BULLETIN OF MARINE SCIENCE, 86(4): 785–801, 2010 doi:10.5343/bms.2009.1077

REPRODUCTIVE CYCLE OF THE PENSHELL seminuda (: ) IN NORTHERN WATERS OF VENEZUELA

L. Freites, C. Cordova, D. Arrieche, L. Montero, N. García, and J. H. Himmelman

ABSTRACT We examined reproduction of the pinnid bivalve (Lamarck, 1819) in tropical waters off northeastern Venezuela and compare it with reproduction of the same species in temperate waters of Argentina, as reported elsewhere (Soria et al., 2002). In Venezuela, a proportion of individuals was reproducing throughout the year, although there was increased spawning activity in certain periods (May–July 2004, September–November 2004, and February 2005). Stepwise multi-regression analyses correlating the frequency of the different gametogenic stages with various environmental factors suggested that reproduction was likely controlled primarily by temperature and secondarily by food abundance (as measured by chlorophyll a). During the first part of the year gamete production appeared to be supported by food resources obtained from the environment (indicating an opportunistic reproductive strategy), whereas gamete production during a period of relatively low phytoplanktonic food availability (June–September) suggested that it was supported by tissue reserves (a conservative reproductive strategy). The above reproductive pattern contrasts with A. seminuda in Argentina where gametogenesis is highly synchronized among individuals and follows an annual cycle. Also, A. seminuda in Argentina displays only the conservative reproductive strategy, with gamete production occurring during the period of food scarcity in the autumn and probably supported by reserves from the adductor muscle (Soria et al., 2002). Together, these data provide the first comparison of reproduction of the same species in both tropical and temperate waters (separated by 50° in latitude).

Reproductive timing should vary in different regions for marine invertebrates with wide geographical distributions (particularly for species distributed along a latitudi- nal gradient), as individuals in different regions are exposed to contrasting environ- mental conditions. The degree to which conditions vary seasonally increases towards the poles and at high latitudes, thus the period when conditions are appropriate for gamete production and larval survival is likely to be restricted (Lalli and Pearson, 1993). Temperate species usually show highly synchronous reproduction with an an- nual cycle. Often active gametogenesis takes place in the spring and early summer (when phytoplankton is abundant), spawning occurs in summer, and then there is reproductive rest during the winter. Such patterns are demonstrated by the mussel Mytilus galloprovincialis (Lamarck, 1819) in Galicia, Spain (Villalba, 1995), and Myti- lus edulis (Linnaeus, 1758) in Newfoundland, Canada (Thompson, 1984). In contrast, bivalves in tropical regions generally show continuous and asynchronous reproduc- tion, with some individuals producing gametes and spawning in every month of the year. Examples are Anadara grandis (Broderip and Sowerby, 1829) (Cruz-Soto, 1987), Polymesoda radiata (Ruiz et al., 1998), Arca zebra (Swainson, 1833) (Lista et al., 2006), Atrina maura (Sowerby, 1835) (Angel-Pérez et al., 2007), and Nodipecten

Bulletin of Marine Science 785 © 2010 Rosenstiel School of Marine and Atmospheric Science of the University of Miami 786 BULLETIN OF MARINE SCIENCE, VOL. 86, NO. 4, 2010 nodosus (Linnaeus, 1758) (García et al., 2007). Since few species extend from temper- ate to tropical waters, it is not clear how a species with a wide distribution will adapt to the major shifts in environmental conditions. Changes in reproductive patterns within a species across latitudes have only been documented for a small number of species (Urban and Campos, 1994; Angel-Pérez et al., 2007; Cardoso et al., 2007). One example is the study of Bricelj et al. (1987) on the reproduction of the Argopecten irradians (Lamarck, 1819) in widely separated regions. They show that scallop size-specific fecundity is seven fold greater on the coast of New York, USA (40°N), than near the southern limit of its distribu- tion in Florida, USA (27°N). Further, reproduction in A. irradians has been shown to occur later in the year and at higher water temperatures at more southern latitudes. In Massachusetts, USA (42°N), gonad growth in A. irradians begins in April when temperatures attain about 10 °C, gonad size peaks in July to August, and spawn- ing occurs during August and September (Sastry, 1970); whereas in North Carolina, USA (35°N), gonad growth begins in June when the temperature attains 20 °C, go- nad size peaks in September, and spawning occurs in October (Sastry, 1970, 1979). Other studies have indicated differences in the timing of reproductive activities in latitudinally separated populations of the same species of bivalve, and food avail- ability appears to be an important factor in explaning the observed shifts in the re- productive cycles (Newell et al., 1982; Barber and Blake, 1983; Cardoso et al., 2007). Such changes in reproduction presumably represent adaptations to environmental changes (Cantillanez et al., 2005; Avendaño et al., 2008). Reproductive strategies displayed by bivalves, even those within a given geo- graphical region, often vary according to the manner in which energy is provided for reproduction (Bayne, 1973). Some species, such as the cockle Cerastoderma edule (Linnaeus, 1758) (Navarro et al., 1989), show a “conservative” reproductive strategy. Energy reserves are built up over a period and later mobilized to support gamete production. Such reproductive activity is strongly linked to energy transfers between tissues. In contrast, other species, such as the clam Venus mercenaria (Linnaeus, 1758) (Ansell and Lander, 1967) and the cockle Glycymeris glycymeris (Linnaeus, 1758) (Galap et al., 1997), show an “opportunistic” strategy. Gametes are only pro- duced during periods when food is available; otherwise, there is reproductive rest. Interestingly, the mussel Mytilus galloprovincialis Lamarck, 1819 appears to show the conservative strategy in some periods and the opportunistic strategy in others (Villalba, 1995). Atrina seminuda (Lamarck, 1819) is a member of the family , which in- cludes bivalves commonly known by such names as callo de hacha, cucharón, cholga paleta, and fan shell. Atrina species generally live in shallow bays and coastal la- goons, on sandy bottoms or in beds of the seagrass Thallassia testudinum Banks & Soland. ex König (Kuhlmann, 1998; Lodeiros et al., 1999; Munguia, 2004). Although A. seminuda has a very wide geographical distribution in the western Atlantic, from North Carolina in the United States to Patagonia in Argentina (Lodeiros et al., 1999), the only report of its reproduction is by Soria et al. (2002) for a population in Argen- tina where there is a strong seasonal environmental cycle. These authors show that A. seminuda in Argentina has a clearly annual reproductive cycle which finishes in the summer (February). The present study examines reproduction of the penshell A. seminuda in tropical waters in northeastern Venezuela and compares it with the pattern observed for A. seminuda in temperate waters in Argentina, as reported by Soria et al. (2002).

freites et al.: reproductive cycle of atrina seminuda 787

Materials and Methods

We studied a population of A. seminuda at Guayacán, on the north coast of the Araya Pen- insula, northeastern Venezuela (Fig. 1). The collection site was at 3–7 m in depth on a sandy bottom and this area supported a bed of the seagrass T. testudinum. There is considerable resuspension of sediments in the area due to wave exposure and strong tidal currents (semi- diurnal tides with an average amplitude of 0.82 m). To examine the reproductive cycle, we collected 30 adult individuals (160–220 mm in shell length) by snorkel at fortnightly intervals from March 2004 to February 2005. After removing epibionts, the were placed in insulated boxes and transported to the Aquaculture Laboratory of the Fisheries Department of the Oceanographic Institute of Venezuela for analysis. For 14–18 of the individuals, we de- termined shell length, shell dry mass, and total dry tissue mass (drying for 72 hrs at 80 °C) and calculated the condition index (tissue dry mass as a percentage of dry shell mass). We did not determine gonad mass because it is difficult to separate the gonad from the digestive gland. For each of the remaining individuals (n = 12–16; at least six individuals of each sex), we made visual notes on the condition of the gonad and then fixed it in Bouin’s solution for 8 hrs. The gonads were later rinsed in running water, dehydrated by transferring them to increasing concentrations of ethanol (70%, 80%, 90%, 95%, and 100%, progressively), and finally embed- ded in paraffin. We used a microtome to make 5–9 µm sections of portions of each gonad and the sections were stained in Harris’ haemotoxyline and eosine. The sections were examined under a Carl Zeiss optical microscope to classify each gonad according to its developmental stage. We used the stages (1) early active, (2) developing, (3) mature, (4) spawning, and (5) spent, as defined by Soria et al. (2002). We did not use the stage “indifferent” of Soria et al. (2002), as we did not encounter gonads with a total absence of gametes. To characterize the different reproductive stages, we measured the diameter of follicles and the size of oocytes using a calibrated micrometric ocular. Oocyte size was estimated by recording the longest distance through the cell crossing through the nucleus, as per Soria et al. (2002). We calculated an index of gonad size by adopting the system described by Heffernan et al. (1989). For each individual, we assigned a rank for gonad size based on the expected gonad

Figure 1. Map of Guayacán, on the northern coast of the Araya Peninsula, northeastern Venezu- ela, where we sampled Atrina seminuda. 788 BULLETIN OF MARINE SCIENCE, VOL. 86, NO. 4, 2010 size given its observed histological stage. Then, a gonad index was calculated for each date as the average rank for the individuals collected on that date. A rank of 1 was assigned to the stage “spent,” 2 for “early active,” 3 for “developing,” and 4 for “mature.” A rank of 2 was also assigned for the stage “spawning.” On each sampling date, temperature and salinity were measured using an environmental data recorder (YSI, model 30/10 FT). In addition, we measured chlorophyll a and seston con- tent in water samples taken 1–2 m above the bottom with a Niskin bottle. The samples were transferred to the laboratory in an insulated container. In the laboratory, we first removed larger particles using a 250-μm filter and then collected seston using a Millipore™ apparatus with 0.45-μm Whatman GF/C filters. Chlorophyll a was determined using the method de- scribed by Strickland and Parson (1972), and the organic and inorganic portions of the seston were calculated from measurements of the mass of the filters taken before and after ashing at 450 °C for 4 hrs. Stepwise multiple regression analyses were used to examine the relationship between environmental factors (temperature, chlorophyll a, salinity, and organic seston) and (1) the relative frequency of each gametogenic stage for the testis (early active, developing, mature, spawning, and spent) (2) the relative frequency of each gametogenic stage for the ovary (early active, developing, mature, spawning, and spent), and (3) for the condition index. Mass values for chlorophyll a and seston were log-transformed and relative frequencies of sexual stages were arcsine-transformed to obtain the maximum r2 value (Zar, 1984).

Results

Gonad Histology As our histological observations indicated the development pattern for the ovary and testis closely resembled that reported by Soria et al. (2002) for A. seminuda in Argentina, we will here focus on aspects that differed between the two populations. We did not encounter any hermaphrodic individuals. Ovary Development Early Active.—For this stage, we observed both follicles beginning to develop and fully developed follicles indicating variation in development within the ovary (Fig. 2A). Mean follicle diameter was 54.1 µm (SD ± 17.4). Some follicles were probably recovering from egg release, as they contained some small type I oocytes measuring 4.1 ± 2.7 µm as well as type II oocytes measuring 14.9 ± 2.2 µm. The type II oocytes had a well-defined nucleus, were either attached by a peduncle or detached from the acinus walls, and were strongly stained by hematoxyline. Developing.—The follicles had a mean diameter of 95.7 ± 23.1 µm. They contained type II oocytes (Fig. 2B), which were pear-shaped with a prominant peduncle attach- ing them to the follicle wall. The nucleus of each oocyte was large and transparent and it was stained with hematoxiline. Mature.—The follicles were large and well-developed with a mean diameter of 119.2 ± 29.1 µm. They were filled with mature oocytes that measured 20.96 ± 1.59 µm in size (Fig. 2C), which were more strongly stained with eosine and polyhedrical in shape (related to being compressed). Spawning.—The follicles had a mean diameter of 71.06 ± 16.25 µm and contained pear-shaped type II oocytes that were either attached to the follicular wall or free in the lumen (Fig. 2D). Spent.—This stage was similar to that described by Soria et al. (2002) (Fig. 2E). freites et al.: reproductive cycle of atrina seminuda 789

Figure 2. Photomicrographs of developmental stages of the ovary of Atrina seminuda. 790 BULLETIN OF MARINE SCIENCE, VOL. 86, NO. 4, 2010

Atrophy.—This stage can be present during vitelogenesis and also during postvi- telogenesis, when the female gonad is readsorbing gametes. The oocytes had an ir- regular shape and the cytoplasm becomes less dense that early active or spent stages (This stage was observed in January; not shown in Fig. 2). Testis Development Early Active.—The follicles measured 94.16 (SD ± 23.56) µm in diameter and were somewhat separated from one another. There was less connective tissue than in the spent stage (Fig. 3A). Developing.—The follicles were larger, mean diameter 128.83 ± 17.10 µm, and more developed but still separated from one another (Fig. 3B). Mature.—The follicles were distended (181.10 ± 30.91 µm in diameter) and com- pletely developed. The spermatogonia layer was well-defined and spermatozoa were abundant (Fig. 3C). Follicular spaces were reduced to a minimum and connective tissue was absent. Spawning.—The mean diameter of the follicles was reduced to 121.5 ± 25.20 µm and there was more space between follicles (Fig. 3D). Empty spaces were present in the lumen, due to the loss of spermatozoa, and some spermatocytes were present. Spent.—The testis was brown in color and very flaccid. The follicles had fragmented walls and contained some residual spermatozoa (Fig. 3E). Seasonal Variations in Reproductive Activity and Gonad Index The histological observations showed that females (Fig. 4A) and males (Fig. 4B) with mature gametes were present throughout the year, except for males in January 2005 (Fig. 4B). Nevertheless, there was evidence of seasonal changes in the intensity of gamete production and spawning. The frequency of individuals in advanced ga- metogenic stages (developing and mature) tended to increase during March to May 2004 and no individuals (males or females) in early gametogenic stages were found in May. For both sexes, individuals with gonads classified as “spawning” first appeared in late April and they predominated during June, September, late November, and early December, when there were decreases in the gonad index (Fig. 4A,B). There was also evidence of renewed gametogenesis during June, as individuals in early game- togenesis reappeared. These individuals were more frequent in July, when the gonad index increased. Individuals that were spawning or had spent gonads were then pres- ent from August 2004 until the end of our study in February 2005; and the propor- tion was particularly high for females during most of July, September, October, and November 2004 (Fig. 4A); and for males in late February 2005 (Fig. 4B). At the same time, females with gonads classified as in early to advanced gametogenic stages were present throughout the extended spawning period, except in late September (when the proportion of spent individuals peaked). In brief, we observed gonads in histo- logical stages leading up to spawning during the first months of our study, March and April 2004, and there was evidence of spawning activity from June until late February 2005, with intense spawning in June and from September through Novem- ber 2004. Gonad index reflects closely the variations in the frequency of the different reproductive states, in both sexes (Fig. 4C). freites et al.: reproductive cycle of atrina seminuda 791

Figure 3. Photomicrographs of developmental stages of the testis of Atrina seminuda. 792 BULLETIN OF MARINE SCIENCE, VOL. 86, NO. 4, 2010

Figure 4. Relative frequency of different gametogenetic stages of the (A) ovary and (B) testis, and (C) changes in the gonad index for Atrina seminuda at Guayacán, northeastern Venezuela, based on sampling at 2-wk intervals from March 2004 to February 2005.

Condition Index The condition index, representing the dry mass of all tissues as a percentage of shell dry mass, was around 15% from March to late October 2004 (Fig. 4C), except for a drop between May to late June (Fig. 5) that coincided with the first spawning peak. A second drop occurred during November 2004 and coincided with the end of a prolonged spawning period. Thereafter, values varied from 10% to 12%. freites et al.: reproductive cycle of atrina seminuda 793

Figure 5. Condition index (dry tissue mass as a percentage of dry shell mass) for Atrina seminuda at Guayacán, northeastern Venezuela, based on sampling from March 2004 to February 2005. Vertical bars represent standard deviations.

Environmental Factors and Their Relationship with Gonad Development Temperature conditions at Guayacán showed a seasonal pattern (Fig. 6A). Low val- ues (24–25 °C) persisted from March through August 2004, reflecting the period of wind-driven upwelling (Okuda et al., 1978; Ferráz-Reyes, 1989) and a sharp rise in September indicated the easing of upwelling. The annual temperature maximum occurred during October–November 2004 (28–29 °C), and temperatures fell from December 2004 through February 2005. The seasonal pattern of salinity was less clear but tended to vary inversely with temperature (Fig. 6B). Chlorophyll a concentration, indicative of phytoplankton biomass, followed an ir- regular pattern that showed little relationship with the temperature cycle (Fig. 6C). Values were relatively high during March–May 2004, when temperatures were low, but then fell progressively from June to mid-August, even though temperatures were stable. Subsequently, chlorophyll values were intermediate, except for high values in early October and mid-December 2004. Total seston and the organic content of the seston generally varied in parallel (Fig. 6D). Overall values tended to increase from March to October 2004 (although with sudden, short-lived decreases in late March, August, and October), and fell precipitously to low values in November 2004 through February 2005. The stepwise multiple regression analyses indicated that temperature was the only factor significantly associated with the relative frequency of the various ovary stages (P < 0.05; Table 1). The association was negative for early active (explaining 26.9% of the variation) and mature (30.4%) stages, and positive for spawning (43.3%) and spent (71.7%) stages. No environmental factor was correlated with the frequency of gonads in advanced gametogenesis. The parallel analyses applied to the testis stages revealed that temperature was the only factor significantly correlated with two of the stages, mature (27.1% of the variation) and spawning (42.4%; Table 2). Salinity alone accounted for 40.4% of the variation in the early active stage of the testis, and the value increased to 53.3% when temperature was added to the model. For the developing stage, temperature alone accounted for 39.2% of the variation, and the value increased to 45.3% when chloro- phyll a was added to the model. Finally, for the spent stage, chlorophyll a explained 794 BULLETIN OF MARINE SCIENCE, VOL. 86, NO. 4, 2010

Figure 6. Seasonal variation in (A) temperature, (B) salinity, (C) chlorophyll a, and (D) seston abundance (thin line = total; thick line = organic fraction), from March 2004 to February 2005 at Guayacán, northeastern Venezuela. freites et al.: reproductive cycle of atrina seminuda 795

Table 1. Results of stepwise multiple regression analyses of the correlation between environmental factors and the frequency of different gametogenic stages of the ovary of Atrina seminuda. SE = Standar error; F ratio = Fisher ratio.

Parameter Coefficient SE F ratio r2 P Early active Constant 228.81 Temperature −147.40 −0.27 4.98 0.27 0.029

r² = 0.27; n = 66; F1, 64 = 4.980; P < 0.029. Mature Constant 319.14 Temperature −206.81 −0.30 6.51 0.30 0.013

r² = 0.30; n = 66; F1, 64 = 0.013; P < 0.013. Spawning Constant −380.36 Temperature 287.16 0.43 14.79 0.43 < 0.001

r² = 0.43; n = 66; F1, 64 = 14.791; P < 0.000. Spent Constant −509.10 Temperature 366.55 0.72 67.75 0.72 < 0.001

r² = 0.72; n = 66; F1, 64 = 67.749; P < 0.001.

Table 2. Results of stepwise multiple regressions analyses of the correlation between environmental factors and the frequency of different gemetogenic stages of the testis of Atrina seminuda. SE = Standar error; F ratio = Fisher ratio.

Parameter Coefficient SE F ratio r2 P Early active Constant 2,219.66 Salinity −257.66 −0.38 10.66 0.40 0.002 Temperature −1,165.33 −0.56 22.95 0.53 < 0.001

r² = 0.54; n = 66; F2, 63 = 8.584; P < 0.001 Developing Constant 217.54 Temperature 79.60 0.23 4.04 0.39 0.049 Chlorophyll a −170.70 −0.28 5.33 0.45 0.024

r² = 0.45; n = 66, F2, 63 = 5.334; P < 0.05 Mature Constant 233.74 Temperature −141.24 −0.27 5.06 0.27 0.028

r² = 0.27; n = 66; F1, 64 = 5.334; P < 0.05 Spawning Constant −381.12 Temperature 285.97 0.42 14.01 0.42 < 0.001

r² = 0.42; n = 66; F1, 64 = 14.011; P < 0.001 Spent Constant −1,003.58 Chlorophyll a 45.71 0.23 4.25 0.32 0.043 Temperature 129.83 0.36 9.23 0.43 0.003 Salinity 492.85 0.45 14.09 0.54 < 0.001

r² = 0.54; n = 66; F3, 62 = 8.584; P < 0.001 796 BULLETIN OF MARINE SCIENCE, VOL. 86, NO. 4, 2010

32.3% of the variation, and the value increased to 43.4% with the addition of tempera- ture and to 54.2% with the addition of salinity. Stepwise multiple regression analysis indicated that the temperature was the only factor that explained variation in the condition index, accounting for 29% of variance and its effect was negative.

Discussion

The reproductive pattern of the tropical population of A. seminuda that we studied in Venezuela contrasted with the temperate population in Argentina studied by Soria et al. (2002). First, the Venezuela population was reproductively active throughout the year, although there were periods of increased spawning (June–July, Septem- ber–November, and February). In contrast, gametogenetic events in the temperate population in Argentina were highly synchronized among individuals and followed an annual pattern with spawning restricted to the spring (Soria et al., 2002). Atrina maura shows a similar shift in reproductive pattern along a latitudinal gradient, as it has various spawning periods during the year in Oaxaca State (16°N), southern Mexico (Angel-Pérez et al., 2007), and only spawns from January to March in Mag- dalena Bay (25°N), northern Mexico (Rodríguez-Jaramillo et al., 2001). The change in the reproductive timing of A. seminuda with latitude is likely related to the shift in oceanographic conditions. Seasonal changes are far less pronounced in Venezuela than in Argentina. The environmental changes in Venezuela are mainly related to the effect of the trade winds. Increased winds during December/January to June cause increased upwelling and supply nutrients for increased phytoplankton production, whereas reduced winds during July/August to October/November lead to stratifica- tion of the water column and a drop in phytoplankton production (water tempera- tures only vary by 4–5 °C) (Okuda et al., 1978; Ferráz-Reyes, 1989). Despite these seasonal changes, reproduction of A. seminuda in Venezuela occurs throughout the year. Whereas spawning in the Argentinean population of A. seminuda ends with the gonads being spent (Soria et al., 2002), there is a renewed production of gametes at the same time as spawning in the Venezuelian population. The rapid renewal of ga- metogenesis after spawning appears to explain why the indifferent stage was absent in the Venezuelian population. Renewed gametogenesis at the same time as spawn- ing (and the absence of a post-spawning stage) was also reported by Angel-Pérez et al. (2007) for a southern population of A. maura in Mexico. Gonads with gametes at different stages have similarly been reported for A. maura in Bahía Concepción, Mexico (Angel-Pérez et al., 2007), and for A. zebra (Lista et al., 2006), Perna perna (Linnaeus, 1758) (Arrieche et al., 2003) and Anadara notabilis (Roding, 1798) (Freites et al., 2010) on the north coast of the Araya Peninsula, Sucre State, Venezuela. In Venezuela, we observed individuals with follicles both beginning to develop and fully developed. We further observed oocytes varying in size and characteristics in the same follicles during early oogenetic development, suggesting differential devel- opment and probably partial spawning. In our study, type I oocytes measured 4.1 ± 2.7 µm and were smaller than the type I oocytes observed in Argentina (Soria et al., 2002). Type II oocytes were not observed in the early active stage in the Argentin- ean population (Soria et al., 2002), but were present in the Venezuelan population. In addition, mature oocytes in our study measured 20.96 ± 1.59 µm, half the size freites et al.: reproductive cycle of atrina seminuda 797 reported by Soria et al. (2002) for mature oocytes (45.6 ± 23.7 µm) in Argentina. The latter contrasts with the shift in oocyte size with latitude reported by Cardoso et al. (2007) for the oyster Crassostrea gigas (Thunberg, 1793) in Europe: mature oocytes increased in size with decreasing latitude (from France to Holland). Cardoso et al. (2007) suggest that increasing gonad mass together with decreasing oocyte volume can increase reproductive output (in terms of the number of eggs produced) from France to Holland. Unfortunately, we cannot compare the gonad mass of A. semi- nuda in Venezuela and Argentina because of the difficulty in separating the gonads from other tissues for this species. The gonad index data further illustrated the difference in the timing of reproduc- tion of A. seminuda between Venezuela and Argentina. We observed drops in the in- dex during four periods of the year, indicating major spawnings in different seasons. In contrast, in Argentina there was only a single gonad index decrease during the spring, thus a restricted spawning period (Soria et al., 2002). As for many temper- ate invertebrates, the increase in the index leading up to spawning in Argentina was progressive and prolonged, extending through autumn and winter. In Venezuela, the renewed gametogenic activity in A. seminuda following spawn- ing between November and December coincided with the onset of upwelling. The upwelling period occurs from January until May each year and is characterized by low temperatures and an abundance of phytoplankton. The association of renewed gametogenesis with increased food abundance suggests an opportunist reproduc- tive strategy (sensu Bayne, 1973). The greatest spawning activity occurred during June and coincided with the onset of increasing temperatures. However, individuals with gonads classified as “spawning” and “spent” continued to be found throughout the period of high temperatures, even through there was reduced primary produc- tion due to stratification of the water column. Gamete production during June–Sep- tember was thus likely supported by tissue reserves. This suggests a conservative reproductive strategy during the warm period (sensu Bayne, 1973). A conservative reproductive strategy is also found in A. seminuda in Argentina, as gamete produc- tion during the autumn, when food is scarce, appears to be supported by energy reserves from the adductor muscle (Soria et al., 2002). The presence of these two strategies for A. seminuda demonstrates the physiological plasticity of this species. A number of changes in the condition index of A. seminuda appear to be related to reproductive activity. High indices were associated with periods when the gonads were mature, whereas drops in the index were associated with periods of spawning. Solano et al. (1997) indicated that the condition index is an appropriate parameter for determining the maturity of Pinctada mazatlanica (Hanley, 1856) because of the strong correlation between the index and maturity. In the present study, drops in dry tissue mass (shown by the condition index) in late May/June and in October/No- vember coincided with the increased frequency of individuals with gonads classified as “spawning” for both sexes. Araya-Núnez et al. (1991) similarly observed that the condition index decreased with the spawning of Pteria sterna (Gould, 1851). For many marine species, reproductive activities coincide with particular tem- perature conditions, suggesting that temperature plays a role in controlling repro- duction (e.g., Giese and Pearse, 1974; MacDonald and Thompson, 1985; Lodeiros and Himmelman, 1994; Arsenault and Himmelman, 1998). The stepwise multiple regression analyses in our study indicated that, for both males and females, gonad state was most often associated with temperature. Gametogenesis and maturation 798 BULLETIN OF MARINE SCIENCE, VOL. 86, NO. 4, 2010 were associated with periods of low temperatures and spawning with temperature increases. Thus, temperature likely plays a role in synchronizing reproductive activi- ties in A. seminuda. Angel-Pérez et al. (1997) also showed that gamete production (gametogenesis and maturation) in A. maura in Oaxaca, Mexico, is associated with low temperatures and spawning with high temperatures; and Cendejas et al. (1985) found that spawning of rugosa (Sowerby, 1835) is associated with a temperature increase. Likewise, Soria (1989) suggests that temperature controls reproduction of A. maura in Acapulco, Mexico. In the present study, the availability of phytoplankton food particles was positively related to the frequency of males with testis classified as “developing” and “spent”, suggesting that food along with temperature contributed to the timing of gamete production. Temperature together with food conditions have similarly been reported to influence reproductive activities in a variety of marine bivalves (Bayne, 1973; Giese and Pearse, 1974; MacDonald and Thompson, 1985; Ceballos-Vásquez et al., 2000), including Euvola ziczac (Linnaeus, 1758) (Lodeiros et al., 1998), Pinna carnea (Gmelin, 1791) (Narváez et al., 2000), P. perna (Narváez et al., 2009), and A. zebra (Lista et al., 2006) in Venezuela.

Acknowledgments

This project received financial support from the Consejo de Investigación de la Universidad de Oriente (project no. CI-5-1802-1175/ 04) and the work was facilitated by logistical support from the Centro de Investigaciones Ecológicas Guayacán (CIEG), Universidad de Oriente. We are greatful to N. Aguado for his advice on the histological analysis.

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Date Submitted: 14 December, 2009. Date Accepted: 28 May, 2010. Available Online: 28 June, 2010. FREITES ET AL.: REPRODUCTIVE CYCLE OF AtrinA SeMinudA 801

Addresses: (L.F., C.C., L.M.) departamento de Biología Pesquera, instituto Oceanográfi co de Venezuela, universidad de Oriente, C. Postal: 6101, A. Postal: 245, Cumaná, edo Sucre, Venezuela. (D.A.) instituto de investigaciones Biomédicas y Ciencias Aplicadas, universidad de Oriente, Cumaná, estado Sucre, Venezuela. (N.G.) Centro de investigaciones ecológicas Guayacan, universidad de Oriente, Cumaná, edo Sucre, Venezuela. (J.H.H.) département de Biologie, université Laval, Quebec City, Canada G1V 0A6. Corresponding Author: (L.F.) telephone: +582934002370, Fax: +582934002243, e-mail: .