Most bottom-dwelling invertebrate animals have complex life cycles that include pelagic larvae. Many of these larvae feed in the plankton as they develop, grow, and disperse to habitats where they settle and metamorphose to benthic juveniles. The duration of the larval period is central to any model of transport and recruitment. Similarly, the size at metamorphosis can contribute to the success of newly settled recruits. The rates of larval development and growth have long been known to be influenced by larval nutrition, and recent data on small-scale patchiness in the plankton underscore the prevalence of short-term food variability in nature. Previous efforts to quantify the effects of food variability as larvae develop in the plankton have, however, been limited. In contrast, ecologists studying the life cycles of amphibians and insects have created several theoretical models and performed comprehensive experiments to test the effects of environmental variability on the timing of and size at metamorphosis. This study will apply lessons learned from ecological studies of amphibian and insect metamorphosis to test the effects of short-term food variability during the development of marine invertebrates.
Lab experiments will be performed on diverse larvae (polychaetes, bivalves, crustaceans, echinoderms) to test the quantitative predictions of a theoretical model of larval development and growth in the context of short-term food variability. The theoretical framework and empirical data will foster a comprehensive understanding of the effects of food variability, which then can be incorporated into oceanographic models of dispersal and recruitment. In addition to measuring the timing of and size at metamorphosis in variable feeding regimes, the temporal expression of microRNAs that are known to regulate the timing of development also will be measured, with the aim of acquiring new tools to assay larvae in situ.
Education and outreach will be integrated explicitly into the research program by having teams of undergraduate students perform many of the experiments as part of a year-long capstone course ("Senior Research Experience: Marine Invertebrate Larvae") and by developing a peer-mentoring program involving high-school teachers and their students (LEARN: Larval Ecology And Research Networking). In the first semester of the capstone course, undergraduates will learn the diversity of invertebrate larval forms, practice techniques for culturing larvae, and develop individual research proposals focused on a species of interest. Students will review proposals written by their peers and evaluate the proposals in a panel-review setting. In the second semester, teams of four students will collaborate to revise the proposals into group projects and then perform the experiments. Most larval-feeding experiments are not technically difficult, but they are very labor intensive. Working and learning in cooperative teams will distribute the workload and enrich the educational experience. Undergraduate teams will compile video journals during their research and will present their projects in four formats: a scientific paper, an oral presentation, a poster, and a presentation to a high-school biology class as part of the LEARN program.
LEARN will be a program involving the PI and his undergraduate students collaborating with high-school biology teachers and their students. The program will target inner-city and ethnically diverse high schools. The program's centerpiece will encompass high-school biology classes performing larval feeding experiments on hardy crustaceans that have larval periods lasting just a few days. These simple experiments will parallel the more elaborate ones performed by the senior undergraduates, creating a common theme to facilitate discussion and peer mentoring between the high-school and college students. The program's design will be refined each year during workshops involving the PI and the high-school teachers, and additional workshops will aid the undergraduates in mentoring the high-school students. Furthermore, the LEARN program will include opportunities for the high-school teachers and students to participate as summer interns in the PI's research lab.
This project examined how daily and weekly environmental variability, especially shifts in food, affect the young, larval life stages of marine invertebrates. Most marine invertebrates that live on the ocean bottom begin life as microscopic larvae drifting and feeding in surface waters for weeks or months before growing and eventually undergoing metamorphosis to become a bottom-living juvenile. This sort of life cycle involving larval growth and metamorphosis is ecologically similar to a tadpole becoming a frog or a caterpillar becoming a butterfly. The consequences of larval nutrition for the success of later stages in the life cycles of marine invertebrates are generally unknown and have implications for the conservation and management of marine populations and ecosystems. We performed a series of controlled laboratory experiments on diverse invertebrates to examine how food variability affects ecologically important parameters such as the growth rate of larvae, the time it takes them to metamorphose, their sizes immediately before and after metamorphosis, and the growth rate of juveniles soon after metamorphosis. The most important and conclusive of these experiments involved larvae of a reef-building honeycomb worm, Phragmatopoma californica. These larvae must grow and develop to a stage called "metamorphic competence", which means they are able to respond to chemical stimuli that initiate metamorphosis. Larvae that experienced a switch from low food to high food at a young age reached metamorphic competence earlier than and at larger body sizes than larvae that experienced increased food a few days later in the larval period. In addition, the size of larvae when they first became metamorphically competent was very sensitive to changes in the amount of food 1-4 days before they reached metamorphic competence. This 4-day period appears to be critically important for determining a larva’s size at metamorphosis. Data also show that body size immediately after completing metamorphosis was strongly correlated to size immediately before metamorphosis. We also performed an experiment in which larvae that were metamorphically competent were either fed or starved for an additional month before being stimulated to actually metamorphose. Larvae previously fed a high food concentration before being starved did not lose their ability to metamorphose when chemically stimulated to do so, but 80% of larvae initially fed a low food concentration lost their ability to metamorphose if they were starved for more than 10 days. Starvation also led to smaller body size after metamorphosis compared to continuously fed larvae, with longer starvation periods leading to greater declines in size. We also performed several experiments to determine if the feeding history of larvae before metamorphosis extended to the growth rate of juveniles during their first week after metamorphosis. We raised larvae on high food or low food and also starved subsets of each group for 1 week after they reached metamorphic competence. After stimulating larvae to metamorphose and build their juvenile sand tubes attached to microscope slides, we transplanted slides containing the post-metamorphic juveniles to a sandy beach and recovered and re-measured these individuals 1 week later. Low-food and starved juveniles that had smaller body sizes immediately after metamorphosis remained smaller than high-food juveniles that were initially larger, but the relative growth rate of all juveniles was similar during the week-long field transplantation. In terms of the growth rate of juveniles, present conditions in the juvenile’s environment are much more important than the earlier conditions experienced by larvae. The absolute size of recently metamorphosed juveniles might, however, be ecologically important for longer term survival and success. We also performed a series of laboratory experiments to determine how well microscopic larvae of the sand dollar, Dendraster excentricus, can actively swim to and aggregate near thin, 1-cm patches of water enriched with food or avoid thin patches containing chemicals associated with predatory fishes. Thin patches were created in salinity-stratified water columns in cylindrical aquaria, and the vertical positions of hundreds of larvae were observed in each cylinder. We consistently found that the number of larvae actively aggregating near salinity thin layers increased as larvae aged. Surprisingly, sand dollar larvae did not actively aggregate near food-rich patches more than food-poor patches. Larvae did, however, avoid thin patches that contained seawater previously inhabited by a fish, suggesting predator-avoidance behavior on a small spatial scale. The project provided research experience for 37 undergraduate students, many of whom completed a new two-semester course "Marine Larval Ecology Research Experience" (Biol 516A and 516B). These college students visited local high schools annually to share live invertebrate specimens, engage high school students in hands-on activities like chemically inducing adult sand dollars to spawn, and discuss their Biol 516B research projects. Two Ph.D. students and three M.S. students were supported by the project. Four high school students gained research experience through internships, and a high school teacher participated in a summer-long experiment as a paid intern.
