REPRODUCTIVE CONDITION AND SPAWNING INDUCTION IN THE SOUTH AFRICAN SCALLOP PECTEN SULCICOSTA TUS.
The scallop Pecten sulcicostatus (Sowerby, 1842) is endemic to the inner shelf of the south and southwest coasts of South Africa. As one of the 29 species of Pectinidae occurring off the southern African coast, it is the only species large enough to be considered for commercial harvest (Dijkstra & Kilburn 2001). Exploratory fishing for P. sulcicostatus in 1972 showed exploitable animal densities in False Bay, but the total population size was considered insufficient to support a sustained fishery (De Villiers 1976). The average shell length of scallops in False Bay was 94 mm, with a few animals exceeding 120 mm. Scallops were found to a depth of 70 m, but catches were highest at around 40 m depth. The meat of scallops of 90 mm shell length was approximately 15 g, which is considered a marketable weight. These observations along with the considerable success of scallop culture in other areas of the world triggered an interest in the culture of P. sulcicostatus on the South African coast.
An understanding of the reproductive cycle of scallops is important for effective management of commercially important species, and much that is learnt of the processes that regulate reproductive physiology can be applied to the commercial culture of scallops (Beninger 1987). Knowledge of reproductive physiology is important for successful hatchery production, specifically for the conditioning and spawning of broodstock, as an understanding of the regulation of gametogenesis and spawning will lead to a more consistent supply of seed organisms (Barber & Blake 2016). This is particularly important if hatchery production is reliant on artificially induced spawning (Yuan et al. 2012).
Cycles of gametogenesis include a vegetative period followed by periods of differentiation, cytoplasmic growth, vitellogenesis or maturation, the release of gametes or spawning, and the resorption of unspawned gametes. The cycle of these events is genetically controlled in response to the environment (Sastry 1979). For a particular species and location, the timing and duration of these events are determined by various exogenous and endogenous factors. Seasonal cycles in temperature and levels of food are the exogenous factors most often cited as influencing gametogenesis in bivalves (Sastry 1979, Rodman & Capuzza 1983, Barber & Blake 2016). In terms of endogenous control, neuroendocrine activity (i.e., neuronal and hormonal) is considered important in coordinating the physiological processes responsible for the reproductive response relative to the environment (Barber & Blake 2016).
Once sexual maturation has been reached, environmental factors may also serve to stimulate spawning, which is often synchronous within a population. Potential triggers may again include not only temperature and food but also salinity, pH, dissolved oxygen, light, lunar phase, etc. (Barber & Blake 2016). Because periods of spawning represent the culmination of the reproductive process and are easily definable, they provide a convenient focus for comparing gametogenic cycles (Barber & Blake 2016). In this regard, the observation of Sause et al. (1987) that pectinid species in the Northern and Southern Hemispheres spawn during the same months (primarily June to October, i.e., summer-autumn in the Northern Hemisphere and winter-spring in the Southern Hemisphere) suggests that the gametogenic response of pectinids to temperature and other related environmental parameters is not consistent.
The present study builds on the initial work of Arendse et al. (2008) on the reproductive cycle of Pecten sulcicostatus in False Bay. This work demonstrated peak spawning during winter and early spring (June-September), although oocyte development appeared to occur through the entire year, indicative of minor spawning events throughout the year. The study of Arendse et al. (2008), however, did not make any assessment of the environmental conditions important in controlling gametogenesis and spawning and made no attempt to spawn animals. The present study, therefore, served to investigate (1) the reproductive cycle of P. sulcicostatus in relation to the key environmental parameters of temperature and food as determined by chlorophyll concentration and (2) the induction of spawning by four different methods on broodstock collected monthly over an annual cycle. These findings provided the opportunity to compare the spawning season of P. sukicostatus with 18 commercially important scallop species from the Northern and Southern Hemispheres.
MATERIALS AND METHODS
To assess the reproductive cycle of Pecten sukicostatus, 30 scallops within the size range of 70-90 mm shell height were collected monthly between August 2010 and November 2011. Collection was undertaken by scuba divers at 20-30 m depth off Miller's Point in False Bay, South Africa. Fifteen scallops were processed the same day for calculation of the gonadosomatic index (GSI) and for histological analysis, whereas the remaining 15 scallops were placed in holding tanks for use in spawning induction trials.
Assessment of Reproductive Cycle
Gonad condition in scallops collected monthly from the wild was assessed by means of the GSI and by histological examination. To calculate the GSI, scallops were shucked and the soft tissue and gonad individually weighed using a Denver Instrument APX-602 scale. GSI was calculated as follows (MacDonald & Bourne 1987):
GSI = [Gonad weight (ww) / Somatic tissue weight (ww)] X 100.
For histological studies, gonads were fixed in Dietrich's fixative (Yevich & Barszcz 1977) and slides were prepared for the measurement of oocytes, as described in Arendse et al. (2008), using a Nikon imaging system and basic research package version 3.2. A total of 50 oocyte areas, rather than diameters, from each individual were measured as they are less influenced by sectioning techniques, providing more reliable data from which the oocyte diameter may be calculated (Dukeman et al. 2005). Gonad stages were also determined by qualitative histological analysis and classified into six different categories: early-maturation, mid-maturation, mature, partial spawn, spawn and recovery, and spent (Navarte & Kroeck 2002).
Environmental conditions at the site of scallop collection were assessed for the duration of the study through measurements of temperature and chlorophyll a (Chl a). An Ecofluorometer and Starman temperature recorder were moored off Miller's Point at 10 and 30 m depth, respectively, and set to record at 10-min intervals. Instruments were serviced monthly and measurements of in situ fluorescence were calibrated through comparison with extracted Chl a concentrations as detailed by Parsons et al. (1984).
Following collection, each month 15 scallops were acclimated for 2-3 days in filtered seawater at temperatures within 2[degrees]C of those measured at the site of collection. The scallops were acclimated before spawning induction to reduce stress following harvesting and transport. A 40-[micro]m mesh sieve for egg capture was placed at the outflow of broodstock tanks to monitor any spawning activity before controlled spawning induction.
Scallops were exposed sequentially to four spawning induction techniques: (1) temperature shock, (2) feeding after initial starvation, (3) desiccation, and (4) hormone injection. For the temperature shock treatment, all 15 scallops were placed into a large tank where the temperature was raised from 12[degrees]C to 17[degrees]C for a period of 1 h. Scallops that failed to spawn were then subjected to a shock-feeding treatment for 1 h through addition of a feed mixture of Chaetoceros, Pavlova, and Isochrysis at a ratio of 1:1:1 and a total concentration of 15 X [10.sup.6] cells [L.sup.-1]. Scallops that failed to spawn were then individually placed into glass bowls containing seawater, from which they were removed every 10 min and returned following a 5-min period of desiccation. This treatment was also repeated for 1 h. Scallops that again failed to spawn were then subject to intragonad injection (in both the male and female areas of the gonad) of 0.2 mL serotonin (2 mM) hormone solution and placed back in the glass bowls. The number of scallops that released sperm and eggs following each spawning induction method was recorded. The number of eggs and D-veligers obtained per spawning induction method was counted following the methods of Helm et al. (2004).
Statistical analyses were conducted using Statistica version 6.1 (Statsoft, Inc.). The Shapiro-Wilk test was used to establish normality in both mean GSI and mean oocyte diameters. Analysis of variance was used to test for differences in both mean GSI and oocyte diameters between months, and the Tukey HSD test was used to establish which months were different. The significance level (a) was set at 0.05.
Assessment of Reproductive Cycle
A seasonal pattern was evident in the GSI (quantitative assessment), which was highest (>11%) from May to September (Fig. 1 A). The minimum mean GSI value of 5.8% ([+ or -]1.7 SD, n = 16) was recorded in November 2011 and the maximum of 17.8% ([+ or -]4.8 SD, n = 16) in August 2010. Qualitative assessment of the reproductive cycle as provided by the GSI frequency showed a similar seasonal cycle to that of the GSI. From October to April, the vegetative stage was common (>40%), whereas from May to September, vitellogenesis was evident (Fig. 1B). Mean oocyte diameter demonstrated notable variability in that the minimum mean diameter of 34.5 [micro]m ([+ or -]3.2 SD, n = 15) in July 2011 and the maximum of 53.4 [micro]m ([+ or -]7.3 SD, n = 15) in August 2010 were significantly different (Fig. 1C).
Qualitative histological analysis showed that scallops of different stages of gonad development were present throughout the year, except in October 2011 when all gonads were in the spawn and recovery stage (Fig. 1D). Gonads in the early maturation stage were present only from November to January (Fig. 1D), whereas gonads in the mid-maturation stage were present in most months, with the highest incidence in September 2011 (Fig. 1D). The dominance of the spawn and recovery stage in December 2010 and in April and October 2011 (>66% of gonads) provided evidence of spawning in these months (Fig. 1D). Gonad histology indicative of partial spawning characterized the gonads of 13%-47% of scallops, except during the months of July, September, and October 2011 when this stage was absent (Fig. 1D).
During the period of study, bottom temperatures at the site of scallop collection in False Bay ranged from 9.7[degrees]C to 17.6[degrees]C, with a mean of 13.1[degrees]C ([+ or -]1.69 SD, n = 69,198) (Fig. 1E). Monthly mean bottom temperatures were lowest in midsummer [11.4[degrees]C ([+ or -]1.2 SD, n = 4,464) in January 2011], increasing through winter with the highest mean temperatures in spring [15.6[degrees]C ([+ or -]1.3 SD, n = 4,320) in November 2011]. Short-term variations as high as 6[degrees]C were superimposed on this seasonal trend (Fig. 1E).
Chlorophyll a concentrations also displayed a seasonal cycle, with highest values in summer and early autumn (December-March) declining in late autumn, winter, and spring (Fig. 1F). The highest monthly mean Chl a value of 14.1 mg [m.sup.-3] was recorded in February 2011 and the lowest mean value of 2.9 mg [m.sup.-3] in June 2011 (Fig. 1F).
The only method that successfully induced spawning was hormonal injection of serotonin. The number of scallops that released sperm was notably higher than those that produced eggs (Fig. 2A). During the course of the experiment, 17.2% of the scallops released eggs and 79.6% released sperm. Whereas spermatozoa were released in all months, oocytes were not released in either November or December (Fig. 2A). D-veligers developed in 11 of the 13 mo in which eggs were released (Fig. 2B).
The reproductive cycle of Pecten sulcicostatus as determined in False Bay for the period 2010 to 2011 exhibited a similar seasonal trend to that described by Arendse et al. (2008) for the same species during the period 2004 to 2005. The GSI, supported by histological analysis, showed spawning to occur primarily from June through September, followed by a decrease in the mean GSI and oocyte diameter coincident with the highest number of spent individuals in July. The maximum GSI for P. sulcicostatus of 17.8% reported in this study is slightly higher than the 14.4% reported by Arendse et al. (2008). This variation might be attributed to the smaller size of the scallops analyzed by Arendse et al. (2008). The GSI of P. sulcicostatus is somewhat lower than that reported in many other scallop species, for example, Nodipecten subnodosus (28%; Arellano-Martinez et al. 2004), Patinopecten yessoensis (28%; Kawamata 1988), Pecten maximus (23%; Magnesen & Christophersen 2008), and Placopecten magellcmicus (28%; Parsons et al. 1992).
Few scallops in False Bay exhibited totally spent gonads (0%-27%), indicating the general absence of a postspawning resting period. This finding is similar to that of the 2004 to 2005 study, which also reported a low frequency of spent gonads in the False Bay population, indicating either the development of gametes immediately after spawning or the reabsorption of gametes (Arendse et al. 2008). In July 2011, observations of oocytes with smaller than expected diameters are conceivably attributed to low June temperatures, indicating the possible impact of exogenous factors on the reproductive cycle.
Considerable individual variability in the GSI and oocyte diameters each month implies a low level of reproductive synchrony in Pecten sulcicostatus. In False Bay, P. sulcicostatus is found between 22 and 70 m depth, with the densest populations at approximately 40 m (De Villiers 1976). The temperature data from 30 m shows high variability which might trigger minor spawning events throughout the year, leading to low synchrony. Despite the low synchrony, a primary spawning event is still evident and the diameters of oocytes during that period (June-September) indicate the presence of mostly mid-mature and mature gonads. A similar observation was found in the scallop Placopecten magellanicus, where individuals at greater depths demonstrate reduced rates of gamete development and consequently lower reproductive synchrony (MacDonald & Thompson 1986, Barber et al. 1988, Schmitzer et al. 1991).
The reproductive cycle of scallops is understood to be controlled primarily by water temperature and food availability (Barber & Blake 1981, Cruz & Villalobos 1993). Spawning activity in some scallops has been documented when large temperature differences occur between seasons (Barber & Blake 1983, Strand & Nylund 1991, Young et al. 1999). Temperature at 30 m depth in False Bay dropped by around 6[degrees]C with the transition from spring to summer and the intrusion of cold bottom water into the bay. False Bay forms part of the southern Benguela upwelling regime (Shannon 1985), and the water is stratified in summer because of the heating of surface waters and the inflow of cold bottom water (Boyd et al. 1985, Swart & Largier 1987). Increasing bottom temperatures following mixing in winter and spring could be important, among other factors, in stimulating the main spawning event in Pecten sulcicostatus at this time.
Phytoplankton biomass in False Bay is generally lower in winter (June-August) and peaks in summer and early autumn (December-March). Food in the form of sinking phytodetritus is likely to be at a maximum in autumn as phytoplankton blooms decline and may explain the GS1 maxima in winter. The relatively high phytoplankton biomass throughout the year could contribute to the low synchrony of gonad development, as is evident from the low number of spent individuals, and the year-round presence of newly developing oocytes.
Reproduction is definable by activation, growth, gametogenesis, the ripening of gonads, spawning, and a resting period (Sastry 1979). Although scallop populations are often synchronous spawners (Barber & Blake 2016), differences in the timing and frequency of spawning events do occur in response to inter- and intraspecific variations, and exogenous and endogenous influences (Sastry 1979, Barber & Blake 2016). Consequently, the primary spawning periods in the reproductive cycle of scallop species differ globally. The period of spawning for Pecten sulcicostatus is compared with that of 18 commercially important species from both the Northern and Southern Hemispheres (Fig. 3). The observation of Sause et al. (1987) that scallops from both hemispheres spawn during similar calendar months appears to hold true for this comparison with spawning in the south commonplace in winter and spring, whereas spawning in the north is mostly a summer and autumn activity. These observations by Sause et al. (1987) were considered to indicate the absence of a uniform reproductive response to temperature or photoperiod by the pectinids. Acknowledging the limitations of the comparison made as part of this study, in terms of the number of species compared, the observations of winter-spring spawning in P. sulcicostatus on the South African coast are certainly in line with those of Sause et al. (1987) made on the Australian coast.
Various triggers, including thermal shock, food deprivation, desiccation, and injection with the hormones dopamine and serotonin, are used to induce spawning in scallops (Culliney 1974, Matsutani & Nomura 1982, Gibbons & Castagna 1984, Velasco et al. 2007), all of which were applied sequentially during this experiment. Although the hormone serotonin was the only trigger to induce spawning activity during this experiment, it is possible that the sequential stress treatments before injection of serotonin also contributed to spawning. The rate of success of spawning induction in Pecten sulcicostatus was nevertheless low and a likely consequence of the low synchrony of gonad development, with scallops in False Bay seldom in the same reproductive phase, as is the case for scallops that undergo a resting period.
The staging of gonads during this study showed that most scallops were in developing stages and that mature gametes were present in most months. This suggests that spawning should be possible during most months of the year. Success with a combination of spawning induction techniques has been achieved in other scallop species (Parsons & Robinson 2006). For Pecten sulcicostatus, injection of serotonin appeared to be the only method that triggered the release of both spermatozoa and eggs, although a combination of spawning techniques could have contributed to spawning. The release of both spermatozoa and eggs following injection of serotonin has been achieved in species such as Patinopecten yessoensis and Pecten albicans (Matsutani & Nomura 1982, 1984,Tanaka&Murakoshi 1985), but in other species, such as Pecten ziczac and Argopecten ventricosus, serotonin has been shown to induce only the release of spermatozoa (Velez et al. 1990, Monsalvo-Spencer et al. 1997). Serotonin has also resulted in the release of immature gametes, which may lead to reduced fertilization and high larval mortalities (Braley 1992). Nonetheless, larval development in the present study was considered successful based on the number of larvae relative to the number of eggs released. The development of spat from released eggs throughout the year still needs further investigation. The fact that most eggs and D-stage larvae were produced in June and August reinforces this as the primary spawning period for P. sulcicostatus in False Bay.
It has been demonstrated that Pecten sulcicostatus is most likely to spawn between June and September, which in turn designates the best time to collect, condition, and spawn this species. Environmental data indicate the possible role of both temperature and food availability in reproduction, as spawning followed maximum food production in summer and autumn, and rising bottom temperatures in winter and spring. The results further indicate that it may be possible to spawn P. sulcicostatus throughout the year because of low synchrony and the absence of a resting period. It is advisable that prior conditioning be considered before spawning induction, as some primary oocytes may require further development before spawning can take place.
We would like to thank Winston April for preparation of the histological slides and Lee-Ann Jacobs, Brett Lewis, Alick Hendricks. Mark Goodman, and Andre du Randt for their technical assistance.
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DALE C. Z. ARENDSE, (1,2*) GRANT C. PITCHER (1,2) AND CHARLES GRIFFITHS (2)
(1) Fisheries Management, Department of Agriculture Forestry and Fisheries, Private Bag X2, Rogge Bay, Cape Town 8012, South Africa; (2)Department of Biological Sciences and Marine Research Institute, University of Cape Town, Rondebosch 7701, South Africa
(*) Corresponding author. E-mail: email@example.com
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|Author:||Arendse, Dale C.Z.; Pitcher, Grant C.; Griffiths, Charles|
|Publication:||Journal of Shellfish Research|
|Date:||Dec 1, 2018|
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