Pacific Science (1974), Vol. 28, No.3, p. 275-295 Printed in Great Britain Molluscan Distribution Patterns in Fanning Island Lagoon and a Comparison of the Mollusks of the Lagoon and the Seaward ReefsI E. ALISON KAy2 AND MARILYN F. SWITZER3 ABSTRACT: Lagoon molluscan assemblages at Fanning Island are described in terms of three topographical areas: the lagoon reef flat, the patch reefs, and the lagoon floor. Among the large mollusks, CtJIpeomorus brevis, Rhinoclavis asper, Pupa su/cata, Pyramidel/a sp., and two bivalves, Fragum fragum and Tel/ina robusta, are the principal components of the fauna of the reef flat; Cypraea moneta and Trochus histrio are the dominant epifaunal mollusks of rubble on patch reefs; and sessile bivalves, Carditavariegata, E/ectroma sp., Ostrea sandvichensis, and Tridacna maxima, are associated with coral. The micromolluscan assemblages of the lagoon reef flat are dominated by Trico/ia variabi/is, and patch reef and lagoon floor assemblages by Dia/a ftammea. Obtortio sulci/era is the second most abundant mollusk on the patch reefs and O. pupoides the second most abundant mollusk on the lagoon floor. The patch reef and lagoon floor assemblages are distinguishable into assemblages associated with turbid water and clear water areas of the lagoon. Standing crops of micromollusks are greatest on the windward or southeastern periphery of the lagoon reef flat. The lagoon mollusks are distinguished from the seaward reef mollusks in terms of species composition, modes of life, and feeding habits. The lagoon assemblages are predominantly herbivores and suspension feeders among the macrofauna, and are epifaunal herbivores among the microfauna. The seaward reef macrofauna is dominated by carnivores and herbivores, and the microfauna by faunal grazers. Standing crops of seaward reef micromollusks are less than those in the lagoon and the species diversity index is higher. A SURVEY of the littoral marine mollusks of A second expedition to Fanning Island in July Fanning Island in 1970 reported a faunal list of and August 1972 gave opportunity to make a 305 species and showed a clear distinction be­ detailed survey of the distribution of the lagoon tween the species composition of the seaward mollusks. In this report we explore the relation­ reefs and that of the lagoon (Kay 1971). Distri­ ships between molluscan assemblages and bution patterns for the mollusks of the seaward lagoon parameters such as topography, sedi­ reefs indicated discrete assemblages associated ment distribution, salinity, and algal distribu­ with various features of topography. It was tion, and compare aspects ofthe species compo­ suggested that species composition and distri­ sition and modes oflife oflagoon and seaward­ bution of the lagoon mollusks would also reef mollusks. reflect the topographical structure ofthe lagoon. METHODS I Hawaii Institute of Marine Biology contribution no. 439. This study was supported by National Science During July and August 1972, the 42-km Foundation grant no. GA-313111 and by an NSF Pre­ perimeter of the Fanning Lagoon reef flat was doctoral Fellowship to Marilyn F. Switzer. Manuscript surveyed on foot and observations and collec­ received 1 September 1973. tions of mollusks and sediments were made at • University of Hawaii, General Science Department, 98 stations. Twenty-three patch reefs were Honolulu, Hawaii 96822. 3 University of Hawaii, Department of Zoology, sampled by snorkel diving and estimates were Honolulu, Hawaii 96822. obtained of distribution and abundance of the 275 276 PACIFIC SCIENCE, Volume 28, July 1974 NORTH MOllUSCAN COllECTIONS: • LAGOON • SEAWARD REEF 6. SEDIMENT ANALYSIS STATIONS MIL ES o 1 I I I II o 1 2 3 KILOMETERS FIG. 1. General distribution of the stations sampled at Fanning Island. Molluscan Distribution Patterns-KAY AND SWITZER 277 mollusks by throwing a 0.25 m2 quadrat Samples from 29 stations were utilized in on the surface of the reefs, counting all large sediment analysis. A representative portion of mollusks visible in the quadrat, and recording each sample was sorted through a series of substrate types. Fifteen stations on the lagoon sieves, each 20cmin diameter. Table 2 shows the floor were sampled by SCUBA diving and phi (¢) and mesh sizes of the sieves used. Each clam shell dredge. The general distribution of fraction obtained was weighed and its percen­ the stations sampled is shown in Fig. 1. tage of the sample calculated. Cumulative dry­ Sediments from all stations were put into weight percentages were plotted on probability plastic bags upon collection and returned to the paper. The graphic mean, Mz' was plotted, the laboratory where they were placed in open mean defined as Mz = ¢16 +¢50 +¢84/3 (Folk containers and air-dried. Sediment samples were 1968). Theformula<T = ¢84-¢16/2was used to processed in the laboratory for micromollusks, determine graphic standard deviation. that is, for shells less than about 10 mm in greatest dimension. Shells were sorted under a binocular dissecting microscope from standard LAGOON TOPOGRAPHY 25 cm3 volumes. This volumeproduced standing cropS as great as 77 shells per cm3, and an The lagoon molluscan assemblages are con­ average of 10 shells per cm3. The samples were veniently described in terms of three general deemed sufficient for subsequent analysis, as topographical areas: the lagoon reef flat, the larger samples would have been difficult to deal patch reefs of the lagoon basin, and the lagoon with in a routine manner. Although live/dead floor. Details of topography, salinity, nutrient shell ratios were not determined, it is assumed, concentrations, etc. are found in Maragos tl and because living specimens of most species were b, Smith and Pesret, and Stroup and Meyer (all, found, that the dead biofacies patterns reflect this issue). the general occurrence of living populations. The lagoon reefflat is a predominantly sandy A list of mollusks recorded from Fanning flat extending from 50 to 300 meters in width Island is presented in Table 1, and the most around the lagoon shore. Most of the reef flat is commonly encountered micromollusks of the exposed at zero tides. Patches of turf-forming lagoon and seaward reefs are illustrated in Fig. 4. algae such as I-[ypnea, Polysiphonia, and Schizo­ Species counts are based on all mollusks except thrix occur on the flat, as do occasional patches for triphorids and some small rissoids which of rubble and exposed sections of fossil reef were lumped togetherin each ofthose categories studded with Tridacna valves in situ. The lagoon­ respectively because of difficulties in identifi­ ward edge is marked by the occasional occur­ cation. rences of colonies of Acropora and/or Porites Several techniques were utilized to gain a and clumps of the brown alga Turbinaria. Two quantitative insight into the distribution of types of discontinuity interrupt the otherwise individual species. Relative abundance values, continuous reef flat: passes between the islands pr percentage composition of the assemblages; of the atoll and brackish water inlets. Three .frequency, the number of occurrences of a passes dissect the lagoon reef flat, a single deep species in a habitat type; and standing crop, the pass (ca. 8 m in depth) at English Harbor and number of shells per cm3 of sediment, were two shallow passes (or intertidal reefflats ca. 1 m computed. Trophic structure was determined in depth) at North Pass and Rapa Pass. The reef by counting the number of individuals associ­ flat in the shallow passes is paved with coralline ated with a particular feeding habit and dividing algae and mats ofHypnea and Polysiphonia (Doty by the total number ofindividuals in the sample. and Russell 1973). The lagoon shore is dissected Species diversity was calculated using the by inlets which effect changes in salinity in the function surrounding lagoon (Guinther 1971, this issue). H = - ~Pi log2Pi, The lagoon reef flat north of English Harbor is wherePi equals the fraction of the total number distinguished by a short stretch of rubble and of individuals represented by each species shingle shoreline. (Pielou 1969). Thelagoon basin is divisible into a clear-water 278 PACIFIC SCIENCE, Volume 28, July 1974 TABLE 1 MOLLUSKS RECORDED FROM FANNING ISLAND, JULy-AuGUST 1972' AMPHINEURA RISSOIDAE CHITONIDAE Alvania sp. cf. kenneyi Ladd, 1966 Ischnochiton sp. Alvania sp. Merelina (two spp.) GASTROPODA Parashiela beetsi Ladd, 1966 SCISSURELLIDAE Pyramidelloides sp. cf. miranda (A. Adams, 1861) Scissurella coronata Watson, 1886 Rissoina ambigua Gould, 1849 Scissurella sp. R. exasperata Souverbie, 1866 Rissoina sp. cf. incisa (Laseron, 1956) FISSURELLIDAE R. miltozona Tomlin, 1915 Diodora granifera (Pease, 1861) R. plicata A. Adams, 1851 Emarginula bicancellata Montrouzier, 1860 R. tenuistriata Pease, 1867 E. dilecta A. Adams, 1851' R. turricula Pease, 1860 Hemitoma sp. Zebina semiplicata (Pease, 1863) PATELLIDAE Z. tridentata (Michaud, 1830) Patella flexuosa Quoy & Gaimard, 18343 ASSIMINEIDAE TROCHIDAE Assiminea nitida Pease, 1864 Pease, Euchelus angulatus 1867 VITRINELLIDAE Monilea nucleus (Philippi, 1849) Monilea sp. Teinostoma (two spp.) Trochus histrio Reeve, 1848 ORBITESTELLIDAE (= Vitrinellids of Kay 1971) (two STOMATELLIDAE spp.) Stomatella rosacea (Pease, 1867) SKENEIDAE Synaptocochlea concinna (Gould, 1845) Parviturbo minutissimus (Pilsbry, 1921)6 TURBINIDAE RISOELLIDAE Astraea calcar (Linnaeus, 1758)4 Risoella sp. Leptotf!yra sp. cf. wellsi Ladd, 1966 Turbo argyrostomus Linnaeus, 1758 TRUNCATELLIDAE PHASIANELLIDAE Truncatella