Size Matters (At Least When it Comes to Brains), by Jessica Fitzpatrick

            Fish are the largest and most diverse group of animals. They come in all different shapes, sizes, colors, and habitats. Each family of fish has different adaptations within that have evolved over millions of years to allow them to live in such diverse habitats. Brain size is one characteristic that has adapted especially different over the years. Different brain sizes are helpful in different living situations; however, they come at a cost. Fish that experience moderate to extreme encephalization must compensate for the large metabolic energy costs of brain tissue by performing tradeoffs with energy use of other organs or by increasing overall energy intake. This paper will explain how the fish brain metabolizes energy and how they deal with the increased energy requirement.

            The brain is one organ that usually takes up about 1% of overall body weight in vertebrates and utilizes between 2-8% of energy consumption (Soengas and Aldegunde, 2002). The teleost brain obtains its energy from the oxidation of glucose while the elasmobranchs oxidize ketones and amino acids. Many factors can affect the main metabolic processes for the brain such as an absence of food, increase in water pollution, and water hypoxia. These situations are characteristic of an unhealthy environment that puts stress on the fish and therefore the fish must undergo hormonal or behavioral changes to maintain its health. Fish with unusually large brains must do even more to meet their needs in these stressful areas. The fish brain needs a specific amount of energy to function and when aspects of the environment do not meet the fish’s needs, adaptation must occur to continue living under those conditions.

            A larger brain is associated with increased oxygen intake or metabolic rate. Metabolic rate can be determined by measuring the rate of oxygen consumption over a specified period of time (Sukhum et al., 2016). Most fish that experience only moderate encephalization perform energetic tradeoffs between their organs and their brain; however, these tradeoffs are insufficient for fish that experience extreme encephalization. These fish’s increased metabolic rate require a much greater energy intake which is often satisfied by spending more time foraging for food and adaptations to assist in finding the food. Due to the increased need for oxygen to support the unusually large brains that some fish have adapted, these fish have a very low tolerance for hypoxic environments. Some fish deal with this limitation by migrating to areas of higher oxygen concentration. When distribution is limited, having a larger brain can be more costly than it is beneficial.

            There are many fish that experience extreme encephalization and therefore must deal with the tradeoffs associated with a larger brain; however one species has an especially large brain with extremely costly energy demands. One species of the mormyrid electric fishes from Africa, Gnathonemus petersii, has a brain that makes up about 3% of its overall body mass (Sukhum et al., 2016). This may not sound like much but when compared to human brains which only make up about 2% of body mass, it becomes clear that their brains are relatively larger than what most animals experience. Human brains are responsible for about 20% of oxygen consumption while the Gnathonemus petersii brain uses an incredibly high proportion of oxygen at 60% (Nilsson, 1996). The energy intake of the brain is so significant in this fish because it is ectothermic and therefore is much more expensive when compared to the energy budget of the endothermic human. When this fish finds itself in hypoxic environments it resorts to gulping air in an attempt to increase oxygen intake. When in an ideal environment, this species has dealt with its greater need for energy by adapting a flexible chin appendage which utilizes electrolocation to find food; however this sensory process is supported by the large brain (Sukhum et al.,2016). Encephalization in fish is an interesting concept because the costly large brain is paid for by increasing energy intake but the appendage responsible for the energy intake is only possible due to the capabilities of the large brain. Another fish that experiences extreme encephalization is the Carcharhinus falciformis (Linsey, 2006). This shark has a relatively large brain compared to other sharks and teleosts and this large brain lends many benefits. For example, the brain’s telencephalon is very large allowing it to communicate with other group members and form pairs. This shark also has a large cerebellum which assists in sensory-motor integration. This makes sense for this species as it is one of the most powerful swimmers of all sharks. Sharks rely on smell to find food so it is common for them to have a well-developed olfactory bulb; however this shark species has a larger than average bulb to assist in more wide-range hunting. Although the large brain requires more food and therefore more time spent hunting, Carcharhinus falciformis, an apex predator in ocean, is obviously having no problem meeting the increased energy needs and is enjoying the many capabilities that a large brain provides.

            Different classes of fish utilize their resources in different ways to support the energy needs of their bodies. Fish have a diversity of adaptations that allow them to live most efficiently in their niche and often times these adaptations lead to an increased need for energy which must be dealt with through either physiological or behavioral changes. Large brains require significant energy inputs. The fish that experience extreme encephalization usually rely on behavioral changes to meet their energy requirements as tradeoffs between organs would not provide enough energy. How they deal with that incredibly expensive brain shapes how and where they live and is therefore evolutionarily significant.

References

Linsey, T. J. 2006. Brain morphology in large pelagic fishes: a comparison between sharks and teleosts. Journal of fish biology 68:532-554.

Nilsson, G. 1996. Brain and body oxygen requirements of Gnathonemus petersii, a fish with an exceptionally large brain. Journal of Experimental Biology 199:603-607.

Soengas, J. and Aldegunde, M. 2002. Energy metabolism of fish brain. Elesevier 131:271-296.

Sukhum, K., Freiler, M., Wang, R., and Carlson, B. 2016. The costs of a big brain: extreme encephalization results in higher energetic demand and reduced hypoxia tolerance in weakly electric African fishes. Proceeding of the Royal Society B 283:2016-2157.