REPRODUCTIVE CYCLE OF THE SOFT-SHELL CLAM,MYA ARENARIA, AT SKAGIT BAY, WASHINGTON

RUSSELL G. PORTER'

ABSTRACT

The annual reproductive cycle of the soft-shell clam. Mya arenaria L.. was studied'll Skagit Bay innorthern Puget Sound. Wash. Spawning occurred from late May to early September in both 1971 and1972 with peak spawning in July and June respectively. Small clams (less than 60 mm in length) had aspawning peak that coincided with other size classes although the spawning period was shorter induration. The single yearly spawning period at Skagit Bay corresponds with east coast populations inCanada and Maine.

The soft-shell clam, Mya arenaria L., is found onvirtually all coastlines in the northern hemi-sphere (Hanks, 1963) and is still extending itsrange as evidenced by its recent movement intothe Black Sea (Zambriborshch, Marchenko, andTelegin, 1968; Ivanov, 1969). On the NorthAmerican continent, it is native to the east coastfrom which it reportedly was accidentally intro-duced into San Francisco Bay, Calif., about 1874(Fitch, 1953>' However,' there is some evidencefrom Indian middens that the soft-shell clam isalso native to the west coast (Craig, 1927\. Itsrange on the west coast presently extends fromCalifornia to Alaska (Morris, 1966l.

The reproductive cycle of the soft-shell clamhas been described from a variety of locations onthe east coast, but no data have been presentedfor west coast populations except for one briefnote from Oregon (Edmondson, 1920). The firstdetailed study on the histology of the gonad ofMya arenaria was conducted by Coe and Turner(1938) in New England. They found that spawn-ing occurred in the summer. At Martha's Vine-yard in Massachusetts spawning was found tooccur over a 6-mo period from spring throughearly fall (Deevey, 19481. In northern Mas-sachusetts, spawning occurs in late summer andearly fall

PORTER: REPRODUCTIVE CYCLE OF MYA ARENARIA

in surface water temperature at Strawberry Pointwas 4°C.

METHODS AND MATERIALS

The study began in November 1970 and wascompleted in November 1972. Samples were col-lected once a month from November thru Feb-ruary and twice a month from April thru Octoberin 1971 and March thru October in 1972. Thebimonthly samples were taken at 2-wk intervals:generally during the first and third weeks of themonth. No sample was collected in January 1971because river flooding prevented access to thestudy area. Each sample consisted of 50 clamswhich was separated from a larger random sampleto represent five size classes: clams less than 60mm in length, those in the 60-,70-, and 80-mmlength ranges, and those larger than 90 mm. Asfar as possible the 50 clams selected for each sam-ple were equally distributed between the five sizeclasses. The samples taken during the first 3 mo ofthe study consisted of only 10, 10, and 15 clamsrespectively. A total of 1,785 clams were collectedwhich ranged from 22 to 105.5 mm in length. Ofthis total 2.6% were immature, leaving a total of1,739 mature clams that were utilized in theanalysis of the reproductive cycle.

The samples were returned to the laboratorywhere they were measured and weighed and thegonadal mass removed and preserved inDavidson's acetic acid fixative (Shaw and Battle,1957). In smaller clams the entire visceral masswas preserved and sectioned; for larger clams acube of gonadal tissue was removed from themid-lateral portion of the visceral mass. Usuallydissection and preservation were accomplishedthe day ofcollection. Clams not dissected until thefollowing day were held in a refrigerated saltwa-ter system overnight.

Slides were prepared by standard histologicaltechniques: tissues were dehydrated in alcohol,cleared in xylene, embedded in paraffin, sectionedat 5-8 microns, and stained with Mayer'shematoxylin and alcoholic eosin (Galigher andKozloff, 1971).

The number of gonadal stages used to describebivalve reproductive cycles varies widely. Lam-mens (1967) distinguished 11 stages and meas-ured the nuclear-cytoplasmic ratio. Previous in-vestigations on Mya reproductive biology gener-ally have recognized five phases of development(Shaw, 1962, 1965; Ropes and Stickney, 1965);therefore the following five phases were used: in-

active, active, ripe, spawning, and spent. Thesefive phases were distinguished by the followingcharacteristics.

Males

Inactive (Figure la)

During this phase the alveoli are filled withfollicle cells which contain the typical male typeinclusions as described by Coe and Turner (1938).Primary spermatocytes may be visible along thealveolar wall, but are not abundant.

Active (Figure Ib-d)

This phase is typified by the proliferation andmaturing of the spermatocytes. In staging theslides, an early active, middle active, and lateRcbve stage were identified. The early active stage(Figure Ib) is characterized by the proliferation ofprimary spermatocytes at the basal membrane ofthe alveoli and the appearance of some sper-matids. The middle active stage (Figure lc) ischaracterized by the disappearance of the folliclecells and the migration of spermatids toward thecenter of the alveoli where they begin aligning inradial columns. The late active stage (Figure Id) ischaracterized by the greatly increased number ofradially aligned spermatids and the formation ofacentral lumen in the alveoli.

Ripe (Figure Ie)

In the ripe male clam, the sperm are distinctlybunched in radial columns around the alveoli withtheir tails, which stain pink with eosin, projectinginto the central lumen.

Spawning (Figures If and 2a)

When spawning commences a single row offol-licle cells form at the alveolar membrane (Figure10. These follicle cells contain the typical inclu-sions of the male, and the number of rows in-creases as spawning proceeds (Figure 2al.

Spent (Figure 2b)

In the spent clam most all sperm have beendischarged, but a few may remain. The alveoli arealmost completely filled with follicle cells.

Females

Inactive (Figure 2c)

In the inactive phase the alveoli are filled withfollicle cells which contain the distinctive female

649

I'ISHERY BULLETIN VOL. 72. NO.3

FICURE i.-Gonadal stages of the soft-shell clam, Mya arenana, at kagit Bay, Wash. a) Inactive male (160x). 22 Sept. 1972.Follicle cells wIth inclusIOns fill the alveoli. bI Early active male

PORTER, REPRODUCTIVE CYCLE OF MYA ARENARIA

FIGURJl: 2.-Gonadalstages of the soft-shell clam, MyCl arenaria, at Skagit Bay, Wa h. a) Spawmng male (100 xl, 6 Aug. 1971. Thefollicle ceJ1s reappear as spawning progress s. b) pent male (100x), 8 pt. 1972. c)lnactive female (100x), 12 Oct. 1971. Folliclecells containing the typical female inclusions fill the alveoli. dl Early active female (145x), 3 Mar. 1972. The primary ovocytesbegin enlarging forming talked ovocyte.. e) Active female Cl36x), 8 May 1972. Th follicle cells and their inclu ions havedisappeared. 0 Late active female (240x), 14 June 1972 The nucleolus and amphinucleolus of the ova have appeared, but mostova are still attached to the basal membrane of the alveoli

651

type inclusions (Coe and Turner, 1938). A fewprimary ovocytes are visible along the alveolarmembrane.

Active (Figure 2d-f)

As in the male, three stages were identified forthis phase: early, middle, and late active. Theearly active stage (Figure 2d) is characterized bythe proliferation of primary ovocytes and theirelongation producing stalks which protrude to-ward the center of the alveolus between the folliclecells. In the middle active stage (Figure 2e) thefollicle cells and their inclusions have disappearedleaving a central lumen in each alveolus. An in-creasing number of stalked ovocytes attached tothe alveolar wall protrude into this lumen. In thelate active stage (Figure 2D the ovocytes are begin-ning to become spherical with slender stalks,and in many the nucleolus and amphinucleolusare readily visible.

Ripe (Figure 3a, b)

In the ripe phase a majority of the ova are free ofthe alveolar wall and have taken on sphericalshape (Figure 3al. In some individuals the ova arequite abundant, and almost all will be free of thealveolar wall (Figure 3b).

Spawning (Figure 3c)

The spawning phase is characterized by theemptying of the alveoli of ripe ova, leaving behinda few ovocytes that are still attached to the alveo-lar wall.

Spent (Figure 3d)

In the spent clam the alveoli are empty, andfollicle cells begin to fill in the alveoli from thebasal membrane inward. Inclusions reappearwith the follicle cells, and some of the primaryovocytes are visible.

Immature

The immature gonad (Figure 3e) has a muchsmaller number of alveoli which are filled withfollicle cells df'void of any inclusions.

Each clam was identified as to sex and staged inaccordance with the above phases. The percentageof clams in each phase was then calculated. Forthe purposes of analyzing the reproductive cycle(Figure 4), the three stages of the active phase:early, middle, and late were combined under thesingle term active phase. In addition, meanmonthly percentages were utilized in analyzing

652

FISHERY BULLETIN: VOL. 72, NO.3

the reproductive cycle (Figure 4) for those monthsduring which two samples were collected,

In the presentation of results, the terms 1971and 1972 reproductive cycle refer to the cyclewhose spawning phase occurred during that re-spective calendar year. However, the reproductivecycle as a whole does not necessarily coincide with,nor is it restricted to, a particular calendar year.The reproductive cycle was assumed to begin withthe active phase and end with the inactive phase.

RESULTSThe histological examinations revealed a single

yearly spawning period which occurred from lateMay to early September. This was true for bothsexes and for both the 1971 and the 1972 reproduc-tive cycles although the period of peak spawningvaried slightly (Figure 4). The sex ratio of the1,739 clams utilized in the analysis of the repro-ductive cycle was 48% males (837) and 52%females (902).

1971 Reproductive CycleDuring the 1971 reproductive cycle (Figure 4)

clams in the active phase were encountered fromFebruary through July for males and Februarythrough June for females. Active clams were un-doubtedly first present in January although nosamples were collected that month. Individuals inthe early active stage (Figures Ib and 2d) firstappeared in February for both sexes, while thosein the middle active stage (Figures Ie and 2e) firstappeared in March and the late active stage (Fig-ures Id and 2f) in ,early April.

Ripe clams of both sexes were first observed inlate April. Ripe males (35%) were most abundantin May and ripe females (47"/(,) in June.

Clams in a spawning condition were first en-countered in the later part of May, peaked inJuly, and were last observed in the early Septem-ber sample. During July 75% of the males and55% of the females were in a spawning condition.

Spent clams were present from July to Octoberwith the highest percentage occuring in Augustwhen 38'% of the males and 65% of the femaleswere in this phase.

There was no observed difference in reproduc-tive cycle with size class, except for clams under 60mm in length. In general the period of peakspawning for those clams was the same as otherfiize classefi, but the duration of the spawningperiod was shorter. It began about 1 mo later thanother size classes and ended a month earlier.

PORTER REPRODUCTIVE CYCLE OF MYA ARENARIA

•

••••0'

e •. "": .... . ...•

.' _.. .. ..~ .~o.

••0; . .!.

•I .. •• .•

FIGURE 3.-Gonadal stages of the soft-shell clam. Mya arenaria, at Skagit Bay, Wash. a) Ripe female (180xl, 14 June 1972. Theova are now free ofthe alveolar wall and have taken on a spherical shape. bl Ripe female (145X), 14 June 1972. cl Spawning female(180xl. 1 June 1972. dl pent female (100XI. 24 Aug. 1972. The ova have been di charged. and follicle cells with inclusions areforming along the basal m mbrane ofthe alveoli el Immature clam (l60xl. 24 Aug. 1972. Follicle cells devoid of inclusions. butcontaining th small black follicular nucleii. fill the alveoli. From an individual 38.4 mm in length. nH rmaphrodite (40 I in aspawning condition (14 June 19721.

653

FISHERY BULLETIN: VOL. 72. NO.3

[~J INACTIVE D ACTIVE ~ RIPEMALE

II SPAWNING [(I] SPENT

100 - ,.- .-

ao-

50 -

§cra. 40-

20

~ ~ ":' ? .-.;

~.

~~

~. ~

FEMALE

~.

fD r100ao

~ 50

~... 40-20

NOV DEC I1970

H::B MAR APR MAY JUN JUl

1971

AUG SEP OCT

1HJ

NOV DEC IJAN HB MAR

,--

APR MAY JUN JUl AUG SE P OCT NOV

1972

FIGURE 4.-Male and female reproductive cycles of the soft-shell clam, Mya arenaria, during 1971 and 1972 from Skagit Bay, Wash.The length of each shaded area represents the percentage frequency of clams in each reproductive phase. A total of 1,739 clams (48%males and 52% females) were examined during the 2-yr study.

The smallest mature male clam examined dur-ing the 1971 reproductive cycle was 22.9 mm inlength, and the smallest female 31.0 mm. Bothwere taken in the September sample and were inthe inactive phase. The largest immature clamwas collected in late May and was 45.2 mm inlength.

1972 Reproductive CycleThe 1972 reproductive cycle (Figure 4) was

similar to 1971 with the exception that the cycle asa whole began earlier and the active phase was oflonger duration. Active female clams first ap-peared in November 1971 and active males inDecember 1971. The active phase lasted until May1972. During the period from February until Aprilthe majority of all clams sampled of both sexeswere in the active phase.

Ripe male clams first appeared in April and ripefemales in May. Ripe clams of both sexes, 78% ofthe males and 49';( of the females, were mostabundant in May.

Spawning commenced in late May and peaked

654

in June for both males (86%) and females (65%)and then continued at a diminished rate untilSeptember.

Spent clams of both sexes were present fromJune through October. They were most abundantin September when 45°,{, of the males and 7'2P1" ofthe females were in the spent phase.

Inactive male clams first appeared in July(lO'ki J, while inactive females first appeared inSeptember (13%). The highest percentage of inac-tive clams occurred in October with 790k of themales and 7O'k; of the females in the inactivephase.

As in 1971, the spawning cycle for most of theclams under 60 mm in length commenced about 1mo later than the normal cycle and ended 1 rnaearlier. The smallest mature clam collected was a36.3-mm spawning male obtained in July. Thesmallest mature female was 38.9 mm in lengthand was in the active phase in March. The largestimmature clam was 51.5 mm in length and wascollected in March.

In 1971 spawning was quite complete in both

PORTER: REPRODUCTIVE CYCLE OF MYA ARENARIA

sexes, while in 1972 many of the females failed tospawn completely although the discharge of malesex products seemed complete.

DISCUSSIONThe gonadal inclusions of male and female

soft-shell clams are distinctive. Coe and Turner(1938) state that the origin of these inclusions ispartly from cytoplasmic activity of follicle cellsand partly from cytolysis of gametes. The fact thatall immature clams in the process of sexual dif-ferentiation were found to be developing inclu-sions characteristic of their sex seems to verifycytoplasmic activity of the follicle cells as oneorigin of these inclusions. In older male clams, themethod of formation of the multinucleated cellsfirst described by Coe and Turner (1938) as pycno-tic nucleii and later by Shaw (1965) as sperm balls,needs further study. If the unspawned sperm areretained by the male clam as sperm balls as re-ported by Shaw (1965) and as my observationsindicate (Figure 1c), then perhaps cytoplasmic ac-tivity of the follicle cells is the major method bywhich the inclusions are formed. In female clamsthe exact relationship between cytolysis of un-spawned ova and the formation of inclusions is notknown. The single row of follicle cells which formalmost immediately at the basal membrane of thealveoli in spent female clams already contain anumber of inclusions (Figure 3d) before any of theunspent ova have undergone cytolysis. The originand function of gonadal inclusions in both sexesrequires further investigation.

The gametogenic cycle of the soft-shell clam atSkagit Bay is identical to that reported for clamsfrom the east coast (Coe and Turner, 1938; Shaw,1962, 1965; Pfitzenmeyer, 1965; Ropes and Stick-ney, 1965). The single spawning cycle per year,from late May to early September, is similar tothat described for studies in eastern Canada (Staf-ford, 1912; Battle, 1932; Sullivan, 1948) and theNew England area (Welch, 1953

3; Ropes and

Stickney, 1965).The slight variations noted between the spawn-

ing cycles of 1971 and 1972 and the incompletespawning of females in 1972 cannot be explainedat present.

Few hermaphroditic Mya are reported from

'Welch. W. R. 1953. Seasonal abundance of bivalve larvae inRobinhood Cove. Maine. Fourth Annual Conference on ClamResearch. U.S. Fish and Wildlife Service. Clam Investigations.Boothbay Harbor. Maine. p. 4-6.

other areas (Coe and Turner, 1938; Shaw, 1965). Asingle hermaphroditic specimen was collected atSkagit Bay (Figure 3D.

ACKNOWLEDGMENTS

I wish to extend my thanks to the WashingtonCooperative Fishery Unit for providing transpor-tation and equipment for the study. Also, toPreston E. Porter for his dutiful help with fieldcollections and to the Department of Ocean-ography, University of Washington for provid-ing environmental data.

LITERATURE CITED

BAITLE. H. I.1932. Rhythmic sexual maturity and spawning of certain

bivalve mollusks. Contrib. Can. BioI. Fish., New Ser.7:255-276.

BELDING, D. L.1930. The soft-shelled clam fishery ofMassachusetts. Mass.

Dep. Conserv., Div. Fish Game, Mar. Fish. Servo 1,65 p.

COE, W. R., AND H. J. TURNER.1938. Development of the gonads and gametes in the soft-

shell clam (Mya arenarial. J. Morphol. 62:91-111.CRAIG, E. L.

1927. Some mollusks and other invertebrates from thenorthwest. Univ. Colo. Stud. 16(1):63-74.

DEEVEY, C. B.1948. The zooplankton ofTisburry Great Pond. Bull. Bing-

ham Oceanogr. Collect. Yale Univ. 12(1):1-44.EDMONDSON, C. H.

1920. Edible mollusca of the Oregon coast. Occas. Pap.Bernice Pauahi Bishop Mus. 7(9):77-201.

FITCH, J. E.1953. Common marine bivalves of California. Calif. Dep.

Fish Game, Fish Bull. 90, 102 p.GALIGHER, A. E., AND E. N. KOZLOFF.

1971. Essentials of practical microtechnique. 2d ed. Leaand Febiger, Phila., 531 p.

HANKS, R. W.1963. The soft-shell clam. U.S. Fish Wildl. Serv., Circ. 162,

16 p.IVANOV, A. I.

1969. Immigration of Mya arenaria L. to the Black Sea, itsdistribution and quantity. [In Russ., Engl. summ.]Okeanologiya 9:341-347.

LAMMENS, J. J.1967. Growth and reproduction in a tidal flat population of

Macoma balthiea (L.) Neth. J. Sea Res. 3:315-382.LANDERS, W. S.

1954. Seasonal abundance of clam larvae in Rhode Islandwaters, 1950-1952. U.S. Fish Wildl. Serv., Spec. Sci. Rep.Fish. 117, 29 p.

MORRts, P. A.1966. A field guide to shells of the Pacific coast and Hawaii

including shells of the Gulf of California. 2ded. Houghton Mimin Co., Boston, 297 p.

NELSON, T. C., AND E. B. PERKINS.1931. Annual report of the department of biology July 1,

655

1929 - June 30, 1930. N.J. Agric. Exp. Stn. Bull.522:1-47.

PFlTlENMEYER, H. T.1962. Periods of spawning and setting of the soft-shelled

clam, Mya arenaria, at Solomons, Maryland. ChesapeakeSci. 3:114-120.

1965. Annual cycle of gametongenesis of the soft-shellclam, Mya arenaria, at Solomons, Maryland. ChesapeakeSci. 6:52-59.

ROPES, J. W., AND A. P. STICKNEY.1965. Reproductive cycle of Mya arenaria in New

England. BioI. Bull. (Woods Hole) 128:315-327.SHAW, W. N.

1962. Seasonal gonadal changes in female soft-shell clams,Mya arenaria, in the Tred Avon River, Maryland. Proc.Nat!. Shellfish. Assoc. 53:121-132.

1965. Seasonal gonadal cycle of the male soft-shell clam,Mya arenaria, in Maryland. V.S. Fish. Wild!. Serv., Spec.Sci. Rep. Fish. 508, 5 p.

656

FISHERY BULLETIN: VOL. 72. NO. 3

SHAW, B. L., AND H. I. BATTLE.1957. The gross and microscopic anatomy of the digestive

tract of the oyster Crassostrea virginica (Gmelinl. Can. J.Zoo!' 25:325-347.

STAFFORD, J.1912. On the recognition of bivalve larvae in plankton

collections. Contrib. Can. BioI. Fish. 1906-1910:221-242.SULLIVAN, C. M.

1948. Bivalve larvae of Malpeque Bay, P.E.!. Fish. Res.Board Can., Bull. 77, 36 p.

V.S. DEPARTMENT OF THE INTERIOR.1971. Water resources data for Washington, 1970. Part 1.

Surface water records. V.S. Dep. Inter., Geol. Surv.,448p.

ZAMSRISORSHCH, F. S., A. S. MARCHENKO, AND O. N. TELEGIN.1968. New findings and distribution of Mya arenaria L. in

the northwest part of the Black Sea. [In Russ.] Gidrobiol.Zh.4(6):48-51. Eng!. abstr. in BioI. Abstr. 50:8638, Abstr.89709.