300 TRANSACTIONS

AGE DETERMINATION IN MOLLUSCS*

B y HAROLD H. HASKIN**

Age determination in molluscs has been largely concerned with spec ie s of economic importance, including such bivalves a s clams, scallops, oysters, and fresh-water mussels, and such predatory gastropods as the boring snails. Such s tudies have gener- ally been designed to provide growth and mortality da ta necessary for the manage- ment or control of the populations.

T h e methods of age determination em- ployed to da te may be conveniently grouped into three categories as follows: (1) size- frequency study; (2) the interpretation of growth interruption l i nes on s h e l l s or other par t s of the animal; and (3) experimental methods involving the re lease and recovery of marked individuals. Age determinations of molluscs based on (1) or (2) alone a re frequently doubtful, and check determina- t ions using at least one of the other methods a re desirable.

T h e s e methods may best be explained by illustrating their use. A very recent u se of the size-frequency method is that of Posgay (1954) described in h i s s tud ie s of t h e growth of the sea scallop, Pecten grandis, in Cape Cod Bay. Collections from the scallop beds f i shed by commercial scallopers yielded animals ranging in length from approximate- l y 5 mm. to 170 mm. T h e s e specimens were grouped in five mm.-s ize classes and plot- t ed in a size-frequency diagram. F i v e modes were evident at 20, 50, 75, 100, and 130 mm. Assuming the addition of a new generation to the population each year, the collection then apparently contains five year-classes, and the difference between success ive modes indicates the annual increment in length.

*This paper, illustrated with lantern s l ides , was the aecond of four papers in a symposium on Age Determination in Animals presented a t the meeting of the Section on Msrch 8. 1954.

**Department of Zoology and N. J. Oyster Research Laboratory, Rutgers University. New Brunswick, N. J.

T h i s size-frequency method h a s been much used in fishery investigations. I t h a s the principal advantage of enabling the in- vestigator to estimate the age composition of the population directly from h i s catch without more complex studies. There are, however, several difficulties and weak- n e s s e s in th i s method: (1) the complete absence of a year-class, o r i t s poor repre- sentation in the population, may lead to error in the estimated ages; (2) in the ab- s e n c e of other information the age of the smallest year c l a s s is unknown. For ex- ample, as regards the sea scallop, is the 20-mm. year-class zero, one, or two years old?; (3) the older year-classes show a greater spread of sizes, overlapping of classes and, finally, a complete obscuring of the modes. The weaknesses of the size- frequency method may be partially over- come by its repeated application t o the population at intervals so spaced that changes in the modes can b e followed. Posgay in h is sea-scallop study also em- ployed the third method, the re lease and re- covery of tagged individuals. T h i s pro- cedure was followed in the case of indi- viduals measuring 80 mm. and over. T h e increases in length during the known periods of time between re lease and re- covery (approximately one year) were meas- ured directly. These measurements provided additional data on one-year length incre- ments, agreeing well with the deductions from the size-frequency determinations for the larger sizes of scallops.

Posgay s t a t e s that growth interruption l ines, method two, on sea-scallop she l l s a r e unreliable indicators of age, principally because they may be caused by such dis- turbances as dredging, or by warming to approximately 16O C. or cooling to 3 O C. T h i s occurrence of a summer growth check, in addition to the more usual winter check, follows from Posgay's interesting discovery that the sea scallop h a s an unusually low optimum temperature for growth at about 9' C.

THE NEW YORK ACADEMY OF SCIENCES 301

A classical example of the use of growth interruption l i nes in age determination of bivalves is the study by Weymouth (1923) of the Pismo clam, Tivela stultorum. Wey- mouth demonstrated that a growth check occurred regularly every winter and was clearly evident on the shell. The validity of the growth checks a s indicators of age w a s carefully established by a direct study of t he Pismo clam population over a period of several years, and by the recovery of marked specimens. T h i s paper i l lus t ra tes well the precautions to be taken in applica- tion of the growth-interruption l ine method. Many observers have assumed that in various bivalve spec ie s one interruption l ine is added per year. It i s necessary t o establish th i s assumption by supplementary observations of the given spec ie s in t h e particular situation before the interruption l i nes can b e equated to annual rings. For example, Turner (1954) h a s shown that in the moon shell , the predatory gastropod Polynices duplicata, shell-growth checks a re completely unreliable as indicators of age. Growth in t h i s animal is correlated with available food supply a s shown both by field observations and laboratory experiments.

Growth indicators other than checks on the surface of bivalve she l l s may appear i n molluscan structures. Nelson (1942) h a s used the l ines on the ligament s c a r s of Crassostrea virginica she l l s as indicators of age. According to Nelson the oys te rs in Delaware Bay shift their ligaments posteri- orly once each winter, thus leaving one l ine per year on the ligament scar. More recent- ly, Kubo (1953) h a s determined age in the marine gastropod, Babylonia japonica, t he Japanese topshell, by use of the operculum. T h e operculum is enlarged by marginal growth centered on i t s “nucleus,” and the cessation of growth during each winter re- s u l t s in an annual check. According t o Hubendick (1948) opercular rings in the gastropod Viviparus viviparus also corre-

spond to periods of hibernation. Crozier (1918) established the reliability of trans- verse growth l ines on the p l a t e s of t he amphineuran, Chiton fuberculatus, a s an index to age.

The application of the third of t h e s e methods, the re lease and recovery of mark- ed indi,-iduals, to the problem of age de- termination in Venus mercenaria, the quahog clam or hard clam of our e a s t coast will be discus,?d in some detail, In 1947 a program to explore the feasibility of hard-clam cul- ture Nas begun a t Rutgers. A primary prob- l e m lyas the determination of growth ra tes in the a reas potentially suitable for such culture. Preliminary study of the native populations indicated: (1) that size-fre- quency relations were an unreliable index to age because of sporadic “ se t s” of juveniles and the depletion of various size classes owing to heavy fishing pressure; and (2) that growth-checks on the s h e l l s could not be interpreted with confidence, at

A

PERMACETI COVE

J A R V I S S O U N D

FIGURE 1

302 TRANSACTIONS

! b f

5 100

50

CAPE 5 H O R E

R E L A T I V E G R O W T H

1950

5 200

quahog dimensions may b e used.

w = 0.671 I. where w = weight i n grams

3

(or milligrams)

(or milimeters) 1. -. length i n cent imeters

CALCULATED GROWTH CURVES

1950 I I \\< I I I I 1 0 -

200-

50 100 I 5 0

i'so:io speed. Clams c a n b e drained and weighed on a pan ba lance i n t h e f ie ld , handl ing groups of 10 or 20 as quickly a s two or th ree clams c a n b e measured. For compari- s o n of d a t a with r e s u l t s of other workers t h e following empirical re la t ion between

la t ions. Belding (19 12) successfu l ly em- ployed t h e growth interruption l ine method i n h i s ex tens ive s t u d i e s of the hard clam i n Massachuse t t s waters. T h e growth-rate problem i n our waters w a s then a t tacked pr incipal ly by t h e frequent s tudy of marked, seques te red populat ions i n t h e var ious areas .

Experimental plant ings were made i n s i x a r e a s along the New J e r s e y C o a s t from Rar i tan Bay t o Delaware B a y (FIGURE 1). At e a c h locat ion t h e in i t ia l plant ing w a s from 1 to 2 thousand marked hard clams sorted in to arbitrary size classes. All plant ings covered t h e range of sizes readily avail- a b l e through commercial sources, i.e. from 4 to 10 cm. in length. Smaller sizes were p lan ted as they became ava i lab le from s p e c i a l sources . T h e most commonly used measure of size i n bivalve mol luscs is the length, measured along the antero-posterior ax is . After the f i r s t year of our work, i n which the u s e of weight and volume as well a s t h e l inear measurements w a s s tudied, i t w a s decided to u s e weight as a measure of size. T h e principal advantage of t h i s i s

1 2 3 4 5 6 7 8 YEARS AFTER P L A N T I N G

F"1GURE 3

F o r each arbitrary size of clam i n e a c h planting, annual average increments i n weight were obtained. From t h e s e incre- ments t h e percentage i n c r e a s e s i n weight were calculated. T h e la t te r plotted aga ins t t h e in i t ia l weight gave a re la t ive growth curve, FIGURE 2. T h i s t y p e of curve is par- t icular ly valuable, for from i t may then b e

I CAPE SHORE CALCULATED GROWTH CURVES

200 -

150- v)

0100-

YEARS A F T E R P L A N T I N G

FIGURE 4

derived a cumulative growth curve for the

(upper curve, FIGURE 3) .FIGURES 2 and 3 are

the growth curves expected if all the years

ra tes in the various a reas and with the

season for the hard clam. FIGURE 3 shows

were l ike 1950. T h i s method, then, obviously does not provide reliable long-term growth curves for the a reas indicated, but the method is useful in comparing 1950 growth

entire size range covered by the plantings $

based on data covering the 1950 growth f,.-

I 2 1 4 5 1 ) 7 8 P I I I I I I I I I - 700

R E L A T I V E G R O W T H

CAPE S ~ O R E ,950 -600

M A I N E - 500

400 ; - M4z;Ri,,,,

-

300 $

200 L :LL+ . 05 .I0 . IS .20 .25 .30 .35 I00

200-

mlm-

N 2 O -

tion, t he lower portion of the growth curves may be safely constructed.

T h e growth curves and the direct obser- 2 vations of the experimental plantings also

- 4 0 2 provide a b a s i s for t he interpretation of the z growth-interruption l ines on the shells. 5 FIGURE 7 shows s i x hard-clam s h e l l s on

&-o<<m> 2 0 5 which been accentuated the major growth by pencil. interruptions “A” have is a

native of Spermaceti Cove, “B” is a native of Edge Cove. Both of these a reas a re im-

CAPE SHORE

1947-8-9-50 COMPOSITE GROWTH CURVES

-I0 5 . / I I I I I I I I 1 2 3 4 5 6 7 8

304 TRANSACTIONS

interruption l i n e s a r e annual rings, t h e Spermaceti Cove clam at about s i x y e a r s i s much larger than t h e Edge Cove nat ive at a t l e a s t 11 years. T h e crowding of t h e r ings beyond t h e 10th makes an accura te count impossible . Without other ev idence o n e would be skept ical of the va lue of t h e s e rings. FIGURE 3, b a s e d on experimental plant ings, ind ica tes that t h e deduction of ages based on the interruption l i n e s of clams i n t h e s e two a r e a s i s reasonable. C l a m s “C” and “D” in FIGURE 7 a r e part of a population of Maine s e e d t ransplanted to Spermaceti Cove i n t h e spr ing of 1950. T h e y were harvested three years la ter and t h e three known s e a s o n s of growth check wel l with the expected growth based on the ca lcu la ted curve (FIGURE 3). An additional interruption ring w a s apparently formed dur- i n g 1952. Clams “E” and “F” i n FIGURE 7 a r e from a n experimental plant ing on the C a p e Shore of Delaware Bay from July 1947 t o September 1950. T h e s e clams clear ly show a n ex t ra interruption l i n e formed dur- i n g t h e summer of 1949. In all f ive summers s tud ied at t h i s locat ion, growth w a s s lower than i n e i ther spr ing or f a l l but the summer of 1949 w a s t h e only one i n which a clear- c u t interruption ring w a s formed. I t appears from s u c h observa t ions as t h e s e t h a t

B - D F

growth-interruption l i n e s on hard c lams i n New J e r s e y waters a r e of limited va lue in a g e determination.

Summary

T h e a g e of mol luscs h a s been determined by three principal methods. All of t h e s e methods have their s p e c i a l advantages and weaknesses . T h e size-frequency method and t h e u s e of growth-interruption l i n e s a r e par t icular ly useful in providing information rapidly on t h e a g e composition of t h e popu- la t ion. T h e rel iabi l i ty of t h e information so obtained must b e checked independently. T h e r e l e a s e and recovery of marked indi- v idua ls provides completely rel iable growth information within t h e s i z e ranges of t h e an imals recovered. From s u c h information, size-age re la t ionships of t h e population may b e determined. T h i s method i s com- parat ively time-consuming and i s recom- mended for a g e determinat ions only where good recoveries may b e expected or where c h e c k s on f indings by t h e other methods a r e desired.

References

1. BELDING, D. L. 1912. The quahaug fishery of Massachusetts, Marine Fisheries Series 2. Massachusetts Dept. of Conservation.

2. CROZIER. w. J. 1918. Growth and duration of life i n chiton tuberculatus. Proc. Natl. Acad. Sci. U. S. 4(11): 322-25.

3. HUBENDICK, B. 1948. Uber den Bau und des k o n zentrischen Operculartypus bei Gastropoden. Archiv. 2001. 40(A 10): 1-28.

4. Ku8o.I. 1953. Age determination of the Babylonica j W n k a ( R e e v e ) an edible marine gastropod, bas- ing on the operculum. J. Tokyo Univ. Fisheries.

5 . NELSON, T. c. 1942. The oysters. In the Boylston Street Fishweir. 2. chap. 3. Papers of the Robert S. Peabody Foundation for Archaeology.

6.POSGAY. J . A. 1954. Private communication from Woods Hole Oceanographic Institution, Woods Hole, Mass.

7 . TURNER, H. 1954. Private communication from Woods Hole Oceanographic Institution, Woods Hole, Mass.

8. WEYMOUTH, F. w. 1923. The life history and growth of the Pisrno Clam, nvela shrltonnn, Mawe. Fish Bulletin No. 7. Calif. Fish & Game Commission.

3 4 199.

FIGURE 7