Journal of Marine Biology & Oceanography ISSN: 2324-8661

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Research Article, J Mar Biol Oceanogr Vol: 4 Issue: 1

Mytilus galloprovincialis: Reproductive Cycle of Fields Mussels Close to a Lagoon (North Atlantic, Moulay Bousselham, Morocco)

Sanaa Bhaby*
University Hassan II, Casablanca, Laboratory of Aquatic Ecology and Environment, Casablanca, Morroco
Corresponding author : Sanaa Bhaby
University Hassan II, Casablanca, Laboratory of Aquatic ecology and Environment, Casablanca, Morroco
E-mail: [email protected]
Received: October 17, 2014 Accepted: April 20, 2015 Published: April 23, 2015
Citation: Bhaby S (2015) Mytilus galloprovincialis: Reproductive Cycle of Fields Mussels Close to a Lagoon (North Atlantic, Moulay Bousselham, Morocco). J Mar Biol Oceanogr 4:1. doi:10.4172/2324-8661.1000140

Abstract

Mytilus galloprovincialis: Reproductive Cycle of Fields Mussels Close to a Lagoon (North Atlantic, Moulay Bousselham, Morocco)

This work concerns the gametogenesis of a mussels’ deposit located in the Atlantic northern shoreline of Morocco, in Moulay Bousselham. The concerned coast is characterized by specific hydrodynamic conditions: it is subject to freshwater input from rivers that are supplying the surrounding lagoon. The results are discussed on the basis of adequate statistical inferences. The winter and spring seasons are characterized by high emission of gametes, and throughout the year, there is a significant proportion of mature gametes. To achieve this result, we considered the following two biological parameters: the gonadic index and the condition index. The study took place over an unusually long period, from March 2009 to March 2011, at a rate of one sample per month. In conclusion, given the permanent spawning activity, this area deserves further exploration in order to implement the mussel farming within.

Keywords: Mytilus galloprovincialis; Reproductive cycle; Lagoon; Condition index; Gonadic index; Histology; North Atlantic

Keywords

Mytilus galloprovincialis; Reproductive cycle; Lagoon; Condition index; Gonadic index; Histology; North Atlantic

Introduction

Currently, with the exception of a few dozen tons of bass fish, oysters and mussels, there is almost no aquaculture marine production in Morocco [1]. In order to fund this shortfall, the Moroccan fisheries sector adopted, in 2009, a development strategy so-called ‘Halieutis program’ [2,3]. This strategy has been implemented and coupled within the institutional framework of the cooperation with European Union [4].
The mussel is a marine resource whose aquacultural potential remains to estimate. It became imperative to know better the life cycle of mussels and their environmental conditions. That is the object of the present article which is dedicated to the cycle of reproduction at the site of Moulay Bousselham. This site has a particular hydrodynamic condition, because it is both affected by the marine phenomena and by the fresh water of the rivers. Indeed, these rivers feed the neighbouring lagoon [5].
More exactly, this work consists of studying reproductive cycle of Mytilus galloprovincialis, the only species of this genus in Morocco [6]. Furthermore, it will be necessary to plan an aquacultural device capable to resist the powerful swells (up to ten meters of hollow in the wide), accompanied with the strong tidal wave, which rage from November till May in this region of the North Atlantic [7].
The histology is necessary for the description of the events relative to the development of gametes, while the quantitative estimations are important because they eliminate the subjectivity and the problems associated to the description [8]. The condition index is one of the methods retained for the evaluation of the potentialities of weight growth bound to the reproductive cycle. Indeed, at the ripe individuals of the mussel, the reproductive tissue can represent more than 59% of the weight of the soft tissue [9], or more than 95% of the tissue of the mantle [10].
This study investigates in the annual successive cycles of the reproductive activity of gametes in the Atlantic coast of Morocco. The status of gonad development and condition index are analyzed (Figure 1).
Figure 1: Location map showing the geographical position of the site; sampling was carried out from a deposit adjacent to the lagoon; this lagoon encompasses two broad elements: a shallow marine lagoon, named for this reason Merja Kahla (black lagoon) and a deep water lagoon, named for this reason Merja Zerga (blue lagoon), whose surfaces are respectively 3 km2 and 27 km2.

Materials and Methods

Sampling site
Moulay Bousselham is situated approximately 120 km from Rabat (GPS 34°53’ N ; 6°17’ W). It is a small rocky coast spread on approximately 1 km to 500 m of the North of lagoon Marja zerga (site RAMSAR). It is a part of Gharb’s coast, constituted largely by dead cliffs parallel to the bank and by rectilinear sandy beaches, interrupted by the rocky outcrops of stoneware dunaires at the level of Merja Zerga (lagoon). It is located in the north end of the upwelling. Characterized by a biodiversity and a remarkable productivity, this rocky station is characterized by an intense activity of mussels collects of both species Mytilus galloprovincialis and Perna perna.
Samples
Monthly in two years (March 2009 - February 2011), 30 bivalves were sampled per month with shell length equal or over 40 mm were measured (4 0.01 g) from the umbo. The temperatures were recorded at the time of sampling in 30 cm of depth. While samples in January 2010 and January 2011 weren’t taken due to environmental conditions.
The M. galloprovincialis samples were individually examined in the laboratory. The size (shell length) of the mussels was measured with calipers, and the total wet weight (Wt, g), wet meat weight (WMW) and shell weight (SM) were recorded by analytic balance to the nearest 0.001g. To determine the state of physical stress of host, condition index (CI) was calculated as used by Bhaby et al. [11], given by the formula (1):
Histologic inspection was used to determine the sex and gametogenic stage. Gonads were cut at the central portion of the mantle, fixed in Davidson’s solution for 24 hours, dehydrated in ascending ethanol concentrations and embedded in paraffin blocks. Histologic sections (2 μm) were stained in hematoxylin-eosin Y and studied under light microscope (Leica DM2500). To examine the reproductive states of the M. galloprovincialis, gonad development stage was classified into seven stages using the method defined by Lubet [12] and applied in Ojea et al., Benomar et al., and Bhaby et al., [11,13,14]. For convenience in analyses and further histopthalogical interpretation, Gonadic index (GI) was assigned to each stage. GI were calculated as the following formula of Seed (2) [14]:
Where, ni: number of individuals at each stage of reproductive cycle, Si: score of the stage and N: total number of individuals.
Monthly mean of the GI were calculated, and the time series of GI was analyzed through the seasons.
Gametogenic development was indicated by increases of the monthly GI while decline in the time series indicates that occurrence of spawning (Table 1).
Table 1: Mytilus galloprovincialis cycle of reproduction [12] scoring gonadic index (IG).
Statistical analysis
All statistical tools (graphic representations, distribution function, cumulative frequency and hypothesis testing) were provided by the R version 2.15.2 software (R. Core Team) on Windows.
The Chi-square test allowed comparison of observed proportion female numbers/males numbers to theoretical proportion 1; The Shapiro and Wilk test was used to check the fit of the distribution of a quantitative variable observed at a normal distribution. We presented this test through checking the normality of a series of gonad index observations (GI).
Kruskal Wallis test was used for the comparison of the condition index (quantitative variable) sampling on four seasons.

Results

Temperature
The water temperature recorded variations according to the season. For three successive years from 2009 to 2011, the lowest level of temperature was recorded in November 2010 (14°C). The highest temperature was recorded in summer (23°C) in July and August (Figure 2).
Figure 2: Monthly variation of the temperature of the seawater in the site Moulay Bousselham, the measures were made on surface in 30cm of depth between March 2009 and February 2011.
Sex-ratio
On the total reviewed sections, 317 correspond to females, 339 to males; there were four with indefinite sex.
Thus, the number of females (48%) was lower than the number of males (52%). The sex ratio calculated was not significantly different from the theoretical sex ratio 1/1 (p=0.390, according to the Chisquare test). However, the sex ratio varied from one time to another; females had preponderance in the month of February 2010, September 2010 and October 2010, while males were in majority in the month of August 2009 and February 2011.
Reproductive cycle
Gonads of male and female M. galloprovincialis were histologically examined based on observed states of connective tissues and contents of gonad acinus (Figures 3-5). Stage 0 was observed in small percentage of individuals (less than 6%, Figure 6A) in March 2009, August 2009, November 2009 and February 2011. There was no observation of primordial follicles or gametes, so we could not determine sexes. In August 2009 and November 2009 (Figure 3A), connective tissues in mantles in stage 0 were filled with reserve cells. On the other hand, in March 2009 and February 2011, the mantle had infections of sporocystes of trematodes, and reserve cells in connective tissues were hardly observed (Figure 3B).
Figure 3: Photomicrographs showing resting of gametogenesis. (A) stage 0, (B) the mantle with heavy infection by sporocystes of trematodes.
Figure 4: Photomicrographs showing the gametogenics developement in females, (A) stage IIID, oocytes degenereted and heavy hemocytic infiltration (B) stage I, oogonia (C) stage II, development (D) stage IIIA, ripe (E) stage IIIB, partly spawned (F) stage IIIC, redevelopment. Ihm: hemocytic infiltration, oog: oogonie, opr: previtelogenic oocytes, opv: vitelogenic pedunculated oocyte, om: mature oocyte.
Figure 5: Photomicroraphs (X10) showing different gametogenic stages observed in males. (A) stage IIID connective tissue packed of reserve cells, residual follicle and two primordial follicles, (B) stage I, primordial follicles, (C) stage II, development (D) stage IIIA, ripe, (E) stage IIIB, spawning, (F) stage IIIC, connective tissue poor of reserve cells and new germinal line sequence in follicles. Pf: primordial follicle, ihm: hemocytic infiltrations, rc: reserve cells, spg: spermatogonie, spc: spermatocytes, spt: spermatids, spz: spermatozoîds.
Figure 6: Monthly distribution of Mytilus galloprovincialis gametogenic stage in females (A) and males (B) between March 2009 and February 2011.
Gonadal tissues in the stage of degeneration and end of the gametogenesis (stage IIID) were observed in both females and males (Figures 4A and 5A). The proportion of females was higher than that of males in the total specimen in this stage. Stage IIID of female individuals was observed from June, one month before the observation of males in 2009 and the observation was progressively increased until September (73%, Figure 6A). This stage was highly observed in November and December in 2009.
We observed regenerations of the connective tissue, abundant reserves cells and heavy hemocytic infiltrations in female’s gonadal tissues of stage IIID individuals (Figure 4A). We also observed lysis of gametes and mature follicles indicating reduced sexual activity in the regenerated parts of the mantle for new sexual cycles with the primordial follicles (Figure 5A).
The individuals with only the primordial follicles with spermatogonia or oogonia (stage I) appeared as a relatively low percentage. In female, stage I was observed in 2009 from August (5%) to November (12%). In males, this stage was appeared from June to October (6%, Figure 6B).
From August to November, stage II was most apparent in males, which the percentage progressively increased from June (6%) to December (until 60%). In female, this stage was observed in 2010 from May (12%) to June (8%); and from November (46%) to December (20%). In males and females the stage II was recorded in February 2011 in 5% and 6% respectively. In stage II, gametogenesis was more developed in some individuals of the both sex, and the size and the number of follicles were increased in the mantle. Follicles were observed sparsely in connective tissues.
This phase was followed by a maturation of gametes (st. IIIA) (Figures 4D and 5D). In stage IIIA, the gametes was morphologically ripe, the connective tissue which had in this stage lost most of its reserves cells may be almost completely obscured by the swollen with spermatozoa in male and with oocytes in females follicles. In males the Stage IIIA was particularly strong in spring from February to June (83%). In this timing, the majority of females were started the emissions of the oocytes (stage IIIB) (Figure 4E). The emission of gametes, stage IIIB (Figure 3D) showed peaks above 50% in both sexes. It was for 2009, in April (56%), May (60%), June (50%), and it was for 2010 in May (53%).
Then the spawning continued in August and September by the restarting up of the gametogenesis (stage IIIC), this rapid period of gamtogenesis was finished in both sex by a spawning and emptying of reserve tissus in October (43%). The stage IIIC was very developed in March 2009 (86%); 2010: February (56%), March (76%), April (80%), July (66%), August (60%) and December (50%) and in February 2011 (50%).
Gonadal index
The gonadal index illustrated in Figure 7, was showed a temporal variation. The values were included between 2.6 and 1.2. The highest values were recorded in early summer (June 2010) and winter (February 2011); which the histology indicated an advanced level of maturity (stage IIIA). The minimal values were recorded in automun (September 2009), which reveled the presence of stage IIID. The Kruskal-Wallis test indicated that there were, in relation to the seasons (Figure 8), a significant difference in gonadal activity (p<0.05).
Figure 7: Monthly variation of the gonadal index of Mytilus galloprovincialis in Moulay Bousselham.
Figure 8: Boxplot highlighting the seasonal distribution of gonadal index.
Condition index
Change of the condition index indicated seasonal cyclic pattern of variations in survey years (Figure 9). From April, when the condition index was minimum through survey years, it increased and reached at maximum in early summer, June. During the summer spawning from July to August, the condition index decreased.
Figure 9: Evolution of condition index and the corresponding monthly standard deviatiation of Mytilus galloprovincialis in Moulay Bousselham.
From autumn to winter, the condition index decreased. There was difference of the observed seasonal pattern in autumn. In September 2010, the condition index did not exceed 45%. On the other hand, the condition index was approximately 50% in September 2009 (Figure 9). These differences were most likely caused by the change of the number of individuals in stage IIID (Figure 6).
Seasonal difference of the condition index was observed and statistically significant (Kruskal-Wallis test, p<0.05). Summer was the season with maximum of condition index, and winter was the season with minimum of the index (Figure 10).
Figure 10: Boxplot highlighting the seasonal distribution of Condition index.

Discussion

Reproductive studies of Mytilus galloprovincialis from around the world have shown a wide range of reproductive patterns both between and within populations [11,15]. The site studied divides up in a characteristic environment of the Moroccan Atlantic north coast. To know all the details and the information of this phenomenon is important to make a success of the breeding. This information can be used to master effective techniques for the conditioning, the induction of heavy weight (spawning) and the embryonic development [16].
The variability of condition index and the histologic analysis of the gonad reveal four major spawning (February; April; July–September and November–December), with minor spawning exists permanently. There is no apparent resting period during the two annual studies period, a start development of gametes may take place simultaneously when the last cycle finishes. However, production of gametes decreases in summer during the months of July and August. These results show that the reproductive cycle of Mytilus galloprovincialis from Moulay bousselham (North Atlantic of Morocco) is continuous and there is three principal seasonal spawning in winter, spring and late summer. The continuous reproduction of Mytilus consistent with Suárez et al. [17] who found that there is no resting period transition between the two gonad cycles in cultured population from Atlantic in Spain and with O’Connor [18] and Fearman [19] who studies the mussels in tropical zones. The same report was observed by Idhalla [20] and Bhaby et al., [11] in south of Morocco. The massive spawning are translated by a notable hollow of the condition index, there was synchronous with the spawning of Mytilus in the Atlantic [11] and the Mytilus in other waters in temperate zones [20-24].
The temperature and the food are the major exogenous factors which control and regulate the duration and the synchronization of the cycle of reproduction of the mussel [25-27]. We speculate that the conditions of surrounding water such as food availability based on high primary productivity [28] and preferable thermal conditions in this site allows the template M. galloprovincialis continuously reproduce. Except the strong rise of the temperature in August, 2009, and the decrease in November, the temperatures recorded in Moulay bousselham (between 18°C - 21°C) stay in the superior thermal range activating the stop of cycle. Also, to be noted is that there is a combination of follicles degeneracy in relation to temperature rises (21°C à 26°C).
The peak spawning in early spring coincides with the principal timing of spawning for mussels studied by in Atlantic coast of Morocco [21,29]; in Atlantic Spain [15,26,30,31]. Shafee, Morchid and Andréu [21,30,31] described the possibility of lesser spawning especially in the late of summer and in autumn, when environmental conditions are favorable.
Seed and Ferran [20,30] added that one year to another sexual cycle may variability in the same local. This explains the strong growth seen in September 2010 issue and the monthly shift that we had between the two years of the study.
Be that as it may, the succession of the season punctuates in the background, the overall development was affected by the massive emission of gametes in the last winter.

Conclusion

Our research shows that gametogenesis is active throughout the year, this result leans in favour of the possibility to develop mussel farming in that area. Most significantly, among the mussels examined, that contain active gonads are present continuously with a significant spawning rate. Therefore, deposit of mussels takes full advantage from the neighbourhood of the great the lagoon “Merja Zerga”. Indeed this wetland is a major migratory stopover for many birds (gull, crow, oystercatchers and eider duck); benches surrounding mussels contribute to their diet. Consequently, mussel farming should be integrated in the global ecological system of this zone. In particular, it would be necessary to continue this work by conducting an extensive research on this marine environment characterized by a substantial input of freshwater following the incessant rhythm of the tides.

Acknowledgment

The completion of this work owes a lot to the laboratory of shellfish pathology in National Institute of Fisheries Research (Morocco). That each finds here the expression of my profound gratitude.

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