ON THE RATE OF GROWTH OF POND UNIOS.
BY L. S. FRIERSON.
During the latter part of March 1916, the writer, for the purpose of constructing a fish pond, excavated a barrow-pit near the bank of a small creek, about ten feet wide, and at the time nearly dry. The barrow-pit was perhaps one hundred feet long, fifty feet wide and three feet deep. Early in April, 1916, the pit became full of water, overflowing from the adjacent creek, and together with two subsequent overflows, supplemented with seepage from the newly constructed fish pond, the pit remained more or less full of water, until May 25, 1917, when it was drained by a ditch into the nearby creek. From the dried bottom of this pit some thirty Unios were picked up by the writer. Ten of these were Unio tetralasmus Say, and the rest were T. texasensis Lea. All the specimens were of remarkably uniform size and appearance. The texasensis being about one and a half inches, and the tetralasmus two and a half inches long. Exact dimensions of a texasensis: length 43, height 24, diam. 16 mm.; of tetralasmus 75, 40, and 25 mm.
Both of these species had attained puberty. A female texasensis had its gills fairly full of young glochidia. A tetralasmus had several (three or four) ovisacs with a few (remaining) glochidia. In assigning an age to these shells it is quite sure that the tetralasmus discharges its glochidia in March and early April, so that when picked up on May 25, these shells were just about fourteen months old, from the date of discharge from their mother’s gills.
In the case of the texasensis (which spawns somewhat later) it is possible that these were dropped by fish of which, at least six species obtained access to the pit on May 7, 1916 (on which date an overflow occurred), thus making about thirteen months. At any rate the maximum age of either species is fourteen months from their mother’s ovisacs. One of the U. tetralasmus is shown of natural size in Pl. VII, fig. 4.
Another observation concerning pond mussels might here prove of interest. A large pond was cut into two by a railroad embankment, a culvert preserving the level and providing communication between the two. In the lower and larger pond a half-bushel of Yonkapin (Nelumbium luteum) seed was sown. It was six years before these seeds germinated. These plants, during the summer, cover the entire surface of the pond with their broad peltate leaves. In this pond the writer planted a colony of a dozen Anodonta grandis. Several years after, taking advantage of extreme low water, the writer made a careful survey of these twin ponds, with the result that hundreds of Anodons could be found in the upper pond, but not a single one was found in the lower pond. Either the shade killed the young shells, or else the glochidia-laden fish avoided the shade of the lotus plants and congregated in the upper pond (there are no Nelumbii in the upper pond). Is not this avoidance of shade a reason for the paucity of unios in the tropics?