Salinity tolerance and resistance of the Pacific lion's paw scallop (Nodipecten subnodosus) and the relationships with species distribution and density in a coastal lagoon.

ABSTRACT Salinity tolerance and resistance of the Pacific lion's paw scallop Nodipecten subnodosus, using the median lethal concentration method (L[C.sub.50]) and a gradual change method were measured. We also attempted to demonstrate whether salinity and temperature, pH, dissolved oxygen and turbidity control the distribution and densities of species within a coastal lagoon (Laguna Ojo de Liebre, Mexico). The upper and lower L[C.sub.50] of this scallop were determined in the laboratory at predetermined times over 4 days (96 h) when exposing specimens to salinities of 15, 20, 25, 30, 37, 42, 47, 50 and 60 ppt. The upper and lower salinity resistances were studied by subjecting the specimens to either increases or decreases of salinity by 3 ppt every 3 days, departing from a normal salinity of 37 ppt. Salinity, temperature, pH, dissolved oxygen, and turbidity of the water close to the bottom were measured once a month from January 2001 to March 2004 at 20 stations located along the shore of the lagoon, Results showed no statistical differences at the upper or lower L[C.sub.50], at 24, 48, 72 and 96 h, and the species salinity tolerance varied from 23.5-24.2 to 50.4-53.8 ppt. Salinity resistance, measured by the gradual change method showed no mortality in the 22-52 ppt range, but abrupt mortality occurred with exposure beyond this range. Comparing these results with field data, we concluded that our results provide evidence that salinity is not a limiting factor regulating distribution and density of adult Pacific lion's paw scallop in the lagoon. Food quality and availability, water currents and substrate could be limiting factors.

KEY WORDS: salinity, molluscs, Pectinidae, tolerance, eurihalinity, stenohalinity, scallops, Nodipecten subnodosus

INTRODUCTION

The Pacific lion's paw scallop (Nodipecten subnodosus G. B. Sowerby I, 1835) is a fast-growing species distributed along the Pacific coast from Laguna Ojo de Liebre in Baja California Sur, Mexico to Peru (Keen 1971). Exploitable beds exist in Laguna Ojo de Liebre, where a fishery developed after 1991, with an initial estimated harvest of 5 t, peaking in 1998 with 158 t, and remaining steady to the present with production close to 100 t (Masso-Rojas et al. 2000). This lagoon covers 2,280 [km.sup.2] and water is exchanged with the Pacific Ocean through a wide 3.7 km mouth by tidal action. There are many irregularly distributed channels, with depths from 5-33 m. The water in this lagoon is hypersaline in some areas as a result of high evaporation rates coupled with strong winds (Alvarado et al. 1986, Contreras 1988). Hypersalinity represents an advantage for the Compania Exportadora de Sal that pumps water from the head of the lagoon into evaporation ponds in one of the largest international salt-producing operations. In other areas, salinity in the lagoon is lower than normal levels (37 ppt) found in other protected areas of Baja California.

Scallops are regarded as stenohaline bivalves, lacking the capacity to withstand wide salinity changes. This was demonstrated in Argopecten irradians (Mercaldo & Rhodes 1982, He et al. 1999, Wang et al. 2001), Chlamys opercularis (Paul 1980), Mimachlamys asperrima (O'Connor & Heasman 1998), Nodipecten nodosus (Rupp & Parsons 2004), Patinopecten yessoensis (Sakurai et al. 2000), Pecten maximus (Strand et al. 1993, Christophersen & Strand 2003), and Placopecten magellanicus (Frenette & Parsons 2000, Frenette et al. 2001). Therefore, it seems peculiar that exploitable populations of Pacific lion's paw scallop develop in this lagoon under such variable saline conditions, even when an estimated 13.5-22.5 million scallops were harvested between 1996 and 2004 (Masso-Rojas et al. 2000).

Such a contrast makes the study of salinity tolerance and resistance of this species and the influence of salinity on stock distribution and densities significant. Tolerance and resistance to environmental variables could be determined by two general procedures suggested by Fry (1947). The L[C.sub.50] tolerance method consists of maintaining containers at various constant salinities in which samples were introduced and the time of death noted. The method for resistance exposes the organism to increasing or decreasing levels of a given variable at a constant rate until it died. The first method covers ecologic conditions, which the species may encounter, and the second, although it has the disadvantage of involving two variables, time and salinity, allows investigators to follow the internal changes occurring in the organism throughout the range of change until death. In this study, results using both methods are presented. We also attempt to demonstrate whether salinity and other variables such as temperature, pH, dissolved oxygen or turbidity control the distribution and densities of species in the lagoon.

MATERIALS AND METHODS

Experimental Organisms

Specimens of Pacific lion's paw scallop (6.0 [+ or -] 0.3 cm shell height) were obtained from a scallop farm at Laguna Manuela, B.C.S., Mexico (28[degrees]07'51.8" N, 114[degrees]04'04.1" W; Fig. 1). Salinity at the time of collection was 37 ppt. Specimens were transported to the laboratory at CIBNOR in La Paz, B.C.S., Mexico following the method described by Maeda-Martinez et al. (2000). Before the experiments, specimens were kept in the laboratory for 15 days in 500-L plastic containers with seawater at 22[degrees]C, 37 ppt, and constant aeration. The scallops were fed a 1:1 mixture of Isochrysis galbana and Chaetoceros calcitrans in two daily rations of 1.0 x [10.sup.9] cells/scallop/day.

Salinity Tolerance

The upper and lower salinity tolerance of the Pacific lion's paw scallop were determined by the median lethal concentration method (L[C.sub.50]) (Sprague 1973) for 4 days. Ten scallops were transferred in triplicate to 70-L tanks containing 40 L of 1-[micro]m filtered and aerated seawater at 22[degrees]C and one of the salinity treatments (15, 20, 25, 30, 37, 42, 47, 50 and 60 ppt). Hyper-osmotic media was prepared by diluting salt obtained from Compania Exportadora de Sal in Guerrero Negro, B.C.S. in filtered seawater. Hypo-osmotic medium was prepared by adding distilled water at 22[degrees]C to filtered seawater. Chemical composition analyses showed that the proportion of major ions in both media was similar to that of seawater. Salinity of the media was adjusted to a resolution of 0.1 ppt with a calibrated YSI model 85 salinometer. The water was completely changed daily and the specimens were fed as described earlier. Mortality was recorded every 60 min during the first 24 h, and every 4 h to 8 h until the end of the experiment (96 h from the start of the experiment). Death was determined using the criteria of a gaping shell, complete mantle retraction and with no response from mantle or adductor muscle prodding using a dissecting needle (Ledwell 1995, Frenette 2004). The upper and lower L[C.sub.50] values and their 95% confidence limits for salinity were calculated for 24, 48, 72 and 96 h with a computer program based on a method described by Finney (1971), which consists of transforming the mortality data in probit mortality. With this method, the estimated probit line and results of a [chi square] test for goodness of fit were determined. A z-test for the comparison of two L[C.sub.50] values at the 5% level of significance was carried out.

Salinity Resistance

Salinity resistance was determined by a gradual change method, where the specimens were exposed to either increasing or decreasing salinities from normal salinity (37 ppt). Seven groups of 20 organisms were transferred to seven 70-L plastic tanks, containing 40 L seawater at 22[degrees]C and 37 ppt and fed the same diet as in the previous experiment. The salinity of three tanks was increased 3 ppt every 3 days and another set of three tanks was lowered at the same rate. One tank was maintained at 37 ppt and served as control. Mortality was recorded daily until all scallops died. During the experiment, care was taken to remove the dead animals from the tanks to avoid deterioration of the water.

Effect of Salinity and Other Environmental Variables on Stock Distribution and Densities

Salinity, temperature, pH, dissolved oxygen and turbidity of the water close to the bottom were measured once a month at 20 stations near the shoreline of the lagoon, with a Horiba multiparameter monitor from January 2001 to March 2004 (Fig. 1). Maximum monthly values at each station were averaged and plotted using the Surfer 8 (Golden Software Inc., Golden, Colorado) program to draw salinity isolines on a map of Laguna Ojo de Liebre. Scallop populations in this lagoon are clustered in 17 stocks, occurring in discrete areas previously located with a GPS device. The location of these stocks did not show changes during the period of study. The density of each stock was recorded once each year over the last 10 y by direct counts of specimens of all sizes in an area of 10 [m.sup.2]. The effect of salinity on scallop distribution was determined by overlapping salinity isolines with the distribution of scallop stocks. The effect of salinity on stock density was estimated by correlation analysis. The effect of the other variables on scallop distribution was done by finding how heterogeneous these variables were within the lagoon, comparing extreme values at the mouth and the head of the lagoon.

[FIGURE 1 OMITTED]

RESULTS

Salinity Tolerance

Accumulated mortality at different salinities is shown in Figure 2. Survival of N. subnodosus was not affected by sudden salinity changes within the salinity range of 30-47 ppt. Exposure of scallops at 25 and 50 ppt produced a gradual mortality starting at 24 h in the 25-ppt treatment and at 60 h in the 50-ppt treatment, until 40% mortality was reached in both treatments at 96 h. Total mortality occurred at 15, 20 and 60 ppt in less than 24 h. With these results, the calculated upper L[C.sub.50] at 24, 48, 72 and 96 h (Fig. 3a) varied from a mean of 50.4-53.8 ppt. The lower L[C.sub.50] varied from 23.5-24.4 ppt from 24-96 h of exposure (Fig. 3b). The z-test showed no statistical difference between L[C.sub.50] values at different exposure times in both the upper and lower salinity tolerance experiments.

[FIGURES 2-3 OMITTED]

Salinity Resistance

Salinity resistance with gradual increases or decreases of 3 ppt every 3 days, (Fig. 4) indicated no deaths from exposures between 22 and 52 ppt, but abrupt mortality was produced with exposure beyond this range.

[FIGURE 4 OMITTED]

Effect of Salinity and Other Variables on Stock Distribution and Densities

Spatial variations of salinity based on mean minimum (Fig. 5) and maximum (Fig. 6) salinity data for each month from January 2001 to March 2004 confirm that Laguna Ojo de Liebre is a hyposaline lagoon in the vicinity of the mouth and hypersaline in the vicinity of the head. From both figures, the range of monthly salinity values varied from 31 ppt at the mouth of the lagoon to 45 ppt at the head. Relating salt concentrations against stock distribution (Figs. 5, 6), we concluded that stocks develop in an area where salinity remains within the range of 31-37 ppt, and that none of the stocks are exposed to salinities greater than 37 ppt.

[FIGURES 5-6 OMITTED]

Average scallop densities of the 17 stocks (Fig. 7) increased from 0.2 to around 0.7 ind [m.sup-2] from 1995 to 1996. Since then, densities have remained steady at the higher values. Relating stock density with salinity (Fig. 8), a negative but not significant correlation (r = -0.558; P > 0.05) was obtained.

[FIGURES 7-8 OMITTED]

The influence of temperature, pH, dissolved oxygen and turbidity variations on distribution and densities of stocks was also studied. As mentioned, their effect on scallop distribution was estimated by comparing extreme values at the mouth versus the head of the lagoon. Maximum, average, or minimum monthly values, rather than only the average values, were chosen for this purpose, focusing on the most stressing (limiting) conditions, such as maximum temperature (Fig. 9a), average pH (Fig. 9b), minimum [O.sub.2] (Fig. 9c), and maximum turbidity (Fig. 9d). Maximum monthly values of temperature varied between 15[degrees]C and 25[degrees]C. Differences between the mouth and the head of the lagoon were never greater than 3[degrees]C (Fig. 9a). A similar situation occurred for pH and dissolved oxygen, with negligible maximum differences of 0.29 pH units and 1.8 mg [L.sup.-1] [O.sub.2]. Monthly average pH remained steady throughout the study, ranging from pH 7.4 to 8.7 (Fig. 9b). Minimum dissolved oxygen varied from 2.9-7,5 mg [L.sup.-1] (Fig. 9c), with lower concentrations in the summer, Maximum monthly turbidity fluctuated between 0 and 2 NTU (Fig. 9d) most of the year, and only small differences were observed between the head and the mouth of the lagoon. However, from March to June 2001, high turbidity, >3 NTU, was measured at the head whereas negligible values were measured at the mouth of the lagoon.

[FIGURE 9 OMITTED]

DISCUSSION

Our results indicate that upper and lower tolerances of salinity were similar to the upper and lower salinity resistance values in the Pacific lion's paw scallop. Salinity tolerance in N. subnodosus was only 2 ppt higher than the resistance level towards lower salinity, from 24-22 ppt, and was practically the same at the upper end (52 ppt). A difference of greater magnitude and in both directions was expected because the gradual method used for salinity resistance allows time for adaptation to a new level (Fry 1947). This, together with the wide range of tolerance and resistance to salinity changes of about 28 ppt between 24 and 52 ppt at an optimum temperature of 22[degrees]C (Gonzalez-Estrada 2001), makes it difficult to consider this scallop a stenohaline species. Singnoret-Brailovsky et al. (1996) determined the salinity resistance limits at 27 and 47 ppt in Argopecten ventricosus, results in a range of 20 ppt. According to the literature (Table 1), the Pacific lion's paw scallop possesses the highest upper salinity limits of all scallop species, although this is based on the only paper reporting upper salinity tolerance in scallops (Singnoret-Brailovsky et al. 1996). In contrast, N. subnodosus is one of the most sensitive species to low salinity, after N. nodosus and A. ventricosus, with minimum values of 23.6 and 22 ppt respectively (Table 1). The mechanisms governing the capacity of N. subnodosus to cope more efficiently with high salinity and be more sensitive to dilute media than other species are not yet known, but this may result from long-term adaptation to hypersaline conditions. With these results, we concluded that it was a mistake to regard this scallop species as a stenohaline bivalve, considering solely its susceptibility to dilute media and not taking into account both limits of tolerance and resistance of the species to salinity.

Relating salinity tolerance to conditions in the lagoon, this variable does not seem to be the limiting factor regulating the distribution and density of Pacific lion's paw scallop stocks in Laguna Ojo de Liebre. The highest salinity at the head of the lagoon was 45 ppt, while the upper salinity tolerance and resistance was 52 ppt. No stock, even those located in the deeper part of the lagoon (Stocks 15-17) were exposed to salinities higher than 37 ppt, which indicates participation of other limiting factors. Because salinity readings were done once a month, there is the possibility that we did not detect hypersaline water masses moving towards the mouth. Therefore, it would be necessary to increase the sampling frequency to draw definitive conclusions. The other variables do not seem to have that role, because values fall within the limits tolerated by the species. The highest temperature recorded in the lagoon (25[degrees]C) was lower than the lethal temperature of 29[degrees]C reported for the species (Gonzalez-Estrada 2001). The pH remained at a similar level as normal pH of seawater. Although there were periods of low dissolved [O.sub.2], bivalves have a high capacity to withstand hypoxic or even anoxic conditions (Bayne 1976), Sakurai et al. (2000) found that Patinopecten yessoensis withstood 16 h in anoxic conditions and at least 2 days in hypoxic conditions at 20% oxygen saturation and 13.5[degrees]C.

Food quality and availability, water currents and substrate type could be limiting factors. Arellano-Martinez et al. (2004) found that Laguna Ojo de Liebre has low food availability (maximum 1.2 [micro]g [L.sup.-1] chlorophyll a) year-round, which agrees with previous works (Millan et al. 1987, Delgadillo et al. 2002). Additionally, the food index (FI), which is equivalent to the ratio of organic mass of seston to total mass of seston x 100, had a maximum value of 17%, and is considered indicative of low nutritional quality of the food available to suspension-feeders, since sites with FI values between 8% and 11% are considered nutritionally poor (Soniat et al. 1984, Luna-Gonzalez et al. 2000). Sites with FI values as high as 55% are nutritionally rich environments (Navarro & Thompson 1995), Poor quality food in bivalves is deleterious because of the low energy input from the food coupled with energy losses in preingestion particle selection (Bricelj & Shumway 1991, Bacon et al. 1998). This situation could have been exaggerated towards the head because of the higher seston concentrations found there, in contrast with the mouth. With regard to water currents, Garcia-Esquivel et al. (2000) found maximum growth rates at flows between 5 and 10 cm [s.sup.-1] for N. subnodosus. Water currents in Laguna Ojo de Liebre have not yet been determined, but because currents are produced by tidal exchange, it is likely that current velocities near the head would not be sufficient to meet the species' requirements. In addition, if lower currents occur close to the head, higher deposition of organic materials would be expected, making the substrate unsuitable for the species. Keen (1971) and Pena (2001) described the natural habitat of N. subnodosus as places with prevailing high currents with a substrate of sand and shells. There are three additional potential limiting factors that have to be considered. One is related to the effect of salinity in early life stages. Although this has not been studied in N. subnodosus, there is evidence that salinity tolerance changes with ontogeny. In Mimachlamys asperrima, salinity optima remained unchanged from embryo to adult stages, but the lower salinity tolerance tended to increase with ontogeny from embryo to larvae to juvenile stages, only to decrease at the onset of maturity (O'Connor & Heasman 1998). The other potential limiting factor is the effect of salinity on byssal synthesis. Rupp and Parsons (2004) demonstrated a reduction of byssal attachment at salinity lower than 29 ppt in N. nodosus, indicating that scallops were physiologically stressed. A similar situation may occur at elevated salinity, affecting larval recruitment. The last limiting factor could be the effect of elevated salinity on living substrates, such as seagrass Zostera marina, which is used for pediveliger larvae recruitment. This seagrass is found at salinities from 6 to 36 ppt (Santa-Maria 1996), but information on the upper salinity tolerance is still lacking. It is important to point out that sublethal effects affecting metabolism, growth and reproduction could also be playing a role on distribution, because hypersaline conditions may not be lethal, but be suboptimal for the establishment of stocks.

With regard to stock densities, contrary to the general pattern occurring in mollusk fisheries around the world, densities have not dropped in Laguna Ojo de Liebre in the last 10 y of exploitation. This is probably the result of an appropriate harvesting program based on stock evaluations prior to the opening of the season by the management authorities. This includes a minimum catch size (14 cm shell height or 9-10 meats [kg.sup.-1]) reported by Masso-Rojas et al. (2000). Baqueiro et al. (1982) mentioned that this species normally forms stocks of 1 individual per 25 [m.sup.-2], and that it was rare to find beds of 1 ind [m.sup.-2]. Densities in Laguna Ojo de Liebre fluctuated around 0.7 ind [m.sup.-2], but several stocks reached a maximum of 1.7 ind [m.sub.-2]. Densities of N. subnodosus stocks are similar to those reported for other species of large scallops, such as Pecten maximus, Placopecten magellanicus and Patinopecten yessoensis (Table 2), but these are lower than stocks of smaller species be longing to Argopecten and Chlamys. Apart from the low densities reported by Baqueiro et al. (1982), there were no official reports of potentially exploitable populations of Pacific lion's paw scallop in Laguna Ojo de Liebre before 1991. It is possible that the populations of N. subnodosus currently being harvested developed recently as a result of favorable environmental conditions that were not present in the past, A good monitoring program of environmental variables is recommended, together with continuing stock evaluation.

ACKNOWLEDGMENTS

The authors express their gratitude to Compania Exportadora de Sal S.A. de C.V. and especially to Pedro Martin Dominguez and Martin Garcia for providing environmental data of Laguna Ojo de Liebre. To CIBNOR personnel: the staff editor for improving the English text, Oscar Armendariz for drawing the map of Laguna Ojo de Liebre, and Jesus Bautista for calculating the salinity isolines. Thanks to Dr. Domenico Voltolina (Laboratorio de Microalgas UAS-CIBNOR) for his critical review of this paper and Dr. Martin Frias Espircueta (FACIMAR, Universidad Autonoma de Sinaloa) for providing assistance with the calculation of probit values. Thanks to Dr. Leslie-Anne Davidson (Department of Fisheries and Oceans, Moncton, N.B. Canada) and to an anonymous reviewer for improving the quality of this manuscript. This research was supported by the National Council for Science and Technology (CONACYT Project G33593-B).

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LUCELLY M. ROLDAN-CARRILLO, (1) ALFONSO N. MAEDA-MARTINEZ, (1) * ANTONIO MASSO-ROJAS (2) AND MARIA TERESA SICARD-GONZALEZ (1) (1) Centro de Investigaciones Biologicas del Noroeste (CIBNOR), Mar Bermejo 195, Col. Playa Palo de Santa Rita, La Paz, B.C.S. 23090, Mexico; (2) Centro Regional de Investigacion Pesquera (CRIP), Km. 1 Carretera a Pichilingue, La Paz, B.C.S. 23020, Mexico

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

TABLE 1.
Lower and upper salinity tolerance of selected scallop species
and the methods employed for the determinations.

                                   Salinity

                            Minimum     Maximum
Species                     (ppt)       (ppt)

Argopecten irradians           10         --
Argopecten ventricosus         27         47
Chlamys opercularis            16         --
Nodipecten nodosus          20.1-23.6     --
Nodipecten subnodosus          24         51
                               22         51
Patinopecten yessoensis        18         --
Pecten maximus                 20         --
Placopecten magellanicus       20         --

Species                              Method

Argopecten irradians        Gradual change
Argopecten ventricosus      Gradual change (5ppt/day)
Chlamys opercularis         L[C.sub.50] 24h
Nodipecten nodosus          L[C.sub.50] 48h
Nodipecten subnodosus       L[C.sub.50] 96h
                            Gradual change 3ppt/3days
Patinopecten yessoensis     L[C.sub.50] 24h
Pecten maximus              Gradual change
Placopecten magellanicus    L[C.sub.50] 96h

Species                              Reference

Argopecten irradians        Mercaldo & Rhodes 1982
Argopecten ventricosus      Singnoret-Brailovsky et al. 1996
Chlamys opercularis         Paul 1980
Nodipecten nodosus          Rupp & Parsons 2004
Nodipecten subnodosus       This study
                            This study
Patinopecten yessoensis     Sakurai et al. 2000
Pecten maximus              Strand et al. 1993; Christophersen &
                              Strand 2003
Placopecten magellanicus   Frenette & Parsons 2000

TABLE 2.
Stock densities reported for different scallop species.

                                                      Density
        Species                  Location         (ind [m.sup.-2])

Argopecten gibbus          Florida, east coast    mean = 43;
                                                    max. = 108

Argopecten irradians       Long Island, N.Y.      0.2-4.4
                           Niantic River, Conn.   up to 76
                           Newport River, N.C.    19.5

Argopecten purpuratus      Chile                  0.2-5.0

Chlamys islandica          Balsfjord, Norway      means 56.0-69.9

Chlamys opercularis        English Channel        mean = 0.46;
                                                    max. = 4.46
                           Guernsey               5.7
                           W. Scotland            500-1000

Chlamys varia              Ireland, various       means = 1.8-28.0
                             areas
                           Bay of Brest, France   mean = 2.3

Nodipecten subnodosus      Laguna Ojo de          mean = 0.7;
                             Liebre, Mexico         max. = 1.7

Patinopecten yessoensis    Mutsu Bay--adults      1.5-6.0
                                    --spat        50-250

Pecten maximus             Strangford Lough       0.67
                           Claonaig Bay,          0.12
                             Scotland
                           English Channel        mean -0.16;
                                                    max. = 2.33
                           Isle of Man            0.04-0.13

Placopecten magellanicus   Bay of Fundy           max. = 6.3
                           Gulf of St. Lawrence   1.4-6.6
                           Gulf of Maine, USA     0.98
                           Newfoundland           0.19-0.86

        Species                  Location              Reference

Argopecten gibbus          Florida, east coast    Cummins 1971

Argopecten irradians       Long Island, N.Y.      Bricelj et al. 1987
                           Niantic River, Conn.   Cooper & Marshall
                                                    1963
                           Newport River, N.C.    Thayer & Stuart 1974

Argopecten purpuratus      Chile                  Illanes-Bucher 1987

Chlamys islandica          Balsfjord, Norway      Vahl 1981

Chlamys opercularis        English Channel        Franklin et al. 1980
                           Guernsey               Askew et al. 1974
                           W. Scotland            Mason 1983

Chlamys varia              Ireland, various       Rodhouse & Burnell
                             areas                  1979
                           Bay of Brest, France   Shafee & Conan 1984

Nodipecten subnodosus      Laguna Ojo de          This study
                             Liebre, Mexico

Patinopecten yessoensis    Mutsu Bay--adults      Imai 1980
                                    --spat

Pecten maximus             Strangford Lough       Hartnoll 1967
                           Claonaig Bay,          Mason et al. 1979
                             Scotland
                           English Channel        Franklin et al. 1980
                           Isle of Man            Murphy 1986

Placopecten magellanicus   Bay of Fundy           Dickie 1955
                           Gulf of St. Lawrence   Caddy 1968
                           Gulf of Maine, USA     Langton & Robinson
                                                    1987
                           Newfoundland           MacDonald & Thompson
                                                    1986