Effects of water flow and prey behavior on coral feeding. K. Sebens and K. Heidelberg (University of Maryland).
This project is specifically designed to quantify, using video and other techniques, the small-scale mechanisms that affect how corals feed and capture prey.
Usually, this work is conducted in the laboratory, but Aquarius provides computers, power, and the necessary bottom time that allows scientists to do this work directly on the reef. The advantage is that measurements are made under natural conditions, and the corals are not stressed by removing them from the reef into the laboratory for study. Additionally, a detailed study of coral morphology will be conducted to help explain feeding strategies used by different coral species.
Several factors affect the health and distribution of corals within a reef. Water motion has large impacts on coral biology, affecting processes such as distribution, competition, larval dispersal, fragmentation, activity of predators, and sedimentation. Water motion also delivers prey to corals and enhances uptake and exchange of nutrients, oxygen, carbon dioxide, and other ions. Previous work by Sebens and his students documented that the shape of the coral , and the size and density of polyps affects how corals feed. Polyps of the mounding coral Montastrea are relatively large, and correspondingly less dense(1.2 polyps per square centimeter). On the other hand, polyps of the branching coral Madracis mirabilis are reatively small and densely packed (22.4 polyps per square centimeter)- almost 20 times as dense as Montastrea cavernosa. A typical reef experiences a wide range of flows, and detailed measurements of water motion using various instruments will be collected during the mission. A special current meter is used to measure flow speeds at Conch Reef. Data are posted live during the mission for current speed and direction, temperature, and salinity. Even over a few meters, flow differs substantially causing substantial differences in coral growth rates.
Recent advances in technology have allowed researchers to conduct research that traditionally was possible only in the lab. In situ (e.g. in the natural environment) coral feeding experiments and detailed measurements of water flow will be conducted in 24 hour runs, using the extended bottom time available from Aquarius. High resolution video analysis will be used to record zooplankton interactions with coral tentacles . The proposed research is essential to determine how corals obtain nutrition required for growth and reproduction. While corals are predators on zooplankton, they also contain single celled algae in their tissues that provide energy compounds through photosynthesis. One of the long-standing questions in coral reef science relates to understanding the relative importance of photosynthesis and predation as a means of energy capture by corals. This basic information is related to understanding the distribution and abundance of corals on reefs, especially in Florida where substantial human impacts occur and significant natural system variation in water quality frequently occurs. Changes in zooplankton densities and composition, combined with changes in water clarity, which affects the potential for photosynthesis, are fundamental factors that affect coral growth and reproduction.
Previous results from work conducted by Sebens and his students include:
Several coral species grow significantly faster (20-40%) exposed to zooplankton concentrations about three times what they normally receive (Witting, Sebens).
Feeding also significantly enhances skeletal growth and tissue growth (more than three times). When skeletal growth (where the coral produces its calcium carbonate framework) is limited by reduced photosynthesis, or low concentrations of calcium and bicarbonate, it can be overcome by added amounts of prey capture.
Water flow enhances zooplankton capture overall, although some categories of zooplankton prey have relatively successfully escape behaviors - and can avoid capture. Special underwater "flumes" were used to measure flow rates, feeding rates, and adapters were used to attach high resolution close-up video cameras. It turns out that many corals are food and flow limited, and could be growing faster than they are. Some corals do not depend on prey capture to meet the majority of their energy requirements. Instead, photosynthesis by their symbiotic algae is more important. Growth in these species can be reduced when photosynthesis is limited, as can occur with declining water clarity or during coral bleaching events (Heidelberg, Sebens).
Sediment can be a food source for corals, as well as a stressor. Generally, sediment is thought to stress corals because extra energy is required to remove sediment when it settles on live tissues. However, some coral species are very good at sorting through fine sediments that fall on their surfaces, and they can pick out the most nutritious particles. Nitrogen isotope analysis shows that significant amounts of nitrogen are incorporated into coral tissue from sediment sources (Mills, Sebens).
Interesting facts about corals
Coral tentacles are armed with tiny, poisonous stinging cells, called nematocysts. The nematocysts discharge into potential prey upon contact and help to immobilize and hold prey until transfer into the coral mouth. The nematocyst types and abundances differ amoung coral species. Possibly, these differences help explain why some corals are better zooplankton feeders than others.
Corals feed on zooplankton, primarily at night. We are interested in evaluating how much zooplankton corals consume. These are some of the most common zooplankton types on reefs. They all exhibit marked behavioral differences which either increase or decrease their susceptibilty to capture after an encounter by corals.
Some corals harbor small algae cells in their tissues. These algae cells are able to use sunlight to photosynthesize, similar to terrestrial plants. The coral host then receives some of the "fixed carbon" to use in their own metabolism. During the day, the corals look hard and rocklike.
However, at night, these corals extend their tentacles and are able to capture tiny, microscopic animals in the water, called zooplankton. The zooplankton provide nutrients for coral growth that are not provided by the algal symbiont. Zooplankton capture by corals is what we are primarily interested in studying during our saturation mission.
To study zooplankton capture by corals, we will conduct most of our experiments at night, using special infrared lights, so as not to affect the behavior of the corals or the zooplankton.