Reproductive cycle of the purple snail Plicopurpura pansa (Gould 1853) from two locations at Baja California Sur, Mexico.
KEY WORDS: reproductive cycle, purple snail, Plicopurpura pansa, Muricidae
Most marine snails of the family Muricidae, if not all, produce in the hypobranchial gland a colorless secretion that turns on exposure to air and light to "Tyrian Purple" (Fretter & Graham 1994). For its dye the carnivorous, gonochoristic muricid Plicopurpura pansa (Gould 1853), which inhabits intertidal rocky shores exposed to high impact waves of the open sea, has been exploited on the Pacific coast from Central America to Mexico for hundreds of years by indigenous communities for dyeing threads of cotton to be woven afterwards into traditional dresses. However, in recent years in Mexico the commercial exploitation of the purple snail for dyeing kimonos with "Tyrian Purple" had reached such levels as to threaten the survival of the species, and in 1988 the Mexican government declared P. pansa a protected species (Anonymous 1988, 1994).
Despite the recent concerns about the state and recovery of P. pansa, little is known about the principal life-history features. The impeded accessibility, only during extreme low tides, is the main reason for the difficulties of P. pansa field research. Detailed information on the reproductive strategy however, is a prerequisite for the development of techniques for restocking natural populations and to facilitate effective management.
In Mexico, some information about the gonad reproductive cycle of the purple snail P. pansa is available from Oaxaca (Hernandez-Cortes & Acevedo-Garcia 1987, Acevedo-Garcia et al. 1993), a histological study from Nayarit (Quiroz-Rocha 1992), and a detailed study about the reproductive cycle from Sinaloa (Gonzalez-Flores 1997). Especially, the understanding of the reproductive cycle and the spawning period of P. pansa will provide the necessary information needed for the determination of the recruitment period of natural populations. Up until now data are missing about the reproductive cycle of P. pansa from Baja California Sur, the most northern area of its distribution. The objectives of this study were to determine over a period of 20 mo the reproductive cycle of P. pansa at two locations in Baja California Sur. Additional information on minimum size at sexual maturity and sex ratio is needed to facilitate efficient management of this natural resource.
MATERIALS AND METHODS
Preliminary trials were started from January to May 2000 to determine the best location and period for the collection of P. pansa, whether egg capsules can be found, and to assess the method of sampling to obtain statistically valid data. From intertidal rocks at days during extreme low-water spring tides at Playa Cerritos at the Pacific coast (23[degrees]19'54"N and 110[degrees]10'38"W) and at Punta Perico at the Gulf of California (24[degrees]01'54"N and 109[degrees]48'21"W; Fig. 1), 20-30 specimens from 100-300 animals of unexploited populations of P. pansa were randomly selected from June 2000 to January 2002, monthly, with the help of a table of random numbers. At Playa Cerritos a total of 592 were randomly selected during 20 mo and at Punta Perico a total of 596 specimens.
[FIGURE 1 OMITTED]
During sampling the water temperature was recorded, whether egg-capsules could be found and whether copulas were observed. Injecting a 10% neutral buffered formalin solution into the snails preserved the tissue of the animals. Subsequently about 24 h later in the laboratory the total SL of the animals was determined from the apex to the end of the siphon channel with the help of a digital caliper with a precision of 0.01 mm, the shell was broken and the gonads and the digestive gland were removed. Table 1 shows the number of gonads examined, the average total SL with standard deviation, and the size range. Sex determination was accomplished by visual inspection for a penis, and verified by histological examinations of the gonads.
Observations by Light Microscopy
The gonads were dehydrated in the alcohol series, cleared with butylated hydroxanisole, and embedded in paraffin. The sections (7-9-[micro]m thick) were prepared from the thickest part of the gonads, and subsequently, according to the methodology described by Luna (1968), stained with Harris hematoxylin and contrasted with eosin. This method was adopted after verifying in four specimens, that gonadal maturity was uniform in different parts of the gonad. Both sexes were confirmed by light microscopic examination of histological preparations of the gonads. Light micrographs were taken with a digital camera (Olympus Camedia C-5060) mounted on a light microscope (Olympus BX 41).
Reproductive Cycle With Gonad Developmental Phases
To determine the seasonality and duration of the reproductive cycle from the histological preparations the development of the gonads is classified arbitrarily: in females it is divided into four successive stages (developing, ripe, spawning, recovering), in males only into 3 (developing, spawning, and recovering) stages.
In female gonads in the developing stage can be found oogonia and developing oocytes (Photo la).The size of the gonads is increasing through the accumulation of growing ovocites accumulating yolk. The oocytes are predominantly previttelogenic. In male gonads can be observed the spermatogenic follicles, spermatogonia, spermatocytes, and spermatozoa arranged in characteristic bands (Photo 2a).
In females, all follicles are filled with ripe vitellogenic oocytes of polygonal shape containing mature yolk granules (Photo 1b). No empty space was observed between the follicles. In males, the ripe stage cannot he distinguished from the developing stage, and is therefore omitted.
In females, because most oocytes in the follicles are discharged into the environment, the lumen of the follicles becomes essentially empty. Spawned ovaries are characterized by the presence of spaces between the free oocytes in the lumen (Photo 1c). In males, the lumen of the follicles is partially empty with spaces inside, and a decrease in the quantity of spermatozoa can be noticed. The spermatocytes remain at the follicular walls (Photo 2b). In this stage the spermatic ducts are filled with spermatozoa (Photo 2b and 2c).
In males and females, after spawning the gonads are collapsed, and each follicle is empty. In females few residual oocytes are present, being phagocytized by amoebocytes. The gonads are reduced to a thin layer of tissue adjoining the hepatopancreas (Photo ld). No active oogenesis is evident. In males phagocytosis occurs of the residual spermatozoa by amoebocytes, and no active spermatogenesis are evident. However, not in all samples in the recovery stage are the gonads visible because they have been completely absorbed and therefore it is only possible to determine anatomically the sex of the animals by the presence or absence of a penis
Mean Maturity Index
Grant and Tyler (1983) proposed using the term "maturity index" for examinations of gonad histological sections and restricting the use of "gonad index" to the quantitative analysis of relative gonad weight or size. We obtained the mean maturity index for each monthly sample, according to the method described by Seed (1976) and Kautsky (1982) for the bivalve Mytilus edulis (Linne 1758). The number of animals in each stages of maturity was multiplied by the numerical ranking of the stage (recovering = 1; developing = 2; ripe = 3; spawning = 4) and by dividing the sum of these products by the number of individuals in the sample. The index varies from one, if the female gonads of the entire population are recovering and 4 if the whole female population is spawning. In the male population, because the ripe stage is missing, the index will be 3 if all animals are in the spawning stage, and 1 if all animals are recovering.
Biological Minimum Size at Sexual Maturity
To determine the minimum size at sexual maturity we selected from all collected animals the 10 smallest females and males from each location and determined their stage of gonadal maturity.
Sex Ratio of the Histologically Examined Specimens
The total number of males and females collected at Punta Perico and Playa Cerritos was subjected to the X2 test to examine whether statistically significant differences exist between the numbers of the different sexes examined histologically.
Copulation, Egg Capsule Deposition, and Water Temperature
During the collection of the snails we observed at Playa Cerritos and at Punta Perico year-round copulations, whereas egg capsules could be found on both sites only during February to May. During this period at Punta Perico water temperatures of 24[degrees]C to 26[degrees]C were measured. The water temperature in summer reached up to 32[degrees]C. At Playa Cerritos slightly lower water temperatures were measured: January up to May of 19[degrees]C to 20[degrees]C, and during summer up to 28[degrees]C (Fig. 2). No relation could be determined between the water temperature and the copulation and capsule deposition.
[FIGURE 2 OMITTED]
Color of the Gonads
Marked variations in the color of the gonads could be observed during different sampling dates. During September to October the gonads were dark brown, and no sexual differences could be observed. In contrast from November until April the female gonads were yellow, whereas the male gonads were brown. The histological observation that the gonadal maturity is uniform in different parts of the gonad confirmed the uniform coloration of the formol-fixed gonads.
In this 20-mo study we examined histologically 544 gonads from snails collected at Playa Cerritos (260 females, 284 males) and at Punta Perico 543 specimens (221 females, 322 males). Applying the [chi square] test we examined the hypothesis that the sex ratio is 1:1. For the animals from Playa Cerritos the calculated P value was 0.3 showing that no statistically significant differences existed between the number of males and females. In contrast at Punta Perico were found statistically significant differences in the sex ratio (P < 0.001). More males than females were found.
Gonad Reproductive Cycle
In gonads from female snails collected at Punta Perico the developing stage was found nearly during the whole sampling period, with the exception of September to October 2000, and October 2001. In December 2001, 90% of the nine examined female gonads were in the developing stage. Female gonads were found in the ripe stage from June to August and November 2000, and from February to June 2001 and in January 2002, with the highest percentage in June 2000 (40% from 7 animals) and in February 2001 (36% from 11 animals).
Female gonads in the spawning stage were observed from June to August 2000, January to September 2001 and in January 2002. The highest frequency of spawning occurred in August 2000 and March 2001 (41% from 12, and 60% from 10 animals, respectively). Female gonads in the recovery stage were observed during nearly the whole sampling period, with the exception of June 2000 when no gonad was found in the recovery stage. In September to October 2000 and in October 2001, 100% of the 32 samples studied were in the recovery stage (Fig. 3a).
[FIGURE 3 OMITTED]
Male gonads from snails collected at Punta Perico were in the developing stage during the whole observation period, with two peaks of 100%: one in November 2000 (12 gonads studied) and another in December 2001 (19 gonads examined).
Individuals in the spawning stage were found during two clearly defined periods, one from June to September 2000, and another one from December 2000 until June 2001. Also in January 2002 the beginning of another spawning stage was seen. One spawning peak was found in February to March 2001 with 95% from a total of 39 male gonads studied. In September 2000, 87% of the 16 male gonads were examined in the recovery stage, and in September 2001 80% from a total of 15 gonads (Fig. 3b).
Female gonads in the developing stage could be observed during the whole sampling period, with the exception of October 2000 and October to November 2001. Two peaks occurred, one in January and another in June to July 2001. In January 2001 all 15, and in June to July 82% from the 23 examined female gonads were in the developing stage.
The ripe stage of the female gonads showed two peaks of activity: one in June 2000 with 50% (12 gonads examined) and another in April 2001 with 40% from 12 examined gonads.
The spawning stage was observed between June up until August 2000 and March up until September 2001. In August 2000 from 17 examined gonads, 47% were in the spawning stage; in March 2001 from 14 examined gonads, 71% were in the spawning stage, and in August 2001 from 17 observed gonads, 24% were in the developing stage.
The recovery stage was observed during the periods; from August to December 2000 with a peak in October (100% from 12 examined gonads), and June 2001 to January 2002 with a peak in October to November 2001 (100% from 26 examined gonads) (Fig. 3c).
The male gonads in the developing stage were observed for the whole sampling period, with the exception of September 2000 and during October 2001. Two peaks of activity were observed: in November, December 2000, and January from 38 examined male gonads were all 100%, and in December 2001 from 14 gonads 86%, in the developing stage.
Spawning took place from June to August 2000, with a peak in February 2001 (100% from 15 examined gonads), and from February to September 2001, with a peak in April 2001(85% from 14 examined gonads).
The recovering stage was mainly observed from August to October 2000, with a peak in September 2000 when all (100%) of the 10 examined gonads was in the recovering stage. In 2001 the recovering stage was observed from July to November with peaks in September (64% from 14 examined gonads) and October (100% from 19 examined gonads; Fig. 3d).
Mean Maturity Index
Insight into the reproductive cycle of P. pansa was obtained from the combination of the monthly assessments of the gonad reproductive cycle and the monthly mean maturity index.
The mean maturity index for female gonads collected at Playa Cerritos had values above 2 during January to August, with peaks in February and March, demonstrating that the majority of the snail population was in the ripe and spawning stage. In September to October the majority of the gonads are found in the recovering stage (Fig. 4b). The situation of the mean maturity index of the male gonads is very similar. From December to August the majority of the population is in the spawning stage, and from September to October the gonads are recovering.
[FIGURE 4 OMITTED]
The majority of the female gonads from snails collected at Punta Perico had during January to August a mean maturity index >2.0, demonstrating that they were, like at Playa Cerritos, in the ripe and spawning stage. An index value below 2.0, as observed from September to December, showed that they were in the recuperating stage (Fig. 4a). The same situation can be found in male gonads: the majority of the population was in the spawning stage during November to July, and only during August to October in the recovering stage. The data on the mean maturation index agreed well with the histological results described previously.
Size at Sexual Maturity
The 10 smallest female snails from the 260 females collected at Playa Cerritos had a total SL between 13.0-19.08 mm, and the size of the 10 smallest males from 284 animals SL ranged from 14.8618.56 mm. In the females the gonad development stage was up to 18.77 mm SL, either recovering or developing. Spawning could be observed in females larger than 18.84 mm. In males, spawning could only be observed in animals larger than 18.00 mm. A similar situation could be observed from the snails collected at Punta Perico. The size of the 10 smallest females from 221 animals collected ranged from 16.37-21.00 mm and the size of the 10 smallest males from 322 animals ranged from 14.8-19.6 mm. Only one female of a size of 17.3 mm showed gonad development, larger animals were in the developing, ripe, spawning, or recovering stage. The gonads of the males were either in the developing (7) or in the recovering (2) stage. One male snail with a size of 18.94 mm had a gonad with signs of having spawned.
Reproductive Cycle of P. pansa
The timing and duration of the reproductive cycle of P. pansa from gonad development through developing, ripe, spawning, and recovering is controlled by an interaction of environmental and endogenous factors. Activation of the gonads is probably controlled by endocrine hormonal factors being initiated in connection with lowered water temperature. With increasing air and water temperatures gonad development is starting and rapidly leading to maturation. Such effects were noticed by Chung et al. (2002) during studies of the reproductive cycle of the Korean muricid Rapana venosa (Valenciennes 1846). Kautsky (1982) made similar observations with the bivalve Mytilus edulis. Detailed information about the reproductive cycle of P. pansa is only sparingly available, mainly as thesis work, and it is difficult to compare one study with the other. Hernandez-Cortes and Acevedo-Garcia (1987) mention that copulation of P. pansa occurs at the Pacific coast of Oaxaca during the hot season from March until July, with the peak of occurrence in May, and from June to July the females are laying their egg capsules. Similar observations from Oaxaca are reported by Turok et al. (1988), who additionally mention that the peak of capsule deposition is in June, and that this continuous activity leads to a constant production of young animals during July, up until September.
In a histological study about the gonad development of P. pansa collected in Nayarit, Quiroz-Rocha (1992) reported that during December to January the gonads are in the resting (immature) stage and that spermatozoids can be scarcely found in male gonads. During May to July the gonads are in the developing stage, and reaching the ripe stage during August to September. In a more detailed histological study about the gonad development cycle of P. pansa further north from Sinaloa, Gonzalez-Flores (1997) reported that the recovering period occurs from September to October, but can be extended up until November to December. The developing stage can be observed in January to February but possibly can be extended during the whole year.
Copulation and the ripe stage of the gonads can be observed from March until July. Spawning starts in May and lasts until July. Acevedo-Garcia et al. (1993) report, that females larger than 2 cm lay egg capsules during December up until September. In our study however, we could show that P. pansa can reproduce during the whole year with the highest frequency of spawning occurring in March and August. For adequate natural resources management of P. pansa in Baja California Sur, strict prohibitory measures should be enforced for exploiting animals during the developing, ripe, and spawning stages of the gonads, and only after special considerations should collections be permitted from September to October, when the animals are in the recovering stage.
Influence of the Water Temperature on the Reproductive Cycle
In most prosobranchs the reproductive cycle is in synchrony throughout a population, controlled by some kind of exogenous and/or endogenous controlling mechanisms (Webber 1977). Important exogenous factors for the gonad development of marine gastropods are considered to be temperature, nutrition, and photoperiod (Webber 1977, Martel et al. 1986). In temperate climates the seasonal changes of the temperature together with changes in the photoperiod are considered important factors controlling the gonad development, however in tropical zones where seasonal temperature fluctuations are less pronounced, temperature does not have such a marked impact. Many species have a seasonal spawning cycle spawn during springtime, and some even in winter (Webber 1977). In Argentine, in the southwestern Atlantic Ocean (Mar del Plata), Zidonia dufresnei (Donovan 1823) spawns during spring and summer (Gimenez & Penchaszadeh 2002). In Coquimbo, North Chile, the subtidal gastropods Priene scabrum (King 1832) spawn in winter, and Sinum cymba (Menke 1828) during almost the whole year (Romero et al. 2003). At the Pacific coast of Bahia Tortugas, Baja California Sur, Mexico, the wavy turban snail Astrea undosa (Wood 1828) spawns during the whole year, with spring and autumn as the major intensive spawning seasons (Belmar-Perez et al. 1991). In Oaxaca Hernandez-Cortes and Acevedo-Garcia (1987), as well as Gonzalez-Flores (1997) observed in Sinaloa the main spawning season of P. pansa is during the summer months of May to July. In Baja California Sur P. pansa has a spawning season from January to July until August, however, mainly in February to April during relative cold, and July to August during warm water and air temperatures. The gonad reproductive cycle of P. pansa seems therefore to be correlated with the temperature. The time of spawning of P. pansa might also be correlated with periods of decreasing differences between low and high tides. A decrease in the tidal range is observed in January to March and June to July up until September. During these periods the laid egg capsules are located either a short distance under the water surface or in water-splashed areas.
Differences in the sex ratio of a mature population of gonochoristic species can be used to check whether there is differential growth or mortality rate between the two sexes (Gimin & Lee 1997). In this study we examined the gonads from 284 males and 260 females collected at Playa Cerritos and no statistically significant difference was detected in the ratio between males and females. Also in Jalisco no difference in the male to female ratio could be found (Leon-Alvarez 1989). However, at Punta Perico a significant statistical difference in the male/female ratio (1:0.74) was observed. At this location we examined the gonads from 322 males and 221 females. The difference in the male/female ratio could be explained by the different conditions of the rocky shore at Playa Cerritos and Punta Perico. At Playa Cerritos, even at high tides, all the rocks are never completely submerged the water. In contrast, at Punta Perico, during high tides a great part of the shore is totally submerged and offering only limited hiding places to the snails, which need protection against displacement by the strong impact of the waves. To avoid the displacement by strong wave actions the snails prefer protective areas where they can hold fast onto the substrate (Hernandez-Cortes & Acevedo-Garcia 1987). Because females reach larger sizes, they are more endangered to be displaced by wave actions and currents than males (Denny et al. 1985). Although the reason is not mentioned, both Hernandez-Cortes & Acevedo-Garcia (1987) and Alvarez-Diaz (1989) reported that on the Pacific coast of Michoacan, males can be found in greater numbers than females (1: 0.83) and (1:0.75), respectively. Similar results were obtained by Turok et al. (1988) (1:0.78), Hernandez-Cortes, and Acevedo-Garcia (1987) in Oaxaca; and in Jalisco by Reyes-Aguilera (1993) (1:0.84), Fonseca-Madrigal (1998) (1:0.72), and by Michel-Morfin et al. (2000) (1: 0.75). Another explanation for the occurrence of more anatomical males than females in a sample could be imposex, caused by the organic metal compound, tributyltin, which is used as a antifouling paint in shipyards, and which is leading to erroneous classification of males and females, resulting in a higher number of males (Gooding et al. 1999, Penchaszadeh et al. 2001). The comparison of our visual anatomical inspection Of the animals with the histological examinations of the gonads show that the differences in the dominance of males at Punta Perico cannot be explained by imposex. The collection of the purple snail by fishermen as a bait and even as a specialty food could also be a reason for an unequal sex ratio. Fishermen prefer larger animals, and for this reason female snails are more sought after. In Punta Perico the population of P. pansa is not exploited by fishermen, and only environmental conditions could explain the unequal sex ratio observed there.
Minimum Size at Sexual Maturity
The determination of the minimum size at sexual maturity of any species of commercial interest is an important parameter for considerations of management and exploitation. Only fragmented results are available about the life history of P. pansa. After spawning, intracapsular development until hatching takes six to eight weeks at 22[degrees]C (Naegel et al. 2003, Naegel 2004, Naegel & Gomez del Prado-Rosas 2004). The duration for settling and metamorphosis of the veliger larvae after spawning takes at least the same length of time (Naegel et al. 2003). The growth, environmental optimal range, and food requirements of the settled juveniles smaller than 6 mm are as yet unknown. Certainly, for a snail to reach this size after spawning more than one year is needed. In this study we show that juveniles of P. pansa do not reach sexual maturity until they reached a size of more than 18 ram. It can be concluded that the purple snails begin to spawn when they reach a size of at least 18 mm. At this size they are at least two or three years old (Ramirez-Rodriguez & Naegel 2003). This extremely slow growth and long period before reaching sexual maturity could be caused by the harsh intertidal environmental conditions, such as the risk of desiccation and overheating through several hours of daily exposure to air, sun, and current reversal during the tidal cycle. Similar observations were made with the mangrove snail Thais kiosquiformis (Duclos 1832), which showed a growth of only ~1 mm/year, and reaches the onset of sexual maturity at a SL of ~24 mm (Koch & Wolff 1996).
In view that P. pansa is in Mexico, because of overexploitation in the past, today a species under special protection, and in view of the extremely slow growth rate, collecting snarls <18 mm can potentially cause a drastic reduction in recruitment, and for this reason should not be permitted.
The authors thank Jorge Lopez- Rocha for his valuable help during the snail collections and for his excellent histological work. The authors also thank Oscar Armendariz lot drawing the figures and Chris Cooksey (London) for proof reading of the manuscript. The study was financially supported by a grant from the Conchologists of America, Inc., US-MEXUS, and by CONACYT (Project 31566 N). Both authors received grants from the Instituto Politecnico Nacional (COFAA, EDI).
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LUDWIG C. A. NAEGEL * AND FEDERICO A. GARCIA-DOMINGUEZ Centro Interdisciplinario de Ciencias Marinas, Instituto Politecnico Nacional Apdo. Postal 592, La Paz, B.C.S. 23000 Mexico
* Corresponding author. E-mail: firstname.lastname@example.org
TABLE 1. Males Avg. Total Stand. Deviat. Punta Perico Number Length (mm) [+ or -] (mm) Range (mm) June 2000 11 30.28 3.92 23.9-40.7 July 14 35.35 4.36 26.1-45.0 August 14 30.20 5.71 19.6-45.02 September 16 31.04 4.24 26.82-38.33 October 19 32.17 6.49 20.88-48.93 November 12 32.18 5.31 24.23-43.6 December 11 33.09 5.41 25.62-47.83 January 2001 19 30.84 4.48 17.63-41 February 19 31.73 4.56 25.11-41.01 March 20 30.35 5.75 22.67-43.68 April 18 29.17 5.56 20.82-47.42 May 20 31.59 5.87 22.8-45.82 June 20 27.51 7.37 18.76-44.03 July 15 27.42 3.7 21.84-34.63 August 11 29.64 6.05 22.48-45.21 September 15 29.58 4.95 19.95-39.02 October 12 30.12 5.57 20.83-40.36 November 18 29.36 5.27 17.49-39.46 December 19 30.01 4.9 20.49-40.05 January 2002 19 27.83 3.47 14.8-34.67 Total 322 30.38 5.43 Females Avg. Total Stand. Deviat. Punta Perico Number Length (mm) [+ or -] (mm) Range (mm) June 2000 7 37.03 10.62 23-58.7 July 14 35.71 4.26 26.0-44.2 August 12 33.33 7.21 23.16-47.46 September 3 36.9 4.31 32.75-42.55 October 10 34.19 6.32 23.3-45.68 November 8 34.18 4.73 24.75-41.55 December 16 35.45 4.18 25.08-47.14 January 2001 9 31.25 4.40 19.2-40.03 February 11 38.11 2.86 21.95-44.59 March 10 33.57 4.36 21.99-42.37 April 12 33.62 7.96 21.17-50.33 May 9 34.87 6.58 23.36-49.22 June 15 36.22 8.91 18.44-50.38 July 10 33.48 5.29 16.37-45.11 August 15 30.19 3.27 24.51-37.17 September 9 32.88 4.77 25.9-40.53 October 19 32.29 5.18 22.45-46.31 November 13 27.75 4.64 19.09-36.88 December 9 33.92 4 23.13-40.37 January 2002 10 32.24 5.72 23.06-42.11 Total 221 33.64 6.00 Avg. Total Stand. Deviat. Playa Cerritos Number Length (mm) [+ or -] (mm) Range (mm) June 2000 11 27.53 3.39 22.3-31.1 July 16 26.76 3.58 21.0-43.5 August 12 29.75 2.51 25.06-32.55 September 10 28.42 2.96 23.46-31.59 October 17 28.54 4.25 21.62-35.66 November 11 27.89 2.93 17.18-27.64 December 12 23.31 3.18 17.18-27.64 January 2001 15 25.28 2.8 19.01-29.05 February 11 26.17 4.82 19.01-29.05 March 16 24.86 4.36 18.31-34.73 April 14 28.05 3.91 21.25-35.38 May 12 29.25 4.33 23.53-40.81 June 19 26.48 3.81 20.74-32.76 July 17 23.4 3.07 18.56-29.4 August 14 25.58 4.7 18.34-36.44 September 14 21.5 3.28 15.87-35.82 October 19 23 3.8 14.86-28.4 November 15 24.52 2.33 19.84-28.57 December 14 24.63 2.69 18.98-29.95 January 2002 15 26.79 3.54 20.21-33.43 Total 284 25.93 4.11 Avg. Total Stand. Deviat. Playa Cerritos Number Length (mm) [+ or -] (mm) Range (mm) June 2000 12 31.17 3.72 24.2-36.3 July 8 27.16 5.68 20.5-40.6 August 17 34.39 5.86 24.81-48.38 September 9 31.37 6.78 23.89-46.55 October 12 29.08 6.16 23.07-41.12 November 10 29.85 5.81 24.41-42.93 December 17 29.85 5.38 24.41-42.93 January 2001 15 29.71 5.83 21.97-44.96 February 7 29.12 6.2 21.1-41.77 March 14 27.51 6.69 18.84-41.71 April 15 28.43 4.68 23.08-42.47 May 16 29.36 2.69 23.97-33.78 June 11 29.64 4.58 20.28-36.45 July 12 26.54 3.57 21.7-34.17 August 17 27.32 5.53 20.65-38.31 September 13 25.14 5.39 13-33.42 October 12 25.83 4.33 16.71-31.52 November 14 28.81 7.14 15.94-41 December 17 28.28 5.71 15.88-38.61 January 2002 12 28.78 5.04 17.74-36.89 Total 260 28.92 5.62
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|Author:||Garcia-Dominguez, Federico A.|
|Publication:||Journal of Shellfish Research|
|Date:||Dec 1, 2006|
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