in a hatchery at Ferrara (Italy), but in this initial set of experiments the survival did not go beyond the 25th week. In Greece, the reproduction has been studied by Galinou-Mitsoudi et al. (1997) in populations from the Gulfs of Thessaloniki and Termaikos.
0165-7836/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0165-7836(03)00106-1
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In southern Spain, V. verrucosa, locally known as “bolo”, is a much-appreciated shellfish, with an illegal market of the same order of magnitude as the official catch, not taken into account by statistics. These latter show an annual average catch of 8.5 t, with a collapse from ca. 41.7 t in 1987 to 0.67 t in 1995 that points to an overexploitation of this resource. This fishery is located in the littoral of Málaga, being very important for the local economy. The lack of a closed season since 1990 in Málaga province is a consequence of the absence of biological studies on the reproductive cycle of the species in the area. The overexploitation of this resource and the other shellfish from southern Spain, has lead the regional fishery authorities (Consejer´ıa de Agricultura y Pesca) to promote a research project on the reproductive cycles of the most important shellfish of Andaluc´ıa. The main objective was to establish a closed season for each species and a better management of these fisheries (Tirado and Rodriguez de la Rúa, 2000). The aim of this paper, as part of the above project, is to provide information on the reproductive cycle of
V. verrucosa in the littoral of Málaga and to suggest a closed season for a better management of this fishery.
2. Materials and methods A total of 3409 specimens of V. verrucosa was examined, from which 2933 specimens were used for the analysis of flesh dry weight variation (ca. 185 specimens/sample) and 476 specimens for histological study (usually 30 per sample). The samples were collected between Fuengirola and Marbella (36◦ 29.5 N–4◦ 48 W) (Fig. 1), at 10 m depth, on a sandy bottom, from June 1999 to May 2000, with monthly frequency from October to March and with fortnightly frequency in the other spring and summer months. The specimens were captured using a dredge with a toothed aperture, with teeth 6 cm long. The mesh was 7.5 cm, usual among the fishermen of the area. The length (L) of every specimen was measured to the nearest millimeter, and the soft parts were then
Fig. 1. Sampling area. The sampling site is indicated with
.
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Table 1 Different stages of development of the gonad, according with the scales of De Villiers (1975), Seed (1969) and Boyden (1971) (G: gonad) Authors
Stages of gonad (G)
De Villiers (1975) Seed (1969) Boyden (1971)
Cytolized Resting G (0) Indeterminated G (I)
Preactive Developing G (1, 2, 3) Developing G (II)
extracted and dried in an oven at 100 ◦ C for 24 h, and weighed to the nearest milligram (flesh dry weight, FDW). Two different indices of condition were applied, FDW/L3 variation, and that proposed for Crosby and Gale (1990) as FDW × 1000/volume of the internal cavity of the shell (considered the milliliters of water as milligrams) which is referred to as CI. The changes in weight could be related to somatic growth and/or variations in the size of the specimens between successive samples. Because of that, the variation of flesh dry weight was estimated for a standard individual of 52 mm long, size close to the mean size of the population, for which the changes in weight must be related with the development of the gonads. For the estimation of the monthly weights of the standard individual, we calculated the monthly regression lines for the weight–length relationship, after a logarithmic transformation of Ricker’s function W = aLb (Ricker, 1975), where W is the weight, L the length, a the ordinate at origin and b the slope. For histological processing, specimens were anaesthetized with MgCl2 , fixed in 10% formaldehyde, embedded in paraffin, sectioned at 10 mm and stained with haematoxylin of Carazzi and eosin, and a trichromic staining (VOF according to Gutiérrez (1967)) of haematoxylin 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) 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 either actively increasing and enlarging.
Active Ripe G (5) Ripe G (III)
Spawning Spawning G (4, 3) Spawning (IV)
Postactive Spawning G (2, 1) Resting G (V)
Spawning. The alveolar pattern is disturbed and the alveolar walls are often broken. The alveoli are often flattened and show an orientation towards 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 equivalent stages from Seed (1969) and Boyden (1971) are given in Table 1. To evaluate the possible influence of environmental factors on the cycle, the temperature of the seawater at 10 m depth was measured simultaneously with the collection of the individuals. Samples of water (2 l) were taken from the bottom for determination of chlorophyll a. Pigment analyses were carried out by filtering the water through Whatman GF/C glass filters. The pigments of the retained cells were then extracted with acetone for 12 h in cool, dark conditions, following the recommendations of Lorenzen and Jeffrey (1980). Concentrations of chlorophyll a were calculated using the trichromatic equations of Jeffrey and Humphrey (1975).
3. Results 3.1. Size composition and sex-ratio The shell length of V. verrucosa sampled ranged from 23 to 76 mm. The sex in V. verrucosa cannot be distinguished macroscopically by the color of the gonads. Therefore, sex determination must be made microscopically and this was done on 476 specimens. It was, however, impossible to determine the sex of four individuals from two monthly samples, so the 56 individuals of these 2 months were not considered for the sex-ratio estimation. From the remaining 420 individuals, 228 (54.29%) were males and 192
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Fig. 2. Monthly average flesh dry weight (FDW)/length (L3 ) ratio of V. verrucosa throughout the year of study (June 1999–May 2000). Bars show standard deviation.
(45.71%) females. The sex-ratio obtained was 1:1.19. According to the chi-square test (χ2 = 3084; d.f. = 1; P < 0.05) the different proportion of sex can be considered as not significant. 3.2. Sexual cycle 3.2.1. Flesh dry weight analysis The variation of flesh dry weight–length ratio [FDW/L3 ] during the annual cycle is shown in Fig. 2. The mean monthly values of both variables, flesh dry weight (FDW) and size (L3 ), were considered. The
standard deviations range between 14.94 and 25.15%, being broader than the standard deviations observed in the monthly average size (between 8 and 11%) (Fig. 3). The monthly average weights show strong standard deviations that range between 20 and 36% (Fig. 4). V. verrucosa shows from September to April three decreases of the above ratio, followed by an increase in the summer months (Fig. 2). The other index of condition (CI) shows a similar pattern (Fig. 5). According to the monthly regression lines for weight–length relationship (Table 2), the monthly
Fig. 3. Monthly average length (L) of the specimens of V. verrucosa throughout the year of study (June 1999–May 2000). Bars show standard deviation.
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Fig. 4. Monthly average flesh dry weight (FDW) of the specimens of V. verrucosa throughout the year of study (June 1999–May 2000). Bars show standard deviation.
variation of flesh dry weight for a standard individual of 52 mm long (mean size of the studied population), were estimated (Fig. 6). There are three decreases from September to April that are coincident but more pronounced than those in the indices of condition (Figs. 2 and 5). The most continuous increment of the flesh dry weight of the standard individual was registered between June and September. 3.2.2. Gametogenic cycle The data from the histological study are presented in Fig. 7. The gametogenic cycle is asynchronous in
the population, such shown by the presence of at least two development stages in nearly all the samples. An asynchrony is also detected in the individuals, due to the coexistence of areas with different stages in the same gonad. The population of V. verrucosa showed continuous spawning throughout the year. The whole population was spawning in August, while the lowest percentage of spawning (20% of the sample) was in December. Activation of the gonads begins in February, with an increment of active individuals, while their regression begins in September, with the occurrence of
Fig. 5. Monthly average CI values (flesh dry weight × 1000/volume of the internal cavity of the shell considered in equivalent grams of water) of V. verrucosa throughout the year of study (June 1999–May 2000).
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Table 2 Monthly linear regression for weight–length relationship, after the logarithmic transformation of Ricker’s function W = aLb (Ricker, 1975)a Month
Lm
Regression lines
1 June 2 June 1 July 2 July April 1 September 2 September October November December January February March 2 April 1 May 2 May
57.37 51.27 50.10 51.11 50.57 57.09 51.59 51.16 52.18 52.08 51.37 49.97 57.23 49.94 50.99 51.89
y y y y y y y y y y y y y y y y
= 1.8409x + 0.1266 = 2.8107x − 1.547 = 3.1404x − 2.0832 = 2.8773x − 1.624 = 3.015x − 1.8493 = 2.8498x − 1.5393 = 3.4475x − 2.6058 = 3.4580x − 2.6630 = 3.6641x − 3.0302 = 3.2511x − 2.2795 = 3.2637x − 2.3323 = 3.3491x − 2.4944 = 2.9081x − 1.6514 = 3.4451x − 2.6648 = 3.4345x − 2.598 = 3.251x − 2.2847
R2
R
N
W (L = 52 mm)
0.588 0.766 0.725 0.705 0.667 0.541 0.763 0.805 0.829 0.799 0.717 0.747 0.696 0.842 0.808 0.813
0.767 0.875 0.851 0.840 0.817 0.736 0.873 0.897 0.910 0.894 0.847 0.864 0.834 0.917 0.899 0.902
61 178 198 199 200 201 191 200 197 200 194 194 195 190 135 200
1930.12 1888.69 2021.74 2058.00 2110.77 2243.66 2042.26 1866.07 1808.82 2243.56 1854.42 1789.19 2182.22 1766.00 1975.16 1967.96
a Lm: average length; R2 : coefficient of determination; R: coefficient of correlation; N: number of observations; W (L = 52 mm): weight of a standard individual of 52 mm long (=the mean size of the studied population).
individuals in cytolized and postactive stages. There is not, however, a true resting (or inactive) period due to the existence of spawning in the population at any time of the year and the absence of high percentage of individuals in cytolized stage. Several cohorts of ovocytes can be detected throughout the year, together with the direct step from postactive to active stage, without the previous cytolized and preactive phases in many individuals.
3.3. Environmental factors The maximum seawater temperature at 10 m depth (Fig. 8) was registered in August (20 ◦ C) and the minimum (14.8–14.9 ◦ C) in April and January, respectively. The maximum of chlorophyll a (Fig. 8) levels occurs in the first half of August 2000 and the second peak in the first half of May 1999. Between these extremes, we can observe several minor peaks, at the
Fig. 6. Monthly variations in flesh dry weight in a standard animal 52 mm long (the mean size of the studied population from M´alaga) of V. verrucosa.
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Fig. 7. Monthly cumulative frequencies of different stages of development of the gonads in V. verrucosa. C: cytolized; Pr: preactive; EA: early active; A: active; S: spawning; Ps: postactive.
Fig. 8. Seawater temperatures and changes in concentration of chlorophyll a in seawater throughout the year of study.
end of summer (September), in autumn (October) and winter (March). Because of problems during sampling procedures, the data series on temperature and chlorophyll a levels in seawater are incomplete, so the correlation analyses have been omitted. 4. Discussion 4.1. Sex-ratio V. verrucosa does not show sexual dimorphism, and has a sex-ratio of 1:1. Our data on the sex-ratio are
similar to those of Galinou-Mitsoudi et al. (1997) from the Aegean Sea. We did not find any hermaphrodites. 4.2. Reproductive cycle According to the histological data, V. verrucosa in Málaga spawned throughout the year (Fig. 7), whereas in the Gulf of Trieste (northern Adriatic) the activity of the gonads lasts from December to September (Valli et al., 1988). In the lower Adriatic the spawning period extends from June to October (Marano et al., 1980, 1982), whereas in the Aegean Sea it lasts from May to November (Galinou-Mitsoudi et al., 1997; Arneri
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et al., 1998). The lower winter seawater temperatures in the Adriatic and Aegean Sea (between 7 and 8 ◦ C in December to 12–15 ◦ C in April) could explain these differences in the spawning period, since the milder seawater temperature in the littoral of Málaga (Fig. 8) would favor a long reproductive cycle. The absence of a resting (or inactive) period in the littoral of Málaga, as in the Adriatic populations, could be related to the high amount of phytoplankton in both areas (Valli et al., 1994), while low amount of phytoplankton and low seawater temperatures during the winter months in the Gulfs of Thessaloniki and Termaikos, could explain the existence of a resting period (cytolized stage) in these populations. Although spawning occurs throughout the year, two peaks of high percentages of spawning were observed from the histological study, one from March to April and another from June to August (Fig. 7). Of the three decreases in weight, reflected by the condition indices, FDW/L3 (Fig. 2) and CI (Fig. 5), and the standard individual (Fig. 6), only one (from March to April) coincides with a high percentage of population in spawning (Fig. 7). The two remaining decreases seem to be related with regression of the gonads, with a high percentage of individuals in postactive and cytolized stages (Fig. 7). August, with 100% of the histological sample in spawning (Fig. 7) did not show any loss of weight, which seems to indicate very poor release of gametes (partial emission). The above data show that the most important spawning period (maximum amount of released gametes) was March–April. Another species with a similar spawning period in the littoral of Málaga is Callista chione, for which the most important release of gametes appears between February and March (Tirado and Salas, 2002). It is interesting to observe that the peak of spawning of V. verrucosa occurs when the seawater temperature is relatively low (14.8 ◦ C in April), but there is, at the same time, a continuous increase of chlorophyll a in the seawater (Fig. 8). This could indicate some relationship between levels of phytoplankton and spawning, as found in Galicia with Crassostrea gigas (Ruiz et al., 1992). Galinou-Mitsoudi et al. (1997) also pointed that the transparency of the water and the decrease of dissolved oxygen affect the spawning time in the Aegean populations.
In the population of V. verrucosa from Málaga, there are new activations of the gonads, with repeated small partial releases, during the spring and summer months that did not coincide significant loss of flesh dry weight (Figs. 2, 6 and 7). Valli and Cester (1980) also observed the existence of several periods of emission during the reproductive cycle in the Gulf of Trieste. The asynchronic gametogenic cycle in the population is reflected by the high standard deviations of the flesh dry weights (Figs. 2 and 4), the existence of several cohorts of ovocytes and the coexistence of several stages of development in the same gonad. The coexistence of different stages has been found in many bivalves from temperate areas, among them, Chamelea striatula (Ansell, 1961), D. serra (De Villiers, 1975), Tapes rhomboides (Morvan and Ansell, 1988), D. trunculus (Tirado and Salas, 1998), D. venustus and D. semistriatus (Tirado and Salas, 1999). Although the total of captures of V. verrucosa is relatively low in Andaluc´ıa according to official statistical data, they are obtained in totality from the littoral of Málaga, and therefore this fishery is important in the local economy. Because there are no previous studies on the biology of this species in southern Spain we do not know the evolution of the average size in the Málaga population. However, the reduction of the volume of catches suggests an overexploitation of this resource, and indicates the need for a closed season. According to the data of this study the closed season should be established from March to April, when there is the most intense release of gametes (high loss of weight together with high percentage of the population in spawning).
Acknowledgements The project has been supported by the Consejer´ıa de Agricultura y Pesca (Department of Fishery) and entrusted to D.A.P. Enterprise (Tirado and Rodriguez de la Rúa, 2000). The authors wish to thank David López, Daniel Gómez and Ma José Garc´ıa-Patiño for helping in the laboratory work. This study is part of a project supported by the Junta de Andaluc´ıa (Department of Fishery). We thank Ma Dolores Atienza, General Manager of the Department of Fishery for her support and permission to publish. We are grateful to Manuel Castañon (Provincial Manager of Fishery) for
C. Tirado et al. / Fisheries Research 63 (2003) 437–445
encouraging this research. The project was entrusted to D.A.P. Enterprise, which is thanked for the facilities given for the conduct of the work and its publication. We are grateful to J. Ignacio López and Manuel Aguilar for their management and the facilities provided. Eva Garc´ıa is thanked for helping with the bibliography.
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