U1

Indian Journal of Marine ScletlcesVol. 5, June 1976, pp. 98·102

Ecology of Sandy Bea~h at Sancoale, Goa: Part II- Population Model &Production of Emerita holthuisi Sankolli

C. T. ACHUTHANKUTTY & M. V. M. WAFAR

National Institute of Oceanography, Dona Paula 403004

Received 10 December 1975; revised received 19 March 1976

Beach population of E. holthuisi was constituted by 0, I and 2 yr classes. Individuals were

reoruited abundantly in June and July. Berried females were of the size range 10 to 15 mmcarapace length. Spawning extended from December to June with a peak in March. Annualinstantaneous mortality coefficient was high and only a small fraction of the 1yr class enteredthe 2 yr. Production was high during the premonsoon months suggesting that stable physicalconditions are conducive for production.

\

MOLE crab, Emerita holthuisi, recorded fromt~~ sandy beach of Mirya, Ratnagiri1,forms a major component of the macrofauna

inhabiting the sandy beaches of the west coastof India and contributes significantly to the beachproduction. Apart from its larval development2and some notes on its size-frequency distribution,length.weight relationship and larval recruitmentfrom 2 sandy beaches in south-west India3, no otherstudies hf-ve been made either on the biology orproduction of this species. Hence, the presentwork has, been undertaken to study these aspectsin this species from a sandy beach in Goa whereit occurs in abundance.

Materials and Methods

The topography, physical conditions and otherrelated aspects of the area of study are described4.Specimens of E. holth1tisi were collected froma single transect at 5 m intervals from a fixedreference Ipoint above the high tide mark to thesurf wash~d zone using a quadrat of 0·1 sq. m areaand a 0.5 mm mesh seive. Collections were madeat fortnightly intervals over a period of 12 months(October 1973 to September 1974) covering all3 season~, viz. post monsoon (Oct.-Jan.) - periedwhen the physical conditions of the beach becomegradually stabilized after annual monsoon, pre­monsoon (Feb.-May) - period when the physicalconditions remain largely unchanged and monsoon(June-Sept.) - when rainfall severely alters the

physical qonditions of the beach. Carapace length(mm), wet weight (mg) and the sex of all specimenscollected :were recorded. Age and growth studieswere made by the length-frequency method andprobability plot technique. Fecundity estimateswere made by counting all the eggs from 9 maturefemales.

Results

Age and growth - Length frequency distributionof monthly samples collected (Fig. 1) indicates thepresence of more than 1 mode except in Oct. '73,Nov. '73 land Sept. '74. The validity of length

98'

frequency distribution in dettlminil1g He age andgrowth of tropical fishes has been shown by Qasim5.According to him, either slow growth or prolongedbreeding or a combination of both will render thetracing of the progress of modes difficult. Thefact that the modes in different months (Fig. I}do not follow a regular pattern indicates H.e influenceof either or both cf these factors. Therefore, various.modes obtained for different months are arranged(Fig. 2) in order to trace the progress of modes.Moulting or ecdysis, a characteristic phenomenonby which growth in crustaceans is effected is alsotaken into consideration while tracing the move­ment of the modes. Immediately after the moult­ing, there is a rapid increase in the body proportions'resulting in a large size which is followed by aninter-moult period dUling which the hard exoskeletonprevents growth until the next moulting. The3 mm mode of Nov.' 73 is traceable to 5 mm modein Dec. '73, an increase in length after moulting.This 5 mm mode remains the same until Feb. '74.indicating the inter-moult period, when the nextmoult leads to an increas.e in length to 7 mm inApril '74. This is followed by a 'no growth' periodtill June '74, and the next moult increases thelength to 9 mm in July '74. Likewise, the 11 mmmode of Dee. '73 is traceable to 15 mm in May '74through 2 growth periods, 11-13 rum and 13-15mm with an inter-moult period from January to­April '74. The 3 mm mode persisting in May.June and July '74 reflects an inter-moult periodwhich would have led to an increase in size, thoughthe subsequent mode is not represented in the presentcollection.

Breeding in E. hoZthuisi extends from Decemberto June with a peak in March (see maturity andspawning). On average, it takes 27 days to completethe larval development in E. hoZthuisi, the 6thzoea moulting into a megalopa of approximately1·7 mm carapace length2• Considering the plank­tonic phase (27 days) and the time that would havebeen taken by the megalopa (1·7 mm) to grow to3 mm carapace length, the 3 mm mode of May '74is probably the result of spawning in the preceding

ACHUTHANKUTTY & WAFAR: ECOLOGY OF SANDY BEACH AT SANCOALE: PART II

MONTHS

MODES IN mm

lsJ1

2)45 89

OCT.7)

...

NOV.

A

".I-_a

.;

...

~", ,,

'It*'•••

I I. II

I8REED!~:G PC:RiOO j

PEAK IN 8RE~~iNG:

O""-N"'-N""OOOOOOOOO"'CD~-.O' CJl. C?ooo - NC"'I •••• IHett- CD ell 0 ~ca . art

'" '" CUMULATIVE FREQUENCY % g: g: g:

Fig. 3 - Probability. plot of the. length frequencydistti-. bution of E. holthuisi ..

1J.

E.§1i':I:...'"~9..•

Fig. 2 - Monthwise position of growth modes in E. holthuisi

months (1974 spawning season). Likewise, the3 mm mode of Nov. '73 comes from the breedingoccurring in the preceding months of 1973 spawningseason. This 3 mm mode of Nov. '73 progressesto 9 mm mode in July '74 which representsan average growth rate of 0,75 mm/month. Takingthis average growth rate into consideration, the11 mm mode of Dec. '73 can be traced back to thebreeding in 1972. Thus, it is evident that thepresent collection is constituted by 3 yr classes,namely 0, 1 and 2, products respectively from thebreeding seasons of 1974, 1973 and 1972. Thisindicates that though the breeding is prolonged itresults in only 1 distinct year class. Cassie's6graphical method of separating age groups in apolymodallength frequency distribution also showedthe presence of 0 and 1 yr classes (Fig. 3).

With the growth rate of 0·75 mm/month, ascalculated from the progress of modes in 1 yr class,it is evident that the 3 mm sized individuals wouldhave taken 4 months to grow to that size. Furtherincrease in lengths at monthly intervals is obtainedfrom Fig. 2 and summarized in Table 1. Thesevalues are utilized in estimating Bertalanffy para­meters (Lrt., K, to) and in fitting the growth equation.

NOV.

n=43

FEB.n=29

APR.n=19

MAR,n=34

DEC.n=41

MAYn=68

OCT·n=83

JAN.

n =42

SEP.1i=3

JUNE""126

JULYn=59

.4 6 810 12 14 16CARAPACE LENGTH

( mm)

20

40

60

10

20

80

10

10

(I)...I 20«:EZ«&&. 10oa::tIJ 20m:E::)z

Fig. 1 - Length frequency distribution of E. holthuisi inSancoale beach from October 1973 to September 1974

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INDIAN J. MAR. Se1., VOL. 5, JUNE 1976

TABLE 1- GROWTHIN E. holthuisi SANKOLLI TABLE 2 - CORRELATIONCOEFFICIENT(r) ANDDEVIATION(tfFROM ISOMETRICGROWTHIN THE LENGTH-WEIGHTAge Length

Berta··Age . LengthBerta-RELATIONSHIPOF E. holthuisi(months) frequency

lanffy(months) frequencylanffy(mm)

equation (mm)equationSeason Sexr(mm) (mm)

PremonsoonMales0'92881-50951 I-- --1178'91 Females0,989721·8863*

2--1299,72Monsoon Males0'97236'1806*

3-1-9613-10''34 Females0'978113'1392*

433'1 141111·08

553,83151311·64Postmonsoon Males0'94933'6867*

654-8316-12'19 Females0'96903,6312*

755-84171312'84

8-6·61181313'33Prcmonsooil andFemales

977'53191513-69monsoon <10 mm0'979515·3671 oF

1078·23 length

>10 mm0,87843·0393*

length

IThe von Bertalanffy growth model and Walford­

plot techhique are being widely used to study thegrowth of various organisms. Bertalanffy's growth

equation, Ibased on the concept that growth is thenet resuIt of anabolism and katabolism, producesa growth I curve in length that explains well thegrowth rates of many species7• This equation isexpressedl as

1, = LIX[I-e-K('-t,l]

where L ex: is the theoretical average maximumlength, K is the growth constant indicating thp.rate at which length approaches LIX, It is the lengthat time t 'and to is the calculated time at which 1 isequal to zero.

In Ford and Walford method, for lengths obtained

at equal Jime intervals, the length at a given age(1,) is plottted against the length at the next inteIvalof time (l~+I) and a straight line is fitted by meansof least squares. The relationship between It+ 1and 1t m~y be expressed as,

1,+1=Dx(l-e-K) +l,e-K

The length at monthly intervals (Table 1)were usedlin fitting the regression. The point wherethis straight line intersects the line of equality gavethe value pf Lcc K was calculated from the valueof the slope (0'957). Thus,

LIX=25·5 mm and K=O'044

to was calculated by the follwing equation:to-t=lfK[log e LIX-log e(LIX-1,)]-t

and was found to be 1·119Substituting the values of K, LIX and to in the

Bertalanffy growth equation, growth in E. holthuisimay be expressed as

I, =25·5[I_e-0,0440(t-1·1190)]

Length-weight relationship - All the 507 specimenscollected were utilized to study the length--weightrelationship of this species. All the specimens weresexed and further subdivided into groups representingthe 3 monsoon periods. The exponential equationW=a1b was used to study this relation where W isweight, 1 1s length and a and bare constants. Thec{)efficient of correlation a in all cases was above

0,9 indica~ing a high degree _of relationship betweenlength and weight (Table 2). Weights of females

*Significant at 0·01 level (t1% = 2'58).

increased at a faster rate than those of males duringpremonsoon and monsoon months.

The exponents in all the regressions were testedagainst the exponent in isometric growth by t test,applying the formula of Snedecors.

b-~t=S;;

The t values are included in Table 2. In all thecases, except in the males of premonsoon, b wassignificantly different from 3 suggesting a deviationfrom isometric growth and that the weights of thecrabs increase at a higher rate than the cube oflengths. The deviation was more significant inthe females collected during the spawning period(premonsoon and monsoon months). To testwhether such a high deviation is brought about byspawning, 2 length-weight regression equations'were derived separately for the females below andabove 10 mm carapace length (10 mm is the mini­mum length at maturity in females). The b values'in both cases were tested against ~ and the results(Table 2) are quite surprising in that the deviationfrom isometric growth is more pronounced inimmature females «10 mm) than the ovigerousones (>10 mm). Though we are not able to explainthe reasons for this, the results themselves indicatethat there are remarkable variations in the length­weight relationship of males and females and alsofrom season to season.

Maturity and spawning - Microscopic examina­tion of gonads of all specimens collected showedthat females of E. holthuisi attain maturity at lo.mm carapace length. Egg bearing females werefound in the range of 10-15 mm carapace length.To arrive at the spawning periodicity and time,the percentage occurrence of berried females wereplotted against respective months (Fig. 4). It isapparent from the figure that there is a single majorspawning season, extending from December toJune with a peak in Matth and a supplementarypeak in October.

Fecundity - Fecundity estimations were made byenumerating all the eggs from 9 mature specimensrepresenting different size groups. Fecundity

V/

100

1'1 •.,

Fig. 5 - Production of E. holthuisi in the Sancoale beach.

100

ONDJFMAMJJ

40

:]:::;~i\; i\ i:"EalGOz

monsoon season when the physical conditions werestable. Due to the presence of berried female&in the collection, wet weight was fairly high fromDecember '73 to May '74 (2·5 to 14·3 gfm2) althoughthe abundance was low. The wet weight level&in Fig. 5 are in good agreement with tho~e of Fig. 4during this period, indicating tl:at the higher biomass.was mainly contributed by the· berried females.Although the numerical abundance was more inJune and July '74 (266 and 112fm2 respectively), thewet weights were low (8·35 and 4·59 gfm2 respectively}due to the presence of newly recruited individuals.Discussion

Information on the biology of E. holthuisi2.3'is very much limited. Some interesting studies.have, however, been made on other species of thisgenus9-l4. Ansell et al.3 have suggested the possi­bility of continuous breeding within the populationof E. holtuisi along the Kerala coast, but that recruit­ment to irdividual beaches is concentrated at scmeparticular period as observed in ShertalJai andCochin whele although there is an year-roundsettlement, two main periods of recruitment couldbe seen; in the former, in the premonsoon (Feb.­Mar.) and monsoon periods and in the latter, duringpremonsoon period only. However, inteme re­cruitment of E. holthuisi to the population of San­coale beach is in June and July, although, as in.

Fig. 4 - Monthwise percentage occurrence of berried femalesof E. holthuisi

ACHUTHANKUTTY & WAFAR: ECOLOGY OF SANDY ~EACH ArSANCOALE: PART II

ranges from 102 eggs in a specimen of 12·7 mm cara­pace length to 2309 in. another with a length of13·9 mm .. Individuals above 13'9 mm were found tocarry less number of eggs (14,2 mm specimen having1194 eggs and 15·3 rum with 557 eggs) indicatingthat they are spawning ones.

Regressions fitted for fecundity with length andweight show that the fecundity bears much betterrelation to weight than length. The equations are,for length, log F=-1'9857+4'2923 log L (r=0·3684)and for weight, log F=-0·7541+1·2918 log W(r=0'4927) .

The regression fitted for the ovary weight onlength is

log Ow =1'2492+3·3357 log L (r=0·5320)

The fecundity increases at almost an equal rate(b=4'29) with the weight of the spawning femalesto length (b=4'5977 and 4,1234). The ovary weightincreases at a rate lower (b=3·3357) than the weightof the spawning females to length.

The regression of fecundity on ovary weight wasfound to be

log F=1·5131+0·9155 log Ow (r=0·9643)

The exponential value (0·9155) indicates thatthe fecundity increases at a rate less than the bodyweight and ovary weight to length.

Recruitment - Individuals are recruited to thebeach population when they attain an averagecarapace length of 3 mm (see, age and growth andFig. 2). As seen from Fig. 4, the major spawningseason extends from December to June with a peakin March and supplementary minor peak in October.As a result of this, individuals are being recruitedalmost throughout the year. The main period ofrecruitment is June and July (Fig. 1).

Mortality - Among the several methods avail­able for the estimation of the annual instantaneousmortality coefficient (Z), the simplest is by compar­ing the abundance at successive ages5 when Z isobtained by the following formula:

eaz =S=N2fNl

where N 1 and N 2 are the number of individualsin the year t and t+ 1 respectively and 5 is the annualsurvival rate. Based on this, Z in E. holthuisiwas calculated by comparing the abundance of1 yr old specimens with that of 2 yr old ones andwas found to be 1·85. As there is no fishing forE. holthuisi in Sancoale beach, the total mortalityrepresents only natural mortality due to death bynatural causes including predation.

The low annual survival rate (0·1564) indicatesthat most of the individuals die at the end of 1 yritself and only a small fraction of the populationpasses on to the spawning ages as the females werefound to attain maturity at 10 mm carapace length(just after 1 yr age).

Production - Achuthankutty4 has descrihd indetail the standing crop of E. holthuisi at differentstations during each observation and the importanceof this species in the macrofauna production of theSancoale beach. The numerical abundance andrespective wet weights for each month are givenin Fig. 5. The biomass was high during the pre-

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INDIAN J. MAR. SCL, VOL. 5, JUNE 1976

Rerala coast, individuals are being recruited insmall numbers throughout the year (Fig. 1).

Like most of the other Clustaceans, the adultmales ofl E. holthuisi are smaller in size than adultfemales. According to Sankollil, adult males andfemales did not vary much in size, males rangingfrom 11·5 to 17 mm and berried females from 12·5to 18 mm. In Sancoale, adult males seldom attain­ed a caliapace length of 10 mm or above whereasall the ovigerous females were between 10 and 15mm, the largest being 15·3 mm. This clearlyindicates' that there is considerable size differencein the adult male and female of E. holthuisi andthat the individuals of San coale beach attain matu­rity at a smaller size. Having been recruited tothe beadh population at 3 mm carapace length,the individuals take about 9 months to grow tothe adult size. This almost agrees with the observa­tion of Ansell at al.3 in south-west Indian beaches,where they grow to the adult size of 12 mm in 8 or9 months which again confirms the fact that the<:rabs of Sancoale beach attain maturity at a smallersize.

Ansell at al.3 have fOund that crabs settled duringmonsoon weighed less for any given carapace lengththan those of the premonsoon settlement. Thedata from Sancoale beach showed no weight reduc­tion in the specimens of the monsoon season butthe weights of the females increased at a faster ratethan those of males of premonso<.;n and monsoonmonths.

Spawning periodicity or fecundity studies havenot been, made for E. holthuisi of the scuth-westIndian Beaches. However, spawning in E. analoga14begins in late spring and early summer and mostof the females are found to be ovigerous as thesummer progresses. Similarly, spawning in E.holthuisi begins by December and reaches the peakin March when the temperature is high and otherphysical conditions are stable. Sankolli2 has alsoobserved berried females from late September toearly June and is of the opinion that this speciesmay b,e l)feeding throughout the year except forthe rainYlseason. This is nearly in conformity withthe present observation.

Gradual disappearance of males of E. analogaduring th~ summer months has been attributed byEffordll to their death after mating in their 2nd yr.But, Bames and Wenner14 have suggested thattheir disappearance may also be consequence ofsex-reversal ,(protandric hermaphroditism). Sex­reversal rhay hold good in E. holtht.{,isialso sinceadult males seldom attained a carapace length of10 mm whereas all the mature females were in therange of 10-15 mm.' Further evidence is essentialto confirm this.

102

Reberrying in E. analoga has been reported byBoolootian et al.9 Barnes and Wenner14 are of theopinion that if reberrying occurs in the field, somefemales may even rebeny for 4 times in a season.Reberrying does not exist in E. holthuisi of Sancoalebeach because individuals do not complete their2nd yr of life as the maximum size of the ovigerousfemales collected was only 15·3 mm. Althoughthe theoretical maximum carapace length attain­able by this species is 25·5 mm it never grows tothat length in this beach and instead dies at an earlyage.

The causes and details about mortality occurringat different stages of the life history of E. holthuisifrom other west .Indian beaches are lacking. Thelow annual survival rate indicates that most of theindividuals die at the end of 1st yr of life due tonatural causes, especially the unfavourable physicalconditions of the monsoon season.

At present, there is no fishing for Eo holthuisiin Sancoale beach. It is suggested that if this speciesis to be fished for human consumption or as bait,which is a regular phenomenon in near by beaches,the suitable period is the late' postmonsoon and th epremonsoon seasons when most of the individualsattain the adult size. But since this is also thespawning period, over exploitation may adverselyaffect the subsequent population.

AcknowledgementThe authors are thankful to Dr S. Z. Qasim,

Director, for his valuable comments and criticism.They express their thanks to Shri M. Devaraj,CMFRI, Mandapam Camp, for his help in theanalyses of data and to Dr (Miss) Ann Stirling forher suggestions.

References1. SANKOLLI, K. N., Crustaceana, 8 (1965), 48.2. SANKOLLI, K. No, Proceedings, Symposium on Crustacea

(Marine Biological Association of India), 1965, Part II,1967, 744.

3. ANSELL, A. Do, SIVADAS, P., NARJ\.YANAN, B. & TREVEL-LION, A., Mar. Biol., 17 (1972), 311.

4. ACHUTHANKUTTY, C. T., Indian J. mar. Sci., 5 (1976)5. QASIM, S. Z., Indian J. Fish., 20 (1973), 351.6. CASSIE, R. M., Aust. J. mar. Freshw. Res., 5 (1954), 515.7. BEVERTON, R. J. H. & HOLT, S. J., Fish. Invest. London,

Sero 11, 19 (1957), 553.8. SNEDECOR, G. W., Statistical methods aPPlied to experi­

ments in agriculture and biology (Allied Pacific PrivateLtd, Bombay), 1961.

9. BOOLOOTIAN, R. A., GIESE, A. C., FARMAN-FARMIAN, A.& TUCKER, }., Physiol. Zoot., 32 (1959), 213.

10. EFFoRD, I. E., J. animo Ecol., 34 (1965), 63.11. EFFoRD, I. E., Crustacean a, 13 (1967), 81.12. EFFoRD, I. E., Crustaceana, 16 (1969), 15.13. EFFoRD, I. E., Crustaceana, 18 (1970), 293.14. BARNES, N. B. & WENNER, A. M., Limnol. Oceanogr••

13 (1968), 465.

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