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Feeding and growth of prosobranch veligers

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Title: Feeding and growth of prosobranch veligers
Authors: Bell, Janice Lee
Issue Date: 1993
Abstract: For marine invertebrates, larval growth rates in the plankton impact dispersal, recruitment and the timing of metamorphosis. Previous laboratory studies of gastropod larvae have suggested that adequate growth is achieved only when concentrations of algal food are much higher than phytoplankton concentrations in the ocean, especially those in oligotrophic environments. The few studies of larvae feeding on food in natural seawater have suggested two possibilities: larvae are food-limited or larvae are growing at near-maximal rates. The effects of larval concentration, food diversity, nannoplankton concentration and picoplankton abundance on larval growth and survival were evaluated for the prosobranch gastropods Crepidula aculeata and Crucibulum spinosum. Larvae were grown on particles in natural seawater or on single or multiple species diets of the laboratory-cultured algae Tahitian Isochrysis galbana, Nannochloropsis oculata, and Chaetoceros gracilis. Larval growth rates increased with the number of algal cells available per larva. Highest growth rates were achieved at the lowest larval density (20 larvae/liter), and larvae in this low concentration grew well, even with algal concentrations as low as those in ambient seawater. This larval density, unusually low for laboratory culture, was designed to simulate ocean conditions. When algal concentration and diversity of algal cell types were compared, larval growth rates were not influenced by diversity; total algal biomass as indicated by dry weight had a significant effect on growth rates. Two factors in natural seawater, picoplankton and availability of patches, may contribute to its superiority as a food resource. Larvae grown on particles from natural seawater demonstrated higher growth rates and percent survival than larvae grown on cultured-algal diets of the same particle concentration as the seawater. When natural seawater was enhanced with cultured algae, larval growth rates were significantly higher than in natural seawater alone. Thus growth was limited by food available in ambient seawater. Particles in seawater were separated into size fractions by reverse filtration, and ambient phytoplankton particles ( > 2 μm) were concentrated (3X) to simulate patches. Larval growth rates in the concentrated fraction were higher than those in ambient seawater. Thus the food-limited growth resulted from insufficient phytoplankton biomass in ambient seawater, not from a nutrient deficiency corrected by enhancement with laboratory-cultured algae. Larvae grew at rates close to maximum when reared with particles from a naturally occurring patch of plankton. Nannoplankton particles in the patch were 30 times more abundant than those in average ambient seawater. Since larvae are food-limited at average ambient phytoplankton concentrations, larval growth and survival are enhanced when phytoplankton patches are available. Significant larval growth resulted from a diet of only picoplankton particles ( < 2 μm). Larvae cultured in the picoplankton fraction enhanced with cultured algae grew significantly faster than larvae in Millipore-filtered seawater enhanced in the same manner. Thus the major contribution to increased growth was picoplankton and not dissolved organic matter (DOM). The lack of significant growth in the 0.22 Millipore-filtered flow-through control also demonstrated little contribution from DOM. In Hawaiian coastal waters 25-50%of total chlorophyll a was derived from picoplankton particles. In offshore waters, picoplankton accounts for 6080% of the chlorophyll a. In oligotrophic oceans, larvae may derive a considerable portion of their nutrition from picoplankton. Most information about larval growth and life-span has come from laboratory studies. However, laboratory conditions (temperature, light, food and antibiotics) probably do not closely simulate conditions encountered in the plankton. Our knowledge of larval life-span would be enhanced if the age of field-collected larvae could be assessed. For gastropods, the statolith, an integral part of the statocyst sensory apparatus, may furnish the age of a larva. Statoliths of C. spinosum, C. aculeata, and Littoraria scabra were examined with light microscopy and scanning electron microscopy (SEM). In laboratory culture, the number of incremental layers in the statoliths was highly correlated with larval age in days. The statolith diameter also increased in a linear relationship with larval age. Knowledge of the age of field-caught larvae will allow us to answer questions about larval growth, metamorphosis, dispersal and longevity in the plankton.
Description: Thesis (Ph. D.)--University of Hawaii at Manoa, 1993.
Includes bibliographical references (leaves 159-172).
Microfiche.
xiii, 172 leaves, bound ill. 29 cm
URI/DOI: http://hdl.handle.net/10125/10329
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.
Appears in Collections:Ph.D. - Zoology



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