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Title: Variations in larval density and settlement in space and time : important determinants of recruitment in sessile marine invertebrates? 
Author: Hurlbut, Catherine Jane
Date: 1990
Abstract: Recruitment of new individuals can be an important determinant of community structure for sessile marine invertebrates. In order to identify factors influencing recruitment, temporal and spatial variation in settlement and juvenile mortality was measured in the field for the colonial ascidian Didemnum candidum. a member of a fouling community encrusting docks in Pearl Harbor. Hawaii. Some comparisons were made with co-occurring species. The hourly pattern of larval release was measured by counting larvae released from adults in traps. Acrylic settling plates were suspended from the dock for varying periods of time and at different angles and depths, and sampled non-destructively to census settlement and monitor juvenile mortality over time. The daily pattern of settlement and mortality differed among seven species. Larval release and settlement of the colonial ascidians Didemnum candidum, Diplosoma listerianum and Diplosoma sp. only occurred during the daytime; larvae thus were settling when light was available as a cue to favorable sites. The effect of light level on recruitment of Didemnum candidum was determined by examining larval photo- and geotaxis in the laboratory. comparing settlement intensity between light and dark surfaces in the field, and monitoring survival of juveniles exposed to or protected from direct light. Settling larvae were photo- and geonegative. Settlement and survival were greatest on downward-facing opaque surfaces. The Diplosoma species also settled abundantly on downward-facing surfaces. but subsequent exposure to light did not affect survival. For both genera, settlement and survival were greater on downward-facing than on vertical or upward-facing opaque surfaces, probably due to decreased siltation. Settlement and mortality was more abundant at shallower depths for both genera. However, experimental movement of plates to different depths after settlement revealed that mortality, per se, was independent of depth. Rather, for D. candidum, rate of mortality was a density function, and the higher mortality near the surface was due to the greater number of larvae settling there. For these three ascidians I settlement varied temporally and spatially. The time of larval release and settlement affected recruitment, since light was an important environmental cue used to locate settlement sites on downward-facing surfaces where juvenile survival was enhanced.
Description: Typescript. Thesis (Ph. D.)--University of Hawaii at Manoa, 1990. Includes bibliographical references (leaves 124-130) Microfiche. xv, 130 leaves, bound ill. 29 cm
URI: http://hdl.handle.net/10125/10315
Rights: All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Keywords: Sea squirts -- Larvae, Marine invertebrates -- Larvae, Animal colonies

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