Female Gnathia marleyi (Isopoda: Gnathiidae) feeding on more susceptible fish hosts produce larger but not more offspring.

The reproductive success of female Gnathia marleyi (Crustacea: Isopoda) was examined among individuals from eight species belonging to five families of common Caribbean reef fishes of St. John, US Virgin Islands that differ in susceptibility to G. marleyi infestation. Fish were placed in cages during times of peak gnathiid activity. Gnathiids were recovered from host fishes and reared to adulthood in the laboratory. Ovigerous females were then placed in individual containers until offspring were released. Measures of reproductive success used were the number of hatched zuphea 1 larvae that emerged from the brood pouch, "brood size," and the average total length of newly emerged zuphea larvae, "average z-length." Among the five host fish families tested, females that fed on the two families most susceptible to gnathiid infestation (Haemulidae and Lutjanidae) produced longer larvae than those feeding on the three less susceptible host families. However, there was no significant difference in the number of viable offspring produced. To our knowledge, this is the first study to examine the relationship between the source of the blood meal and any measure of reproductive success in female gnathiid isopods. These findings open the door to future research on potential differences in host blood quality and defense mechanisms that may cause variations in susceptibility to gnathiid among different host fishes.