“Dense dwarfs versus gelatinous giants: The trade-offs and physiological limits determining the body plan of planktonic filter feeders”
Julia Dölger, Thomas Kiørboe, and Anders Andersen (Aug 2019)
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A theoretical model explains why gelatinous plankton are gelatinous
Gelatinous plankton form a special group of marine organisms due to their distinctly watery bodies with low carbon content. We find the gelatinous body plan mainly in large, centimeter-sized plankton who feed by filtering their micron-sized prey out of the water instead of perceiving it at a distance. Why do such planktonic giants tend to be gelatinous while microbes (dwarfs) that filter feed on the same prey stay dense? A marine biologist and two physicists in the Centre for Ocean Life at the Technical University of Denmark have resolved this question by developing a new theoretical model.
By modeling the energy budget of filter feeders, the authors show that the interplay between gain and cost of energy has the key implication that a minimum filter area is necessary to collect enough food to sustain a living. Through comparison of their model predictions and existing data on filter feeders, the authors conclude that filter feeders need to be either small or if they are large to increase the filter area by being gelatinous in order to survive in dilute oceanic environments. Large plankton that are non-gelatinous also exist, but they either live in prey-rich environments or compensate by sensing their prey at a distance.
Such understanding of planktonic survival strategies is essential to model and predict the global distributions of plankton. It directly connects to the main goal of the Centre for Ocean Life where marine ecologists, physicists, chemists, and mathematicians are working jointly to build a trait-based description of life in the ocean.
Most marine plankton have a high energy (carbon) density, but some are gelatinous with approximately hundred times more watery bodies. How do those distinctly different body plans emerge and what are the trade-offs? We address this question by modeling the energy budget of planktonic filter feeders across life forms from micron-sized unicellular microbes like choanoflagellates to centimeter-sized gelatinous tunicates such as salps. We find two equally successful strategies, one being small with high energy density (dense dwarf), and the other being large with low energy density (gelatinous giant). The constraint that forces large – but not small – filter feeders to be gelatinous is identified as a lower limit to the size-specific filter area, below which the energy costs lead to starvation. A further limit is found from the maximum size-specific motor force that restricts the access to optimum strategies. The quantified constraints are discussed in the context of other resource acquisition strategies. We argue that interception feeding strategies can only be accessed by large organisms if they are gelatinous. On the other hand, organisms that use remote prey sensing do not need to be gelatinous, even if they are large.