Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Biological Sciences

First Advisor

J. Michael Fitzsimons


Fundulus olivaceus and F. euryzonus are sister species that commonly occur syntopically in the Amite and Tangipahoa River drainages in Louisiana and Mississippi. Although the habitats of the two species are quite similar, and the timing of their reproductive activity overlaps extensively, no natural hybrids between them have been documented. I examined physiological, as well as behavioral, factors that could potentially serve as effective isolating mechanisms between them. The courtship and spawning behaviors of the two species were relatively generalized and showed a number of similarities. Under forced mating conditions in which females were allowed to associate with heterospecific males only, hybrids from reciprocal crosses were produced, and hatchlings grew at rates similar to rates of hatchlings from conspecific matings. F$\sb1$ hybrids were fertile and produced F$\sb2$ hybrid hatchlings that also grew as well as conspecific hatchlings. Although both combinations of parental types produced hybrid offspring, fertilization success was significantly lower in crosses involving F. euryzonus females and F. olivaceus males. Additionally, eggs produced from that same combination hatched less successfully than eggs from either the other heterospecific combination or the two conspecific combinations. In laboratory experiments, the observed frequency of spawning sites was non-random for both F. euryzonus and F. olivaceus pairs. Fundulus euryzonus pairs tended to spawn in faster-moving water (mode = 10 cm sec$\sp{-1}$) than F. olivaceus pairs (mode = 1.4 cm sec$\sp{-1}$). Examination of mate preference revealed that although F. olivaceus females were not more likely to spawn with conspecifics, a significantly higher proportion of F. euryzonus females spawned with conspecific mates. Reproductive isolation between F. olivaceus and F. euryzonus is probably maintained in nature by a combination of behavioral (mate preference and spawning site selection) and ecological (small-scale distributional patterns) factors.