Brain like a pumpkinseed
January 20, 2019
There are over 30,000 species of marine and freshwater fish on the planet that can be found in a vast array of different habitats. Within this diversity of species there is a variety of brain sizes and this is observed even when body size is taken into account. If there is a link between brain size and cognitive capacity, could brain size vary with habitat complexity? In other words, does a fish inhabiting the relatively homogeneous open waters of lakes have a similar brain size and cognitive abilities as fish inhabiting the near-shore where there is greater physical complexity?
Axelrod et al. 2018 used pumpkinseed sunfish (Lepomis gibbosus) to address this question. Pumpkinseeds are native to northeastern North America and can typically be found in warm and cool water lakes. Within a lake, this species exhibits different ecotypes, one that lives in the near-shore or littoral habitat of lakes and the other that is in the open-water or pelagic habitat.
In 2016 and 2017, the research team collected 131 adult pumpkinseeds from four littoral and four pelagic areas of Ashby Lake in Ontario, a mid-sized Canadian shield lake. Fish were sacrificed for this study and measurements of body weight and length as well as jaw width were taken. Jaw width adjusted for body length was determined since even within a habitat, fish can specialize to some extent on either benthic or pelagic prey (with benthic prey being characteristic of littoral habitat). Whole brains were weighed and measurements were obtained for each of five brain regions: the cerebellum, optic tectum, telencephalon, olfactory bulb, and hypothalamus.
Their results demonstrated that brains were on average 8.3% larger in the littoral versus pelagic ecotype. Individuals that preferentially fed on benthic prey in either habitat, as indicated by jaw size, also had larger brains. Conversely, the sizes of the five brain regions were unrelated to habitat, indicating that habitat differences may not affect these regions or that they simply cannot change in size.
Based on available evidence, the researchers determined that the larger brain size was not due to the presence of more or higher energy food in the littoral zone leading to more energy for brain growth and not due to differences in cranium size. Their results support that the size differences reflect cognitive requirements for living in each habitat. Studies on other species of fish have also found a similar difference in brain size between individuals that occupy different habitats. Axelrod et al. 2018 study results were novel in that it found brain size differences within a single species rather than between species.
A number of questions remain to be explored around the mechanisms underlying the brain size differences – Do pumpkinseed with a certain sized brain select one habitat over the other? Could brain size be evolving to an optimum size for each habitat? Or, is brain size phenotypically plastic? In other words, as pumpkinseeds grow, do they develop a brain size that reflects the habitat they are inhabiting? These questions are important to further our understanding of how organisms, in general, can colonize different habitats and become ecologically separate populations.
Read the full article here (open access):