Chumby, that depends. Are the scientific names in a separate field or already present in the caption? In the latter case, short of enhancing the exporter script (this means coding) of your choice to recognise and reformat the caption, there is no solution. If, however, they are in a separate field (e.g., keywords), you could add the necessary html around the text that goes into the title of your photo on the gallery.
For instance, using the PM Classic exporter template, set the Title for your images as such: {caption} (<em>{keywords}</em>)
This will put the keywords (the scientific names) in italics in brackets behind the text of the caption of the image.
Hope this helps,
Hayo