Environmental parameters
Water temperature (◦C) ranged from 19.7 ◦C (winter) to 35.5 ◦C (spring and autumn) and had higher records at GBL than that at TL (Fig. 2A). Mean water temperature significantly varied with seasons (P= 0.025, Table 1). There were no significant differences between the studied areas in terms of temperature. Temperature data were significantly different in winter compared to autumn and spring (Tukey test, Table 1). The highest and lowest values of the pH data were detected in winter (~8.5) in TL, and summer (~8.0) in GBL, respectively (Fig. 2B). Data analysis revealed no significant variations among categorical factors. Salinity distribution fluctuated between 29.1‰ to 33.2‰ and from 40.5‰ to 43.6‰ at TL and GBL, respectively, and indicated a hypersaline pattern in GBL (Fig. 2C). The mean salinity data revealed significant differences (P = 0.004) between the areas. (Table 1). The range of dissolved oxygen data (DO) was between relatively 6.2, 6.18 mg/L to 11.9 mg/L and 9.91 mg/L, respectively, at TL and GBL (Fig. 2D). Mean DO data showed significant seasonal variations (P = 0.016) and data between the areas were not meaningful (Table 1). A Tukey test indicated significant differences in DO data in autumn and spring vs. summer and winter (Table 1). The highest chlorophyll a concentrations were 34.6 µg/l and 17.1 µg/l, respectively at TL and GBL, whereas the lowest concentration were 15.1 µg/l and 1.2 µg/l, respectively, at TL and GBL (Fig. 2E). The chlorophyll a distribution pattern between TL and GBL was significantly different (P = 0.02, Table 1(.
Sex Ratio
The total number of individuals whose sex was assigned was 1242 and 1228 at TL and GBL, respectively. The male contribution was 55.64% and 56.03%, whereas the percentage of females was 44.36% and 43.97% at TL and GBL, respectively (Table 2). The overall sex ratios (M: F) were 1:0.8 in TL and 1:0.78 in GBL. Results of Chi-square (χ2) rejected the null hypothesis and revealed deviations in the ratios from the standard value (1:1), as the χ2 values were 15.78 and 17.84 at P < 0.001 at TL and GBL, respectively. A significant monthly deviation in the sex ratio was recorded at both TL and GBL (Table 2). Excepting July at GBL, the data showed a male-skewed distribution.
Condition Index
The CI values of P. textile at TL and GBL tracked each other during the sampling period, except for July and October (Fig. 3). The range of P. textile CI at TL was between 9.15 ± 1.23(October) and 19.41 ± 3.22 (March). While in GBL, the highest and lowest values were recorded in March (22.24 ± 10.99) and April (10.99 ± 1.61), respectively. There were no significant variations in the mean CI values between TL and GBL. However, the mean data for CI differed significantly among months (Table 3). The statistically significant varied data were scattered and detected during January and February (winter) vs. April and May (spring), August (summer), and October and November (autumn), as was shown by the least mean difference (T) in Table 3. The P values for January's mean differences were 0.017, 0.019, 0.005, 0.011, and 0.012, respectively, and February's least different means were at P = 0.007, 0.008, 0.036, 0.023, 0.002, 0.027, 0.005, and 0.005, respectively, vs. April, May, June, July, August, September, October, and November (Table 3). Similarly, March (spring) had significantly different mean data for CI from April, May, June, July, August, September, October, and November (P = 0.001, 0.001, 0.004, 0.003, 0.0001, 0.003, 0.001, 0.001, respectively, Table 3).
Sexuality
Paphia textile lacks external sexual dimorphism, and the histological sections prove that it is dioecious; no hermaphroditism was observed. Both male and female P. textile gonads have cream to yellow in color. A single gonad is located above or adjacent to the ventral muscular foot of both sexes. Gametes are discharged through an exhalant siphon, and fertilization is completed externally.
Gametogenic Maturity Stages
The reproductive cycle includes the earliest detection of sex cells, the growth and multiplication of the sex cells, and finally, the filling of the gonad tubules and the disappearance of the sex cells from the tubules during the spawning period. The stages of gonad development in P. textile are described in Table 4, and Figures 4 and 5.
Reproductive Cycle of Paphia textile:
The gonadal maturity stage distribution and the percentage of frequency (occurrence) revealed a wide range of monthly variation and lacked periodicity for both sexes(Fig. 6). The resting stage distributional range was narrow, and data did not locate stage I in some months (April, June, and October, Fig. 6), The highest frequency of stage I was 13.3% (TL), and 26.7% (GBL). There were only slight variations between the histograms of males and females (Fig. 6).
The developing stage (II) was highest between January and April at TL and in January and February at GBL for female clams (Figs. 6A and C). January’s highest contribution fluctuated between 52% at TL and 64% at GBL, whereas the developing stage disappeared in September and October at TL and November at GBL. The pattern of developing stages of the male clams was inconsistent throughout the study year. However, the highest contribution was detected during the early winter at TL and in winter and spring at GBL, with few exceptions (Figs. 6B and D).
The ripe females (stage III) were the most frequent in April (52%) and September (56%) at TL and during March (60%) and April (65%) at GBL (Figs. 6A and C). Ripe males wererecorded year-round with a wide frequency range (Figs. 6B and D).
The ripe stage was followed by two spawning peaks (stage IV) for females at both studied sites. In TL, the first and second spawning peaks occurred in May (40%) and October and November (36%). In GBL, the two peaks were detected in June (60%) and September (48%). Male spawning stages were found throughout the year and tracked their females in both study sites (Figs. 6B and D).
The highest contribution of the completely spawned females (stage V) was observed in October (52%) and November (36%) at TL and GBL, respectively. The spent-stage males were found from May to December, with the highest contribution in November (26.7%), at TL (Fig. 6B). At GBL, the proportion of spent males (stage V) was recorded throughout the study year, except for February and August, and peaked in October (33.3%, Fig. 6D).
Gonadal development stages were found to significantly vary by month (P ≤ 0.001, Table 5). However, the results of the 3-way ANOVA did not reveal significant differences between the sampling areas, sexes, and the interactions among categorical factors, except for the mean data of stage III, which was significantly different between TL and GBL (P =0.005, Table 5).
Results from the Tukey test (Table 6) detected that stage I data had the most significantly frequent least mean differences year-round. The distributional frequency of stage I was significantly higher in January and February than in April (T =1.21 and 1.04, respectively Table 6). The T values were significantly higher in June, October, and November than in January and February. The data revealed significant variations in spring, April vs. May, July, and August, and May vs. June, October, and November. Further, the in-between data frequencies during the warm months were significantly different in June vs. July, August, October, and November, and in July and August vs. October and November (Table 6). The trend of monthly least mean differences in the distributional data of the other four stages was less clear. The least statistically significant differences in stage II data were observed in winter (January and February) vs. summer (July and August) and Autumn (September and November). Significant least mean differences in stage III were detected in January, March, and April vs. April, October, and October-November, respectively. The significantly different least mean stage IV data were recorded in January vs. May, summer, and autumn months, and February’s stage IV data differed considerably from June and August. The January and March least mean stage V data differed significantly from October-November and October, respectively (Table 6).
The relationships between the data of maturity stages and the measured environmental factors differed in sign and magnitude, as indicated by the Pearson correlation analysis (Table 7). The temperature of the water affected the distribution of stages I and II negatively (r = -0.69 and -0.72 at P = 0.003 and 0.002, respectively), and had a mildly significant positive effect on stages IV and V (r = 0.58 and 0.57, P = 0.018 and 0.019, respectively).In contrast to the temperature effect, salinity correlated significantly positively with data from the first two stages, since their r values were 0.572 (P =0.02) and 0.866 (P < 0.0001), respectively. Relatively high salinity negatively affected the last two stages (r = -0.743 and -0. 551, at P = 0.001 and 0.027, respectively). The effect of the dissolved oxygen was confined to stage I with relatively strong positive significant relationship (r = 0.68 at P = 0.004). Surprisingly, chlorophyll a data revealed strong negative (r= -0.799 at P < 0.0001) and positive (r =0.815 and P <0.0001) correlations with the first and last stages, respectively. However, the pH data did not reveal significant correlations with different maturity stages.
Clam Size at Sexual Maturity
The calculated mature clam shell length sizes (mm) at SM50 were 28.60 and 31.70 mm SL for females and males in TL, respectively. At GBL, SM50 was 31.50 mm for females, and 34.10 for males (Fig. 7), indicating the size of mature individuals was smaller at TL than at GBL. The observed results of the microscopic examination revealed that the length of the shell of mature clams was between 27.55 mm for females and 25.60 mm for males in TL. At GBL, the SL values at the maturity were 25.55mm and 27.70mm for females and males, respectively.
Gonad index
Gonad indexresults revealed fluctuation during the sampling period between December, 2019 and November, 2020 at TL (Fig. 8A) and GBL (Fig. 8B). The highest TL scores were detected in April (3.47 for males and 3.28 for females), and the lowest values were recorded in September (2.27 for males) and in October (1.72 for females). Furthermore, female values were slightly higher in GBL than at TL and vice versa for the male data. However, mean GI data at TL was not significantly different from the data at GBL (Table 8). The scoring of the GI data varied markedly among months (F-ratio= 4.423 at P < 0.0001; Table 8). The Tukey analysis indicated that GI data were significantly different in March vs. July, October, and November (P = 0.023, 0.003, 0.030, and 0.027, respectively). Mean GI data in April was significantly different from July and October (P = 0.012 and 0.002, respectively Table 8).