While travel restrictions have become part of the new normal for people all around the world, a recent study has found that the distance travelled by marine larvae is dictated by both biological and physical constraints.

Marine invertebrates face many challenges when it comes to reproduction. Sperm and sometimes eggs are released into the water where they must meet-up to allow fertilisation to take place. These fertilised embryos develop into larvae and remain in the water column until they find a suitable spot to settle. The amount of time they spend in the water column and the distances they travel can be vastly different for different species.

It is not easy to measure how far larvae travel in real-time so, instead, biologists often use genetic information to work out the relatedness of populations as a proxy for dispersal distance. An alternative approach gathers data on larval characteristics to estimate the time spent in the plankton and so the potential for dispersal.

Mariana Noriega and Dustin Marshall from the Centre for Geometric Biology have been working with colleagues from the United States to examine existing data to help them grasp how larval dispersal distance changes on a global scale. Recent exploration of this question has focused on the role of latitude (or temperature) on larval development, developmental mode (feeding or non-feeding larvae), maternal investment into egg size and hydrodynamics. Often these factors are considered separately rather than all together.

Here’s what we know. Higher temperatures speed up larval development so larvae in the tropics may spend less time in the plankton and disperse less far. But to complicate things, larvae in the tropics are more likely to be feeding larvae which means they tend to spend more time in the plankton than their non-feeding counterparts. Plus, mothers in cooler climes tend to invest more energy into their eggs which for non-feeding larvae means more time in the plankton for those that live at higher latitudes.

Mariana and her colleagues were particularly interested in understanding whether these life-history traits that change with latitude will combine with ocean current information to support their prediction that dispersal distances are shorter in the tropics.

The team have looked at data from 766 marine invertebrate species and classified the larvae into feeding or non-feeding. They extracted data on egg size and the time spent in the plankton, plus the latitude and longitude of the recorded observation.

They were then able to use statistical models to estimate planktonic duration at different latitudes by incorporating their data on development mode and egg size. Having the location of the record also enabled Mariana and the team to estimate local current speeds using the publicly available Mercator-Ocean modelling system. Finally, the expected planktonic duration for the ‘average larvae’ was then multiplied by current speed at each location to estimate dispersal potential.

To the team’s surprise, they didn’t find that dispersal distances were shorter in the tropics.

Instead, they found that the faster surface current speeds in the tropics overcame the effects of temperature on larval development time. So, even though larvae spend less time in the plankton they still have the potential to disperse further than the team predicted due to the faster current speeds.

In fact, the team found that larvae travel further at high and low latitudes, that is, the tropics and the poles. Dispersal distances were shortest in temperate regions where the time spent in the plankton is intermediate and current speeds are slower.

Species richness is greater in the tropics but it seems as if this pattern is not driven by larval dispersal as has been previously suggested. If species richness were driven purely by dispersal distance, this study suggests we would find similar species richness at high latitudes and in the tropics, yet this is not the case.

Understanding patterns in larval dispersal is essential for understanding patterns in marine biodiversity and managing our marine systems. Without this, we will struggle to adequately design marine protected areas, effectively manage biological invasions and predict the consequences of climate change.

This research was published in the journal Nature Ecology & Evolution.