Most fishes have small brains, at least when compared with warm-blooded vertebrates. However, holding this against them would be as silly as trying to draw inference about the worth of different groups of people from the (mismeasured) size of their brains (Gould 1981).
Rather, we should realize that fish have evolved the brain size they need, and then use the brain size difference among species of fish to draw inferences on their ‘needs’, i.e., on their niche (see, e.g., Bauchot et al. 1989). The brain size database assembled by Roland Bauchot and his collaborators and kindly made available for inclusion as a table of FishBase allows inferences of this sort. The following describes, based on Bauchot and Bauchot (1986), how this database was created.
Over 2,800 brains were dissected from over 900 species of teleost fishes (see Fig. 49). Many of the fishes were collected at tropical and subtropical localities such as the Hawaiian and Marshall Islands, New Caledonia, Queensland, Australia, the Philippines, southwest India, Mauritius and Réunion, Gulf of Oman, northern Red Sea, Senegal and the Caribbean, but also in France and the North Atlantic. All fish were weighed before removal of the brain and their standard and/or total length taken. The brain was cut from the spinal cord at the first spinal nerves, the meninges and blood vessels removed, blotted and weighed, and then preserved in Bouin solution.
With a large dataset on relative brain size at hand, we were tempted to test some obvious hypotheses. Fig. 50 shows a first attempt to link the BRAINS table with other physiological data, here the OXYGEN table. Both datasets present measurements on individual fish, which in both cases are strongly correlated with weight.
Therefore, we used the slope of the log-log relationship of oxygen consumption vs. body weight and relative brain weight vs. body weight, respectively (plotting for all available data) to correct the individual values for the influence of body weight. For the plot of brain size vs. O2 consumption, we then took the average of the available values for species with at least three records of both brain size and oxygen consumption. Fig. 50 shows that despite a fair amount of variance, the hypothesis that large brains require more oxygen, and are therefore more common in active fish with higher metabolic rates, cannot be refuted. We expect the variance to be less once the OXYGEN table (see this vol.) has been more thoroughly checked, and its own sources of variance further identified.
Rainer Froese and Daniel Pauly
Fig. 49. Relationship between relative brain weight and body weight. Light dots: miscellaneous records in FishBase; black dots: data for sharks and rays, which have large brains, possibly to support their electrosensing ability. In contrast, 6 of the dots below the cloud belong to lampreys.
Fig. 50. Oxygen consumption vs. relative brain weight in 30 species of fishes. See Box 33 for a discussion of this graph.
Because juveniles have a larger brain relative to body weight than adults (Bauchot et al. 1979), it was mostly adult fishes which were used for comparative studies. However, some series were also obtained which range from juveniles to large adults, thus allowing the study of ontogenic changes in brain size.
The single-fish records thus obtained are presented here under the current species names, and consist of the following elements:
- brain weight (in mg);
- body weight (in g);
- a first encephalization coefficient (a calculated field = brain weight / body weight, see Fig. 49);
- a second encephalization coefficient, standardizing for body weight (a calculated field = brain weight / body weight)2/3; (see Fig. 49);
- body length (SL and/or TL, in cm).
These single records are presented for each species in the form of a table, with at least one, and up to 73 rows.
Subsequent work on this table will include incorporation of over 200 records with species names that we have so far been unable to link with valid FishBase names. One of us (James Albert) from the Department of Anatomy, Nippon Medical School, Tokyo, is developing this table further and has already contributed 77 species records representing 18 new families. A paper that analyzes the extended dataset has been prepared (Albert et al. 1999). Also, Ms. Xiomara Chin, Institute of Marine Affairs, Trinidad & Tobago contributed brain weights obtained during her thesis work (Chin 1996).
You get to the BRAINS table by clicking on the Biology button in the SPECIES window, the Morphology and Physiology button in the BIOLOGY window and the Brains button in the next window.
On the Internet, the BRAINS table can be accessed by clicking on the Brains link in the ‘More information’ section of the ‘Species Summary’ page. You can create a list of all species with available data by selecting the Brains radio button in the ‘Information by Topic’ section of the ‘Search FishBase’ page. If you select the Graphs radio button in the ‘Information by Family’ section of that page, you can create Relative brain weight graphs for different families.
We thank R. Bauchot and his collaborators for entrusting FishBase with their valuable records, and J.-C. Hureau for painstakingly transferring them to a file format that we could read. We also thank Ms. X. Chin for 14 records of Caribbean fish brain weights.
Albert, J., R. Froese, R. Bauchot and H. Ito. 1999. Diversity of brain size in fishes: preliminary analysis of a database including 1174 species in 45 orders, p. 647-656. In B. Séret and J.-Y. Sire (eds.) Proceedings of the 5th Indo-Pacific Fisheries Conference, Noumea, New Caledonia, 3-8 November 1997. Soc. Fr. Ichthyol., Paris, France.
Bauchot, M.L. and R. Bauchot. 1986. Encephalization in tropical teleost fishes and its correlation with their locomotory habits, p. 678-690. In T. Uyeno, R. Arai, T. Taniuchi and K. Matsuura (eds.) Indo-Pacific Fish Biology: Proceedings of the Second International Conference on Indo-Pacific Fishes. Ichthyological Society of Japan, Tokyo.
Bauchot, R., M. Diagne and J.M. Ribet. 1979. Post-hatching growth and allometry of the teleost brain. J. Hirnforsch. 20:29-34.
Bauchot, R., J.M. Ridet and M.-L. Bauchot. 1989. The brain organization of butterflyfishes. Environ. Biol. Fish. 25(1/3):205-219.
Chin, X. 1996. A photographic atlas of brains of common Caribbean reef fishes. University of South Florida. B.A. thesis. 62 p.