American Society of Ichthyologists and Herpetologists, Portland, OR, July 2009.

Phenotypic Plasticity or Hybridization? Meristic and genetic variation in the Australian fish species Retropinna semoni along the Murray River.

T.E. Haynie, M. Adams, P.J. Unmack, and J.B. Johnson

Previous work has shown that meristic counts in Retropinna (Osmeriformes) are lower in northern areas (warmer) than southern areas (cooler). Here we test three hypotheses along an upstream downstream gradient: 1) variation is due to phenotypic plasticity as a result of different environmental conditions during early development. If temperature is affecting meristic counts we predict there should be a gradual change from higher to lower counts as one moves downstream. 2) Previous work found evidence for hybridization in the lower portion of the Murray River in South Australia with Retropinna from coastal streams (which have higher meristic counts). We predict that upstream populations will have similar counts, but in South Australia counts will tend to be higher on average due to hybridization with southern coastal populations. 3) A third possibility is that a combination of both hypotheses may be supported, with higher counts upstream, gradually decreasing until the South Australian border, and then increasing again. Fish were sampled from eleven localities along the length of the Murray River (~2,000 km). Dorsal and anal fin ray counts and total vertebrae were obtained from x-rays from 30 fish per site. Allozyme electrophoresis was conducted on 10 individuals per population using 27 loci. A sharp change in meristic counts was found around the South Australian border. Similarly, allozyme alleles from coastal populations were present in the Murray River in South Australia. This evidence supports the hypothesis that meristic variation is a result of hybridization between resident Murray River fish and coastal populations.