Reproductive Biology of the Cockle Cerastoderma Glaucum (Bivalvia:Cardiidae) from Lake Qarun, Egypt

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Egyptian Journal of Aquatic Research (2013) 39, 249–260

National Institute of Oceanography and Fisheries Egyptian Journal of Aquatic Research

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Reproductive biology of the cockle Cerastoderma glaucum (Bivalvia:Cardiidae) from Lake Qarun, Egypt

Kandeel E. Kandeel a,*, Saad Z. Mohammed b, Afaf M. Mostafa a, Marwa E. Abd-Alla a a Department of Zoology, Faculty of Science, Fayoum University, 63514 Fayoum, Egypt b Department of Marine Biology, Faculty of Science, Suez Canal University, 41522 Ismailia, Egypt

Received 13 September 2013; revised 6 December 2013; accepted 6 December 2013 Available online 8 January 2014

KEYWORDS Abstract The cockle Cerastoderma glaucum is an important component of the benthic community Lake Qarun; in Lake Qarun, Egypt. Reproduction and gonad development in this species were investigated dur- Cockle; ing the period from February 2008 to May 2009. Reproductive maturity was estimated by assess- Cerastoderma glaucum; ment of length–weight relationship, gonad index and microscopic examination of gonadal smears Reproduction; and sections. Results indicate lack of periodicity in the sexual cycle. An annual pattern of four Spawning periods spawnings was recorded. The depletion of ripe gametes during spawning was rapidly compensated by the onset of gametogenesis. Monthly changes in ﬂesh and gonad weight reﬂect reproductive cycle with weight loss during spawning. C. glaucum is a gonochoric species without any hermaphrodites or sex reversals. The overall samples presented a male biased sex ratio (F:M = 1:1.29). The size at which 50% of the population reached maturity was 9.6 and 8.5 mm of shell length for females and males, respectively. ª 2013 Production and hosting by Elsevier B.V. on behalf of National Institute of Oceanography and Fisheries.

Introduction Tunisia, Turkey, Sardinia, Italy, Greece, Portugal, Spain, France (Atlantic and Mediterranean coasts), The Netherlands, The lagoon cockle Cerastoderma (Cardium) glaucum syn. with the British Isles, Denmark, Finland, Norway and in the C. lamarcki has been recorded from the coasts of Egypt, Wadden Sea, Adriatic Sea, Red Sea, Aegean Sea and Caspian Sea (for review see Brock and Wolowicz, 1994; Malham et al., 2012). * Corresponding author. Present address: Department of Biological Cerastoderma glaucum has been studied as a component of Science, Faculty of Science and Arts (Al-Mandak), University of Al-Baha, Saudi Arabia. Tel.: +966 532099507; fax: +966 07 7512344. the macrobenthos of Lake Qarun (Abdel-Malek and Ishak, E-mail address: [email protected] (K.E. Kandeel). 1980; El-Shabrawy, 2001; Fishar, 2000). The cockle represented Peer review under responsibility of National Institute of Oceanography one of the most dominant species of the macrozoobenthos and Fisheries. living in the lake (El-Shabrawy, 2001; Fishar, 2000). The stand- ing crop and biomass of this cockle ranged from 11 to 93 ind. m2 and from 6.9 to 70.7 g fresh weightÆm2, respectively (El-Shabrawy, 2001). Several records of C. glaucum were Production and hosting by Elsevier

1687-4285 ª 2013 Production and hosting by Elsevier B.V. on behalf of National Institute of Oceanography and Fisheries. http://dx.doi.org/10.1016/j.ejar.2013.12.003 250 K.E. Kandeel et al. also undertaken from other Egyptian waters, e.g. Suez Canal May 2009 by dragging from a depth of nearly 2 meters. The Lakes (Barash and Danin, 1972), Lake Timsah (Mohammed sediments were washed out carefully in situ through one mm et al., 1992) and Lake Bardawil (Fishar, 2005). mesh size sieve. The materials retained by the sieve were put Ecological and biological studies on C. glaucum are scarce in in labeled containers filled with 6% neutral formalin. A sub- Egypt (Mohammad, 2002; Mohammad et al., 2006, 2009). sample (14–28 specimens) was fixed monthly in Bouin’s solu- Studies on the reproductive condition of this cockle at the tion for histological studies. southern areas of its distribution in Lake Qarun are needed because no prior information exists from this region. Knowledge Length-weight relationships of the reproductive biology of C. glaucum is crucial for its cul- tivation as a food source for fish and/or as an indicator of the C. glaucum specimens were measured for shell length (SL) with environmental conditions. Thus, the present study aims to eval- a precision of 0.1 mm using a vernier caliper. The length of uate the reproductive cycle, spawning pattern, sex ratio and size cockles was defined as the longest distance between the ante- at onset of sexual maturity of C. glaucum in Lake Qarun. rior and posterior shell margins. The flesh was dissected out of the shells. Gonads were carefully separated from somatic Materials and methods mass. Flesh weight (FW), gonad weight (GW) were determined separately for each specimen using top-loading digital balance Study area (precision of 0.0001 g). Weight measurements were restricted to adult cockles as they are likely to exhibit more variations Lake Qarun (Fig. 1); which is one of the largest lakes of Egypt, (due to spawning activity) than juveniles. occupies part of the basin of ancient Lake Mories, an immense Relationships between SL and weight (W) were estimated freshwater palaeo-lake that persisted until the mid Holocene by linear least-squares fit to log-transformed data: (Hassan, 1986). The basin is fed by the Nile River and occupies the lowest level of the El-Fayoum depression. The lake lies be- log W ¼ log a þ b log SL tween 30340 and 30490 E longitude and 29250 and 29340 N where W is the weight, SL is the length, a is the ordinate at ori- latitude at 44 m below the sea level. Lake Qarun covers, as a gin and b is the slope. whole, an area of about 226 km2 and receives annually about Linear regression equations were calculated monthly for the 470 million cubic meters of drainage water through 12 drains logarithm of flesh weight (log FW) and gonad weight of which ‘‘Bats’’ and ‘‘Wadi’’ drains carry most of the water (log GW) versus logarithm of shell length (log SL). brought to the lake (Mansour et al., 2000). Monthly changes in FW and GW of C. glaucum were investigated by following weight changes for standard-sized individ- Sampling uals, as calculated from monthly length-weight regressions. Standard individual of 15 mm SL was selected. This size is Samples of C. glaucum were collected from Lake Qarun (Shak- close to the mean size of the studied population over the sam- shuk area) at monthly intervals between February 2008 and pling periods.

Figure 1 Location map of Lake Qarun showing sampling site . Reproductive biology of the cockle Cerastoderma glaucum (Bivalvia:Cardiidae) from Lake Qarun 251

Gonad index

Gonad index (GI) is the most widely used quantitative method for estimating the reproductive activity of marine bivalves (Giese and Pearse, 1974). It was calculated individually as the ratio between gonad weight and ﬂesh weight · 100 (Lee and Cho, 1985). GI was calculated monthly and expressed as mean ± S.D.

Histological studies

Gonads of mature individuals fixed in Bouin’s solution were dissected, dehydrated in alcohol, cleared in xylene, embedded in paraffin wax and sectioned (6–8 lm) at a standard point. Tissue sections were stained in Ehrlich’s hematoxylin and Eosin Y according to procedures outlined in Howard and Smith (1983). Histological sections of each specimen were staged on Figure 2 Monthly variations in flesh weight (––) and gonad the basis of the degree of development of germ cells. Female weight (---) of 15 mm standard-length computed from length- specimens were categorized into five stages, whereas only four weight regressions of sexually differentiated C. glaucum collected stages were detectable in males. The recognition of these stages from Lake Qarun. was based upon the classification cited by Drummond et al. (2006) with some modifications based upon preliminary histological observations of the gonad. The description and criteria for each stage for females and males are summarized in the results. The relative frequency of each stage was calculated monthly and graphed for both sexes. Gonadal sections were scanned at random. The diameters of about 50 oocytes and ova, already, sectioned through nu- cleus (i.e. near the egg’s center = maximum diameter) were measured from each female gonad using a compound micro- scope fitted with a calibrated optical micrometer. Oocyte diameter data were expressed as frequency polygons as described by Pearse (1965).

6-Sex ratio and size at onset of sexual maturity

Smears of the sexual products were examined at 100 magnifi- · Figure 3 Monthly changes in mean gonad index (±S.D.) in cation. Each specimen was sexed and the sex ratio (expressed males (....) and females (––) of C. glaucum collected from Lake as the number of males per female, F:M) was determined. Qarun. For both sexes, the percentage of mature individuals in 1- mm intervals was calculated. A logistic curve was fitted to the data in order to estimate the length at which 50% of indi- carried out using MINITAB software (version 13, 2000). viduals were sexually mature (SM50). Statistical significance was considered at P < 0.05.

Statistical analysis Results Length-weight regressions were submitted to an analysis of variance (ANOVA) to estimate variance ratio (F) and the sig- Relationship between shell length and flesh weight nificance level of the determination coefficient (r2). The deviation of the slope (b) value of the regression function from the Monthly linear regressions between SL and FW for sexually isometric value (b = 3) was analyzed by means of a t-test, as mature C. glaucum have coefficient of determination (r2) rang- expressed by the following equation (Monti et al., 1991): ing from 0.526 and 0.950 (Table 1). Slope values (b) fluctuated between 1.96 and 2.86. Student’s t-test revealed that FW ðb 3Þ t ¼ increased slower than SL (negative allometric growth) in s s b February–March 2008, December 2008, January 2009 and where ts = t-test value and sb = standard deviation of the April–May 2009. In the other ten months, FW increased slope (b). A significant deviation indicates a negative (b <3) isometrically with SL. or positive (b > 3) allometric relationship. Flesh weights of 15 mm standard-length computed from Statistically significant deviations from the expected sex ra- length-weight regressions showed seasonal fluctuations tio of 1:1 were assessed by Chi-square (v2) analysis with one (Fig 2) with two major peaks in March 2008 (0.239) and degree of freedom (Bailey, 1995). Statistical analysis was January 2009 (0.239) and two minor peaks in June and Octo- 252 K.E. Kandeel et al. ber 2008 (0.199). The lowest value of flesh weight was recorded Gonad index in May 2008 (0.139). Gonad index (GI) of females and males showed more or less Relationship between shell length and gonad weight similar trends in the mode of monthly variation (Fig 3). It de- creased from the maximum values (10.2 for females and 9.5 for Table 2 shows the results of regression analysis between SL males) in February 2008 to reach the minimum values (4) in and GW for sexually mature cockles collected from Lake summer. Then, fluctuations from 3.1 to 8.7 for females and Qarun. Coefficient of determination (r2) varied between 0.211 from 3.3 to 9.4 for males were recorded indicating two major and 0.940 throughout the study period. Slope values were peaks in October 2008 and January 2009. not significantly deviated from 3 value (P > 0.05) indicating that gonad weight increased isometrically with shell length. Stages of gonadal development Monthly variation in GW of a standard individual of 15 mm SL showed a maximum value of 0.04 g in April 2009 Female gonads (Fig 2). A marked decline (<0.01 g) was recorded in May, Five different stages were assessed in females. These stages are July–September 2008, March and May 2009. described as follows:

50 µm

B C

100 µm 200 µm D E

200 µm 200 µm

Figure 4 A–E: Gonad developmental stages for female C. glaucum. (A) Stage 1: early developing. (B) Stage 2: late developing. (C) Stage 3: ripe. (D) Stage 4: partially spawning. (E) Stage 5: completely spawning. Abbreviations: IFS, interfollicular space; FW, follicle wall; OG, oogonia; EO, early oocyte; GO, growing oocyte; RO, ripe ovum; OS, ovum sac; PS, perivitelline space; ES, empty sac; ROV, residual ovum; DO, degenerated ovum; L, lumen. Reproductive biology of the cockle Cerastoderma glaucum (Bivalvia:Cardiidae) from Lake Qarun 253

Stage 1 (early developing). Follicles are scarce, isolated, and Male gonads small. Interfollicular space (IFS) is large (Fig 4A). Numerous Rapid spermatogenesis in males after completely spawning oogonia (OG) and early oocytes (EO) arise along the wall of makes classifying the gonad into early and late active so diffi- the follicles. Follicle lumen (L) is empty and contains no ripe ova. cult that the two stages were placed together into a single active stage. Stage 2 (late developing). Follicles become evident, increasing in size and number (Fig. 4B). They are occupied with early oo- Stage 1 (developing). The follicles are extended and filled with cytes (EO) and growing oocytes (GO). Ripe ova (RO) appear germinal cells in all stages of spermatogenesis (Fig. 5A). Sper- in the center of some follicles but they are no longer packed. matogonia (SG) and spermatocytes (SC) are the most numerous, occupying nearly all the available space of the lumen. Stage 3 (ripe). The follicles are fully distended and confluent Spermatozoa (SZ) appear in the center of a considerable num- with the absence of interfollicular space (Fig. 4C). Follicle lu- ber of follicles. mens are almost filling of ripe ova compacted into a polygonal shape as a result of packing. Ripe ova are surrounded by an Stage 2 (ripe). Follicles are fully distended and closely com- envelope. There is a great reduction of early stages of oogenesis. pacted. Follicle lumens are filled with mature spermatozoa (SZ) with their tails pointing toward the center of the follicle Stage 4 (partially spawning). Empty spaces (ES) appear within forming concentric bands or plugs. There is a great reduction the follicles due to partial emission of ripe ova (Fig. 4D). Ripe of early stages of spermatogenesis (Fig. 5B). ova have become obviously rounded as the pressure within the follicles is reduced. The interfollicular space becomes wide. Stage 3 (partially spawning). The arrangement of spermatozoa seen in ripe stage was found to be disorganized in appearance Stage 5 (completely spawning). Follicles are empty and col- as a considerable number of them were discharged (Fig. 5C). lapsed because of the major and final release of gametes Follicle size decreases and the interfollicular space (IFS) (Fig. 4E). Interfollicular space (IFS) becomes very noticeable. appears. A few residual ova (ROV) may be present in some follicles and can often be seen undergoing degeneration. Degenerated ovum Stage 4 (completely spawning). Testicular follicles have large (DO) is easily detected by the loss of its round shape. In some lumina (L) with few undischarged strands of spermatozoa cases, the majority of the follicles are completely devoid of oo- (SZ) which loosely fill the follicles. Degenerated spermatozoa cytes so that the sex determination is difficult. were observed in some follicles. Inner walls of the follicles were

A B

Figure 5 A–D: Gonad developmental stages for male C. glaucum. (A) Stage 1: developing. (B) Stage 2: ripe. (C) Stage 3: partially spawning. (D) Stage 4: completely spawning. Abbreviations: IFS, interfollicular space; SG, spermatogonia; SC, spermatocytes; SZ, spermatozoa; L, lumen. Scale bar on D = 100 lm and applies to all figures. 254 K.E. Kandeel et al. lined with narrow bands of spermatogonia and spermatocytes. Oocyte size-frequency Interfollicular space (IFS) is larger than that of partially spawning stage (Fig. 5D). Monthly size-frequency distributions of oocyte diameter (grouped in 3 lm size class) are illustrated in Fig. 7. Large per- Reproductive cycle centages of small oocytes (7–15 lm) occurred during March– April 2008 and February–May 2009 indicating the beginning Monthly proportions of the cockles classified in each gonadal of oogenesis. A marked reduction in the relative frequency stage are represented in Fig. 6A for females and in Fig. 6B for of large oocytes was observed in April–May 2008, September males. Both sexes showed monthly variation in the percentage 2008, and February–March 2009. occurrence of the different stages and lack of periodicity. The Sex ratio gametogenic cycles were asynchronous, as evidenced by the simultaneous occurrence of two or three stages in most of the monthly samples. Out of 1862 cockles examined microscopically, 35.87% were Ripe stage was recorded in 75% of the investigated females sexed as females, 46.46% as males and 17.67% as sexually during February and October 2008. All males examined undefined. The overall ratio of females, to males (1 F:1.29 M) during February 2009 and January 2009 were in ripe stage. was statistically different from the theoretical sex ratio of 1:1 2 However, the occurrence of this stage was 75.0% and 60.0% (X = 25.32, P < 0.005) indicating the prevalence of males. of the total examined males in October and December 2008, respectively. 8-Size at onset of maturity Spawning in both sexes appeared on a large scale, as suggested by the presence of partially and completely spawning The relationship between shell length and the percentage of (spent) individuals in most of the year. mature C. glaucum for both sexes is illustrated in Fig 8. The

Figure 6 Monthly variations in the percentage of gonadal developmental stages for female (A) and male (B) C. glaucum collected from Lake Qarun. (See legend). Reproductive biology of the cockle Cerastoderma glaucum (Bivalvia:Cardiidae) from Lake Qarun 255

Feb 2008 Oct.

Mar. Nov.

Apr. Dec.

May Jan. 2009

Jun Feb.

2 Jul Mar.

0 Frequency (%)

Aug Apr.

Sep. May

Oocyte- ova Size (µm) Oocyte- ova Size (µm)

Figure 7 Relative distribution of oocyte-ova diameter of various size classes in female gonads of C. glaucum collected from Lake Qarun (oocyte-ova size grouped in 3 lm size classes). size at which 50% of the population reached maturity (SM50) 15 mm standard-length had many similarities in the mode of was 9.6 and 8.5 mm SL for females and males, respectively. variation. Four periods of weight loss as a result of spawning For both sexes, the smallest mature cockles were 6 mm SL bouts (April–May, July, November 2008 and February–March and the largest immature ones were 12 mm SL (Fig. 8). 2009) were evident (Fig. 2). Ansell et al. (1972) considered using body weight changes to assess spawning seasons as a method depending on a degree of close synchronization of Discussion spawning activities in the population to be effective. The degree of spawning synchronization seemed to vary among spe- Flesh weight of C. glaucum from Lake Qarun increased rela- cies and within populations of the same species in different tively slower than shell length during February–March 2008, parts of their geographic range (Sastry, 1979). Spawning activ- December 2008–January 2009 and April–May 2009 (Table 1). ity of C. glaucum in Lake Qarun is not closely synchronized. Spawning activity may explain this pattern of negative allom- Many cockles were found to be in partially and completely etry. Monthly changes in ﬂesh weight and gonad weight of spawning stages at nearly all months. 256 K.E. Kandeel et al.

110 Reproduction of C. glaucum occurred throughout the year 100 in a poorly defined pattern with some annual variations in the 90 timing and intensity of spawning (lack of periodicity). Varia- 80 tions in flesh weight, gonad weight and gonad index were quite 70 different in 2008 and 2009. Mass spawning during April–May 60 2008 came earlier (February–March) in 2009. Annual varia- C. glaucum 50 tion in the time of spawning was recorded for and C. edule in Danish waters (Navarro et al., 1989; Yankson, 40 1986a). Jaramillo and Navarro (1995) suggested that spawning 30 % of maturity could be induced by a combination of internal and environ- 20 Female: N = 294 Male: N = 446 mental factors, and that their interaction may vary seasonally, 10 producing annual variations in the onset and intensity of 0 spawning. Lack of periodicity in the reproductive cycle has 2 4 6 8 10 12 14 16 been observed by many authors (e.g. Garcia-Dominguez Shell length (mm) et al., 1994, 1996; Malachowski, 1988; Mohammad, 2002; Seed and Brown, 1977). This mode of reproduction makes a precise Figure 8 Relationship between shell length (mm) and percentage prediction of spawning events, or of subsequent recruitment of female and male maturity for C. glaucum collected from Lake pulses very difficult (Hooker and Creese, 1995). Walker and Qarun throughout the study period. The size of 50% maturity is Heffernan (1994) found that temporal variations in gameto- demonstrated. N = number of individuals examined. genic cycle were detected in populations with either a polymodal or a prolonged spawning season. Spawning periods of C. glaucum and its counterpart C. The annual pattern of four spawnings was confirmed by the edule in different localities are summarized in Table 3. Various presence of polymodal distributions in the oocyte size–fre- spawning strategies, ranging from a short single spawning quency at nearly all months (Fig. 7). The polymodal oocyte event to a continuous gamete release, were recorded. The size–frequency distributions appeared to be a result of produc- marked differences in the spawning times of the same species tion of successive oocyte generations (Morvan and Ansell, among the different localities were attributed to the geograph- 1988). ical differences in sea water temperature (Chang et al., 1985; Monthly changes in gonad index (GI) were considered a reli- Hesselman et al., 1989) and time of food abundance (Chang able tool for measuring the reproductive activity of C. glaucum et al., 1985). in Lake Qarun. The highest values (8.7–10.2) were recorded The pattern of protracted or repeated spawning bouts during February and October 2008 and January 2009 when has been described for the oyster Pinctada mazatlanica the majority of the population (>66.7%) was in ripe stage. (Garcia-Dominguez et al., 1996), the scallop Hinnites giganteus The lowest values of GI (3.1–4.9) were recorded during summer (Malachowski, 1988), the clam Megapitaria aurantiaca (July–September), late winter and early spring (February–April (Garcia-Dominguez et al., 1994) and the mussels Limnoperna 2009) when large proportions of the population (42.9–100%) fortune, Perna perna and Mytilus galloprovincialis (Darrigran were in spawning condition (Fig. 6). et al., 1999; Idhalla et al., 1996). Also, this spawning pattern

Table 1 Monthly variation in regression parameters (loga and b) of shell length (mm) and flesh weight (g) relationships of sexually differentiated individuals of C. glaucum collected from Lake Qarun. Values of Student’s t-test (t), level of significance from isometry (P), coefficient of determination (r2), variance ratio (F), and number of animals examined (N) are also given. Month Loga ± S.D. b ± S.D. t P r2 FN February 2008 3.68 ± 0.07 2.55 ± 0.06 7.5 <0.05 0.935 1688.6 120 March 2.96 ± 0.17 1.98 ± 0.15 6.8 <0.05 0.638 169.3 98 April 4.11 ± 0.16 2.86 ± 0.14 1.0 N.S. 0.793 401.6 107 May 3.19 ± 0.24 1.96 ± 0.22 4.7 N.S. 0.526 78.8 73 June 3.92 ± 0.16 2.70 ± 0.14 2.1 N.S. 0.801 373.2 95 July 3.76 ± 0.20 2.52 ± 0.17 2.8 N.S. 0.709 219.4 92 August 4.07 ± 0.14 2.81 ± 0.12 1.6 N.S. 0.891 588.8 74 September 4.02 ± 0.10 2.79 ± 0.08 2.6 N.S. 0.930 1175.4 90 October 3.92 ± 0.07 2.72 ± 0.06 4.7 N.S. 0.950 1946.5 106 November 3.67 ± 0.15 2.48 ± 0.13 4.0 N.S. 0.780 382.4 110 December 3.65 ± 0.06 2.49 ± 0.05 10.2 <0.05 0.950 2061.4 110 January 2009 3.37 ± 0.09 2.33 ± 0.07 9.6 <0.05 0.914 1071.1 103 February 3.92 ± 0.20 2.70 ± 0.16 1.9 N.S. 0.887 282.7 38 March 3.85 ± 0.12 2.64 ± 0.11 3.3 N.S. 0.932 550.7 42 April 3.41 ± 0.08 2.27 ± 0.07 10.4 <0.05 0.930 1038.1 80 May 3.60 ± 0.08 2.42 ± 0.07 8.3 <0.05 0.938 1173.1 79 All regressions were highly significant (P<0.0001), S.D. = standard deviation, N.S. = non-significant (P > 0.05). Reproductive biology of the cockle Cerastoderma glaucum (Bivalvia:Cardiidae) from Lake Qarun 257

Table 2 Monthly variation in regression parameters (loga and b) of shell length (mm) and gonad weight (g) relationships of sexually differentiated individuals of C. glaucum collected from Lake Qarun. Values of Student’s t-test (t), level of significance from isometry (P), coefficient of determination (r2); variance ratio (F) and number of animals examined (N) are also given. Month Loga ± S.D. b ± S.D. t P r2 FN February 2008 5.23 ± 0.26 3.05 ± 0.23 0.2 N.S. 0.799 179.4 47 March 5.14 ± 0.39 2.95 ± 0.36 0.1 N.S. 0.561 67.6 55 April 3.22 ± 0.50 1.06 ± 0.45 4.3 N.S. 0.940 5.6 56 May 4.05 ± 0.48 1.61 ± 0.44 3.2 N.S. 0.211 13.7 53 June 6.83 ± 0.60 4.23 ± 0.53 2.3 N.S. 0.571 63.8 50 July 6.85 ± 0.97 3.99 ± 0.85 1.2 N.S. 0.312 22.2 51 August 6.22 ± 0.40 3.44 ± 0.32 1.4 N.S. 0.707 113.4 49 September 4.73 ± 0.45 2.26 ± 0.35 2.1 N.S. 0.445 42.2 55 October 5.48 ± 0.34 3.16 ± 0.29 0.6 N.S. 0.714 119.7 50 November 5.30 ± 0.35 2.93 ± 0.28 0.3 N.S. 0.671 108.2 55 December 5.34 ± 0.58 2.98 ± 0.49 0.1 N.S. 0.398 36.4 57 January 2009 5.25 ± 0.30 3.03 ± 0.26 0.1 N.S. 0.725 139.7 55 February 3.95 ± 0.37 1.66 ± 0.29 4.6 N.S. 0.368 32.0 57 March 5.56 ± 0.33 2.83 ± 0.28 0.6 N.S. 0.646 102.4 58 April 4.29 ± 0.17 2.47 ± 0.14 3.8 N.S. 0.848 301.0 56 May 4.55 ± 0.31 2.02 ± 0.26 3.8 N.S. 0.571 59.9 47 All regressions were highly significant (P<0.0001), S.D. = standard deviation, N.S. = non-significant (P > 0.05).

was reported for Lake Timsah population of C. glaucum by controlling gametogenesis (Derbali et al., 2009; Lubet and Mohammed (2002). Protracted spawning season and multiple Mann, 1987). spawnings have been reported to have many advantages in The end of the spawning season of C. glaucum was terms of the survival of the species (Lambert and Ware, usually marked by gamete atresia (degeneration and phago- 1984; Weng, 1995). Multiple spawning with intervals between cytosis) within the follicles. The occurrence of this atresia batches may reduce intraspecific competition by allowing each appears to be a common phenomenon linked to the repro- cohort a virtually independent food supply (Lambert and ductive strategy (Pazos et al., 1996). Darriba et al. (2004) Ware, 1984). enumerated three possible reasons for oocyte atresia: (1) a The reproductive cycle of bivalves usually takes the follow- limited capacity of the follicle and a control mechanism ing forms: a progressive development from a condition in covering cell number; (2) a self cleaning process at the which the gonads are undifferentiated, through differentiation end of gametogenic cycle in preparation for the next cycle; of the gonad (gametogenesis and maturation of the gametes), and (3) a response to environmental or contaminating stress to spawning (with partial or complete release of gametes), condition. and a return to one of these earlier stages (Boyden, 1971). A As with most bivalves, C. glaucum is dioecious with no similar sequence of events has been reported for population external morphological differences between the sexes. Her- of C. glaucum from Lake Qarun. However, gametogenesis maphrodites or even sex reversals were not identified in this and maturation of the gametes were divided into three stages study. The sex ratio (1.00 F: 1.29 M) differed significantly in females (early active, late active and ripe stages) and only (X2 = 25.32, P < 0.005) from the expected 1:1 ratio indicating two stages in males (active and ripe stages). This indicates that that males outnumbered females. Similar observations were re- gametogenesis in males was rapid and continuous. corded for two populations of C. glaucum at Sussex, north-east The depletion of ripe ova in female gonads during April– coast of England (Kingston, 1974). However, sex ratio was not May (spring) 2008 and February–March (winter) 2009 was significantly divergent from parity for C. glaucum from the rapidly compensated by the onset of oogenesis. Oocyte size– north coast of Sfax, Gulf of Gabes, Tunisia (Derbali et al., frequency distribution (Fig. 7) indicated the presence of large 2009), three European populations; the Baltic Sea, the percentages of small oocytes at the periods of ripe ova deple- North Sea and the Mediterranean Sea (Tarnowska et al., tion. This explains the absence of sexual quiescence (inactive 2009) and from Berre Lagoon, French coast, Mediterranean or rest stage) seen in C. glaucum from Crouch estuary, Essex, Sea (Tarnowska et al., 2012). England (Boyden, 1971); South Wales (Yankson, 1986a); Lake Male biased sex ratio for Lake Qarun population might be Tunis, the Bou Grara Sea and Gulf of Gabes, Tunisia (Derbali explained by the following possibilities: (1) the presence of sexu- et al., 2009; Zaouali, 1980). Trotta and Cordisco (1998) found ally undifferentiated individuals at completely spawning stage in active gonads throughout the year for C. glaucum from Lesina females. A similar explanation was recorded for female biased Lagoon, southern Italy. It is common for the same species sex ratio in C. edule (Boyden, 1971; Martinez-Castro and Vaz- from the same geographical area to show differences in its quez, 2012); (2) reflections of differences in developmental tem- reproductive cycle from one site to another. This could be po of the two sexes and (3) females are more sensitive to explained by the environmental variability (temperature, unfavorable environmental conditions. The preponderance of salinity, food availability, etc.) involved in defining and males in Modiolus modiolus (Jasim, 1986) was explained in terms 258 K.E. Kandeel et al.

Table 3 Spawning periods of Cerastoderma glaucum and its congeneric C. edule in different localities. (See below-mentioned references for further information.)

Spawning periods Species Location References JFMAMJJASON D C. glaucum Crouch estuary, Essex, England. Boyden (1971) C. glaucum Cuckmre, Seaford, Sussex, England. Kingston (1974) C. glaucum Widewater Lagoon, Shoreham, Sussex, England. Kingston (1974) C. glaucum England Lagoon, the Wide Water, Sussex, southern England. Ivell (1979) C. glaucum Lake Tunis and the Bou Grara Sea, Tunisia. Zaouali (1980) C. glaucum North coast of Sfax, Gulf of Gabes, southern Yunisia. Derbali et al. (2009) C. glaucum The Gulf of Gdansk, Baltic Sea, Polad. Tarnowska et al. (2009) C. glaucum Lake Veere, North Sea, the Netherland. Tarnowska et al. (2009) C. glaucum Berre Lagoon, Mediterranean Sea, France. Tarnowska et al. (2009) C. glaucum Lake Timsah, Suez Canal, Egypt. Mohammed (2002) C. glaucum Lake Qarun, El-Fayoum, Egypt. Present study C. edule Ria of Vigo, NW of Spain. Figueras (1957) C. edule Danish brakish water areas, Denimark. Petersen (1958) C. edule Crouch estuary, Essex, England. Boyden (1971) C. edule Kent coasts, south-east England. Kingston (1974) C. edule Strangford Lough, Northern Ireland. Seed & Brown (1977) C. edule Tamar Estuary,Cornwall, England. Newell & Bayne (1980) C. edule Ria of Noia, N of Spain. Mejuto (1984) C. edule Guadiana River estuary, SW of Spain. Saavedra (1984) C. edule The French Channel and Atlantic coasts. Guillou et al. (1990) C. edule Saint, Pol-de-leon, North Brittany. Guillou & Tartu (1992) C. edule Dutch Wadden Sea, Northwestern Europe. Cardoso et al. (2009) C. edule The Ria de Vigo,Galicia, Northwest Spain. Martinez-Castro&Vazquez (2012)

of sex-specific mortality. Increased female mortality after References spawning owing to the high coast of gonad maturation and spawning was reported in some bivalves, like Chlamys islandica Abdel-Malek, S.A., Ishak, M.M., 1980. Some ecological aspects of (Brokordt and Guderley, 2004) and Mytilus trossulus (Wolowicz Lake Qarun, Fayoum, Egypt. Part II: production of plankton and et al., 2006). benthic organisms. Hydrobiology 75, 201–208. For Lake Qarun population of C. glaucum, the length at Ansell, A.D., Sivadas, P., Narayanan, B., Trevallion, A., 1972. The ecology of two sandy beaches in south west India. III. Observations which 50% of the population reached maturity (SM50) was 9.6 and 8.5 mm for females and males, respectively. For on the population of Donax incarnatus and D. spiculum. Mar. Biol. 17, 318–332. comparing C. glaucum from the Gulf of Gabes (southern Bailey, N.T.J., 1995. Statistical Methods in Biology. Cambridge Tunisia), female SM50 and male SM50 estimated to be University Press, Cambridge, p. 255. 16.78 and 15.14 mm shell length, respectively (Derbali Barash, Al., Danin, Z., 1972. The Indo-Pacific species of mollusca in et al., 2009), were larger than our records for Lake Qarun the Mediterranean and notes on a collection from the Suez Canal. population. Higher temperature in this Lake; monthly values Isr. J. Zool. 21, 301–374. ranged from 16 to 30 C compared to 12.3–26.8 C for Gulf Boyden, C.R., 1971. A comparative study of the reproductive cycles of of Gabes, is important for an early recruitment into the the cockles Cerastoderma edule and C. glaucum. J. Mar. Biol. breeding population. The rate of filtration by bivalves in- Assoc. UK 51, 605–622. creases by a rise in the temperature (Foe and knight, 1986) Brock, V., Wolowicz, M., 1994. Comparisons of European popula- and it is expected to accelerate the initiation of gamete devel- tions of the Cerastoderma glaucum/C. lamarcki complex based on reproductive physiology and biochemistry. Oceanol. Acta 17, 97– opment and maturation for the first time in life. Maturation 103. of 7 week old C. glaucum (4 mm shell length) laboratory Brokordt, K.B., Guderley, H.E., 2004. Energetic requirements during reared spat has been reported and although no fertilization gonad maturation and spawning in scallops: sex differences in experiments were conducted, gamete morphology was com- Chlamys islandica (Muller 1776). J. Shellfish Res. 23, 25–32. parable to those of adults from natural populations (Yank- Chang, Y.J., Mori, K., Nomura, T., 1985. Studies on the scallop, son, 1986b). Patinopecten yessoensis, in sowing cultures in Abashiri waters – In conclusion, results of the present study indicate that reproductive periodicity. Tohoku J. Agric. Res. 35, 91–105. C. glaucum is well adapted to the specific environmental condi- Cardoso, J.F.M., Witte, J.I.J., Van der Veer, H.W., 2009. Differential tions of Lake Qarun in order to maximize its reproductive suc- reproductive strategies of two bivalves in the Dutch Wadden Sea, cess. The reproductive periodicity is less confined to a specific Esturaine. Coastal Shelf Sci. 84, 37–44. Darriba, S., Juan, F.S., Guerra, A., 2004. Reproductive cycle of the season and the onset of sexual maturity is precocious. This razor clam Ensis arcuatus (Jeffreys, 1865) in northwest Spain and baseline study can serve as a reference for assessment and man- its relation to environmental conditions. J. Exp. Mar. Biol. Ecol. agement of C. glaucum as well as for future research on the use 311, 101–115. of this species as a bio-indicator. However, further research is Darrigran, G.A., Penchaszadeh, P.E., Damborenea, M.C., 1999. needed to obtain data on population dynamics. The reproductive cycle of Limnoperna fortunei (Dunker, 1857) Reproductive biology of the cockle Cerastoderma glaucum (Bivalvia:Cardiidae) from Lake Qarun 259

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