The life cycle of the vast majority of marine fish species begins with a millimeter-size larva lasting from weeks to several months, potentially dispersed far away from the birthplace by oceanic currents. Understanding how far the larvae move from their parents is a major goal of marine ecology because this dispersal can make connections between distinct populations and thus influence population size and dynamics. Studying fish population connectivity networks has thus become critical for sustainable management of fishery resources and more generally for the spatial conservation of marine biodiversity through efficient placement of marine reserves.

 Marine ecologists have used two different approaches to understand how fish populations are connected: direct genetic methods that measure connectivity by parentage analyses, and indirect methods using biophysical models that predict larval dispersal by currents and potential population connectivity. There is, however, a mismatch between the model predictions and the genetic observations. The problem with observations is that they are limited in time and space. In other words, they are merely a snapshot of reality but can be used to validate models. On the other hand, probabilistic biophysical models can be used globally, yet they need to include all possible larval behaviors to match reality. It is thought that this mismatch is caused by some unknown behavior of the larval fish.

 The goal of the collaborative project was thus to study the orientation capabilities of larval fish in the wild throughout development and their response to cues under a variety of environmental conditions to see if the gap between observations and predictions of population connectivity can be resolved. Indeed, understanding the complex behavioral responses of larvae to their environment has become a primary objective for those studying larval dispersal. This was done using the neon goby, Elacatinus lori, an endemic species of Belize as a model system. The choice of study system is motivated by the fact that both direct genetic and indirect modeling methods have already been used to describe the dispersal distance for this species living as adult in tube sponges. The genetic observations indicate that dispersal is less extensive than predicted by the biophysical model. More over,  we were able to rear the neon gobi in the lab from hatching to settlement, providing a reliable source of larvae of all ages for proposed experiments,  and used new technology, the Drifting In Situ Chamber (DISC), allowing measurements of larval orientation behavior at sea. 

 Our results provide compelling evidence that fish larvae have the potential to influence their pattern of dispersal throughout the entire larval phase from hatching through settlement. We demonstrate that the high precision in orientation of larvae observed at sea is not random.  Not only neon gobi larvae oriented early in development, but their bearing changes with environmental factors such as tidal phase, distance from the reef, and depth. Surprinsingly, we discovered that they are capable of detecting the direction of the water mass in which they are transported. Indeed, we find for the first time a significant tendency for pelagic fish larvae to orient against the direction of the current. Yet the underlying mechanism of this large-scale rehotactic behavior is still unknown.

A model of larval transport integrating measured currents with behavioral data produced significantly reduced transport compared to a passive transport model. Orientation behavior early in fish development stages has thus the potential to retain the offsprings close to their natal reef, corroborating observed patterns of dispersal using fingerprint of parents and offspring.  Incorporating such behaviors into biophysical models of dispersal may enable us to better predict patterns of larval dispersal and population connectivity.

Our findings determining the influence of swimming orientation behavior in shaping patterns of larval dispersal has important consequences for our understanding of population dynamics and divergence. Because our results show consistent use of external cues by fish larvae, special effort is therefore needed to resolve the orientation behavior of commercially important and key species if we want to achieve a sustainable management of marine fish populations. Such models will also become an invaluable tool for facilitating the design of marine reserve networks. 

Advances and discoveries in the field of larval ecology have been possible with innovative instrumentation and inter-disciplinary (oceanography, modeling, ecology and evolutionary biology) team work on a focal species. The research results are broadly disseminated to the scientific community and general public via peer-reviewed publications, presentations at scientific conferences, appropriate forms of media, and is also presented as a "Movable Exhibit" on fish larval dispersion in oceanic currents, navigation, and population connectivity at the Frost Science Museum (Miami), for the World Oceans Day event – Make a Splash: For Ocean Conservation. This project helped the career development of two PhD students and three undergradaute students.  

Last Modified: 08/27/2018
Modified by: Claire Paris