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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
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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
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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
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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.
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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
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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
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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.
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