2 G test with William's correction [Sokal and Rolf, 1983]). Therefore .... Rowe and Doty (1977) may have overlooked several species reported in this paper.
BULLETIN OF MARINE SCIENCE, 52(2): 780-791,1993
CORAL REEF PAPER
ABUNDANCE DISTRIBUTION OF HOLOTHUROIDS (ECHINODERMATA: HOLOTHUROIDEA) ON A WINDWARD AND LEEWARD FRINGING CORAL REEF, GUAM, MARIANA ISLANDS Alexander M. Kerr, Emilly M. Stoffel and Rosanna L. Yoon ABSTRACT We used line transects to determine the abundance distribution of holothuroids on a windward (Pago) and leeward (Tumon) reef on Guam. In a total sample space of 11,134 m2 between depths of 0 and 23 m, we recorded 20,283 holothuroids comprising 19 species. Another five species were recorded as single individuals off the transects. H%thuria atra was the most abundant species recorded and comprised 92% of the holothuroids counted at both sites. The two next most abundant species were considerably less common: Actinopyga echinites and H. /eucospi/ota were 3% and 2% of the enumerated fauna, respectively. Each of the remaining 20 species were 1% or less of the total fauna. Species richness and a species' relative abundance between sites appeared related to overall reef exposure. Species richness was slightly less for Pago (20) than for Tumon (22). Pago had much lower densities of the largest, epibenthic holothuroids common at Tumon, Bohadschia argus, Stichopus ch/oronotus, The/enota ananas and H. nobi/is. Conversely, between-site abundances of cryptic holothuroids were similar. These observations suggested that storm-generated waves, which often devastate the windward reef at Pago, may strongly influence the holothuroid community there. Within-site species richness was associated with physiographic zones. The middle reef flats had the greatest number of species (20) of any reef zone, while reef slopes supported ten species. The fewest number of species, four, occurred along the reef margin, and only two species, Actinopyga mauritiana and H. cinerascens. were found there in abundance. Species abundances at smaller scales were less predictable. Pairwise product-moment correlation analyses of species abundances at Tumon showed that most distributions were independent of one another when enumerated within 10-m2 and 2-m2 quadrats. Less often, there were weak but significant positive correlations. Weak but significant inverse correlations existed between H%thuria atra and three holothuroids found mainly on the reef front or reef slope, A. mauritiana. H. nobilis and S. ch/oronotus. Each microhabitat supported more than one species, and species were often found in more than one microhabitat: Rubble-and-sand bottoms and areas under rocks each supported II species, seven taxa were seen on sand, while three species were found in sand. Macroalgae, turfaceous-algae-covered pavement, the surface ofIive coral and bare pavement had five, three, two and two species, respectively.
Tropical holothuroids are primarily epibenthic sediment feeders and a prom· inent element of many shallow reefs. Densities of 35 to 52'm-2 (Bakus, 1973; Lawrence, 1979) or about 5 to 7 kg wet weight· m-2 are not uncommon for one IndoPacific species, Holothuria atra. At these high abundances, holothuroids may have a large influence on sand-dwelling meiofauna (Renaud-Mornant and Helleouet, 1977) and other reefal organisms through the incidental ingestion of recruits (Birkeland, 1989). Yet, despite the dominance and potential impact of holothuroids in the tropical eulittoral and substantial commercial interest in some species (Bakus, 1973), relatively little is known about the ecology of these echinoderms (see review in Bakus, 1973; Birkeland, 1989). The spatial variation of holothuroid populations is of interest as they reveal community-wide patterns from which testable hypotheses about species interactions can be derived. We examined the abundance distributions of24 species ofholothuroids on a windward and leeward fringing reef to depths of 23 m on Guam, the largest island in Micronesia. Specifically, we sought to answer the questions: (1) How does the 780
KERR ET AL.: DISTRIBUTION
OF GUAM HOLOTHUROIDS
781
distribution of each species vary between sites, between physiographic zones within sites, and at scales of a few square meters? (2) What are the factors influencing these distributions? MATERIALS
AND METHODS
Guam is a small (- 540 km2) volcanic and upraised limestone island in the Mariana Archipelago at about 13°N latitude and 145°E longitude in the western Pacific Ocean (Fig. I). The first study site, Pago, is on the windward side of the island. The second study area, the north end of Tumon, is on the northern and heavily populated, leeward side of Guam. There are several differences between the two sites which we, a priori, expected to influence the relative abundances of holothuroids. The reef at Pago is narrower than 500 m along part of its length, lacks a well developed inner moat, and is exposed to storm-generated waves during the wet season (June to November), particularly along the narrow northern section. The bay is bisected by a deep channel and receives rainwater runoff and fine tuffaceous sediment from the Pago River. By contrast, the reef flat at Tumon is about 500-m wide throughout, has a well developed moat, and is relatively sheltered from most storms. Tumon receives freshwater via an underground aquifer that drains along the northernmost section of shoreline. We defined the intertidal zone as the area ooooor--ooo or-
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BULLETIN OF MARINE SCIENCE, VOL. 52, NO.2,
1993
Table 2. G-test summary table of the speeies richness of two guilds ofholothuroids at Pago Bay and Tumon Bay, Guam. G statistic adjusted with William's correction for small sample size. Common 2: 10 individuals seen, rare < 10 individuals seen. Number of species Guild
Pago
Tumon
Epibenthic species Common Rare or absent
3 7
3
Cryptic and burrowing species Common Rare or absent
7 7
9 5
Statistic
7
G.dj = 3.29 0.05 < P < 0.10
G.d, = 0.59 0.50
