Coral-Reef Asteroids of Guam

Abstract

Most coral-reef asteroids of Guam, at least 24 species in total, represent those which are widely distributed in the IndoWest Pacific. The diversity of asteroid fauna in Micronesia and its surrounding region follows a general trend of a faunal center in the rich Indo-Malayan Archipelago area with the number of species diminishing eastward in the scattered and remote oceanic islands. Larval development in many common reef asteroids is oriented toward dispersion by producing planktotrophic and surface-floating larvae which stay in the pelagic environment for three weeks or longer. The major oceanic surface currents flow westward, driven by steady trade winds in the area surrounding Guam and most of the Micronesian Islands. It is argued that local recruitment of asteroid populations might be hazardous under such circumstances where coastal water masses would hardly conserve larval populations inshore. Juvenile populations of reef asteroids have been rarely located in the field, in spite of conspicuous adult populations. Feeding habits, life histories, and other aspects of natural histories of reef asteroids are discussed. THE CURRENT PROBLEM of Acanthaster planci infestations on reef-building corals has called attention to the fact that little knowledge concerning the biology and ecology of coral reefs and their associated organisms has been accumulated to date. Although there are reports on systematic and zoogeographical studies of coral-reef asteroids and other echinoderms in the tropical Indo-West Pacific (see recent monograph by Clark and Rowe 1971), little is known about behavioral aspects of population recruitment or about feeding, growth, predation, and life history. The problem of Acanthaster infestations is essentially a matter of population dynamics. An understanding of the mode of population recruitment of the starfish to the coral-reef environments of oceanic islands is vitally important in evaluating the processes maintaining asteroid populations. The confusion among the many hypotheses which have been developed to explain the causes of Acanthaster infestations seems to derive for the most part from lack of knowledge concerning the manner and processes of recruitment to the coral reefs. Thus there are two extreme predictions concerning the infestation: one expects expanding infestations to occur as long as human interference with nature exists (Chesher 1969), and the other suggests that present infestations are episodic phenomena (Walsh et al. 1971). Although arguments about infestations are concentrated only on Acanthaster planci, the existence of many other conspicuous coral-reef asteroids must be considered. In spite of the great diversity of adult structure, habitat, behavior, and population densities, many common asteroids produce larvae with similar structure and behavior (Yamaguchi 1973a). This situation suggests that environmental pressure to reduce larval populations may work in a similar manner among many species, resulting in stereotyped larval forms. The island of Guam is located in the trade wind zone where steady, continuous winds produce surface currents drifting in one direction for long periods of time over much of the year. Most asteroid larvae, as well as larvae of other shallow-water inhabitants, may be transported by the currents away from the island for a great distance and would be most unlikely to be able to recruit to the mother populations. Many asteroids have a threeto fourweek larval life span as plankton. This period is long enough for their wide dispersion and transportation, for example, over the distance from Guam to the Philippines, if given a current velocity of one and a half knots. Almost nothing is known about water movement in the open ocean nor in the areas near the islands, except for some crude data on prevailing current such as the North Equatorial Current. Eddy formation and movement multiplies problems of understanding larval dispersion. It is difficult to discuss the mode of recruitment and colonization of coastal animals on coral reefs when we lack reliable information on water movement and its effect on the breeding behavior of these animals and on their larval ecology. 1 Supported in part by the Government of Guam Acanthaster research appropriation and NSF grant GA39948. Contribution No. 54, University of Guam Marine Laboratory. 12 BIOTROPICA 7(1): 12-23 1975 This content downloaded from 157.55.39.159 on Sun, 18 Sep 2016 05:49:46 UTC All use subject to http://about.jstor.org/terms GEOGRAPHICAL DISTRIBUTION OF CORAL-REEF ASTEROIDS IN MICRONESIA AND SURROUNDING