As ocean temperatures soar to a one in a million-year event, many are wondering what this will mean for systems that receive less public attention than our iconic coral reefs. The oceans are a huge carbon sink and small planktonic crustaceans called copepods are a vital link in the planet’s carbon cycle.

Copepods eat phytoplankton, and phytoplankton productivity is likely to be affected by global warming in a number of ways. In some areas the boundary layer between the warmer surface waters and cooler, nutrient rich waters will reduce the delivery of nutrients to the surface waters. In other areas large storms will likely increase the nutrients delivered to surface. The result? Vastly different amounts of food available for the copepods that feed on phytoplankton

PhD student, Alex Blake, and supervisor Dustin Marshall compared the evolution of copepods under high and low food regimes. They were interested in life history traits – characteristics that describe the way an organism uses resources to increase the chance of successful reproduction. These types of measures have immediate knock-on effects for population growth rates.

In 2018 Alex randomly assigned copepods to high-food or low-food regimes with high-food cultures receiving 10 times the food supply of low food cultures. After approximately 30 generations of evolution Alex moved the copepods into a common environment where they received intermediate levels of food and he began measuring size, age at reproduction and reproductive output over successive generations.

Unfortunately for Alex this intense period of lab work coincided with one of Melbourne’s lengthy lockdowns. In order to comply with health and safety regulations during the long hours in the lab, fellow PhD students had to take turns keeping Alex company remotely. Alex was always visible to another lab member via Zoom while he worked his way through the copepod measurements.

When Alex and Dustin were able to get together and analyse the data they found that the different food regimes had evoked evolutionary changes in body size, the relationship between size and reproductive output and offspring investment strategies.

But the changes weren’t all straight forward. When Alex measured body size in the initial cultures as they entered the common environment, low-food copepods were slightly smaller than high-food copepods. After two generations in a common environment, low-food copepods were now bigger than high-food copepods. This counter-intuitive change is sometimes called cryptic evolution; copepods were smaller in response to the low-food environment, but this masked a genetic change where copepods had evolved the capacity to be bigger when the environment allowed.

At the same time Alex and Dustin found that larger mothers in the low-food cultures produced larger but slightly fewer offspring while high-food copepods produced smaller but more offspring. When combined, these two components of reproduction result in the low-food copepods having a steeper positive relationship between maternal size and reproductive output. So, for two large mothers, the low-food mother will have a greater reproductive output than the high-food mother.

Alex and Dustin speculated that the evolution of low-food copepods to be bigger was driven by selection on larger mothers having a greater reproductive output. While they can’t say for sure why low-food copepods had evolved the capacity to be bigger when the environment allowed, they do emphasise that this example of cryptic evolution means that evolution in response to changing phytoplankton productivity may alter the efficacy of the global carbon pump in ways that have not been anticipated until now.

This research was published in Evolutionary Applications.