s m i t h s o n i a n c o n t r i b u t i o n s t o t h e m a r i n e s c i e n c e s • n u m b e r 3 8 SERIES PUBLICATIONS OF THE SMITHSONIAN INSTITUTIONEmphasis upon publication as a means of "diffusing knowledge" was expressed by the fi rst Secretary of the Smithsonian. In his formal plan for the Institution, Joseph Henry outlined a program that included the following statement: "It is proposed to publish a series of reports, giving an account of the new discoveries in science, and of the changes made from year to year in all branches of knowledge." This theme of basic research has been adhered to through the years by thousands of titles issued in series publications under the Smithsonian imprint, commencing with Smithsonian Contributions to Knowledge in 1848 and continuing with the following active series: Smithsonian Contributions to Anthropology Smithsonian Contributions to Botany Smithsonian Contributions in History and Technology Smithsonian Contributions to the Marine Sciences Smithsonian Contributions to Museum Conservation Smithsonian Contributions to Paleobiology Smithsonian Contributions to ZoologyIn these series, the Institution publishes small papers and full-scale monographs that report on the research and collections of its various museums and bureaus. The Smithsonian Contributions Series are distributed via mailing lists to libraries, universities, and similar institutions throughout the world.Manuscripts submitted for series publication are received by the Smithsonian Institution Scholarly Press from authors with direct affi liation with the various Smithsonian museums or bureaus and are subject to peer review and review for compliance with manuscript preparation guidelines. General requirements for manuscript preparation are on the inside back cover of printed volumes. For detailed submissions requirements and to review the "Manuscript Preparation and Style Guide for Authors," visit the Submissions page at www.scholarlypress.si.edu. The paper used in this publication meets the minimum requirements of the American National Standard for Permanence of Paper for Printed Library Materials Z39. Foreword N early two-thirds of Earth's surface is covered by the ocean, a global system essential to all life. Impacts on one part of the ocean can have worldwide effects. The ocean moderates our climate, provides valuable resources, and produces at least half the oxygen we breathe: it makes our planet livable. We know little, however, about the physical, chemical, geological, and biological aspects of this crucial life support system. s m i t h s o n i a n c o n t r i b u t i o n s t o t h e m a r i n S M I T H S O N I A N C O N T R I B U T I O N S T O T H E M A R I N E S C I E N C E S MARINE BIODIVERSITY, EVOLUTION, AND SPECIATION S M I T H S O N I A N C O N T R I B U T I O N S T O T H E M A R I N E S C I E N C E S i i i • S M I T H S O N I A N C O N T R I B U T I O N S T O T H E M A R I N E S C I E N C E SThe Smithsonian Institution, in efforts to increase knowledge about the ocean, has established a ...

Developmental failure caused by excess sperm (polyspermy) is thought to be an important mechanism driving the evolution of gamete-recognition proteins, reproductive isolation, and speciation in marine organisms. However, these theories assume that there is heritable variation in the susceptibility to polyspermy and that this variation is related to the overall affinity between sperm and eggs. These assumptions have not been critically examined. We investigated the relationship between ease of fertilization and susceptibility to polyspermy within and among three congeneric sea urchins. The results from laboratory studies indicate that, both within and among species, individuals and species that produce eggs capable of fertilization at relatively low sperm concentrations are more susceptible to polyspermy, whereas individuals and species producing eggs that require higher concentrations of sperm to be fertilized are more resistant to polyspermy. This relationship sets the stage for selection on gamete traits that depend on sperm availability and for sexual conflict that can influence the evolution of gamete-recognition proteins and eventually lead to reproductive isolation. K E Y W O R D S :Fertilization, gamete, polyspermy, reproductive isolation, sexual conflict, sperm availability, Strongylocentrotus.

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The Caribbean corals, Acropora palmata and A. cervicornis, recently have undergone drastic declines primarily as a result of disease. Previous molecular studies have demonstrated that these species form a hybrid (A. prolifera) that varies in abundance throughout the range of the parental distribution. There is variable unidirectional introgression across loci and sites of A. palmata genes flowing into A. cervicornis. Here we examine the efficacy of prezygotic reproductive isolating mechanisms within these corals including spawning times and choice and no-choice fertilization crosses. We show that these species have subtly different mean but overlapping spawning times, suggesting that temporal isolation is likely not an effective barrier to hybridization. We found species-specific differences in gametic incompatibilities. Acropora palmata eggs were relatively resistant to hybridization, especially when conspecific sperm are available to outcompete heterospecific sperm. Acropora cervicornis eggs demonstrated no evidence for gametic incompatibility and no evidence of reduced viability after aging four hours. This asymmetry in compatibility matches previous genetic data on unidirectional introgression.

When organisms release gametes into the sea, synchrony must be precise to increase fertilization and decrease hybridization.