Communities of animals that grow on surfaces in the sea originate and maintain themselves by the steady recruitment of minute larvae produced by animals. The larvae of sponges, clams, snails, worms, etc., swim in the sea for various periods of time and then must settle in the right places to survive, grow and reproduce. In this way, both desired marine animals, such as those important to marine farming (e.g., clams, oysters, shrimp and lobsters) and undesirable ones, such as those that make up the fouling communities on boats, piers, and power plant pipes (e.g., sponges, barnacles, mussels and tube worms) become established and are maintained. This research will focus on members of the fouling community and ask questions about how tiny larvae can recognize specific surface requirements and settle onto them in the kinds of very active water movement that characterize all marine habitats.
The communities of animals that live on the sea floor, from the shallowest intertidal areas to the greatest depths, are established by the recruitment of very small larvae that were released into the water by bottom-dwelling animals and then dispersed by ocean currents. After spending hours to months swimming and growing in the water, these larvae settle onto surfaces, attach, and undergo metamorphosis (a drastic change in shape and behavior from the swimming larval stage to the bottom-dwelling juvenile stage). The larvae must settle in the right habitats if they are to survive and reproduce, so larval settlement is a critical process affecting the composition and health of ocean ecosystems. Although it has been known that most larvae use very specific biological and chemical cues to recognize the right places to settle, it is still a mystery how such tiny organisms manage to land on suitable habitats as they are swirled around in the turbulent water currents that sweep across the sea floor. To solve this mystery, we formed an interdisciplinary team of scientists from different fields, combining the expertise of biologists who understand how larvae develop and behave with the skill of fluid dynamicists in studying how water moves. We investigated how larvae settle into an important type of habitat, the "biofouling community" that grows on docks in harbors around the world, on the hulls of ships (slowing ship speed and greatly increasing their fuel consumption and contribution greenhouse gases), and on the insides of pipes that draw water from the sea to cool power plants (clogging the pipes). As biofouling communities grow, the number of species increases and the topography of the community becomes more lumpy and complex. We measured water flow across fouling communities on docks in Pearl Harbor, HI, and found that they were exposed to turbulent currents and small waves (wind chop), and occasional large waves (wakes from passing ships). We reproduced those flow patterns over biofouling communities in laboratory tanks so that we could use laser systems to measure water velocities on the very small scales (tenths to hundredths of an inch) of microscopic larvae. We discovered that tiny organisms experience turbulent water flow very differently from how large organisms like humans do. When we swim in the ocean, we can be carried by currents or washed back-and-forth by waves, but we barely feel the little swirling eddies in that turbulent water flow. In contrast, our study revealed that tiny larvae swimming through these eddies encounter rapid (fractions of a second) changes in water velocities and are spun around. We also learned that small larvae that have landed on surfaces experience brief bursts of water flow that are much faster than the speed of the current flowing past a dock. The velocities of those bursts depend on how lumpy the boifouling community is and on where a larva is sitting on that rough topography. We developed microfluidic devices that enabled us to video through a microscope the behavior of swimming larvae exposed to swirling eddies, and to measure the adhesive strengths and behavioral responses of larvae attached to surfaces when hit with realistic bursts of water flow. Larvae of various species are different shapes and sizes, so we also measured how those different body designs affect how big the forces are that tend to wash the larvae away when water flows over them. We have used our results in computer simulations to predict which larval shapes and behaviors enhance or hinder landing on surfaces or sticking to surfaces in different water flow conditions. We also did experiments in which we compared what happened to larvae in water flowing across living biofouling communities versus inert surfaces with the same topography as the communities. We discovered that the way that surfaces taste to larvae when they touch down on living organisms also can have a big effect on which larvae end up settling in which locations. The basic principles we have learned about factors that affect where larvae can settle in real-world water flow conditions should be useful to applied scientists who want to prevent the settlement of larvae on ships or in pipes, or who want to encourage recruitment of larvae of edible species into aquaculture farms. Scientific breakthroughs increasingly are being made at the interfaces between different disciplines. Therefore, we assembled teams of undergraduate and graduate students from both physical and biological sciences and guided them as they worked together to solve some of the questions posed in this study. We used this question-driven, experience-based learning approach to enable these young scientists to develop skills for successful communication and collaboration across disciplines. Many of the students who learned how to conduct scientific research by participating in this project have been women and members of underrepresented groups (Pacific Islanders, Hispanics).
