Otoliths or earstones (yes humans have similar structures), used for hearing and balance, are metabolically inert and made of calcium carbonate typically in the form of aragonite, however portions of otoliths can be formed of vaterite or calcite (Campana 1999). The different forms of calcium carbonate are differentiated by crystal shape, and at this point it is unclear as to why otoliths can occasionally be made of forms other than aragonite (Tomas and Geffen 2003). Fish have three pairs of otoliths--saggital, lapilli, and asteriscii—but because saggital otoliths are typically the largest they are most often used for research (Campana and Neilson 1985). Otoliths have long been used in fish ageing studies because of the visibility of daily growth increments. Otoltihs form growth rings, similar to that of a tree, by counting these rings you can make estimates of the fishes age in years, or sometimes in days.
While ageing studies have been quite common in fisheries research, otolith chemistry is a somewhat new field. Otolith microchemistry relies on the assumption that the chemical composition of the otolith is similar to the chemical composition of the water the fish has lived in, thus acting as a sort of natural tag (Elsdon and Gillanders 2003). The process in which otoliths are formed is at the moment not completely understood (particularly by me), and at this point in my life the entire process is somewhat over my head but I will take a shot at briefly explaining what I know. The gills absorb elements from the water and the intestine absorbs elements from food, these elements are then passed into the blood plasma and finally into the endolymph fluid, where they crystallize into the otolith (Campana 1999).
Because otoliths are made of calcium carbonate the most important element in their formation is…you guessed it, calcium. Because of this, uptake of other trace elements is correlated to the concentration of calcium in the water. When high concentrations of calcium are present in the water less trace elements are absorbed than when low concentrations of calcium are present. Elements like, Sr (strontium), Zn (zinc), Pb (lead), Mn (manganese), Ba (barium) and Fe (iron), are less physiologically regulated than other elements and thus are used more often in otolith elemental analysis (Campana 1999).
Because otoliths are metabolically inert (meaning they are not reabsorbed, even during periods of starvation), elements incorporated into the otolith should reflect the environmental history of the fish from its time of hatch to time of death (Elsdon and Gillanders 2003). In combination with age information, otolith elemental analysis can be used to hypothesize about the environmental conditions a fish has experienced throughout its life. For example, the center, or core, of the otolith represents otolith growth of juvenile fish, whereas the outer edges of the otolith represent recent, or current, otolith growth just prior to capture (Elsdon and Gillanders 2003). Therefore, the elemental composition of the center of the otolith gives us an idea of the elemental composition of the water the fish was in when it was born, the elemental composition of the outer edge gives us an idea of the elemental composition of the water the fish was in before it was captured.
Again the easiest way to visualize this is to think of tree rings. The innermost rings represent the growth of the tree when it is was very young. Layers of new wood material form as the tree grows. The outer most rings represent the newest tree growth. The same concept is used in otolith chemistry.
Stay tuned for a post about how otolith chemistry can be used to investigate the movement of anadromous fish!
|On these images of river herring otoliths you can see annual rings near the center and near the edges. The center portion of the otolith represents growth as a juvenile. The outer portion of the otolith represents the most recent growth.|
Campana, S.E., and J.D. Neilson. 1985. Microstructure of fish otoliths. Canadian Journal of Fisheries and Aquatic Sciences 42:1014-1032.
Campana, S.E. 1999. Chemistry and composition of fish otoliths: pathways, mechanisms and applications Marine Ecology Progress Series 188:263-297.
Elsdon, T.S. and B.M. Gillanders. 2003. Reconstructing migratory patterns of fish based on environmental influences on otolith chemistry. Reviews in Fish Biology and Fisheries 13:219-235.
Tomas, J., and A.J. Geffen. 2003. Morphometry and composition of aragonite and vaterite otoliths of deformed laboratory reared juvenile herring from two populations. Journal of Fish Biology 63:1383-1401.