Reproductive cycle of coexisting mussels, Mytilus californianus and Mytilus galloprovincialis, in Baja California, New Mexico.

ABSTRACT The bay mussel, Mytilus galloprovincialis, and the California mussel, Mytilus californianus, are sympatric species in the Pacific coast of Baja California in northwest Mexico. To determine the reproductive cycle of both species in a coexistence area, a study of quantitative stereology was carried out from January to December 1995, M. californianus and M. galloprovincialis adults were collected from Mina del Fraile an exposed rocky shore and M. galloprovincialis adults were collected in a culture area a protected bay. In both species spawning organisms were found all year round. At population level, M. galloprovincialis had one major reproductive season from autumn to early spring in both sites. In late autumn, gonad follicles were full of ripe gametes and developing gametes and some spawning could occur. However, the spawning peak occurred in early winter. Minor spawning conditions were more frequent in M. galloprovincialis from the culture area than in M. galloprovincialis from the Mina del Fraile. These differences were presumably associated with the most stable environmental conditions in the culture area, which could also influence differences in percentage of storage cells found from spring to summer. The reproductive season of M. californianus took place from winter to summer. The major reproductive activity occurred in spring and the resting season took place during autumn. The reproductive season of M. californianus in the studied locality is from winter to summer, whereas the reproductive season of M. galloprovincialis occurs from autumn to early spring.

KEY WORDS: coexistence, reproductive cycle, spawning, Mytilus galloprovincialis, Mytilus californianus

INTRODUCTION

The mytilids, Mytilus californianus (California mussel) and Mytilus galloprovincialis (bay mussel), coexist in the southern part of the west coast of North America. The distribution of M. californianus is from Alaska to Islas Socorro in Mexico, and the distribution of M. galloprovincialis is from central California to Baja California, Mexico (Soot-Ryen 1955, Mc Donald & Koehn 1988, Koehn 1991, Seed 1992, Suchanek et al. 1997). The genetic divergence between M. californianus and M. galloprovincialis and their external morphology have been clearly differentiated and, in spite of the fact that they are sympatric species, there is no hybridization between them (Martinez-Lage et al. 1997, Martinez-Lage et al. 2002, Caceres-Martinez et al. 2003, del Rio-Portilla & Caceres-Martinez, in prep.). Studies on the reproductive cycle of coexisting species aid in explaining their coexistence strategy. Suchanek (1981) from field studies carried out in Alaska and Washington stated that the reproductive strategy of Mytilus californianus is based on continuous spawning at a very low level throughout an annual cycle. Mytilus edulis (knowing now that bay mussels in Alaska and Washington correspond to M. trossulus) is a classic fugitive species that rarely reaches a large size, but matures early and is characterized by a single massive reproductive output each year.

Different techniques have been used to assess the reproductive condition in mussels. A summary on studies of reproduction seasons in mussels from North America is shown in Figures 1 and 2. Seed (1976) from an analysis of reproduction records for several mytilids from various parts of their geographical range noted that mussels from the warmer more southerly waters of the Northern Hemisphere generally spawn earlier than those further north. Southern species usually reproduce later in the year and have a progressively restricted season further north; northern species exhibit the reverse trend, spawning earlier and with a more extended season further south. A wide reproductive period in the southern part of the Northern Hemisphere has also been observed in other studies (Ferran 1991, Villalba 1995, Caceres-Martinez & Figueras 1998).

[FIGURES 1-2 OMITTED]

Minor local variations in the reproductive cycle of the same mussels species has also been studied. Seed (1976) described that the gonadal development of Mytilus edulis in England was faster in mussels from the low intertidal zone, than in those from the upper zone and he related these results to food availability. Local variations in the gonadal cycle of Mytilus galloprovincialis were found in Spain and these differences were associated with environmental conditions (Ferran 1991, Villalba 1995), but the gonadal cycle of wild M. galloprovincialis from the low intertidal exposed rocky shores and from those cultured in floating rafts was similar (Caceres-Martinez & Figueras 1998).

There are no studies on the reproductive cycle in coexisting M. galloprovincialis and M. californianus from the same exposed rocky shore in Baja California, Mexico (the southern distribution area of M. galloprovincialis and M. californianus in the Pacific coast of North America) or on the comparison of the reproductive cycle between M. galloprovincialis from exposed and culture conditions. The objectives of this study are to determine the time and duration of the reproductive period of M. californianus front exposed conditions, and of M. galloprovincialis from exposed and culture conditions through quantitative stereology.

MATERIALS AND METHODS

To determine the reproductive cycle using the proportion of germinal and storage cells in the natural population and the cultured Mytilus galloprovincialis and the natural population of Mytilus californianus, two samplings were planned from January to December 1995. The first sampling was carried out in an exposed rocky shore named Mina del Fraile south of Bahia de Todos Santos (Fig. 3) where M. galloprovincialis and M. californianus coexist in the intertidal zone. Each month, 30 individual adults from each species where recollected from the middle intertidal zone during low tide periods. The morphologic differentiation of both species at the adult stage was easy. See the morphologic criteria established by Caceres-Martinez et al. (2003). A total of 360 mussels from each species were sampled; their mean shell length was 57 [+ or -] 5.8 mm for M. californianus, and 63 [+ or -] 8.2 mm for M. galloprovincialis. The second sampling was carried out taking 30 M. galloprovincialis adults from culture ropes (5 m depth) each month in the culture area located in Bahia de Todos Santos (see Fig. 3). A total of 360 mussels with a mean shell length of 64 [+ or -] 6.3 mm were sampled. Mussels from both samplings were brought to the laboratory in plastic bags and were cleaned with a brush and a jet of seawater.

[FIGURE 3 OMITTED]

The soft tissue from each mussel was fixed in Davidson's fixative (Shaw & Batle 1957). An anterior transverse section was taken, including the mantle tissue. Histologic sections with a thickness of 5 [micro]m were cut and stained with hematoxylin and eosin. The presence of gametes and storage cells were determined by quantitative stereology using a Weibel (1969) graticule following the method described by Briarty (1975), Lowe et al. (1982), Seed and Suchanek (1992), and Caceres-Martinez and Figueras (1998). Five random counts were carried out on each histologic slide. Results are given as a percentage of mantle volume occupied by the gamete development stages and cell types or mantle components (MC) defined as ripe gametes (RG). In the female, it is the stage in which oocytes were free within the follicles and some oocytes remained attached to the follicle wall; whereas in the male, it is the stage when follicles were filled by spermatozoa arranged in characteristic bands. Similar to the females during the developing stage of gametes (DO) when rounded oocytes along with oocytes were found attached to the follicle wall, in the male varying quantities of developing spermatogonic cells were present. Additional observations on the presence of hemocytes around gametes, and conditions of the follicle walls were recorded to determine if gamete reabsorption was occurring. The other cell components in the gonad were vesicular cells of connective tissue (VCT) and adipogranular cells (AC); empty follicles (EF) were recorded when the points of the indicate an empty area inside the follicle.

The one-way ANOVA was used to determine if gametes (RG and DG) were different among mussel species throughout the study period in both localities and species (Sokal & Rohlf 1987).

RESULTS

Variations in percentage of the different cell types in the mantle volume of Mytilus galloprovincialis from the culture area are shown in Figure 4A. RG and DG were found throughout the study period. RG and DG reached their maximum levels in October and November indicating a major spawning period, but remained up to 50% from December to April. EF were from 12.9% in October to 24.2% in March, values of RG and DG in April were around 50%, but gametes were surrounded by hemocytes and follicle walls were in regression, showing a reabsorption period. The reproductive season started in October and ended in March. AC reached their maximum from May to September whereas VCT rose from April to September showing the storage of reserves during spring and summer, which would be used for the next massive gametogenesis. Variations in percentage of the different cell types in the mantle volume of this species from the exposed rocky shore ale shown in Figure 4B. RG and DG were also found throughout the study period with their maximum values in November showing a major spawning period, their prevalence remained up to 40% from December to March, when reabsorbing conditions were observed. reaching minimum values from June to September. EF were recorded throughout the study period reaching their higher values from 18.5% in November to 26.6% in February. At this time, AC were in a lower proportion with minimum values in January and February. VCT were present throughout the study period with maximum values from June to September. The percentage of MC was similar to those described above for this species in culture conditions from November to February; and it was different from March to September, the period when the accumulation of reserves was more intense. In general, differences in percentage of gametes throughout the study period in both localities were significant ([F.sub.(2, 4907)] = 301, P = < 0.01).

[FIGURE 4 OMITTED]

The variations in percentage of the different cell types in the mantle volume of Mytilus californianus from the exposed rocky shore are shown in Figure 4C. RG and DG were found throughout the study period with minimum values from August to December, while maximum values were recorded from January to July (up to 40%). Their maximum values were reached from February to June showing the major spawning activity. EF were recorded throughout the study period at similar percentage values. AC were recorded in all the months sampled, with maximum values in September and October (storage of reserves); their minimum values were reached from March to May. VCT were present throughout the study period with maximum values from September to December. Differences in percentage of gametes throughout the study period in both localities and species were significant ([F.sub.(2, 4907)] = 301, P = < 0.01).

DISCUSSION

The reproductive season of Mytilus galloprovincialis cultured in Bahia de Todos Santos is more extended than those from the same species in the exposed rocky shore from Mina del Fraile. These observations could be related to environmental conditions. Culture mussels feed underwater at all times and they are not exposed to environmental changes such as desiccation or to sharp temperature changes due to tides at midnight or midday. Thus. external conditions during the season of reserves accumulation (i.e. temperature changes, food availability, wave splash, and photoperiod) seem to be more stable in the culture condition than in exposed rocky shore conditions, allowing a more frequent and intense reproduction (see Fig. 4. A,B). Seed (1976) described that the gonadal development was faster in mussels from the low intertidal than in those from the upper intertidal, relating these results to food availability. Ferran (1991) and Villalba (1995) associated local variations in the gonadal cycle of mussels cultured in Galician bays in Spain to environmental conditions.

Comparing the reproductive season of M. galloprovincialis with M. californianus it is observed that in some months the percentage of MC was similar in culture or exposed rocky shore conditions; however, the reserves accumulation period between species shows differences in reproductive cycles. The most intense spawning period in M. californianus occurred from middle winter to spring. It is clear that M. californianus presents a more extended reproductive season from winter to summer, suggesting that endogenous factors, regulating the reproductive cycle in this species, and the exogenous factors act in conjunction in different ways in comparison to M. galloprovincialis.

A comparison of the reproductive season of M. galloprovinicialis found in this study at regional scale showed a similar pattern to those described for the bay mussel from Washington to southern California (see Fig. 1), which shows a reproductive season between October and April, with the exception of the studies by Graham and Gay (1945), Coe (1932) and Young (1946) in the Pier of Scripps Institution of Oceanography and Rocky Point. The first two authors arrived to the conclusion that through spatfall studies; thus it is probable that spawning period could be earlier than stipulated. The last author came to his conclusion from spawning induction in laboratory, which does not represent the reproductive conditions of the mussel population in the wild. There seems to be a pattern of reproduction period from autumn to early spring of bay mussels from Washington to Baja California and from spring to summer in Vancouver Island. Are these differences related to the bay mussel species Mytilus trossulus towards north, and Mytilus galloprovincialis towards south? Or to latitudinal and local conditions? A comparison on reproductive seasons of the bay mussels (M. trossulus and M. galloprovincialis) is necessary using the same technique in the north western coast of North America to determine the reproductive strategy of these two bay mussel species.

On the other hand, reproductive periods of Mytilus californianus recorded in the Pacific coast of North America (see Fig. 2) show spawning periods almost all year round. Our results and those from the mentioned studies in Figure 2, support in part the observations by Suchanek (1981) mentioning that the reproductive strategy of M. californianus is based on continuous spawning at very low level throughout an annual cycle. However, more studies on the spawning intensity or level of this species all year round in comparison to M. galloprovincialis are needed to confirm the observation by Suchanek.

The results of this study show that M. galloprovincialis has a less extended reproductive season in the rocky shore exposed environment than in the culture conditions, and the reproductive cycle of this species in both localities is less extended than those of M. californianus in the exposed rocky shore environment. This period takes place from autumn to early spring, whereas for M. californianus it lakes place from winter to summer, both species having smaller spawning periods all year long.

The mussel settlement season in Bahia de Todos Santos has been recorded in December and January (Curiel-Ramirez & Caceres-Martinez 1999, Curiel-Ramirez 2000), coinciding with the spawning observed for M. galloprovincialis in this study. Current studies on the identification of recently settled mussels sup ports the fact that the majority of the spat from these months correspond to this species (Curiel Ramirez & Caceres-Martinez, in prep), such as it had been mentioned about the necessity of studies on the intensity of spawning in these species. Also, studies are necessary on the settlement intensity of both species by identification of recently settled mussels during the year.

ACKNOWLEDGMENTS

The authors thank S. Guevara from Aquacultura Oceanica for allowing us technical assistance for sampling in their mussel culture facility, The authors also thank Rebeca Vasquez for processing samples. This project was supported by the CICESE project number 623106. Curiel-Ramirez received a grant from Conacyt, Mexico.

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SERGIO CURIEL-RAMIREZ AND JORGE CACERES-MARTINEZ *

Centro de Investigacion Cientifica y de Educacion Superior de Ensenada, Laboratorio de Biologia y Patologia de Organismos Acuaticos del Departamento de Acuicultura, Apdo. Postal 2732, 22800, Ensenada, Baja California, Mexico

* Corresponding author. E-mail: [e-mail]