Prof. Dr. Andrew Hirst

'Marine organisms tell us how the world works'

The Scientific Department of Estuarine & Delta Systems (EDS) of NIOZ in Yerseke is led by Professor Andrew Hirst. Hirst has a background in academic research at the Universities of Southampton, Liverpool and London, as well as with the British Antarctic Survey. “Most of my work is done on the ecology and physiology of zooplankton, as well as their role in the biogeochemistry and food webs of the ocean. However, plankton are also very diverse in their size, form and function, and they can make excellent models to explore fundamental theories and rules proposed to explain many aspects of all life.”

Plankton in a warming ocean

In our warming climate, we see three universal responses by organisms. There is a shift in distribution, with many cold-blooded (‘ectothermic’) organisms moving towards the Poles. We also see a shift in phenology, where, for example, spring blooming species shift forward in time in warmer years. And, last but certainly not least, we see many organisms become smaller in warmer waters. This temperature-size rule is near ubiquitous and is seen in organisms ranging from single-celled plankton through many invertebrates to fish. This tendency is also seen in cold-blooded animals living ‘in air’, but it is particularly strong for organisms living in marine and freshwaters. We have worked on all three of these responses, and looked to see if the strength of one of these responses might reduce the need to perform the others, as a trade-off.

Smaller bodies in warmer water

A strong hypothesis on why we see temperature-dependent shrinking, looks at the oxygen consumption. Increasing temperature makes it harder to meet an organism’s demand for resources, including oxygen. This may limit the size many marine species can grow to under warmer conditions . Being smaller helps offset the increase in the demand for resources such as oxygen, but can also increases the surface area-to-mass ratio, which may help in its supply.

Sustainable harvest

At the NIOZ department of Estuarine and Delta Systems, we hope to keep on looking at these phenomena, as they may have strong implications for species such as fish, cephalopods like squid and octopus and bivalves like cockles and mussels. These are not only important organisms in marine ecosystems, but also important sources of protein on our plates as well. Understanding the effects of climate change, may therefore ultimately help us to find ways to protect the marine environment better, and stimulate sustainable harvesting.

Bigger fish are more efficient

A second important branch in our research is the so-called metabolic scaling. Metabolism, often measured as oxygen consumption, is the ‘fire of life’, as it fuels the engine of activity and maintenance that all organisms must undertake. How is  energy consumption related to an increase in body size? When a fish, for example, increases its size by two-fold, the demand for energy does not double. Understanding these relations for different sizes and species, and under differing conditions, tells us critical aspects of life and indeed solutions that are possible or optimal in energy use. We have used diverse planktonic species, cephalopods, fish, amphibians, and even spiders and insects, to explore these pace-of-life-issues.

Tiny males

A third pilar of our work involves sexual dimorphism. In many mammals where the males directly compete with other males for females, territory or resources, the males are often bigger than the females. In contrast, in many invertebrates, females are the larger sex, which is often related to the fact that bigger females produce more or bigger young or eggs. But once you start looking at, for example, the more than ten thousand different species of copepods, a small shrimp-like animal, you get a much more diverse picture. Here we see males that are sometimes thousands of times smaller than the females, depending on their way of living. We use a wide range of aquatic organisms, from plankton to fish, to test various hypotheses related to sexual size differences.

Applied science

Questions about sexual dimorphism or metabolic scaling are often of a very fundamental or even esoteric nature. I wish, however, to make this research applied as well. These fundamental principles have consequences for conservation practices, fisheries, and even how we may develop strategies to mitigate against the negative aspects of environmental change. By understanding how the world works, not in the least under the ocean’s surface, we get the chance to improve the outcome of our actions.