Effects of Darkness on Egg Deposition by Etheostoma lepidum Females

Abstract

Etheostoma lepidum females lay eggs more rapidly in the when isolated with homospecific males and more often in the dark when isolated with heterospecific males. It is concluded that visual cues are a major factor in dis- crimination in these fishes. It is also likely that natural hybrids are produced at night in habitats in which one parental species is rare and the other common. Isolating mechanisms can involve a variety of factors. Premating mechanisms are most efficient in that they do not involve wastage of gametes (Mecham, 1961). The isolation mechanisms of darters have been investigated and at least two factors are documented, partial or complete hybrid sterility (Hubbs, 1958) and gametic attraction (Hubbs, 1960). Gametic compatibility does not seem to be involved (Hubbs and Strawn, 1957a). All of these analyses suggest that etheo- logic isolation is exceedingly significant in species separation. Males had been isolated with females of a second species several times and no eggs were laid. This investigation was initiated in an attempt to detrmine if no- choice natural matings could occur and what factors may be involved in the occurrence of such matings. Preliminary investigations showed that visible was exceedingly significant in discrimination by Etheostoma lepidum females. MATERIALS AND METHODS. Stocks of darters were obtained from various Texas localities and brought into the laboratory. Ripe males were placed in 3-5 gallon containers and glass wool added. Females were checked for ripeness daily and when ripe placed with appropriate males. Females were used only once but ripe males were used for up to 3 experiments. Some of the experimental containers were covered, others left uncovered. The covered containers are as- sumed to have been dark and the uncovered containers light. Fluores- cent room lights were kept on all 24 hours so that dark periods did not occur in the light experiments. The experimental containers