Exploring The Science of Larval Dispersal

In a world where charismatic megafauna often capture the majority share of attention, microscopic marine larvae can sometimes end up overlooked. However, Dr. Steven Morgan,  a professor emeritus of marine ecology at the UC Davis Bodega Marine Laboratory and the Department of Environmental Science and Policy has spent much of his career studying oceanography and the ecology, evolution and behavior of marine life through the lens of planktonic larvae. He tackles challenging questions using interdisciplinary state-of-the-art approaches while blending intensive field studies with laboratory experiments. He has been particularly interested in how the timing of larval release by females and vertical swimming by larvae relative to environmental cues increase larval survival. Tracking the fate of microscopic larvae developing for weeks at sea is intractable, spurring Morgan and his colleagues Drs Tom and Donna Wolcott at North Carolina State University to devise and field-test a robot that simulates the vertical swimming behaviors of larvae.

In 2016, UC Davis Media Relations Specialist Kat Kerlin interviewed and accompanied Dr. Morgan and his team to sea to learn more about how they were using the robots to demonstrate that minute larvae really can, and do, exercise control over their own dispersal (see the resulting UC Davis blog, and the video below). The bright yellow, tube-shaped robots - which will likely remind you of a pack of beloved, if slightly bizarre, characters from a series of children’s movies - are officially called Autonomous Behaving Lagrangian Explorers, or ABLEs, that mimic larval swimming behaviors in the water column. With the advent of new technology and the application of knowledge gained from decades of research, these ABLE’s were programmed to simulate larval vertical migration behaviors of different species. Now, five years later, Dr. Morgan is retiring and we’re circling back to this project to find out where the research stands today, and what paths of exploration it may lead to in the future.

Swimming Against a Current Viewpoint

At the heart of this work sits a long-standing debate. It was long thought that when larvae were released by females, they were pushed out into the open ocean by currents and it was just a matter of fortune’s favor who survived and made the journey back to adult habitat to metamorphose into juveniles. But, a growing body of research surveying the horizontal and vertical distributions of larvae relative to currents in the field and observing vertical swimming of larvae in response to stimuli in the laboratory is showing that larvae may regulate their transport. As Dr. Richard Grosberg, Director of the Coastal and Marine Sciences Institute and a Distinguished Professor in the Department of Evolution and Ecology, points out, what we’re really seeing is natural selection in action. 

“A larva doesn’t have to be a genius, natural selection just has to work. And if there’s variation in behavior, and it’s heritable variation, then natural selection is going to eventually favor the evolution of what appears to be very complicated traits and responses. But they’re bits and pieces that are assembled over time. It’s not a question of how can a larva exhibit such complicated behaviors, it’s really a question of just understanding how natural selection works.” 

Morgan explains that natural selection was not often considered in our former understanding of larval dispersal. “It was patently obvious to me as a beginning graduate student,” Morgan explains, but adds that “people have trouble accepting that a weakly swimming, microscopic larva can have any real impact over where it’s going”. Witnessing this tacit and unfounded assumption greatly influenced the trajectory of his career.

One thing that makes this work so significant is the breadth of species it applies to. Morgan has studied the dispersal patterns of invertebrates and fishes along all three coasts of the US. Helen Killeen, a doctoral candidate in the Morgan lab and member of the Graduate Group in Ecology, is demonstrating the role behavior plays in the dispersal patterns of a diverse array of coastal fishes as part of her dissertation and in her Bilinski Fellowship at Bodega Marine Laboratory project. Using two years of data to map the vertical and horizontal distributions off the coast of the Bodega Marine Laboratory, Killeen is finding that fishes effectively regulate larval dispersal in a dynamic coastal ocean. She notes that 

“Steven’s work with the ABLEs has allowed him to draw very clear connections between behavior, dispersal patterns, and populations and ecosystems. That has inspired me to try to do the same with fish larvae. Now that we know what their behaviors are, let’s see if we can take that next step to map out what that means for the dispersal trajectories and say something more concrete about patterns for population distributions and abundances.” 

 

Why Bodega Bay?

Although larvae disperse in oceans all over the world, conducting this work in the vicinity of Bodega Marine Laboratory was important for Morgan because it is located in one of four upwelling centers on earth. Currents are quite strong, leading to the prevailing view that loss of larvae from offshore transport is especially high, which provides an ideal location to investigate the effectiveness of larval behavior in regulating transport. Morgan collaborated with  Dr. John Largier, a Distinguished Professor of Oceanography in the Department of Environmental Science and Policy who is based at Bodega Marine Laboratory, to characterize dynamic ocean currents in the area. As Dr. Largier explains “In Bodega Bay, we’ve studied the movement of the water and the stratification, and we’ve particularly studied it in the context of how and where larvae move. That knowledge base gave us a great hypothesis to work from going into this research.”

ABLEs simulating different vertical swimming behaviors behaved as Morgan hypothesized, demonstrating that  larvae can  regulate larval transport. Unexpectedly, however, most robots remained tightly clustered rather than dispersing away from those programmed with the same behavior. He notes that this has some potential implications related to kinship and the tendency for larvae from a certain group to stay together, travel together, and then wind up in the same place to reproduce.

Tiny Larvae and Big Robots

Each robot weighs 3 kg and is 20 cm tall with a 15-cm antenna, which begs the question of how such a large robot can faithfully represent and mimic microscopic larvae. Larvae primarily regulate transport by vertically migrating in stratified currents flowing in opposite directions. The robot is a neutrally buoyant body moving with the surrounding water and simulates the vertical movements of larvae by adaptively changing the depth of the water parcel it is tracking as a function of time while taking measurements of the physical microenvironment, such as depth, temperature, light, and displacements by turbulence. The vertical change in currents occurs at a scale that is much larger than both a larva and robot so that dispersion is similar for them.

When it comes to understanding how larvae move through the water column, making and releasing behaviorally programmed robots into the ocean sounds like an ideal solution … until it’s time to find them again. While the programmed behaviors demonstrated broad dispersal patterns that were hypothesized by Morgan, variances in ocean current or timing makes them tricky to find. Helen Killeen echoes this sentiment, adding that “The robots have some personality to them. They’re all slightly different and can be temperamental in their different ways.” When it came time to collect the data, the team used satellite tracking to figure out where they were and, once they had reported back on the locations, they’d all climb into the research vessel and set out to find them.

In order to retrieve the robots, Morgan and Killeen, along with Grant Susner, an Electronics Technician at the Bodega Marine Laboratory, and Connor Dibble, then a doctoral student in Morgan’s lab (now a Data Engineer at Scoot Science), would have to spot a gray antenna topped by a tiny flashing LED light  - no easy feat against a backdrop of blue-gray water. To initially home in on it, they used the GPS location transmitted via satellite and unique hydroacoustic signals below water and radio frequencies in the air. Once they spotted one, they’d come up alongside it and grab it out of the water with a net and download data in the lab. Not all recoveries were successful, unfortunately. Occasionally, an ABLE would malfunction and wash ashore, as to be expected when equipment is deployed at sea. 

From a Dozen to Dozens

When it comes to understanding why some larvae survive and others don’t, Morgan has no shortage of interest and questions.  He points out that “You have to know what the basics are before you go diving deep into these questions”, but what are some lines of questioning he’d like to see pursued on this topic? Well… an army of robot larvae would be a good place to start. With the funding to do it, being able to have the sheer numbers to put them into a really diverse array of currents and situations while simulating different behaviors could offer incredible insight into larval dispersal. The problem is, these robots aren’t simple drifters. They mimic larval behavior, so the equipment isn’t cheap to create. This particular project was only able to deploy 12 robots at a time, three each simulating four behaviors, but even more could be discovered by deploying more robots.

Findings and Implications

Through many 12-robot deployments, the Morgan lab compellingly demonstrated that larvae really can exercise control over their own dispersal. Decades of research suggested that larvae may regulate transport, but deploying robots provided the first experimental test of the ability of larvae to effectively do so at sea. This offers real-world applications in fisheries management, marine protected area planning, invasive species response, and understanding and predicting the impacts of climate change.

In fisheries management, for example, Morgan explains that there’s always a need for ways of predicting how oceanographic conditions will impact each year’s harvest. By better understanding how larvae move and disperse, and how they return to the fishing grounds, accurate predictions can be made without the expensive and time-consuming legwork that would normally be involved. He points to Dr. Bill Peterson’s work as an example of this with salmon. Peterson at the Northwest Fisheries Science Center used ocean conditions as an indicator of larval survival to accurately forecast whether or not salmon would be abundant once they were old enough to enter the fishery, even creating a publicly accessible website that offers a “report card” for salmon fishing conditions.

Dr. Morgan also served as co-chair on the Marine Protected Area (MPA) Science Advisory Team, charged with helping to plan the locations, size and spacing of MPAs for the California MPA Network. Protecting a single area large enough to encompass the multitudes and diversity of coastal areas that are home to vulnerable populations would be strategically and politically challenging. To circumvent this, the advisory team worked with stakeholders and the California Department of Fish and Wildlife to establish locations for MPAs along the coast by taking larval dispersal into consideration as one of the criteria. Once the MPA network was established, Morgan’s research then shifted to refining predictions of the connectivity of MPAs along the north-central coast to help evaluate the performance of the network.

From Research to Retirement

As Dr. Morgan transitions from a long, successful career into retirement, his colleagues are acutely aware of how much he’ll be missed in the lab and the classroom. Dr. John Largier commented that 

“(Collaborating with Morgan) has been really good. He’s a smart person with deep insight and active curiosity. He has an ability to question and challenge and try to change paradigms that are misplaced. He was also a very welcoming colleague when I arrived in Bodega Bay and that was super important, to feel like I already had a built-in friend and colleague who was interested in similar ideas and wanted to work together, and was really motivated by understanding the ocean and learning new things. I think part of that is that he’s a marine ecologist and an oceanographer; many marine ecologists are not as aware of how oceanography works. I will miss working with him.”