•

Ecological studies on the Fauna Associated with Economic Seaweeds of South India.3. Food preferences of selected Algivorous Gastropods

Introduction

M. Mohan Joseph" CSMCRl Marine Algal Researcb Station, Maodapam·623 518,

India.

The importance of algivorous animals in the ecology of economic seaweeds has been recognised by many workers (Tilden, 1927; Leighton, 1960; Chapman, 1962; North, 1962, 1963; Boney. 1966). In recent years many studies have been made to ascertain the food and feeding habits of major algivores, the important among which are by Barkman ( 1955), van Dongen (1956), Bakker (1959), Satio and Nakamura (1961). Leighton and Boolootian (1963), Leighton (1966), Paine (1963) and Paine and Vadas (1969). In order to study the role of algivores in the ecology of cultivatable marine algae and to ascertain the nature of discri_ mination in the choice of algal food by them, a study was undertaken during 1968- '71 at tbe Marine Algal Researcb Station, Mandapam. This paper presents tbe findings pertaining to the cbief algivorous gastropods in tbe Gulf of Mannar and Palk Bay region.

Materials and Methods

The usual methods of the study of food and feeding babits by the analysis of gut contents could not be used because-(i) gut contents provided unreliable clues to food habits in algivorous animal., and (il) the identity of the food and estimation of the quantity consumed were dIfficult. Hence. a method modified after that of Leighton (1966) was followed. Individual. (usually ten in number) collected from a variety of habitats we!C conditioned in the aquarium for 3 to 4 days. They were starved for 48 hours prior to the . commencement of the experiments and oOored equal weights of ten species of algae. Amount. ingested were determined by measuring the loss in weight of algae after a feeding period of 24 hours. The individuals used in the experiments were adults of more or less equal size. Freshly collected algae were used as food and the components of tbe food were thoroughly mixed several times during each experiment. A control with algae alone was kept to determine possible deterioration of the algae during the 24 hr. period. All experimental ves",ls were constantly aerated. The following are the spccies of algae and gastropods employed.

·PrciCDt address : U ni1'eflity of Aaricul turaJ Science., Colleae of Filberiel, Maaialoreo'7S 001, 111&

Food Preferences of Selected Aigivorous Gastropods

AJARe

Gastropod.

Ulva lactuca Linn.

Enteromorpha compressa (Linn.) Grev. Padina gymnospora (Kuetz.) Vickers

Sargassum wightii Greville

Stoechospermum maculatum J. Ag.

Graci/arlo edulis (Gme!.) Silva Graci/aria corlicata J. Ag.

Graci/aria verrucosa (Hunds) Papenfuss

Hypnea musci/or",is (Wulf.) Lomour

Gelidiella acerosa (Forsk.) Feldman et Hamel

Pyrene versicolor Sowerby

Pyrene zebra Gray

Trochus radiatus Gmelin

Turbo intercostalis Menker Aplysia lineolata Adams and Reeve

39

At the end of the 24 hr. feeding period all remaining algal fragments were removed from each tank, sorted into species. blotted free of water and weigbts determined. Weight losses, corrected for changes in the controls were taken as seaweed consumed by grazers. Data were tested statistically for consistancy of ranks and for significant differences in means of consump­lion values.

Result.

The means and range of consumption values by Pyrene versicolor, P. zebra and Trocluu radiDlus are given in Fig. I. Similarly, values for Turbo in/ercostalis and Aplysla IIntDlata are presented in Fig. 2. To test Ibe agreement of results, a ranking melbod described by Kendall (1970) was employed. In each experiment. tbe algae were assigned ranks 1 to 10 in the order of decreasing quantities consumed. The various species of al ~ae 8TC ranked according to the sums of ranks allotted to the individuals in each set of experiments with a given species of gastropod (Table I to 5). Values ofW, tbe coefficient of concordance, (see Kendall, 1970) ranged from O. 858 to 0.935 indicating tbat the preferences of food were not at random In all tbe experiments, values of W were significant at 1% level. Tbe order of preference of each grazing species therefore, ~ca D be regarded as consistent and the mean values for consumption afford tbe best estimate of the relative degree of attractiveness of the grazing species to tbe al~ae .

5Mwud Res. Ulil. vol, 3, 1978

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G¥a.cih,fo. Ve, .. ULO$Q.

~ I • • • • • ;; .. • •

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Le~end

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Fig. 1 Means of algae consumption values of Pyrene versicolor, P. z.eb,a and Trochus radiaWs shown by histograms. Vertical IiDCI indicate the raDles. The numbers of individuals employed and numbera of experiments are given in that order hyphenated in brackels.

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Fj • . 2 Mea. otllpe CODlWDptfoD values or TlU'bo /fflercolta/l.J IDd AplYlia lineo/ala sbown by bittoarama Detail. u In fl .. I.

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42 M, Mohan JOtePh

Table I , Species of algae consumed by 'yr'M ffl',kolo, in the order of preference (top to bottom),

'PP, of algot

Graci/aria ,dill;"

Padina OmMIpora

Hypnta mwciformU

SaTgalSUln wlghtii

Gradlaria corticata Graci/aria 'VerrUCOJa

UI.'a lactuca

Stoechosperm"", maculahllll

Enteromorpha comprt •• a

Gelidiella acerosa

W - 0,935

P<O.oJ

Rank totals for 5 ,xperimenll

5,0

10,0

17,0

19,0

25,5

28,5

41.0

41,5

43,0

44,5

Table 2, Species of algae consumed by Py"nt zebra in the order of preference (top to bottom),

Spp, of alga.

Padino gymno.pora

Hypnea mwclformLr

Grac/will edlJi.r

G,acilari4 _OlD

Slotchospennum maculahllll

Sarga .. "", wightii

Graci/aria corticata

Ulva lactuca

Enteromorpha comprtssa

Gelidiella acerosa

Seawud Res. Cltil. vol. J. 1978

Rask totals for 5 experiments

6,0

9,0

15,0

24 ,5

27,0

28,5

36.5

37,0

41 5

50,0

Food Preferences of Selected Aigivorous Gastro.,od. Table 3. Species of algae CODSumed by TrocJ. rtIdIGlU8 in the order of preference (top to

bottom).

Spp. of algae

Padilla gyllllllJSpora

StoccJrosp.rmum I'IIDCIIlallmt

UI,a /Qctuu

Gtlidklla lICerOSlJ

Sargauum wt,1Ilii

Hwnea musd/_Is

Graciltlr/tJ .tbIIi. Enw~ ctIIfIFIUa

Gradlarla rnruct)la

Graci/aria corticata

W-O.8SS P<O.OI

Rank total. for J eJtptriIMllts

7.0 14.0

16.0

18.0

23.0

31.0

34.3

38.0

46.0 47,S

Table 4. Spocles of algae consumed by Turbo '-_Ialis in Ibe order of prefereDCI (top to bottom).

spp. of algae

Enteromorpha compressa and

Hypnea musciformls

Gradlarill etb.li3

Gracilarla YeTrIICOIa

Graci/arla corticala

GelldUlla acerOla

Stoccho.rpermum 1IIIlCIIl4h1111 Sargassum wlghtll

Padino gymnospora

Ulva lacluca

W - 0.862 P < 9.01

Rank totals for 5uper~nll

14.0 24.0

26.S

3O.S

32.S

34.S 4S.S

49.S

44 , • j ,

M. Moban 10seph

Table 5. Species of algie consumed by J/ply!lo li~olota in the order of preference (top to bottom).

Spp. 0/ algae

Graci/aria vemICosa Gracikuia edulis B ypnea musciformis Ulva lactuca E1Iteromorpita comp,essa Grllcikula .ortleata PadinII gytlJNJspora St.elto3pmrtum mtlCUltzt_ Gelld#1Ia IlcerOl1l

SarglWUm wigh/ii

Table 6. Synoplis of preference order ranking •.

Sp. of algae_ .~ " ~ .. C)

~ .:1! ~ ~

C) <> " ;:; .. E E S ~ .. 2 S " Sp. of animal 'y " " 'ii

~ ~ " .. .:: tI:: " Pyrene versicolo, 3 1 2

Pyrene zebra 2 3 1

Troclnu radilltus 6 7 1

Turbo intercoslalis 1.5 3 9

Aplysia llneoillta 3 2 7

Rank total 155 16 20

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5

9

4

I

25

W-0.913

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4 8

2 3

7 10

9 4

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9

9

8

1.5 5

30 31 32.5

Average of Rank Total 275

StGweed Res. Util. }'ol. J . J97~

Rank totals for j e.perlmerru

2

~ .. C)

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\;)

5

7

10

5

6

33

7.0

8.0

16.0

19.0

25.0

34.0 <40.0 41.5

41.0 42.5

'-'-.., .!!> it

§ ., ., " ~ c!l

4

6

5

8

10

33

" ~ !;

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10

10

4

6

8

38

!

Food Prefereoc:es of Selected Algivorous Gastropods 45

The preference order ran kings are summarised in Table 6. The most highly preferred algae are Hypnea musciformis (rank total 15.5) and Graci/aria etlulis (rank total 16). These are followed by Padlna gymnolpora (rank total 20), and Graci/aria verrucos.; (rank total 25). The remaining six species of algae falling above the average of rank total of 27.5 do not seem to be preferred by tbe five species of grazers.

Dlacuaalon

Several species of red algae were consistently consumed in greater relative quantities by all grazers. Species of Hypnea and Graci/aria were generally preferred to other red, brown or JreCII algae. Interestingly the least preferred by all grazers investigated is G.lidielia actro,a, a red alga. Another red alga GrlJl:i/lUia corlic4ta and a brown alga Sargassum wightli are also at a lower level of preference. Leighton (1966) stated that in Southern Californian kelp beds, algivorous invertebrates revealed stronger preferences for the brown alga Macrocystis pyrifera , LamillDria farlowil and Pterygophora cali/arnica. Red algae were preferred only by deep water borbjyores. In the liabt 01 these observations, the present data appear interesting in that the order or preference is reversed, the red alga being preferred to brown and green algae.

The fact that closely allied species like Pyrene versicolor and P. zebra equipped with similar cbewing, biting or rasping organs exhibited divergent preference indicates that the tougboess of algae alollO is not the basis of food selection. A more probable explanation may be tbe selectivity of algal diet on tbe basis of cbemical perception. Paine and Vadas (1969) showed that food preferences of invertebrate herbivores seem to bave evolved in response to availability rather than to absolute food value.

Physical properties like toughness may determine the quantity of food eaten and some­times food preferences as smooth, juicy or fleshy food may attract, incite or stimulate reeding (paine,1963). In the present study. t he best preferred species of algae (H.m.liformls, G ed.IIs, P. gymnospora and G. verrucosa) are relativoly soner or more fleshy than the least preferred one G. acero,a, S. wightli and G. cortlcata) which are tougher.

I am grateful to Dr. V. Krishnamurthy for guidance, encouragement and discussions during the coures of the work. to tbe Director, Central Salt and Marine Chemica" Research Institute for facilities, to Mr. ~ S. Udupa, Asst. Professor of Statistics, College of Fisberies for statistical treatment of tbe data, to Dr. N. R. Menon, Associate Professor of Marine Biology, College of Fisheries for critical comments and to Council of Scientific and Industrial Researcb for financial suppor!.

Summary

Discrimination in the selection of alaal food from ten species of marine algae was studied in five common gastropods of the Gulf of Mannar and Palk Bay region. Species of Pyrtne preferred Graci/aria edulls, Padina gymnospora, Hypnea musciformls to other spp. o( algae, while in Trochus radiatus the preference order was : Padilla gymnospora, Stoecho'permum macu}atwII. Ulva lactuca etc. Turbo Intercostalis exbibited preference to a diet of Enteromorpha compressa and Hypnea musciformis. Red algae (G. verrocoso, G, tdulis,

Stawud Rls. Util, wll. 3, 1978

46 M. Moban Josepb

H. musei/ormil) and green algae (U. laetuca, E. compressa) were best preferred by Aplysia lI"eolata. An order of preference for tbe ten species of algae by tbe five species of gastropods is given.

Bakker, K. 1959

Barkman, J. J. 1955

1IoDeJ, A. D. 1966

Cbopman, V. J. 1961

KeAcIalI, M. G. 1970

Leilhlon, D. L. 1960

Leiablon, D. L 1966

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