Topsmelt (Atherinops affinis)
By Erika Bland
One of the fish we most often catch at MSI, both on land and on the R/V Robert G. Brownlee, is the topsmelt (Atherinops affinis). Undoubtedly, every student who has visited MSI has had the opportunity to see and t
ouch this fish! Although the young individuals we normally catch are usually only a few inches long, they can grow up to fourteen and a half inches long! Amazingly, the maximum age of these fish is nine years. They are long and thin, and students often describe them as torpedo-shaped, which is a perfect description to help them imagine how easy it is for these fish to swim through the water at high speeds.
When describing an organism’s swimming abilities, scientists use the words neuston, nekton, plankton, or benthic. The only word that cannot apply to topsmelt during one point of it’s’ life cycle is benthic, which describes an organism that lives on the very bottom. All adult topsmelt, and almost all adult fish, are “nektonic” organisms, which describes any animal that can actively swim. On the opposite end of the spectrum are many fish larvae (newly hatched fish), including topsmelt, that are very weak swimmers. For this reason, topsmelt larvae are included in a group of organisms called “plankton,” which means “drifter” in Greek and describes any organism that cannot swim against a current. When it is time for topsmelt to lay eggs, the eggs float to the surface and remain there until they hatch, earning them the title “neuston” which describes all organisms that are found only on the surface of the water.
Swimming together in schools, Topsmelt prefer to live just under the surface of the water, which is a place where most fish do not dare to go because of the danger of birds from above as well as predators from below. Their long, skinny bodies help them avoid being seen, as does their coloration. Topsmelt have silver undersides to imitate the silver light of the sun shining from above, and their blue-gray or green backs help them camouflage with the dark color of the water. This strategy, called countershading, is found in thousands of species of aquatic organisms and is a very effective technique to avoid being seen.
Next time you are near the water, keep an eye out of quick flashes of silver just below the surface, which is the only sign you will see that there is a school of topsmelt hiding right before your eyes!