The Effects of Temperature & Photoperiod on Molting in Seasonal Populations of the Crayfish Orconectes Rusticus Rusticus (Girard)

Immature Orconectes rusticus rusticus (Girard) were collected from Doe Run, Meade Co., Ky. on January 16, June 1, and October 21, 1972, for the purpose of examining the effects of three different temperatures (14C, 18C, and 22C) and three different photo, eriods (6L:18D, 15L:9D, and 24LL) on seasonal molting patterns. For each seasonal experiment, 144 crayfish were placed in 3 x 3 experimental units and maintained for a 90-day period. In Experiment 1 (January 18-April 18), 98 molts were attempted with 98% of the molts being successfully completed. Arrangement of these molts within the separate treatments of this experiment indicated that there was a highly significant linear relationship between increasing temperature and increasing populational molt frequency as well as a significant linear relationship between increasing length of photophase and increasing molt frequency. No crayfish molted during the first 15 days of this experiment and animals in the warmest temperature molted an average of 17.06 days earlier than those in the coldest temperature. Crayfish in Experiment 3 (October 23-January 20) molted in similar patterns with respect to temperature and photoperiod but molted in fewer numbers (43 molts attempted) than those in Experiment 1. Animals molted immediately upon initiation of this experiment and crayfish kept at 22C molted in highest numbers toward the end of the experiment. Ninety five percent of the molts attempted by these crayfish were successful. Crayfish in Experiment 2 (June 3-September 1) molted in the highest total number of all of the experiments (144 molts attempted) but with the lowest percentage of molts successfully completed (30%). Although molt mortalities were high in this experiment, they appeared to occur independent of temperature and photoperiod treatments. Crayfish molt frequency showed a significant quadratic relationship with temperature but no relationship between successful or unsuccessful molt frequencies and increasing photophase was noted. Two auxiliary experiments were also completed in which 46 adults collected in June were tested for molt frequency in three different light intensities (10, 40, and 110 f.c.) and 48 adults collected in October were compared directly with the molt frequency of 48 immatures. In both experiments, adults molted in fewer numbers than immatures tested at the same time, the greatest difference being found in the October experiment (3 adult molts vs. 19 molts by immatures). No relationship between light intensity and molt frequency was found. No sex differences or interactions between treatments were found in any of these experiments. Discussion of these results centered around the characterization of a proposed molt control mechanism for this population of O. r. rusticus. Such a mechanism was characterized as being most sensitive, on a populational level, to temperature and photoperiod during the spring and fall seasons and less sensitive or insensitive to these same signals during the summer. The linear aspect of the relationship between treatments and molt frequencies in Experiments 1 and 3 was interpreted as an indication that either this molt control mechanism is less sensitive to environmental signals than the one previously characterized for more northern populations of O. virilis or that more individual variation exists in this population of O. r. rusticus. The large number of mortalities that occurred in Experiment 2 was discussed but not explained