Microhabitats, Abundance and Food of Conus on Atoll Reefs in the Maldive and Chagos Islands Author(S): Alan J

Microhabitats, Abundance and Food of Conus on Atoll Reefs in the Maldive and Chagos Islands Author(s): Alan J. Kohn Source: Ecology, Vol. 49, No. 6 (Nov., 1968), pp. 1046-1062 Published by: Ecological Society of America Stable URL: http://www.jstor.org/stable/1934489 . Accessed: 31/01/2014 10:28

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This content downloaded from 93.97.37.57 on Fri, 31 Jan 2014 10:28:13 AM All use subject to JSTOR Terms and Conditions MICROHABITATS, ABUNDANCE AND FOOD OF CONUS ON ATOLL REEFS IN THE MALDIVE AND CHAGOS ISLANDS ALAN J. KOHN Dcpartiiicnt of Zoology, University of Washington, Seattle, Washington (Accepted for publication July 1, 1968)

Abstract. Assemblages of up to 17 species and averaging 13 species of Conus were found on topographicallycomplex subtidal coral reefs (Type III habitat) in the Maldive and Chagos Islands, Indian Ocean. Topographically simpler intertidal smooth limestone benches (Type II habitat) supported eight species. At a station with intermediate habitat, 12 species were present. Only two species were found in extensive areas of sand substrate (Type I habitat). Although most of the commoner species occurred in habitat Types II and III, their relative abundances differed strikingly. Species characteristic of the physically harsher, topographically simpler Type II environmentare smaller, sometimes occur in dense populations of up to 1 individual/M2,and are quite specialized predators on herbivorous errant polychaetes. Species characteristic of the more equable but topographically diverse Type III environment are larger, have lower population densities of 0.03-0.15 individual/M2,and are somewhat more generalized predators on deposit-feedingsedentary polychaetes. Detailed analysis of diets supports the conclusion of prior studies that most species of Conus are primary carnivores. The 13 most abundant species of Conus in the Maldive and Chagos Islands feed almost exclusively on polychaetes, which comprised 96% of all food items identified. Species that eat gastropods and fishes were present but uncommon. The nature of the food of eight species was determinedfor the firsttime. Calculations of amount of food eaten, based on rather crude data for five species, gave a mean of about 10% of body weight eaten per day, but some of the estimates seemed unrea- sonably high. Species with more specialized diets ate larger numbers of prey items than generalists, but the larger ratio of prey size to body size of the latter probably compensates, so that specialists are not more successful predators than generalists. In the habitats studied, Conus species are not more specialized predators where larger numbers of congeners co-occur. The topographic uniformityand less patchy nature of Type II habitat probably favor both increased density of the prey and mobilityof Conus, thus making appropriate food more abundant and accessible than in Type III habitat. Conus diets are correspondinglymore specialized in Type II than Type III habitat. These opposite feeding strategies foster increased feeding efficiencyin the two topographically contrasting habitats. It is proposed that topographically simpler, more uniform,intertidal bench habitats favor lower species diversity,specialization, smaller body size, and higher population density, while topographically more complex, patchier, slightly subtidal coral reef habitats favor higher species diversity,somewhat more generalized habits, larger size, and lower population density.

Large numbersof sympatricspecies, high popui- ized by larger numbersof sympatricspecies of lation density,moderately large body size, and Conus than Hawaii. These comparativestudies ratheruniform trophic position as primarycarni- have produced data which,it is hoped, will con- vores characterizethe gastropodgenus Conus on tributetoward an understandingof the variation coral reefs in the tropical IJdo-West Pacific re- in abundanceand diversityof one prominentele- gion. mentin the faunaof Indo-West Pacificcoral reefs. Kohn (1959a) showed that in Hawaii, the Some relevantquestions are: Is observedvaria- northeasternextremity of this region, subtidal tioil in species diversityrelated primarilyto dif- coral reefssupport populations of 9-12 species of ferencesin habitatcomplexity? That is, do larger Conus. Topographicallyless complex intertidal numbersof similar species co-occur where more marine benches support6-9 species, but popula- complexhabitats provide more 'ecologicalniches'? tion densitiesare much higher. Adults of these Or, are these species characterizedby increasingly species differsignificantly with respectto at least specialized habits and stereotypedbehavior, that two of the following:nature of food,nature of and is, are their 'ecological niches' smaller? Or, do relationto substrate,and zonationor distribution theirniches overlap to a greaterextent ? Is there pattern. a relationshipbetween species diversityand abun- More recently,participation in the Yale Sey- dance and, if so, what is its biologicalsignificance ? chelles Expedition and the InternationalIndian It will be recognizedthat there are otherpossible Ocean Expedition has enabled studies of areas determinantsof species diversity(Pianka 1966), nearerthe "faunisticcenter" (Ekman 1953) of the but the data to be reportedare not relevantto all IJdo-West Pacific marine region and character- the hypotheses.

This content downloaded from 93.97.37.57 on Fri, 31 Jan 2014 10:28:13 AM All use subject to JSTOR Terms and Conditions Autumn 1968 CONUS MALDIVE AND CHAGOS ATOLL REEFS 1047 Census data gathered throughoutthe broad (1936a,b). Eibl-Eibesfeldt(1965) gives a recent range (1600 of longitude) of Indo-West Pacific general account of the islands. Conus indicatea gradientof species diversitycor- The Chagos Archipelagoconsists of five atolls relatedwith habitat complexity (Kohn 1967). In and several banks between4?45' and 7'30'S., and severalpapers I shall examinethe otherquestions 710 and 72045'E. Gardiner(1936, see also Gardi- posed above in more detail and within zoogeo- ner and Cooper 1907) gave a generalaccount, and graphic subregions,based on comparativefield Bourne (1888) discussed Diego Garcia, the larg- data collected during intensive,although brief, est atoll. Poisson (1954) gives a more recent visitsto particularhabitats. briefaccount of the geographyof the archipelago. The results do not provide completeanswers Our studies were confinedto two reefsat Ile du to the questionsabove. Very short-termstudies Coin, at the southwestcorner of Peros Banhos dictated by expeditionconditions and restricted Atoll, and were hamperedby stormyweather. to certain life historystages permitonly limited Attentionwas devoted to aspects of compara- and tentativeinterpretations. However, the pau- tive ecology of sympatricspecies, of which food city of quantitativeinformation on the structure and substratewere studied most intensively,and of coral reefcommunities makes it importantthat to abundance and species diversity. Information relevant hypothesesbe subjected to test by the on reproductionand descriptionsof the habitats data available. have beenpublished elsewhere (Kohn 1961, 1964a, This reportis based on fieldstudies made dur- b). The locationand habitattype of each station ing the Yale Seychelles Expedition (YSE) at are summarizedin Table 1. nine stations on lagoon and seaward atoll reef A classificationof Conus habitatsaccording to flats in the Maldive Islands and two stationsat substratetype and heterogeneityand topographic Peros Banhos Atoll, Chagos Islands, in Septem- relief (Kohn 1967) is followed here. Type I ber-October1957. habitatsare extensiveareas of sand substrateat The Maldive Islands forma long, narrow belt or below MLWS in shallowbays or moats. Type of 17 large,complex atolls and manysmaller atolls II habitatsare smooth,intertidal benches of beach- and islands, between 7010'N. and 0'40'S., and rockor reeflimestone, usually covered with a thin between 72?30' and 73'40'E. Accounts of the layer of sand sometimesbound with algae, and structureof Maldive atolls have been given by with somewhatdeeper, loose sand fillingdepres- Gardiner (1903), Agassiz (1903), and Sewell sions. Type III habitatsare slightlysubtidal reef

TABLE 1. Location of stations, with conversion of time-relativedensity of Concus, Maldive and Chagos atoll reefs

No. No. Estimated No. of Conus of Time density Station Habitat of censured species (hr) (1.08m/12t no. Island (Atoll) type censuses m n t =No./lOm2)

MALDIVES: Dense populations-largesamples

17 .| Funidu (North Male) ,IIIII 4 113 11 6.5 1.5 20. Kuredu (Fadiffolu) IIII 3 105 19 7.5 1.3 23. Male (North Male) III 1 20 9 1.5 1.2 25 . 1Gan (Addu) III 5 85 13 7.0 1.1

MALDIVES: Sparse populations-smallsamples

15 . Dunidu (North Male) II 2 22 7 3.0 0.6 21.Dunikolu (South Mahlosmadulu) IIII 3 23 12 7.25 0.3 22. Hulule (NorthMale) III-III 2 15 3 4.5 0.3 24. Hitadu (Addu) II 1 4 2 1.0 0.3 26. Wiringili(Addu) IIII 1 10 5 2.5 0.3

CHAGOS

27. Ile du Coin (Peros Banhos), lagoon III 3 63 8 5.5 1.1 28 . | Ile du Coin (Peros Banhos), seaward II-III 1 188 12 2.5 6.8

Habitattypes: I, Subtidalextensive area of sand substrate. II, Intertidalsmooth limestone bench. III, Subtidalcomplex coral reef platform. II-III, Intermediate betweenII and III. Commasindicate more than one habitat type present and sampled at station.

This content downloaded from 93.97.37.57 on Fri, 31 Jan 2014 10:28:13 AM All use subject to JSTOR Terms and Conditions 1048 ALAN J. KOHN Ecology, Vol. 49, No. 6 platforms(below MLWS to a depth of 1-2 m) Discussion of species representedby one or a few and have more complex substratesof livingcoral, specimensis omittedfrom this report. dead coral heads and boulders,sand, and smooth This sectiondescribes the microhabitatsof the limestonepavement covered with a thin layer of commonestspecies, with emphasis on the substrate sand. Most habitats studied were of Types II and the animals' relation to it. Nine substrate and III, but some were of intermediatecharacter types are distinguished:sand; sand under coral and a few,such as large tide pools and the edges rocks; coral rubbleor rubbleand sand; thinlayer of channels,do not fitinto the classification. of sand on reef limestonepavement or on beachrock; bare reef limestonepavement; bare beach- THE SPECIES OF CONUS PRESENT ON MALDIVE rock; algal turf on reef limestonebench; dead AND CHAGOS REEFS AND THEIR coral heads or rocks; and living coral. Propor- MICROHABITATS tions of each species foundon the fivemost comn- Kohn and Robertson (1968) list the species of monly occupied substratetypes are shown with Conus knownto occur in the Maldive and Chagos othermicrohabitat information in Figure 1. The Islands,based on priorliterature, study of museum microhabitatsof the most abundant species are collections,and collectionsmade duringthe Yale describedbelow. Similar descriptionsof micro- Seychellesand InternationalIndian Ocean Expe- habitatsof the less commonspecies are available ditions. A total of 65 species are listed,of which from the author. Substrate-typediversity was 46 are knownfrom the Maldives and 53 fromthe calculatedfor each speciesby the Shannon-Wiener Chagos Islands. During the presentstudy, eco- informationfunction, H (See Appendix; Shan- logical informationwas recorded for 492 indi- non and Weaver 1949), and is given witha table viduals of 30 species in the Maldives and 252 spe- showing overlap of species with respect to sub- cimens of 13 species at Peros Banhos. Depth, stratetype, R0 (Horn 1966) in Figure 2. Be- substrate,position with respect to reefboundaries, cause observationsduring the animals' nocturnal and time of collectionof each specimenwere re- periods of activitywere not feasible,information corded in the fieldin pencilon the roughenedstur- recordedin the fieldon substratesindicates place face of polystyrenevials containingthe specimens. of refugeor shelter(if any) (luringthe day, when

FIG. 1 b t l s o h m s i fre l

C'4s~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~,

are indicated to the left of species names. Solid bars on ordinate indicate per cent of epifaunal individuals; dashed bars indicate per cent under rocks. The following species pairs do not differsignificantly with respect to substrate type occupancy (X2 tests, with adjacent categories grouped when necessary): C. ernacictus-C. ebraeus; C. emuciatus-C. frigidus; C. rattus-C. mwsicus; C. rattus-C. chaldcteus.

This content downloaded from 93.97.37.57 on Fri, 31 Jan 2014 10:28:13 AM All use subject to JSTOR Terms and Conditions Autumn 1968 CONUS MALDIVE AND CHAGOS ATOLL REEFS 1049

orenatus Con us niusicus Hwass in BrUguikre (75 ;2) was commonly collected species in the choldoeus Ro > 0.75 the second most Maldives; it predominated at sta. 25, where it oristophanesR -.S CU ...... ,, o0507m comprised 25 of the 95 specimens of 13 species musicus ...... emociotus R <0.50 collected. It ranked second inl abundance on sill)-

rattus Significont tidal reefs at stas. 17 and 20, and was present at :::::.::: ::::: ::::: : :.:.. differencein 0 nilioris.nloi ...... -...... b:... :: : . ::...... > \ detdepth all intertidal stations. Like the much larger C...... -i.-.i.-.-.. X friqL dus lividus...~~~~~...... g- g - .. .--.i.Gl.- lividus, this species is usually epifaunal (84%) inl the lower intertidal region (mean level -0.35m, s ebroeus...... s...... iglili =*|ili~ggs . SD 0.35m), and rarely occurs tinder shelter (1 of arenotascthowdaer aoistophones ewsicus emociatus rattus mitions tnigidosIividus ebraeiis 71 specimens). In contrast to C. lividits, how- H 0.6 1.4 0 6 1.1 0.9 1.5 16 .4 1.8 1.5 ever, its characteristic substrate (59% of those Mean e0 0.2 0.5 0.5 06 06 0.6 0.7 07 07 07 Samplesize 18 13 14 69 11 18 47 40 63 55 collected) is dead coral boulders often fixed on the reef in their position of growth; were found FIG. 2. Indices of substrate-typediversity ( H, belowg 23%o matrix) and overlap (R0, body of matrix) of Conuls on1 on smooth limestone pavement or beachrock Maldive reefs. Mean overlap of each species with all covered by a thin layer of sand. Substrate occtu- others (mean R0,) and sample sizes are also given below pancy is most similar to that of C. rattus (R,,- the matrix. Significant differencesin depth, where R., 0.86; Fig. 1, 2). Of the species studied, only > 0.50, were determined by Mann-\Vhitney U test (I' a taxonomic problem. C. wutsi- < .05) . this one presents cus as originally described is a Pacific form (Kohn, in press ) characterized by small but consistent the animals are normally inlactive. In the follow- differencesin shell proportions and color pattern ing paragraphs, the samIple sizes fromtthe M~al- from the Indian Ocean form studied here. Maes dives and Chagos, respectively, are given inl (1967) found both forms at Cocos-Keeling and parentheses, following the species designation. considered them both C. mtnwsicits.I cannot show Convus lizidus Hwass in Bruguiere (89 ;97) is from Maes' few specimens and other limited mia- the commonest species of the genus on M~aldive terial I have studied that the two forms intergrade andl Chagos reefs, ranking first, second, or third in the Indo-Mlalayan region, hut this may well be in abundance at all stations. It is found on a due to the paucity of available specimens. I tenta- greater diversityyof substrates than anly other tively include the Indian Ocean form studied here species (II 1.8; Fig. 2), although the coml- under C. imnsicus; Figure 3 shows examples illus- mlonlestsubstrate is limesanld, either a thlin layer tratillg the prol)lell. onl smooth limestone pavemnelt (27C/oof observa- Conius miliaris Hwass in Bruguiere (56 ;14) tiOnlS), or regions of deep sand( ( 236/). Occu- ranked third iii abundance in the large samples pancy of particularm.st substrate types is similar from stas. 17 and 25 and firstin the smaller sam- to that of C. frigidus (Ro 0.86; Fig. 1, 2). C. ple from sta. 18. Almost half of the specimens liztidulsoccurs predominiantlyinl the loxver inlter- tidal and shallow subtidal zone (mean level was -0.4m, SD.5 2m, and only 6% were collected above datum), as in Hawaii (Kohn 1959a). Conuls ebracufsLinnaeus (71 ;34) was the (lomli- nant species on smooth limestone pavemsient and beachlrockat sta. 17, was common at all intertidal stations, and was absent only from the small collection, (four specimens in all) from sta. 24. C. obraers may tolerate exposure to air somewhat better than C. iividus; nearly half the individuals observed were above the datum (?0.1 5-0.3m; see also Kohn 1959a) and the mean level was -ines; SD 0.2Cg. Specimens were collected on all the substrate types enumerated in Figure 1, FIG. 3. Shells of Conus rnusicus Hwass in Bruguiere. A, Pacific form; shell shape and color pattern charac- but were on limestone pavement or beach- 73%C teristic of Philippine populations. Polloc, Mindanao rock, with or without a thin layer of sand. Occu- (U. S. Natl. Mus. No. 241943). B, Philippine specimen pancy of particular substrate types is most like with shape and color pattern approaching Indian Ocean Nat. Sci. Phila. No. that of C. frigidus (Ro 0.94), C. aristophanes form. Talin Bay, Luzon (Acad. 229562). C and D, Indian Ocean form; specimens from (0.84), C. e0aciatus (0.81) and C. mwiliaris (0.80) the Maldive Islands used in this study (C, #4494; 1), (Fig. 1, 2). #4499;Sta. 25). Scale= 5 mmn.

This content downloaded from 93.97.37.57 on Fri, 31 Jan 2014 10:28:13 AM All use subject to JSTOR Terms and Conditions 1050 ALAN J. KOHN Ecology,Vol. 49,No. 6 (47%o) were collectedat or above MLWS (mean TABLE 2. Population density of Conus on a zone of in- level -0.1M; SD 0.4m) on a substrateof thinsand tertidal beachrock 12 m wide between beach and reef, Funidu Island, North Male Atoll, Maldive Islands (sta. on sloping beachrock or limestone pavement 17) (38%,), amid coral rubbleand sand, oftenin de- pressionsin beachrock(22%), and algal turfon No. per 100 ft.2 (=9.3 M2) beachrock ( 16% ). Substrate diversityis thus high (H 1.6) and most similar to that of C. Distance of frigidus(RO 0.89; Fig. 1, 2). quadrat !E Figure 1 shows that the microhabitatof only from U>. one species, Conus arenatus,is qualitativelydis- beach t2 O O t (ft) 'IQ tinctwith respectto natureof and relationto the substrate. All 18 individuals were found com- 10-20 5 4 3 3 1 0 0 16 10-20 0 3 0 0 1 2 0 6 pletelyburied in sand,and mostoccupied the large 20-30 6 2 1 1 3 0 2 15 areas of deep sand (Type I habitat) typicalof the 25-35 0 0 0 0 0 Qa 0 0 inshore portions of many Maldive atoll reefs 11 9 4 4 (Stoddart, Davies, and Keith 1966). Sample 5 2 2 37 size was limitedby collectingeffort in such areas, Meandensity= 1.00 individual/M2. where only one specimenof another Conus (C. aSome specimensof C. miliariswere seen at this level outside quadrat. tessulatus) was observed. Every other species shows strong overlap (R0 ? 0.75; Fig. 2) with ( 9.3m2) quadratsare shownin Table 2. Popu- one or more congenerswith regard to substrate lation density (1.0/M2) is higher,but substrate occupiedduring the day. The main typesof sub- topographyand prevalenceof C. ebraeusare simi- strate available on reef flats,listed in Figure 1, lar to those of Hawaiian marine benches (Kohn are thus not clearlypartitioned among the species 1959a: Table 2). Most other Conus populations of Conus present. However, species occupying studiedin the Maldives were of much lower den- similar substratesoften had statisticallysignifi- sity. Although some large samples were col- cant differenceswith respect to depth (Mann- lected in Type III habitats,in all cases density WhitneyU test: P < 0.05; circles on matrix in was too low to measure directlywith available Fig. 2). The large standarddeviations and rela- time and facilities. Conversionof time-to space- tivelysmall differencesin level involved,coupled relative density by the factor of 1,200 ft2/hr with the difficultyin makingaccurate field mea- (Kohn 1959a) is biased upward by concentration surementsunder variable wave, wind, and time of collectingeffort in denserareas of patchilydis- conditions,probably make the biological signifi- tributedpopulations. However this bias is less cance of these differencesconsiderably less than importantin the present study,where I had no the statisticaldifferences would suggest. prior knowledgeof Conus dispersion. Surprising Unfortunatelylittle informationwas obtained uniformity(density range of 1.0-1.4 individuals/ on substratesthat Conus cruises over duringnoc- lOM2) characterizedsubtidal reefs (Type III habi- turnal huntingactivities. Seven specimensof C. tats: stas. 20, 23, 25, 27) supportingdenser popu- miliariscollected at nightat sta. 18 were all crawl- lations of Conus (Table 1). At other stations, ing on beachrock,and two had just fed on eunicid includingtwo Type II habitats,time-relative den- polychaetesthat occur in burrows in rock (see sitywas lower,but also veryuniform. below). During the day, most specimenswere Population densitycould not be measured di- partlyburied in a thinlayer of sand on reefrock rectlyat sta. 28 because of stormyweather con- pavement,or were amid rubble and sand. Five ditions. This is unfortunatebecause the substrate C. ebraeus were collectedat the same time on the thereis intermediatein complexitybetween inter- same substrate,where they are also commonly tidal benchand subtidalreef. Conversionof time- foundduring the day (Fig. 1). One had just fed relative density gave 0.7 individual/M2,and I on a burrowingeunicid polychaete. Only one or stated in field notes that densitywas 'probably two individualsof other species were observedat similarto Funidu' (sta. 17). night,and no differencesfrom the typicaldaytime microhabitatwere noted. SPECIES DIVERSITY, RELATIVE ABUNDANCE, AND HABITAT TYPE POPULATION DENSITY It was possible to determineareal population Analyticalmethods densitydirectly only at one location, a transect Ecologistshave expendedmuch effort in assess- across intertidalbeach rock between beach and ing the relationshipbetween numbersof species reef at sta. 17. The results from four 100-ft2 and numbers of individuals in animal communi-

This content downloaded from 93.97.37.57 on Fri, 31 Jan 2014 10:28:13 AM All use subject to JSTOR Terms and Conditions Autumn 1968 CONUS MALDIVE AND CHAGOS ATOLL REEFS 1051 ties. For temporallystable populations,that is r wherethe populationsize at any time ratherthan (r/n) Ll/(n-i+ 1) the rate of increasedetermines population size at wherethere are n species and m individuals. This some later time, MacArthur (1957, 1960) pro- is the ordered random intervalsmodel (Cohen posed thatthe expectedabundance of the rthrarest 1966) and is oftenreferred to as the type 1 or species in a mixed populationof species of similar "broken stick model" of relative abundance, in size and physiologyand occupyinga single,ade- which the stick representsa set of limitingre- quately sampled habitat, and whose ecological sources. The amountof these resourcesavailable niches are contiguous [I use niche here in the to each species,and thus its abundance,or mass or sense of Hutchinson (1958)] is total metabolism(Engelmann 1961), is propor- tional to a lengthof stickbroken by n - 1 points thrownat randomon it; the stick is thus broken 3Mn lSTA. 17 3 Intertidalbeachrock 30 into as many pieces as there are species in the assemblage. This outcome is an expected result

2nm- < 5S2/S 2= 1.2 20 of interspecificcompetition. Many unansweredquestions remain concerning _ H/Hm x=0 83 _ 15 the biologicalsignificance of this distributionand fitsof data to it (Hairston 1959, Engelmann1961, 5 King 1964, Mitchell1965). Nevertheless,I com- A pare patterns of Conus relative abundance at 0 , .- , ,n seven stationswith the expected MacArthurtype 1 distributionsin Figures 4 and 5. This may be i t i q. , justifiedin view of 1) the previouslyreported isomorphismof Conus relative abundance with STA. 18 this model (Kohn 1959a, 1960) and the reserva- m 222275 = tion of Slobodkin (1961: p. 166) that "thereare 31- So 2/Se21.97 = 8 not a sufficientnumber of locations,nor are there 1.7H = H/H3 mo.76_ sufficientcollateral data, to deny the possibility 'I -6 thatagreement with the MacArthurmodel is sim- ply fortuitous." 2) Recent empirical evidence fromstudies consideringthe time dimensionthat assemblagesin stable environmentsapproach iso- l \ -708 STA. 28 - morphismin time,whereas wide departure(in the directionof the type4 distributionof Hutchinson n 1915711)~~~~~ 1961) followsphysical disturbance of the environment (Goulden 1966; Tsukada 1967). Shifts towardthe type4 distributionalso occur whenthe constraintsimposed by other assumptionsof the 40 model are relaxed (Kohn 1959, King 1964). in0 N~~~~~~~X | {Xti'Sn26 3) Graphs of relativeabundance according to the I , * 5 -, , . . , . . C orderedrandom intervals model can be compared directlywith one another; it is easy to ascertain b~lo STA0 2 20 fromsuch a graph whetherone or a few species dominatethe assemblageor whetherthe member species are more equitablydistributed. n 140 Figures 4 and 5 also give the ratio of the variance of observed and expected distributions (S02/S(2) as a crude measure of deviationfrom *20 dahe lie:cluae rmMcrhurHmodel.7 50 fit. For a perfectfit, SO2/Se2 1.00 (King 1964, M_~ ~~~~*~-~~ n~~~~~~I~ ~ but see Cohen 1966). Also given are threemea- sures of species diversityof the censuses: 1) the numberof species,2) the Shannon-Wienerinfor- mation function,H, which increases both with FIG. 4. Relative abundanceof species of Conus on increasingnumbers of species and increasingequi- beachrock and reef limestone (Type II habitats ? sta. 28) of Maldive and Chagos reefs. Solid lines: observed; tabilityof theirdistribution, and 3) H/HUmax.The dashed lines: calculated from MacArthur model 1. last is a measure of the positionof H between0

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5n Sta.20 ______= 98 ~~~~~~~~~m25.7 n =1772 17 4 m = 2.1 4m-n ~~~~Sto. S2/S20 ~~~Subtidalreef platform 8nH 2.e2 m=29 HH 07 = 2.24 3 73--n m N. max=07 fl S2/S2ez 5 fl H=2.25 m H/Hm,,jO.88 2m

q) % 2 - ~ N~%0.%' -114 n ~ ~ ~~~2 n

A 0B0

! ~~~3 .~~~~~~~ 0Z3 0-Z k. r - rn 3 n~f Sta. 25 flSta. 27 3 m= 95 m=63 73 2 5. =78 3rn T~~~~~~~~~_3 _n=8 78825 ~ 3 4 n S~~~~2/S2=1.6 220 3!. S2/S,2= 2.6 0 e6 ~0 %.2 0 H =2.1 N H =1.3 N. H/Hmox=082 -1 5 2 il H/Hmax 0.65 mm-0%10 0 5

fl ~ ~ ~ ~ ~ ~ 0% 0%~~~~~1 fl -

IC 0D

o ~~~~~~ fill0 1

FIG. 5. Relativeabundance of speciesof Conus on subtidalreef platformsin the Maldive and Chagos Is- lands (Type III habitats). Solid lines: observed;dashed lines: calculatedfrom MacArthur Model 1.

(all individualsbelong to the same species) and is not to say that the relativeabundances are the maximumdiversity (individuals of all speciespres- same in adjacent areas of the same habitat. Nearly ent are equally abundant). H/Hmax varies from halfthe data forsta. 17 in Figure 4 are fromquan- o to 1, allowing directcomparison of samples of titativelysampled quadrats, but comparisonof the differentnumbers of individualsand species. It latter (Table 2) withthe total in Figure 4 shows is the "relativeentropy" of Shannon and Weaver shiftsupward in rank of C. miliarisand C. spon- (1949, see Appendix) and is here termedrelative salis. The collectionsgraphed in Figure 5 are diversity. much less complete,somewhat biased samples of Results larger, more patchily distributed populations. There is no evidencethat the distributionof rela- Figures 4 and 5 presentobserved relationships tive abundances in these that between numberand abundance of species. As samples reflects of the larger in it is convenientto consider the results of these populations; fact, this is unlikely (Lloyd and Ghelardi1964, Paine unpublished). analyseswith reference to habitattype, census data Imperfectthough they are, the census data fromType II habitatsare grouped (Fig. 4) sepa- do closely MacArthur'smodel 1. ratelyfrom those of Type III habitats (Fig. 5), approximate De- viationstend toward the type 4 distribution,not as was done previouslyfor Hawaiian populations (Kohn l959a). MacArthur'stype 2 distribution,for assemblages Because of samplingproblems, the data in Fig- of species whose abundances are independentof ure 4 are more appropriateto the MacArthur each other,as can be seen by inspectionof the cor- model than those in Figure 5. Conus individuals rectedcurves given by Pielou and Arnason (1966) on beachrock and reef limestone are typically and Vandermeer and MacArthur (1966). Al- visiblefrom above duringthe day; shelteredplaces thoughthe more completelysampled local popula- that would hide them completelyare rare. The tions of Figure 4 fitthe model somewhatbetter, collectionsgraphed in Figure 4 may thus be con- the reverse was true in Hawaii (Kohn 1959a, siderednearly complete local populations,but this mean variance ratio for populationsof four Type

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II habitats,3.7; for populationsof fourType III mean R0 - 0.64) are more similar to each other habitats,1.5). In the lattercase, one could argue than Type II populationsare to Type III popu- that more heterogeneoushabitats, which reduce lations (mean R = 0.46). the numericaldominance of the most abundant species, are likelyto supply betterfits. SIZE AND SEX RATIO Species diversity(H as well as numberof spe- Informationon shell size of the commonerspe- cies) is lower in populations inhabitingphysio- cies is summarizedin Figures 7 and 8. Shell graphicallysimpler substrates (Fig. 4, 5; see also lengthof males and femalesdoes not differat the Kohn 1967). Since relative diversityis very 0.05 level of significancein any speciesin Maldives similarin the two habitattypes, differences in H or Chagos (t tests). The same is true of shell are attributedto differencesin N. width (maximumdiameter) except that C. lividus All but one (C. aristophanes) of the species found in Type II habitat also occurredin Type 50- III, and all but one (C. rattus) of the eightcom- C. musicus 40- ? monestspecies in Type III also occurredin Type 27dd: 14.8 0.6 x 9.2 0.3 30- 2699:14.6 ? 0.4 x 9.1 ?0.3 II. An index of diversitythat measures only similarityof species composition(Koch 1957, also 20- applied to Hawaiian Conus populationsby Kohn I0 1959a) gave similar values for censuses of all Type II stations (stas. 17B, 18, 24: I 0.32), censuses of all Type III stations (stas. 17S, 20, 205 C. ebrieus 5 * 22(4d:25.9 ?0.7 x16.7?0.4 21, 23, 25, 26, 27: I 0.29), and comparisonof 0 3999: 24.8 ? 0.7 x 16.2? 0.6 all Type II vs. all Type III stations (I 0.33). However, the relative abundances of particular species differedmarkedly between the two types of habitat (Fig. 6). The most abundantspecies in each habitattype are relativelymuch less com- 25- mon in the other,and this is reflectedin the mea- 20 | C. miliaris sure of similarity(R0 of Horn 1966) of species 15- a 26 Ad: 25.0 ? 0.6 x 16.7 ? 0.5 X 30 ?0.7 x17.9 +0.6 compositionand proportion at the more ade- o01 22?92:26.6 quately censused stations. The populations of Type II stations (stas. 17B and 18: R= 0.89) and of Type III stations (stas. 17S, 20, 25, 27: 10- 30 20- 7 C. frigidus 5-1- *27 dd: 30.1 + 0.6 x 17.7? 0.4 -. 20 :29.8? x 18.1+ 1.0 10-C 16+ 1.8

7 6 30 5 25 . aividusC. b 1.: 1 43 31.0 + .8 x 17.8? 0.5 20C 37ay 19.8+90.7 34.2+ 1.2x 15 -

{ t '? t a E D -t 2 S : S ok ~~ti t z Length (mm.) FIG. 6. Relativeabundance of Conus species on inter- Fig. 7. Length and sex frequencydistributions of Mal- tidal benches(Type II stations:17B, 18, 24; solid bars) dive Conus. Sample size, mean shell length and mean and subtidalreefs (Type III stations: 17S, 20, 21, 23, maximum shell width.+~ standard error of mean are given 25, 26, 27; hatchedbars) in the Maldive and Chagos Is- for both sexes. For other species representedby samples lands. Species comprising< 1% of eithersample are sizes :-10, length and width frequencydata in mm are: omitted. These are Type II: C. arenatus; Type III: 14 C. aristophanes, 13.0+ 1.0 by 7.1 -+ 0.4; 13 C. chal- C. arenatus, C. aulicus, C. balteatus, C. capitaneus, C. daeus, 25.7 -+-1.2 by 17.3 +0.9; 18 C. arenatus, 26.9 -+ 1.5 catus, C. litteratus,C. marmoreus, C. moreleti, C. tulipa, by 15.8-+-1.0; 17 C. rattus, 28.4-+-1.6 by 17.9-+-0.9; 10 C. various,and C. Virgo. C. emgaciatus,34.7 -+ 2.5 by 19.6 _+1.3.

This content downloaded from 93.97.37.57 on Fri, 31 Jan 2014 10:28:13 AM All use subject to JSTOR Terms and Conditions 1054 ALAN J. KOHN Ecology, Vol. 49, No. 6 cies occurringin both habitattypes C. milioris (C. miliaris, 5-] 6d:16.3? 1.5x 10.9+1 .0 C. frigidus,C. coronatts,C. mnusicus,C. rattus,C. 7QQ: 15.7 ?0.6 x 11.6 ?0.6 ebraeus) are not significantlylarger in Type III than Type II habitats (Mann-WhitneyU tests: 15, C. coronatus P > 0.1). This contrastswith shiftsin size fre- 10- 8dd: 18.2 +0.6 x 11.4?0.4 quencyof some speciesobserved in Hawaii (Kohn 5- 7 t0.3x 12.1 +0.3 l 18.1 1959a,Fig. 9 and 18). It is possiblybecause wave action is lighterat the Maldive Type II stations 20 with large censuses, 201 C. choldoeus located on lagoon islands, thanat Type II habitatsin general. C. lividusand 15- 15 + 0.5 x 15.5 ?0.3 (d:23.5 C. 16.0 0.5 chaldaeus, however,are smaller in the inter- 0- 23:24.0?0.6 ? mediate habitatof sta. 28 than on subtidal reefs (Mann-WhitneyU test: P < 0.05). 5 There is no correlationbetween shell lengthand 101 _ _ 10- UC. ebroeus abundance on subtidal reefs (Spearman rank- ; 5- * 15 : 24.5?1.1 x 15.9? 0.7 differencecorrelation coefficients (re) range from 16Q99:23.4?I NW . 2 15.2?i0.8 -0.28 to +0.13; in all cases P 0.05). Weak - >> 25- C. lividus inverse correlationson intertidalbeach rock (r, 20 38 dd: 27.6 ? 1.3 x 16.5 ?0.5 -0.30 at sta. 17 and -0.46 at sta. 28) are not 1 22 7.9? 1.2 x 16.4 ? 0.7 significantat the 0.05 level. These findingsagree s5- withstatistics from populations of similarhabitats 10- in Hawaii (Kohn unpublished). The lack of cor- 5- relationis expectedin a group of species which,as will be shown below, belong to differenttrophic subwebs (Paine 1963, see also Hairston 1964). FOOD The unique feedingmechanism of Conus has 5--- been discussedin earlierpapers (see Kohn 1963). 5-. qat.u ' A paralyticvenom is injected into the prey by a Length(mm.) highly modified,needle-like radula tooth. The Fi(;. 8. Length and sex frequencydistribution of Cha- prey is thenengulfed and swallowedwhole. This gos Con us. Sample size, mean shell length and mean facilitatesidentification of prey organismsin the maximum shell width + standard error of mean are esophagus-stomachbefore digestion. Conus feeds given for both sexes. mainlyat night (Kohn 1959a), but a few feeding acts were observedin the fieldduring the day in females in the Maldives are broader than males the present study. Observation at night was (P 0.02). Sex ratios differsignificantly from usuallynot feasible,and most food organismshad unityonly in C. ebraeus in theMaldives (X2 test: to be identifiedfrom remains in the intestinesof 0.025 < P < .05). animals that were kept out of water aftercollec- Conus populationsof intertidalbenches (Type tion and fixedas rapidlyas possible. Some speci- II habitats) consist of smaller individuals than mens were retained alive in small vials of sea those of subtidalreefs (Type III habitats) (Kol- water for collection of fecal matter. As most mogorov-Smirnovtest: P < 0.02; the nonpara- species proved to eat polychaetes,indigestible metrictests used in this paper may be found in setae and jaws permittedidentification. Siegel 1956). Strongerwater movementin the intertidalthan subtidalregion, and lack of shelter Vermivorousspecies on a topographicallysimple and uniformsubstrate Table 3 shows the main food items of the six are selectivepressures for small size in Type II species of Conus in the Maldives fromwhich the habitats. largestnumber of prey organismswere identified. The significantdifference in shell size can be In all, 321 prey organisms,of which 273 were attributedto the presencein Type III habitatsof identifiedto species, were recovered from 446 relativelysmall numbersof large individualsof specimensof 15 species of Conus in the Maldives. species not presentin Type II habitats (C. miles, The 14 mostabundant species of Conus fed almost C. distans,C. emaciatus,C. flavidus,C. zonatus, exclusivelyon polychaetes,mainly of the families C. titlipa,C. moreleti,C. pennaceus,C. capitaneus, Nereidae, Eunicidae, Terebellidae,and Capitelli- C. textile,C. leopardus; see Fig. 4). Most spe- dae.

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TABLE 3. Major prey organisms consumed by six common species of Conus on Maldive reefs. Numbers in body of table indicate numbers of prey taxon at left recovered from alimentary tracts of Conus species at top. Numbers of principal prey item in bold face; secondary items, if important,italicized.

Conus species and no. individualsexamined miliaris musicus chaldaeus ebraeus frigidus lividus Avg. Prey organisms 55 74 12 66 47 82 56 Nereidae Nereis persica Fauvel 7 Nereis sp. cf. N. cockburnensisAugener ...... 5 6 Nereisjacksoni Kinberg...... 5 3 4 Platynereisdumerilii Audouin & Milne Edwards 4 3 Unidentifiednereids ...... 1 6 2 Eunicidae Lysidicecollars Grube...... 50 4 Eunice afra Peters...... 5 10 Palola siciliensis(Grube) 23 64

Capitellidae Dasybranchuscaducus (Grube) 17

Terebellidae Loimia medusaSavigny 12 Otherterebellids (7 species) 9 14 Total identifiedfood"...... 56 45 32 70 28 43 46

No. of preyspecies eaten (N=23) ...... 3 9 3 3 7 13 6.3 Prey species diversity,H ...... 0.38 2.01 0.77 0.29 1.29 2.19 1.15 Index of food specialization,R ...... 0.54 0.12 0.30 0.73 0.31 0.14 0.36

Index offood species overlap,Rob.0.39 0.25 0.87 0.00 0.29 0.13 2 ' 65.7 9.9 8.0 32.0 P . <<0.001 <0.005 <0.01 < <0.001

aIncludesother food items. X2tests and measuresof preyspecies diversity,food specialization, and foodspecies overlap weremade on completesample data, available fromthe author. bOthervalues of Ro>O are: ebraeus-musicus,0.10; lividus-musicus,0.09; ebreaus-miliaris,0.02.

Table 4 containscomparable data on the com- pertinentstatistics are included. The formulae monestspecies of Conus at Peros Banhos. With of these are given in the Appendix. Differences a few minor exceptions,members of the same in species compositionof the diet were tested for familiesof polychaetesare eaten in both the Mal- significanceby the x2 test forhomogeneity of con- dive and Chagos Islands. However, there are tingencytables. The tests were made -on data some differencesin the prey of the same species groupedto conformwith the recommendationsof in the two areas, presumablyowing to differences Cochran (1954, see also Siegel 1956, Lewontin in the suitabilityof the habitatsfor the prey spe- and Felsenstein1965). Tables 3 and 4 emphasize cies. For example,the diet of C. lividus,the most that each of the fivemost abundantspecies in the abundant species in both samples, is 65% Mal- Maldives and at Peros Banhos feedsprimarily on danidae at Peros Banhos, but no maldanidswere a differentspecies or highertaxon of polychaetes, foundin the digestivetracts of 83 C. lividus ex- and the diets are restrictedto small numbersof amined fromseven Maldive reefs. There Tere- preyspecies. bellidaecomprised 61% of the diet. C. chaldaeus, Matrices of the food species overlap index R0 which ate mainlyPalola siciliensis (74%). in the forthe species in Tables 3 and 4 contain25 values Maldives, ate mainly Nereis persica (68%) at (15 + 10, respectively); of these, the 12 values Peros Banhos. Nereids were also moreimportant > 0 are given in the tables. The overall mean in the diet of C. ebraeus at Peros Banhos (31,%) R0 is 0.13, and only two values exceed 0.4. At than in the Maldives (9%o). The thickalgal turf Peros Banhos,although the samplesare smallthey on the reeflimestone of sta. 28 undoubtedlypro- suggestthat the spatial distributionsof C. miliaris vided more suitablehabitat for nereidsof the size and C. rattus (R. = 0.67) differbetween the two required by these two Conus species than was stations,but that at sta. 28 bothspecies are equally presenton Maldive reefs. abundant and their food is essentiallyidentical In Tables 3 and 4, species withthe mostsimilar (Table 5). In the Maldives,93% of the data for diets are arrangedin adjacent columns,and other C. ebraeusand C. chaldaeus (R, = 0.87) are from

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TABLE 4. Major prey organisms consumed by five commmon species of Conus on reefs at Peros Banhos, Chagos Islands. Numbers in body of table indicate numbers of prey taxon at left recovered from alimentary tracts of Conus species at top. Numbers of principal prey item in bold face.

Conus species and no. individualsexamined

chaldaeus ebraeus rattus miliaris lividus Avg. Prey organisms 39 33 38 14 96 44 Nereidae Nereis persica...... 21 1 Nereisjacksoni...... 6 2 Perinereishelleri Orubee 5 Othernereids .3 2 Eunicidae Palola siciliensis 18 Eunice afra 15 2 1 Eunice rubraGrube 7 5 Maldanidae Axiothellaaustralis Augener 15 OtherMaldanidae (2 species) 1 9 Terebellidae (4 species) 7

Total identifiedfoodb...... 31 26 22 12 39 26 No. of preyspecies eaten (N= 17) 5 4 2 5 10 5.2 Prey species diversity,H ...... 0.98 0.89 0.63 1.47 1.84 1.16 Index of food specialization,R. 0.39 0.36 0.10 0.09 0.20 0.23

Index of food species overlap,Roe. 0.22 0.00 0.67 0.28 0.12 x2 ...... 32.3 6.3 25.6 P...... <0.001 <0. 025 <0.001

identificationtentative. bIncludesother food items. x2 testsand measures of prey species diversity, food specialization, and food species overlap were made on completesample data, available fromthe author. oTheonly additional value of R.>0 is: rattus-lividus,0.07.

TABLE 5. Abundance and food of Conus rattus and monly co-occur. Striking differencesin micro- Conus miliaris at stas. 27 and 28, Peros Banhos habitatand nature of food were demonstratedin Hawaii (Kohn 1959a) and the Marshall Islands, Food althoughon atoll reefs in the latter area Palola No. Eunice Eunice siciliensisis the principalfood item of both spe- Station Species observed afra rubra cies (Kohn and Orians 1962). Of the 23 speci- 27 C. rattus 24 3 14 mens of this polychaeterecovered from Maldive C. miliaris 1 0 2 C. chaldaeus,mandibles and other remainsof 11 were foundin one specimenfrom sta. 17. This is 28 C. rattus 14 4 1 C. miliaris 13 5 0 the largestnumber of prey items I have observed in one specimenof Conus, the average here being 0.7 in all Maldive and 0.6 in all Chagos samples. the same stations (stas. 17 and 25). Despite the The higher proportionof nereids in the diet of similaritiesin these RO values, the x2's and proba- C. chaldaeus causes its compositionto differsig- bilitiesin Tables 3 and 4 indicatethat the compo- nificantlyfrom that of C. ebraeus (Table 4). At sitionof the dietof each speciesdiffers significantly Peros Banhos, P. siciliensiswas absent fromthe fromthat of the species withthe mostsimilar food diet of C. chaldaeus,which ate mainlyNereis per- habits, insofar as the data were adequate for sica. These data suggestthat in habitatsfavorable analysis. to nereids, these become more importantcom- AlthoughI examined only 12 specimensof C. ponentsof the diet of C. chaldaeusand to a lesser chaldaeus from Maldives, Table 3 includes the extentof C. ebraeus,and that as in Hawaii these resultsbecause of the very large numberof prey two predatorseat mainlydifferent species of ne- organismsrecovered and the similarityof the diet reids. to thatof C. ebraeus. These two species are taxo- Eunice afra is the predominantfood of C. rattus nomicallyvery similar (Kohn 1959b) and com- in both Maldives (all 12 polychaetesrecovered

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TABLE 6. Size of Coitus species and their prey in the Maldive and Chagos Islands

Means of specimens with measuredprey Estimated Estimated Correlationof predator Estimated Proportion dry weight % of body size and preysize Length dry weight of diet of prey weight Species Location (mm) (mg) Prey species (pi) (mg) eaten/daya rs P N chaldaeus.Chagos 24.0 80 Nereis persica 0.68 2.63 2.6 0.22 >0.10 14 whaldaeus.Maldives 27.8 103 Palola siciliensis 0.74 0.89 2.4 0.68 <0.005 16 ebraeus.Chagos 23.8 96 Palola siciliensis 0.69 2.23 1.8 0.35 <0.005 51 ebraeus.Maldives 27.5 134 Palola siciliensis 0.93 2.17 1.7 12 miliaris.Maldives 24.9 157 Lysidicecollaris 0.90 1.66 1.2 0.47 <0.005 32 - - musicus.Maldives 15.3 16 Eunice afra 0.25 8.2 31 21 0 3 musicus.Maldives 14.7 15 Nereis persica 0.18 2.3 9.2* 1 2 0 . 8 7 attus.Chagos 33.4 168 Eunice afra 0.68 56.0 19.3 <0.05 112 attus.Maldives 28.7 117 Eunicez fra 1.00 29s.0 16.9J 0.50 7

aAssumesother prey eaten are the same size as the major preyspecies measured.

1.8 A. 1.8 B largest numberof prey items in their alimentary tracts (Spearman r, -0.50; 0.05 < P < 0.10). 1.6- 1.6- It is assumedthat feces are completelyvoided 12- 24 hr afterfeeding and that the numberof prey 1.4 * 14 itemsin the alimentarytract in the morning,when nearly all animals were collected,is the number 1.2- 1.222 ~ eaten during the previous night (Kohn 1959a). If specialiststake more prey items per utnittime than generalists,are they then more successful 0.*8't 0.8 '''. predators? To answer this question,one must know whetherthe size ratio of prey to specialist t0.6-t * * *2 0.6 predatoris as large as, or largerthan, that of generalists. Necessary data are difficultto obtain. 0.4- 0.4 . Significantcorrelations between Conus size (shell length) and prey size (jaw length) were found 0.2- 0.2 15 20 25 30 35 15 20 25 30 in four of the five cases where samples of the Shell lengthof Conus ebraeus (mm) Shell lengthof Conus miliaris(mm) prey's remains were adequate to permitan esti- FIG. 9. Examples of the relationship between size of mate of its relative size (Table 6). Figure 9 Conus species and their principal prey items eaten in shows two examples of the data obtained. nature in the Maldive and Chagos Islands. A. Shell length of C. ebraeusand mandible length of Palola sici- Althoughsome of the estimatesof per centbody liensis (Grube). Spearnian r, = 0.35; P < 0.05. B. Shell weight eaten per day by Conus agree with the length of C. miliaris and mandible length of Lysidicc col- 1-5% found by Kohn (1959a) for four species laris Grube. Spearnian rs =0.47; P < 0.005. Dots in Hawaii, some othersare so large as to be sus- represent single observations; multiple points are repre- pect (Table 6, col. sented by the number of observations; x indicates mean. 8). However, estimatesof prey weightare crude and should be taken only as approximate,as they from 17 specimens) and Chagos (15 E. afra, 7 have passed througha series of transformationsfrom E. rubra from 38 specimens). E. rubra predomi- original linear nated in the small sample of C. miliaris from Peros measurementsof jaw length,body length,and Banhos, where no specimens of Lysidice collars, diameter. The estimatedamount of food eaten per day, comprising 90% of the diet of this species in Mal- dives, were recorded from Conus digestive tracts. in termsof per centbody weight,is not correlated Conus lividus, the most abundant species in withthe index of food specialization. In fact,the Type III habitat in both regions, has the most data in Table 6 suggest that some species with diverse diet in both, as measured both by H and more generalizeddiets (small R values), whose by number of prey species eaten (Tables 3, 4). preysize could be estimated,get more to eat than However, the mean numbers of prey items per the specialists. The relativelylarger prey of some digestive tract (0.55 in Maldives, 0.41 ill Chagos) species with more generalizeddiets (C. rattus in were the lowest of any species. Chagos, C. musicusin Maldives) more than com- The species with the most specialized diets pensatesfor their lower feedingrate. It is unfor- (with low N and H and high R) are more charac- tunatethat no comparabledata could be obtained teristic of Type II habitat. They contained the for C. lividusand C. frigidutsbut it is not possible

This content downloaded from 93.97.37.57 on Fri, 31 Jan 2014 10:28:13 AM All use subject to JSTOR Terms and Conditions 1058 ALAN J. KOHN Ecology, Vol. 49, No. 6 to estimatethe size of theirprey fromalimentary tacles (Shaw 1914), not presentin most species tractcontents. of Conus, were just visible. The fishapproached The natureof the food of several less abundant the mouth of the Conus, hestitated,and presum- species of Conus was determinedfor the firsttime ably was stungat thistime, although the proboscis in this study. Examinationof 18 specimensof C. of the Conus was not visible at any time to the arenatus revealed 5 specimensof 2 unidentified unaided eye. The fish hurtled away from the Maldanidae, 2 Dasybranchuscaducus, 1 Lysidice rostrumand showeda silveryspot on its side, but collaris and an unidentifiednereid in the alimen- did not convulse. The rostrumof the Conus ex- tarytracts. Two of 14 C. aristophaneshad eaten tended to a length of 38 mm and expanded in Lysidice collars. Examinationof 22 C. coronatus diameterto 10mm,as is normal in piscivorotis revealed one each Nereis persica, Glycera tesse- species immediatelyafter stinging(Kohn 1956). lata, and Eunice afra,and 3 unidentifiedeunicids The fishwas graduallyswallowed during several and nereids. Remains of 2 Loimia medusa, 1 minutes. The diameterof the expanded rostrum Thelepussetosus, and an unidentifiedunsegmented was severaltimes that of the fish,suggesting that worm were recoveredfrom the alimentarytracts C. tulipais perhapscapable of eatingmuch broader of 13 C. emaciatus. From 6 to 10 individualseach fish. Thirtyminutes later, the meal could be de- of C. distans,C. flavidus,C. leopardus,C. miles, tectedas a slightswelling in the buccal cavity. and C. sponsaliswere examined. In all cases, the diets were similarto those reportedfor the same DISCUSSION species in Hawaii (Kohn 1959a). On the shores of tropical Indo-West Pacific islands, membersof the gastropod genus Conits Molluscivorousand Piscivorousspecies occupy habitatsof three main types: I, extensive In contrastto Hawaii (Kohn 1959a) and Sey- subtidalareas of sand; II, intertidalsmooth lime- chelles (Kohn in prep.), thereis no commonmol- stone platforms,and III, slightlysubtidal coral luscivorousspecies of Conus on Maldive and Cha- reefs with complex substrates; as well as inter- gos atoll reefs. Four species knownor suspected mediatehabitats. In the presentanalysis, obser- to feed on other gastropods were collected in vationswere made on 744 specimensof 30 species Maldives. Only fivespecimens were foundof the of Conus on atoll reefsin the Maldive and Chagos commonestof these, C. pennaceus Born. Unfor- Archipelagoes. Of these, 28 species were found tunately,no remains of prey organisms were on subtidal reefs,with assemblages of up to 17 present in their alimentarytracts. Of the four and averaging13 species on one reef. Eleven spe- specimens of C. textile L. collected,three con- cies were recordedfrom topographicallysimpler tained only used radula teeth and soft unidenti- benches, eight of which could be found at one fiableremains, but one, fromsta. 25, had eaten a station. Two speciesoccurred in the limitedareas C. rattus. One specimenof C. marmoreusL. at of Type I habitatstudied. At the mostintensively sta. 26 was observedafter it apparentlyhad stung sampled intermediatehabitat (between Types II a C. lividusbut beforeit had begun to feed. The and III: Sta. 28) 12 species were presentand H, other C. marmoreusand the one C. aulicus L. a measure of species diversitywas intermediate collectedcontained only used radula teethin their (H = 1.8) betweenmean values for populations intestines. of Type II (H = 1.7) and Type III (H = 2.0) Two specimensof C. catus Hwass in Bruguiere, habitats. These correlationssuggest that topo- a knownpiscivore (Kohn 1956, 1959a), collected graphicdiversity of the substrateis an important at sta. 17 containedno identifiablefood. One of determinantof Conus species diversity. This threespecimens collected at sta. 27 containedfish seems generallyto hold throughoutthe Indo-West bones and scales, and anothercontained a piece Pacificregion (Kohn 1967). of fish skin. The remains could not be further Only Conus arenatus and one specinmenof C. identified. Fish bones and nereid setae were tessulatuswere foundin Type I habitat,C. ebraeus foundin fecalmatter of the one C. tessulatusBorn and C. miliariswere the most abundantspecies in collected(sta. 20). Type II habitat,and C. lividuis,C. musicus, C. The one specimenof C. tulipa L. collected(sta. rattus,and C. frigiduswere most abundant in Type 21) was placed in a vial of sea water aftercollec- III habitat. In general, species that toleratethe tionand examinedfor feces 7 hr later. None were harsherphysical environmental conditions of Type present,and the specimenwas then placed in a II habitat (wave action, lack of shelter,and sub- fingerbowl of seawater to which a small fishof jection to desiccationat low tide) also occur, al- the familyAtherinidae was introduced. The ten- though at much reduced populationdensities, in tacles of the C. tulipa bearingthe eyes extended the seeminglymore favorableconditions of Type immediately. Five minues later the rostralten- III habitat (reduced wave action,topographically

This content downloaded from 93.97.37.57 on Fri, 31 Jan 2014 10:28:13 AM All use subject to JSTOR Terms and Conditions Autumn 1968 CONUS MALDIVE AND CHAGOS ATOLL REEFS 1059 diverse substrate,absence of exposure to air). both habitattypes, but theyare likelyto be more However, in termsof relativeabundance, species accessibleto Conus in Type II, wherethe gastro- that are rare (C. lividus, C. musicus) or absent pods huntthem over a smooth,uniform, hard sub- (C. rattus) in Type II are dominantin Type III strate. Suitable eunicid habitat is more patchily habitat. C. lividus was also the most abundant distributedin Type III habitat,and the mobility species at the two stationscharacterized by inter- of Conus in searching-forprey must be reduced. mediatehabitat. A directrelation between index of food speciali- The patternsof relativeabundance in the census zation (R) and Conus species diversitywould data generally agree with MacArthur's broken mean that diets were more specialized where stick model for trophicallysimilar species with largernumbers of congenersco-occur in the same non-overlappingniches. However, only popula- habitat. Sample sizes were too small to permit tions of adults have been studied,and the factors analyses for individualstations, but the R values limitingpopulation size remainunknown. Other (Tables 3, 4) are not higherthan in Hawaii where shortcomingsof the data, and some of the model, Type III habitatssupport somewhat fewer species presentedabove limit the heuristicvalue of its (Kohn 1966). applicationhere, and generalizationsfrom the fits Within the Maldive and Chagos populations, of Conus censusesmay not be warranted. Never- comparisonof the relative abundances shown in theless, the close agreementto this model, and Figure 6 with data on compositionof diets indi- wide divergencefrom the alternativemodel for cates that the species characteristicof Type II abundances of the differentspecies determined habitatactually have more specialized diets than independently,may suggest relativelystable asso- those characteristicof Type III habitat. For the ciations in which a change in the abundance of former(C. ebraeus and C. miliaris: four values one species is likelyto affectthe abundanceof the from Tables 3 and 4), avg H - 0.8 and avg others,and that the amountof interspecificover- R - 0.4. For the latter (C. lividus,C. musicus, lap of ecological characteristicsmay influencethe C. rattus,C. frigidus:five values fromTables 3 specificcomposition and diversityof Conus assem- and 4), avg H 1.6 and avg R 0.2. More- blages. over,C. ebraeusand C. miliarisare the two species With fewexceptions, Conus species are primary that are relativelymost abundantin both habitat carnivores.In thisstudy, herbivorous and deposit- types, and the diet compositiondata above are feeding polychaetescomprised 96% of all food derivedfrom both. Separationof the feedingdata itemsidentified. Of the species that are common accordingto habitattype shows that both species in Type III habitatbut rare or absentin Type II, have more specializeddiets in Type II: C. lividusand C. frigidusfeed primarily on seden- Prey speciesdiversity (H) tary, deposit-feedingpolychaetes that build tubes in sand (Maldanidae, Terebellidae,Capitellidae). Type II Type III Conus does not exploit membersof these three C. ebraeus 0.13 1.03 familiesin Type II habitat,where such polychaetes C. miliaris 0.00 0.81 are probablyrare or absent owing to the absence of soft substrate. C. rattus,however, eats pri- Thus in Type II habitat,where environmental re- marily Eunice afra, a burrowing,presumably sistanceto movementis less, and increasedaccessi- browsingeunicid. The diversediet of C. musicus bilityof appropriatefood items makes increased consistedof 62% Nereidae and 30% Eunicidae, foodspecialization a moreefficient feeding strategy mainlyE. afraweighing '8 as muchas those eaten (MacArthur and Pianka 1966, Emlen 1966), the by C. rattus. appropriateshift toward increased food specializa- In Type II habitat,eunicids were the main prey tion is observed,both withinand betweenspecies. of the most abundant species: C. ebraeus fed Althoughit was not possible to studythe type mainlyon Palola siciliensis,and C. miliarismainly of substrateoccupied duringthe nocturnalhunt- on Lysidice collars. Both eunicids have very ing activitiesof Conus, the proportionsof differ- large mandiblesand are foundin burrowsin reef ent microhabitatsoccupied by differentspecies limestone. Nereids are more importantprey in during the day differssignificantly (X2 tests; P intermediate(II-III) habitat. Their abundance < .05) for 6 of the 10 species pairs for which probablydepends on the amount of macroscopic data were adequate. The types and proportions algae growingon hard surfaces. Such algae are of substrateoccupied by C. emaciatusdo not dif- more commonin Type II and intermediatehabi- fer significantlyfrom those of C. frigidusor C. tats than in Type III, perhaps because in the ebraeus,but the firstoccurs in deeper water than formerthey are protectedfrom subtidal browsing the othertwo, and the last occurs predominantly herbivores. Eunicids are probably abundant in in a differenthabitat type. C. rattusoccupies sub-

This content downloaded from 93.97.37.57 on Fri, 31 Jan 2014 10:28:13 AM All use subject to JSTOR Terms and Conditions 1060 ALAN J. KOHN Ecology,Vol. 49,No. 6 stratessimilar to C. musicusand C. chaldaeusbut the present argument temporal stabilityis as- occurs in deeper water than the latter. sumed. Type II habitatis characterizedby much more On Maldive and Chagos atoll reefswhere large uniformsubstrate than Type III. This coupled samples were collected, as in Hawaii (Kohn withthe less diversediets in Type II habitat,per- 1959a), high Conus populationdensity and low mits the conclusionthat Conus species are eco- species diversitycharacterize Type II habitat, logicallymore specializedthere than in Type III whereas low populationdensity and high species habitat, at least with respect to two important diversitycharacterize Type III habitat. This in- aspects,nature of food and substrate. verse correlationseems to be a rather general How is overlapof particularspecies withregard phenomenonin nature (Cole 1964). Two alterna- to substratetype related to overlap with regard ive explanationsseem possiblein the presentcase: to food? Tables 3 and 4 indicatevery low overlap Species diversitycould be greater where abun- in species compositionof the diet among most dance is lower because predatorskeep the popu- species,and statisticallysignificant differences even lations below densitiesthat would lead to com- between the membersof species pairs with con- petitiveexclusion (Hutchinson 1961). Secondly, siderableoverlap. The most strikingof these are habitat complexitymay be the most important C. rattusand C. miliarisat Peros Banhos, where factor.The morecomplex Type III habitatwould the combinationof data fromtwo stationshas to enhance niche diversification,while at the same some extentblurred the habitatdifferences (Table timethe increasedpatchiness of its environmental 5), and C. ebraeus and C. chaldaeus in the Mal- mosaic would reduce populationdensity. How- dives. The lattertwo species are extremelysimi- ever,patchiness also fostersat least limitedgener- lar and probablyphylogenetically closely related alization of habits. A choice between these hy- (Kohn 1959b). They co-occurat the same sta- pothesesis speculative;I have no evidencerelevant tions (Fig. 3, 4) and eat primarilythe same prey to the first. It is tentativelyconcluded that habi- species (R = 0.87, Tables 3, 4), although the tat complexityis likelyto be an importantdeter- higherproportion of nereidstaken by C. chaldaeus minantof Conus population(lensity as well as of makes the differencebetween the diets statistically species diversity. significant.The microhabitatsof these two species differwith respect to substratetypes occupied ACK NOWLEDGM ENTS (Fig. 1, 2; R= 0.63). The usually much more Financial support from National Science Foundation abundantC. ebraeusoccurred mainly on limestone grants and State of Washington Initiative 171 Funds for Research in Biology and Medicine is gratefully pavementand beachrock,covered or not with a acknowledged. I completed the manuscriptwhile holding thinlayer of sand. Most C. chaldaeuswere found a National Research Council Postdoctoral Visiting Re- on dead coral heads, rocks,and rubble. search Associateship supported by the Smithsonian In- The substrate-typeand depth distributionsof stitution. The Yale Seychelles Expedition was made C. rattusare similar those possible by the generosity of Alfred C. Glassell, Jr. I very to of C. miusicuts thank W. D. Hartman for collecting some of the speci- (Ro - 0.86) and C. lividus (Ro= 0.79) in the mens, R. T. Paine and R. H. MacArthur for discussion Maldives (Fig. 1, 2). As noted above, however, and criticism of the manuscript,and Mrs. Roberta Mal- the diets are quite different:C. rattus ate only ley and Miss Diane Stonehouse for technical assistance. eunicids; C. musicus, twice as many nereids as LITERATURE CITED eunicids;and C. lividus,mainly terebellids. Other Agassiz, A. The species pairs withsimilar substrate-type and 1903. coral reefs of the Maldives, depth Mem. Mus. Comp. Zool. Harvard, 29: 193 p. distributions,C. ebraeus-C. miliaris. (Ro -0.80) Bourne, G. C. 1888. The atoll of Diego Garcia and and C. lividus-C.frigidus (R = 0.86), also have the coral formations of the Indian Ocean. Proc. Roy. strikinglydifferent diets (Table 3). The same is Soc. London, 43: 440-461. probablytrue for C. emaciatusand C. aristophanes Cochran, W. G. 1954. Some methods for strengthen- ing the common x2 tests. Biometrics, 10: 417-451. (Ro 0.83), but the food samplesare very small Cohen, J. E. 1966. A model of simple competition. (p. 1058). Ann. Computation Lab. Harvard Univ., 41: 138 p. These measuresof overlapcan be interpretedas Cole, L. C. 1964. The impending emergence of eco- indicatingroughly how similarthe substrateand logical thought. BioScience, 14(7): 30-32. Eibl-Eibesfeldt, I. 1965. Land of a thousand atolls. food of co-occurringspecies of Conus may be. MacGibbon and Kee, London. 195 p. The ecological niches of the species studied are Ekman, S. 1953. Zoogeography of the sea. Sidgwick clearly differentiatedwith respectto one or both and Jackson, Ltd., London. of these dimensions. This is of interestif the Emlen, J. M. 1966. The role of time and energy in assemblagesare relativelystable in time. Unfor- food preference. Amer. Nat., 100: 611-617. Engelmann, M. D. 1961. The role of soil arthropods tunately,brief expeditions do not permitevalua- in the energetics of an old-field community. Ecol. tion of the dynamicsof the assemblages,but for Monogr., 31: 221-238.

This content downloaded from 93.97.37.57 on Fri, 31 Jan 2014 10:28:13 AM All use subject to JSTOR Terms and Conditions Autumn 1968 CONUS MALDIVE AND CHAGOS ATOLL REEFS 1061 Gardiner, J. S. 1903. The fauna and geography of the Kohn, A. J., and Robertson, R. 1968. The Conidae Maldive and Laccadive Archipelagoes, Vol. 1. Uni- of the Maldive and Chagos Archipelagoes. Jour. versity Press, Cambridge. 470 p. Mar. Biol. Assn. India, 8, 273-277. . 1936. The reefs of the western Indian Ocean. Lewontin, R. C., and Felsenstein, J. 1965. The ro- I. Chagos Archipelago; II. The Mascarene region. bustness of homogeneity test in 2XN tables. Bio- Trans. Linn. Soc. London, Zool., Ser. 2, 19: 393-436. metrics, 21: 19-33. Gardiner, J. S., and Cooper, C. F. 1907. Description Lloyd, M., and Ghelardi, R. J. 1964. A table for cal- of the expedition. Trans. Linn. Soc. London, Zool., culating the "equitability" component of species di- Ser. 2, 12: 1-55. versity. J. Anim. Ecol., 33: 217-225. Goulden, C. E. 1966. La Aguada de Santa Ana Vieja: MacArthur, R. H. 1957. On the relative abundance An interpretative study of the cladoceran microfos- of bird species. Proc. Nat. Acad. Sci., 43: 293-295. sils. Arch. Hydrobiol., 62: 373-404. . 1960. On the relative abundance of species. Hairston, N. G. 1959. Species abundance and com- Amer. Natural., 94: 25-36. munity organization. Ecology, 40: 404-416. MacArthur, R. H., and Pianka, E. R. 1966. On opti- - 1964. Studies on the organization of animal mal use of a patchy environment. Amer. Natural., communities. J. Ecol., 52(Suppl.): 227-239. 100: 603-609. Horn, H. S. 1966. Measurement of "overlap" in com- Maes, V. 0. 1967. The littoral marine mollusks of parative ecological studies. Amer. Natural., 100: Cocos-Keeling Islands (Indian Ocean). Proc. Acad. 419-424. Nat. Sci. Phila., 119: 93-217. Hutchinson, G. E. 1958. Concluding remarks. Cold Mitchell, R. 1965. Analysis of species abundance in Spring Harbor Symp. Quant. Biol., 22: 415-427. a water mite genus. Amer. Natural., 99: 117-124. . 1961. The paradox of the plankton. Amer. Paine, R. T. 1963. Trophic relations of eight sympa- Natural., 95: 137-146. tric gastropods. Ecology 44: 63-73. King, C. E. 1964. Relative abundance of species and Pianka, E. R. 1966. Latitudinal gradients in species MacArthur's model. Ecology, 45: 716-727. diversity: a review of concepts. Amer. Natural., Koch, L. F. 1957. Index of biotal dispersity. Ecology, 100: 33-46. 38: 145-148. Pielou, E. C., and Arnason, A. N. 1966. Correction to Kohn, A. J. 1956. Piscivorous gastropods of the genus one of MacArthur's species-abundance formulas. Conus. Proc. Nat. Acad. Sci., 42: 168-171. Science, 151: 592. . 1959a. The ecology of Conus in Hawaii. Ecol. Poisson, H. 1954. Contribution a l'6tude de la faune Monogr., 29: 47-90. malacologique marine de l'archipel des Chagos et de - 1959b. The Hawaiian species of ConUs (Mol- l'ile d'Agalega. Considerations generales sur la lusca: Gastropoda). 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On the anatomy of Conuis titlipa Venomous and poisonous animals and noxious plants (Linn.) and Conus textile (Linn.). Quart. J. Micr. of the Pacific area. Pergamon Press, London, p. 83- Sci., 60: 1-60. 96. Siegel, S. 1956. Nonparamnetricstatistics for the be- -. 1964a. Notes on Indian Ocean atolls visited by havioral sciences. McGraw-Hill, New York. the Yale Seychelles Expedition. Atoll Res. Bull., No. Slobodkin, L. B. 1961. Growth and regulation of ani- 101? 9 P. mal populations. Holt, Rhinehart and Winston, New . 1964b. Notes on reef habitats and gastropod York. 172 p. molluscs of a lagoon island at North Male Atoll, Mal- Stoddart, D. R., Davies, P. S., and Keith, A. C. 1966. dives. Atoll Res. Bull., No. 102, 6 p. Geomorphology of Addu Atoll. In D. R. Stoddart . 1966. Food specialization in Conus in Hawaii [ed.] Reef studies at Addu Atoll, Maldive Islands: and California. Ecology, 47: 1041-1043. Preliminary results of an expedition to Addu Atoll . 1967. 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This content downloaded from 93.97.37.57 on Fri, 31 Jan 2014 10:28:13 AM All use subject to JSTOR Terms and Conditions 1062 DONALD E. LANDENBERGER Ecology,Vol. 49,No. 6

APPENDIX Formulae of indices used Index Symbol Formula Reference Diversity H - pilnpi Shannonand Weaver (1949) Relative H - Xpilnpi Shannonand Weaver diversity Hmax InN (1949) Specialization R -1 H Kohn (1966) Hmax (fromH. S. Horn, unpublished) Overlap Ro (xi + yi) ln(xi + yi) - Exjlnxi - Eyjlnyi Horn (1966) (X + Y) In(X + Y) - XlnX - YlnY P Similarity I = ni- N Koch (1957) i=1 N(P -l) N the total numberof species in the sample -i the numberof species in the ith subsample pi the proportionof the ithspecies in the sample P the numberof samples X thetotal number of individualsin the sampleof a population,X xi = the proportionof the ith species in the sampleX Y - the total numberof individualsin the sample of a population,Y yi = the proportionof the ithspecies in the sampleY

STUDIES ON SELECTIVE FEEDING IN THE PACIFIC STARFISH PISASTER IN SOUTHERN CALIFORNIA'

DONALD E. LANDENBERGER Departmentof Biological Sciences,University of California,Santa Barbara, California2 (Acceptedfor publicationJuly 5, 1968)

Abstract. The asteriidstarfish Pisaster gigcmteusand Pisaster ochraceusare important predatorsof mollusks. Experimentswere designedto determinethe extentto whichpreda- tionby starfishon differentkinds of preyis selective,and to learn whetherselectivity varies accordingto 1) numberand relativedensity of various alternativeprey and 2) past history of feedingof the predators. Seven species of intertidalmollusks were used as prey. When alternativeswere presentedin equal abundanceand in pairs, both species of starfishshowed a significantpreference for one alternativein all but one pairing. The hierarchyof prefer- ences was well-definedand consistentamong replicates. When seven species of prey were presentedtogether in equal abundance,the preferenceswere of the same patternbut were, in general,weaker than those exhibitedwhen prey were presentedin pairs. In all experi- ments,mussels were preferredover othermollusks; the strongestpreferences were forMytilus edulis and Mytiluscalifornianus over otheralternatives. Where mussels (M. californianus) and snails (Tegulcafunebralis) were presentedas alternativeprey, changes in their relative densitieshad littleeffect on the preferencefor mussels. When the relativedensities of two species of snails (T. funebralisand Acanthinaspirata, neither preferred when in equal densities) was varied,a slightpreference was shownfor the more abundantform at one extreme densityonly. In this experimentindividual starfish exposed to the same relativedensities of alternativeprey differed in the proportionsof alternativesthey chose. Starfishfed only on snails (T. funebralis)for 3 monthsshowed an increasedpreference for these snails over 1 Supportedby a predoctoralfellowship (1-F1-GM-30, 554-01) fromthe National Institutesof Health. I thank the membersof the Ecology Seminarat UCSB for criticismof the ideas presentedin this paper. 2 Presentaddress: Departmentof Zoology,University of California,Los Angeles,California.

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