Morphological and Molecular Variation in Allopatric and Sympatric Populations of Tidewater Silversides (Menidia peninsulae) and Inland Silversides (M. beryllina)

Tidewater silversides Menidia peninsulae and inland silversides M. beryllina are the most common Menidia species along the Texas coast, but occupy different salinity zones. Morphologically these species are similar, except for a noticeable extension of the gas bladder in the M. beryllina.  In this study, morphometric and meristic measurements of silversides were examined in fishes from freshwater impoundments (Lake Texana, Lake Somerville) and coastal bays (Aransas Bay, Upper Laguna Madre and Cedar Lakes). Genetic data (i.e. mitochondrial DNA haplotypes) accompanied each fish as a means to separate species.  Allopatric populations of M. beryllina were shown to be morphologically distinct from one another (Manova; λ = 0.09; DF = 1, 36; P < 0.0001). Allopatric populations of M. peninsulae were also found to differ based on morphology (λ = 0.23; DF = 2, 56; P = 0.0001), although populations in Aransas Bay and Upper Laguna Madre were found to be very similar. When sites were combined, the two observed species were morphologically distinct (λ = 0.18; DF = 1, 95; P < 0.0001), with a cross-validation error rate of 5 percent. Four measurements were determined to differentiate species, although other than the gas bladder extension, there were no fully diagnostic measurements.