WHAT ARE THE COSTS OF SMALL EGG SIZE FOR A MARINE INVERTEBRATE WITH FEEDING PLANKTONIC LARVAE

Egg sizes of free-spawning invertebrates, fishes, amphibians, and rept iles can reflect an evolutionary compromise between offspring size and number. One explanation for this widespread trade-off in marine inver tebrates is the effect of egg size on performance of the planktonic la rva: smaller larvae from smaller eggs might incur greater risk in the plankton if they feed and grow more slowly. I tested this hypothesis b y experimental manipulation of egg size and measurement of changes in larval feeding performance and growth in the laboratory. In a sea urch in, experimentally halved ''eggs'' became smaller larvae with lower fe eding rates. Preliminary experiments suggested that larvae that obtain less food (either because they develop from halved eggs or because th ey are provided less food) may have longer larval periods or become sm aller juvenile sea urchins. Either cost of reduced egg size might limi t the evolution of large clutches of small eggs. However, halving egg size did not measurably affect time from fertilization to metamorphosi s and had a significant but small effect on juvenile size. Though larv al feeding capability is widely supposed to limit the evolution of ver y small eggs, these experiments suggest that other selective forces mu st also govern the evolution of egg size in species with feeding plank tonic larvae.