Where the larvae go: Connections between reefs and human communities

Contributed by: Emily Conklin


As I basked in the waves of Kailua Bay, enjoying the sun and surf after a long day spent in front of a lab computer, I noticed I had a small companion. The juvenile mamo (Abudefduf abdominalis), identifiable by its bold yellow and black stripes even at this age, stuck close to me as I swam. Perhaps it was taking shelter from a nearby predator, or perhaps it was simply keen on making a home base out of this human-shaped reef. As I tried to avoid disturbing the young fish, I found myself wondering about its origins. Was it a local, spawned on the reef just outside the bay, or an intrepid traveler, born on another island and transported here at the whim of ocean currents?

Juvenile Abudefduf on O‘ahu. Credit: Keoki Stender.

My work in the Toonen-Bowen (ToBo) Lab at the Hawai‘i Institute of Marine Biology seeks to answer that very question. Many reef species stick close to home as adults—invertebrates  such as ‘opihi or coral stay planted where they settle, and many reef fish are known to have a home range of a kilometer or less. The larvae of these species, however, are much more mobile and have the potential to travel tens (or even hundreds!) of kilometers away from where they were spawned. In fact, this long-distance travel is likely how many marine species arrived in the Hawaiian archipelago in the first place.

Figuring out where these larvae come from and where they go can be quite interesting, especially for someone interested in asking questions about heredity and population biology. For example, do these tiny larvae routinely island-hop? Or do strong ocean currents, like the ones we see between O‘ahu and Maui Nui, create a barrier to larval movement and split a  species into distinct populations? These questions are also important from a management perspective: if we are able to identify certain areas that are important for “seeding” other locations with new larvae, we can make an effort to protect those habitats and better maintain breeding populations of key species.

Unfortunately, the properties of marine larvae make them difficult to study in their natural habitat. For one thing, they start off very small, and often look very different than their eventual adult form. This makes marine larvae hard to identify at the species level without costly genetic sequencing–and that’s if you’re lucky enough to find them in the first place. Locating, capturing, and identifying larvae smaller than a fingernail in vast tracts of open ocean quickly becomes a Herculean task. It’s even harder to accurately track the movement of an individual larva, since they are bit too small for electronic tagging. For these reasons, we often turn to the power of computers to estimate patterns of larval dispersal.

Selected larval paths for he‘e larvae (Octopus cyanea),
simulated over a period of several months.

Using a model of the ocean’s physical properties such as current direction and velocity, we can simulate larval spawning and see where they are transported. It’s a bit like a video game where the winners are larvae that successfully find suitable habitat, while losers are swept off to sea. I focus on larval connectivity around the island of Moloka‘i, and have been simulating larval transport for a total of eleven species (eight reef fish and four invertebrates). Specifically, I am interested in the role of Kalaupapa National Historical Park in dictating patterns of connectivity for the area: do these protected waters act as a larval source for the rest of Moloka‘i?

Kalaupapa National Historical Park from above. The park encompasses
the peninsula as well as submerged lands up to a ¼ mile offshore.
Credit: NPS/T. Scott Williams.

Currents are not the only thing that influence the path of travelling larvae. Physiological characteristics like larval behavior, the time it takes before larvae are ready to settle, and the strategies adults use to reproduce (e.g. spawning in the water column vs laying eggs on the ocean floor) can all affect a larva’s fate. We can use many of these traits and behaviors in our model, by incorporating data previously gathered in the laboratory and field. For other important information—such as where adults are found, and when and where they spawn—it helps to turn to local experts.

Lucky for my work around Moloka‘i, we (Dr. Toonen and myself) knew the right local experts to get in contact with. We flew to Moloka‘i on a sunny Monday morning to meet with Kalaupapa managers and members of the fishing community, as well as some of the island’s kūpuna, who have generations of experience and knowledge about the island’s marine community. Our trip served the dual purpose of both relaying our preliminary results and requesting feedback: based on their knowledge, did the results from our model make sense? Were we spawning larvae in the right habitat, at the right time of year, at the right phase of the moon and time of day? With their advice, I was able to calibrate spawning parameters for several species and make the model more realistic.

Now that we have preliminary results in place, the next step is to test how well our model performs. To do so, I and several other members of the lab are currently gearing up to collect samples of cauliflower coral, or Pocillopora meandrina, from several locations along the Moloka‘i coast. Over the summer, we will prepare these tiny samples of coral for genetic sequencing. Using DNA sequence data we can determine how corals are related to each other  around the island, which provides a measurement of the movement or lack of movement of coral larvae among the different areas in which we sample. This information can then be compared to the results predicted by our computer model.

Pocillopora meandrina colony. Credit: Waikīkī Aquarium.

Much like larvae flowing between reef patches over the generations, science communication is most effective as a multi-step process. I hope to continue working on projects that incorporate traditional ecological knowledge, as well as communicate big-data learning outcomes back into the communities they benefit. Conserving habitats that are important larval pathways is one piece of the puzzle for effective reef management, and ensures that traveling larvae can continue to venture across the open ocean to find a home—even if they do occasionally mistake swimmers for a nice coral head.





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