BROWN D B CAMPOS & HJ URBAN 2002 Reproductive Cycle of the Bivalve Clams Semele Solida and Gari Solida From Chile

REPRODUCTIVE CYCLE OF THE BIVALVE CLAMS SEMELE SOLIDA (GRAY, 1828)(SEMELIDAE) AND GARI SOLIDA (GRAY, 1828) (PSAMMOBIIDAE) FROM CHILE

DONALD BROWN,1* BERNARDITA CAMPOS,2 AND H.-JÖRG URBAN3

1Departamento de Biología, Instituto de Ciencias Biológicas y Químicas, Facultad de Ciencias,Universidad de Valparaíso, casilla 5030, Valparaíso, Chile; 2Facultad de Ciencias del Mar,Universidad de Valparaíso, casilla 13-D, Viña del Mar, Chile; 3Alfred-Wegener-Institute for Polarand Marine Research, Section of Comparative Ecosystem Research, Columbusstr. 27568 Bremerhaven,Germany

ABSTRACT Commercial clam landings reached an average of almost 91,000 tons annually in Chile over the last decade. In spite ofthe high value of this resource, few efforts have been made to understand the basic biology of the exploited species, data that mightin the future be needed to aid in their protection or even artificial culture. This study is a contribution to the knowledge of thereproductive cycles of two valuable species, Semele solida (Gray) and Gari solida (Gray). Representative samples of these species werecollected at two widely separated localities in Chile and examined histologically to determine their seasonal reproductive cycles. It wasfound that the species were of separate sexes, and had annual gonadal cycles. In S. solida from northern Chile, the reproductive periodextended from June 1991 to February 1992. In G. solida from southern-central Chile, the reproductive period was relatively short, fromOctober 1991 to February 1992. In both species, most specimens have empty gonads by March. The data obtained are useful indeveloping management plans related to their reproductive periods. Relevant to culture strategies, S. solida has the comparativeadvantage of a lengthy reproductive period, wherein mature individuals may be frequently encountered in nature for spawninginductions. G. solida, with its shorter annual reproductive cycle may have the advantage of being induced to mature in artificialconditioning systems over relatively short periods of time.

KEY WORDS: clam reproduction, reproductive cycle, bivalves, Semele solida, Gari solida, Chile

INTRODUCTION

Chilean coastal waters host very productive and diverse clamfisheries due to the rich coastal upwelling and favorable watertemperatures. The largest clam populations occur in the protectedbays and fjords of southern Chile. Over a number of years officialfisheries records in Chile (SERNAPESCA 1990–1999) consideredall clam species as one generic group (“clams”) among which wereincluded the venerids Protothaca thaca (Molina), Venus antiqua(King and Broderip), Eurhomalea exalbida (Chemnitz), E. lenticu-laris (Sowerby), E. rufa (Lamarck), and the mactrid Mulinia edu-lis. Semele solida (Gray) and Gari solida (Gray) belonging to theSemelidae and the Psammobiidae respectively, and objects of thisstudy are also included in this group. They are primarily exploitedin artisanal fisheries, and commercialized mostly in canned form.G. solida is highly valued from the culinary standpoint. In 1994,the first year of its listing as an individual species, 4613 tons of S.solida were harvested, declining to 2071 tons by 1999; G. solida,recorded separately beginning in 1990, was registered at 31,373tons, which declined to 9931 tons by 1999 (SERNAPESCA 1990–1999). The only regulation for the fishery of these clams is aminimum size limit of 55 mm for S. solida and 60 mm for G.solida (Subsecretaría de Pesca 1996).

Despite their great economic value, not much research has beendone on reproduction in Chilean clams, particularly in S. solidaand G. solida. One recent report (Jeréz et al. 1999) suggested thatG. solida in southern Chile had a continuous reproductive cyclethroughout the year, a pattern apparently common among the heav-ily commercialized clams such as V. antiqua (Lozada & Bustos1984) and P. thaca (Henríquez et al. 1981). This also was true forE. lenticularis (Campos & Brown 1997, Campos et al. 1999) andM. edulis (Jaramillo et al. 1998).

Semele solida (Fig. 1A), locally termed “tumbao”, occurs par-tially buried in sand and gravel bottoms from the intertidal (Osorioet al. 1979) to the subtidal zone (Urban 1994). Its geographicdistribution ranges from 12°S to 47°S. (Viviani 1979). Gari solida(Fig. 1B), locally termed “culengue”, occurs completely buried inbottom sands and gravels, usually at greater depths than S. solidafrom the intertidal to 15-m depth (Urban 1994). Its range of dis-tribution along the Pacific Coast, as given by Viviani (1979) andlater by Guzmán et al. (1998), was between 12°S and 47°S.

Biologic data for species of economic importance is fundamen-tal for proposing regulatory recommendations for sustainable har-vest of these resources over time. The obvious declines in harvestover the years enhances the need for more information on thereproduction and survival of these species to support efforts di-rected towards their artificial culture, repopulation, or managementas a renewable resource in over-exploited beds.

In this study, we describe the reproductive cycles of G. solidaand S. solida by means of histologic observations of gametogen-esis during different seasons of the year. Patterns in reproductivecycles, including gametogenesis and resting gonadal periods werestudied in a population of S. solida from northern Chile and in a G.solida population from central-southern Chile, representing thefirst study of this nature for these two clam species in their re-spective regions.

MATERIALS AND METHODS

Adults of each species were obtained by diving at monthlyintervals from June 1991 to July 1992. S. solida was collected inLa Herradura Bay (29°58�S) and G. solida from Coliumo Bay(36°32�S) (Fig. 2). Maximum anterior-posterior length of the shellwas measured on each specimen, to the nearest 0.1 mm. Matura-tional status of the gonad was determined monthly on around 30animals of each species. Tissue samples 5 mm in thick-* Corresponding author. E-mail: [email protected]

Journal of Shellfish Research, Vol. 21, No. 2, 627–634, 2002.

627

ness were obtained and fixed 24 h in Bouin’s fluid and prepared byroutine histologic procedures as follows: dehydration with gradedseries of ethanol, clearing in xylol and embedding in Paraplast.Five micrometers histologic sections obtained from three levels ofeach gonad separated 500 �m, were stained with hematoxylin andyellowish eosin, dehydrated in graded series of ethanol, cleared inxylol and permanently mounted with Canadian balsam (Gabe1968).

The gametogenic cycles of the two clams were followed bydescribing the histologic appearance of the gonadal sections andclassifying them into different stages of maturity using a modifi-cation of the scale proposed by Lucas (1965). Each individual wasassigned to one of the following stages based on the degree ofmorphologic development of its germ cells: (d1) � initial devel-opment or maturation; (d2) � advanced development or matura-tion; (d3) � complete development or maturation; (r1) � initialregression or evacuation; and (r2) � total regression or evacua-tion. The results were expressed as percentage frequency histo-grams of: (1) males in each gonadal stage; (2) females in eachgonadal stage; and (3) males plus females in each stage, separatelyfor S. solida and G. solida during the sampling period from June1991 through July 1992.

RESULTS

Semele solida specimens measured from 38.9 to 86.0 mm andGari solida from 41.4 to 88.0 mm. The two clam species were ofseparate sexes, with no hermaphroditism and no sexual dimor-phism evident. Histologic analysis of the gonad in both speciesshowed a multilobulate organization of the acini connected toevacuation tubes covered by simple ciliated epithelium similar tothat observed in other bivalves (Sastry1979). The acini consistedof a basal lamina of variable thickness depending on the stage ofgonadal maturity. Its relative thickness was greatest in specimensinitiating gametogenesis, and in those that had spawned. In thesespecimens an intra-acinar reticulum consisting of vesicular so-matic cells and an intra-acinar space containing groups of ame-bocytes may be found (Figs. 3A and 4F).

The cells of the male germinal line that characterize spermato-genesis may be restricted to two zones of the gonadal acinus: (1)a basal region representing the early germinal line that includesspermatogonia and spermatocytes that form a band of circularvoluminous nuclei, and recently formed round spermatids that alsoform a band of small circular nuclei (Figs. 3A, 3B and 4A, 4B),that is evident in G. solida; and (2) a lumen region, representing anadvanced germinal line with spermatids undergoing cytodifferen-tiation with heavy stained elongated nuclei, gathered by their headsin double columns, giving a “feathered” appearance (Figs. 3B, 3Cand 4B, 4C).

Figure 1. Species under study. A: Semele solida, 78 mm total length(maximum anterior-posterior shell dimension); B: Gari solida, 89 mmtotal length.

Figure 2. Location of collection sites for A: S. solida and B: G. Solidaon the Chilean coast.

BROWN ET AL.628

Figure 3. A–E: Light photomicrographs of different histological stages of male and F–J: female gonadal acini of S. solida collected in LaHerradura Bay from June 1991 to July 1992. A–E bar = 100 µm; F–J bar = 200 µm; A and F = initial development of maturation (d1); B andG = advanced development of maturation (d2); C and H = total development or maturation (d3); D and I = initial regression or evacuation (r1);E and J = total regression or evacuation (r2).

REPRODUCTIVE CYCLE OF CHILEAN CLAMS 629

Figure 4. A–E: Light photomicrographs of different histological stages of male and F–J: female gonadal acini of G. solida collected in ColiumoBay from June 1991 to July 1992. A–E bar = 100 µm; F–J bar = 200 µm; A and F = initial development or maturation (d1); B and G = advanceddevelopment or maturation (d2); C and H = total development or maturation (d3); D and I = initial regression or evacuation (r1); E and J =total regression or evacuation (r2).

BROWN ET AL.630

The cells of the female germinal line in the basal region arerepresented by oogonia, previtellogenic and adhered vitellogenicoocytes. In the lumen region they are represented by pedunculatevitellogenic oocytes as well as free oocytes (Figs. 3G, 3H, and 4G,4H). The histologic stages of the gonads of S. solida and G. solidafemales are shown in Figures 3F–3J and 4F–4J, respectively.

The three sampled areas of the gonad from both species allshowed the same degree of gametogenic activity or developmentof the germinal line, indicating synchronic maturation throughoutthe gonad.

Semele solida

The distribution of percentage frequencies of the different his-tologic stages in male gonads, female gonads, and in the popula-tion as a whole for S. solida are given in Figures 5A, 5B and 5C,respectively. This species presented a seasonal pattern of gonadaldevelopment in both sexes. Males and females with developed ormature gonads (d3 stage) as well as in initial regression (r1) (Figs.3D and 3I) were predominant from June 1991 to February 1992. Incontrast, from March to June 1992, there was a greater frequencyof individuals in total regression (r2) (Figs. 3E and 3J). Althoughin both sexes the frequency of samples with gonads in initial stagesof development (d1) (Figs. 3A and 3F) was observed betweenApril and July 1992, the number of females in this stage wasgreater, and predominated over those in total regression (Fig. 5Avs. Figure 5B). However, during the first period, there was a smallpercentage of males in total regression (r2) and in initial develop-ment (d1) (Fig. 5A), which was a condition more apparent infemales (r2-d1; Figure 5B). Some observations not included in thefigures suggested that individuals technically considered to be inregression could show a new wave of initial maturation beginningat the germinal line in the gonadal acini.

The second period was characterized by the total regressionstage (r2), where all individuals had gonads with depleted acini inMarch 1992 (Figs. 5A, 5B). The percentage frequency distributionof the different gonadal stages for the population sample (Fig. 5C)shows this tendency in both males and females.

It is of interest to point out the difference in gonadal conditionsbetween specimens sampled in June to July 1991 compared withthose from the same period in 1992. In 1991 a high frequency ofboth sexes contained elevated numbers of specimens with gonadsin advanced and total development (d2–d3), whereas in 1992 thiscondition was different, with specimens containing gonadal aciniin total regression (r2) or without advanced germinal line (Fig. 5).

Gari solida

In this species the distribution of percentage frequencies of thedifferent histologic stages in males, females, and the entire popu-lation are given in Figures 6A, 6B, and 6C, respectively. There isa periodicity in both sexes with the same general tendency.

There is a well-marked period in which advanced and totallydeveloped gonads (d2-d3) are observed, as well as those in initialregression (r1) (Figs. 4D and 4I) from October 1991 to February1992. In females this period is much shorter (November 1991 toJanuary 1992). This condition persisted in some males until April1992. Some observations not included in the figures showed indi-viduals in regression during this period, which had a new wave ofinitial maturation beginning at the germinal line, as observed in S.solida (see earlier). In a second, more extensive period, the gonadswere characterized by the occurrence of total regression and initial

development (r2–d1) (Figs. 4E and 4A, respectively) in malesfrom June to September 1991, and from March to July 1992 (Fig.6A). In females the r2–d1 period (Figs. 4J and 4F, respectively)extended notably until October 1991, and from February to March1992 (Fig. 6B).

Figure 6C shows the general frequency of the gonadal stagesfor the general population, with a similar pattern to that presentedseparately for males and females. From June to August 1991,February 1991, and April to July 1992 the number of specimens ineach population sample exceeded the males and females togetherbecause included were specimens whose total regression stage wassuch that there were no cells on the germinal line differentiatedenough to permit sex determination. In comparing the gonadalstages of specimens obtained in June to July 1991 with those of thesame period in 1992, stages were observed that were near totalregression and initial development showing an inverse fluctuationwhere regression predominated in 1991 and initial development in1992.

Finally, it was apparent in both species that the stages of ad-vanced or total maturity were observed in water having higherrelative temperature, whereas initial stages of development wererelated to water of relatively low temperature, although our tem-perature measurements were not extensive (Figs. 5C and 6C).

DISCUSSION

The reproductive cycle is characterized by a series of eventsthat in annual species comprises a reproductive period involvingthe gametogenic and spawning phases and a resting period inwhich there is not gametogenic activity.

Present results have shown similar values between the annualreproductive cycles of S. solida and G. solida, where both showedseasonal gametogenesis and spawning, followed by a resting pe-riod without production of gametes.

The reproductive period of S. solida, from June 1991 to Feb-ruary 1992 was longer than that of G. solida. Most of the speci-mens showed gametogenic activity and signs of having spawned.The majority of spawning occurred in February, and in March allspecimens had empty gonads. This point marked the initiation ofthe resting period, indicated by a completely regressive conditionin the gonad, which was more marked in G. solida than in S.solida.

We are cautious to consider the possibility of a second spawn-ing phase during the reproductive period of S. solida because of thelow number of animals (4) sampled and examined in October,notwithstanding that all of them were in advanced development(D2).

In G. solida the reproductive period was relatively short, fromOctober 1991 to February 1992; the spawning phase mostly occursin February. A majority of the specimens had empty gonads inMarch 1992; then the spawning phase mostly occurs in February.The presence of specimens with gonads in the initial state of de-velopment in this period may indicate possible activity in gonialmultiplication and generation of cytes without gametogenic activ-ity that leads to massive production of differentiated gametes. Theresults showed that low temperatures favored the proliferative ac-tivity of the early germinal line, while high temperatures aidedcytodifferentiation of the advanced germinal line. This conditionwas most notable in the reproductive cycle of G. solida fromColiumo Bay, a more southerly location. These events occurred

REPRODUCTIVE CYCLE OF CHILEAN CLAMS 631

simultaneously in both males and females showing (expected) syn-chrony of the reproductive cycles.

Every monthly sampling during the reproductive periodshowed a few individuals having gonadal conditions differing

from the majority of the specimens, a phenomenon more pro-nounced in S. solida. There was a predominance of advancedstages of gonadal maturity, and also those with complete regres-sion as an evidence that spawning had occurred. These observa-

Figure 5. Distribution of different gonadal stages in A. males; B. females; and C. males + females of S. solida colleced in La Herradura Bay fromJune 1991 to July 1992, with sea surface temperature added. The length of each area represents the percentage frequency of specimens in eachhistologic stage of the gonadal acini. N = number of specimens examined.

BROWN ET AL.632

tions suggest intrapopulation asynchrony of gametogenic activity,with partial evacuations of gametes over a longer period. The factthat the specimens showing signs of having spawned showed anew wave of maturation in the germinal layer of their acini, con-

firmed this asynchrony and strengthened the hypothesis of con-tinuous gametogenesis with various cycles of gametogenic activityand spawning by each individual during the reproductive period.

A difference was observed in the degree of maturity of the

Figure 6. Distribution of different gonadal stages in A. males; B. Females and C. males + females of G. solida collected in Coliumo Bay fromJune 1991 to July 1992, with sea surface temperature added. The length of each area represents the percentage frequency of specimens in eachhistological stage of the gonadal acini. N = number of specimens examined.

REPRODUCTIVE CYCLE OF CHILEAN CLAMS 633

population samples of S. solida between June and July 1991 whereadvanced and complete maturity were well represented; in thesame period of 1992, on the contrary, maturity was just beginning.This difference may be explained by normal adaptation to envi-ronmental conditions such as temperature and food availability,which may vary within a limited range from year to year.

Although Urban and Campos (1994) suggested that the repro-ductive cycles of S. solida and G. solida were influenced by tem-perature, Jeréz et al. (1999) working on a G. solida populationfrom the south of Chile found the annual reproductive cycle to becontinuous without a marked resting period. Further studies arerequired to evaluate seasonal variations in gonadal cycles of thesespecies with latitude, as they are distributed over a broad latitudi-nal range from Callao, Peru (12°S) to Chile’s Chonos Archipelago(44°S). The hypothesis here is that the reproductive cycles of theseclams become shorter in populations the farther south they occuron their distributional gradient. Some data available on other clamspecies with extensive distributions support this hypothesis. Popu-lations of Protothaca thaca (Henríquez et al. 1981), Tagelusdombeii (Acuña et al. 1994) and Eurhomalea lenticularis (Campos& Brown 1997) from central and north-central Chile exhibit con-tinuous gonadal activity with various important spawning peaksthroughout the year. Nevertheless, T. dombeii from south-centralChile showed a period of gonadal regression in the fall (Jaramilloet al. 1998). This phenomenon is not clear across other clam spe-cies inhabiting the south-central zone of Chile, such as Venusantiqua, Tawera gayi, Mulinia edulis and Ensis macha that showcontinuous reproductive cycles without resting periods (Lozada &Bustos 1984, Jeréz et al. 1999).

From the practical point of view, regulation of harvesting theseclams should be based on considerations of their reproduc-tive cycles by limiting their harvest during the major spawningseason. Consideration of the reproductive cycles is also importantin obtaining broodstock for aquaculture. Experimental studiesshould prove this a feasible alternative for production or protect-ion of the resource. In studying resource management of theseclams, S. solida shows a comparative advantage in having amore extensive reproductive period, as mature individuals may beencountered over an extended period. This implies that maturebroodstock would be available in nature for artificially inducedspawning (e.g., in aquaculture experimentation) over compara-tively long periods. Although G. solida, in contrast, has a morerestricted reproductive period, it may be a species amenable toartificial conditioning in aquaculture systems, given that its gonadsalmost always contain high numbers of immature gametogeniccells.

ACKNOWLEDGMENTS

The authors thank Ms. T. Jerí for sampling G. solida atColiumo Bay, and Ms. G. Bellolio (U. Católica del Norte at thetime) for providing S. solida from Herradura Bay. We also thankDr. R. Guerra for her supervision of the histologic processing atthe U. de Valparaíso, and Mr. C. Olivares for assistance in histo-logic analyses. Corrections and comments by anonymous review-ers helped to improve the manuscript. The work was financed byFONDECYT Grant 91-502 to B. Campos.

LITERATURE CITED

Acuña, E., Ch. Guisado & M. Berríos. 1994. Ciclo reproductivo de Tagelusdombeii (Bivalvia: Heterodonta: Solecurtidae), provenientes de la bahíaLa Herradura de Guayacán, IV Región. XIV Jornadas de Ciencias delMar, Chile. 131 pp.

Campos, B. & D. Brown. 1997. Aspectos reproductivos de la almeja Eu-rhomalea lenticularis (Sowerby) proveniente de la rada El Algarrobo(V Región). Informe Final Proyecto DIPUV 20–95, Universidad deValparaíso, Chile.

Campos, B., D. Brown, L. Durán, C. Melo & J. Urban. 1999. Estudio deedad y reproducción del recurso almeja en la IV y V Regiones. InformeFinal Proyecto FIP 97-32, Subsecretaría de Pesca, Chile.

Gabe, M. 1968. Techniques histologiques. Paris: Masson et Cie. 1113 pp.Guzmán, N., S. Saá & L. Ortlieb. 1998. Catálogo descriptivo de los mo-

luscos litorales (Gastropoda y Pelecypoda) de la zona de Antofagasta,23°S (Chile). Estud. Oceanol 17:17–86.

Henríquez, R., P. Barboza, R. Ramos, E. Tapia & C. Toro. 1981. Variaciónanual de la gónada de la almeja Protothaca thaca (Molina 1782):análisis histológico. I Jornadas de Ciencias del Mar, Chile. Resúmen.34 pp.

Jaramillo, E., E. Clasing, M. Avellanal, P. Quijón, H. Contreras, P. Rubilar& G. Jeréz. 1998. Estudio biológico pesquero de los recursos almeja,navajuela y huepo en la VIII y X Regiones. Informe Final Proyecto FIP96-46, Subsecretaría de Pesca, Chile.

Jeréz, G., N. Barahona, H. Miranda, V. Ojeda, D. Brown, C. Osorio, A.Olguín & J. Orensanz. 1999. Estudio biológico pesquero de los recur-

sos tawera (Tawera gayi) y culengue (Gari solida) en la X Región.Informe Final Proyecto FIP 97-29, Subsecretaría de Pesca, Chile.

Lozada, E. & E. Bustos. 1984. Madurez sexual y fecundidad de Venusantiqua antiqua King & Broderip, 1835 en la bahía de Ancud (Mol-lusca: Bivalvia: Veneridae). Rev. Biol. Mar. Valparaíso 20:91–112.

Lucas, A. 1965. Recherche sur le sexualité des mollusques bivalves. The-ses Doctorat. Fac. Science, Université de Rennes, France.

Osorio, C., J. Atria & S. Mann. 1979. Moluscos marinos de importanciaeconómica en Chile. Biol. Pesq. Chile 11:3–47.

Sastry, A. 1979. Pelecypoda (excluding Ostreidae). In: A. Giese & J.Pearse, editors. Reproduction of Marine Invertebrates. Vol. V. NewYork: Academic Press. pp. 113–292.

SERNAPESCA. 1990–1999. Anuario estadístico de pesca 1990 a 1999.Servicio Nacional de Pesca, Chile.

Subsecretaría de Pesca. 1996. Medidas de Administración Pesquera. Min-isterio de Economía, Desarrollo y Reconstrucción (Chile). 32 pp.

Urban, H.-J. 1994. Adaptations of six infaunal bivalve species of Chile:coexistence resulting from differences in morphology, burrowing depthand substrate preference. Arch. Fish. Mar. Res. 42:183–193.

Urban, H.-J. & B. Campos. 1994. Population dynamics of the bivalvesGari solida, Semele solida and Protothaca thaca from a small bay inChile at 36 °S. Mar. Ecol. Prog. Ser. 115:93–102.

Viviani, C. 1979. Ecogeografía del litoral chileno. Stud. Neotrop. Fauna &Environ. 14:65–123.

BROWN ET AL.634