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Sea creatures’ senses determined by their size

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Posted November 17, 2015

Why do whales have built-in sonar, and why don’t bacteria have eyes? Because body size largely determines which senses the different organisms have at their disposal, as DTU researchers explain.

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The size of sea creatures helps determine a variety of biological processes in the marine environment, including the creatures’ metabolism and food intake, and the size of their eggs and offspring. A new study published in the Proceedings of the Royal Society of London demonstrates that the sharpness of marine animals’ senses is also defined by their size.

Survival in the marine environment depends on the capacity to use various senses to collect information about the immediate surroundings—in relation to seeking out food and finding a mate, for example; and, of course, avoiding being eaten … To collect this information, different creatures use their senses of smell, touch and sight, their hearing and their sonar capabilities, whereby they emit sounds themselves and then use the returning echo to determine aspects such as their position and direction. But why do most organisms largely use only a selection of their senses?

The smallest organisms, such as bacteria, are thus highly dependent on chemical signals to sense what is happening in their immediate surroundings, while for larger creatures, such as copepods, the ability to detect changes in the flow of fluids (i.e. the sense of touch) also plays a significant role. Among even larger organisms, the senses of sight (crustaceans and fish), hearing (fish) and sonar (toothed whales or ‘odontoceti’) are even more relevant.

“When you look at the diversity of life in the oceans, some of the questions that it seems pertinent to ask include: why don’t bacteria have eyes? Or: why don’t fish use sonar? We wanted to find out whether we could trace a pattern on the basis of the physiological and physical factors that fundamentally determine the way in which the various organisms function,” explains one of the authors of the study, Erik A. Martens, formerly a postdoc at the Centre for Ocean Life at DTU Aqua, currently assistant professor at the University of Copenhagen.

Erik A. Martens therefore teamed up with physicist Navish Wadhwa and a number of colleagues from the Centre for Ocean Life at DTU Aqua to carry out an analysis of the various sensory systems, with a view to examining the size-related boundaries that dictate whether or not the different senses can actually function. One example is the physical size of an eye, which is fundamentally limited by the light-sensitive photorecreptors of opsin molecules. And the answer is clear.

The analysis revealed that size is largely decisive for how marine organisms register their immediate surroundings. Body size determines which senses creatures have at their disposal, and it is thus a crucial factor for aquatic organisms.

“It has to do with physics and physiology. Even though many of our theoretical models are only rough estimates, they are a good match for the conditions in the real world. This knowledge may now contribute to developing better models for the marine ecosystems,” explains Navish Wadhwa from DTU Physics and the Centre for Ocean Life, one of the authors of the report.

Source: DTU

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