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 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology. http://www.jstor.org 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 rela- tive abundances differed strikingly. Species characteristic of the physically harsher, topo- graphically 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