Examining similarities and differences between congeners: Do larval gizzard shad and threadfin shad act as ecologically equivalent units?

Although adult gizzard shad and threadfin shad differ both morphologically and ecologically, their larvae appear quite similar, and myomere counts are required to distinguish them. Because little previous work has distinguish ed between the larvae of these species, little is known about how their lar vae interact with each other or with other members of the aquatic community . In two Alabama impoundments, larval gizzard shad appeared in the limnetic zone before larval threadfin shad, after which they co-occurred for up to 6 weeks. The relationship between mouth gape and fish length was positive a nd similar for both species. Diets of larvae in both lakes were similar bet ween species, consisting primarily of small prey (i.e.. zooplankton eggs, r otifers, and copepod nauplii) in eutrophic Weiss Lake and larger crustacean zooplankton (calanoid and cyclopoid copepods, copepod nauplii, and Diaphan osoma spp.) in mesotrophic Lake Martin. For both species, prey types became more diverse with fish length, and individuals consistently selected prey that were much smaller than their gape. Further, the mean size of zooplankt on prey consumed by both species increased with fish length but was always similar to or smaller than the average size in the lake. Individuals of bot h species that were less than 18 mm total length (TL) positively selected t he smallest and most abundant prey types, while larger shad (greater than o r equal to 18 mm TL) typically exhibited neutral selection for most prey ty pes. We conclude that larval gizzard shad and threadfin shad are similar re lative to gape morphology and feeding ecology and that they probably have s imilar effects on the aquatic community. However, between-species differenc es in larval appearance in the Limnetic zone may alter the timing of the in fluence of both species, as well as modify the outcome of interactions betw een species.