We examined the diet of the alien Nile tilapia and bluegill, redear sunfish, and largemouth bass over a two-year period in coastal Mississippi. Nile tilapia diet was visually separated from the three natives based on group-average linkage cluster analysis. Sequential two-way nested analysis of similarities indicted there was no season effect (Global R = 0.026, P = 24.3%), but there was a moderate size class effect (Global R = 0.457, P = 0.1%) and a strong species effect (Global R = 0.876, P = 0.1%). Pairwise tests indicated species fed on different components of and locations within the environment, with bluegill, redear sunfish and largemouth bass (all R £ 0.683, P = 0.1%) having the most similar dietary components and Nile tilapia (all R ‡ 0.953, P = 0.1%) having the most distinct. Multivariate dispersion indicated that largemouth bass (1.425) and bluegill (1.394) had the most diverse diets compared to redear sunfish (0.906) and Nile tilapia (0.918). Similarities of percentages indicated that diets were separated based on prey: bluegill and redear sunfish consumed chironomids and insects; largemouth bass consumed fish and insects; and Nile tilapia fed most often on sediment resources such as nematodes, rotifers, bryozoans and hydrozoans. Nile tilapia had the highest frequency of mud, sand and detritus in their stomachs, suggesting they fed directly on bottom sediments. These data and the fact that Nile tilapia has a 1.3-7.6 times longer intestine on average than its body length, support our contention that this alien species feeds at the base of the food web and is well adapted to survive and proliferate in non-native environments.

12S rRNA sequences support the existence of a third species of freshwater blackfish (Percicthyidae: Gadopsis) from southeastern Australia. Memoirs of Museum Victoria 61(2): 121-127. Fish of the genus Gadopsis are a distinctive component of the freshwater fish fauna of southeastern Australia. Gadopsis marmoratus and G. bispinosus are the only two species recognised within the genus, with the former of uncertain taxonomic status, as it is thought to be composed of at least two distinct geographical forms based on morphological and allozyme data. The objective of this study was to investigate DNA sequence divergence in Gadopsis, especially in the western portion of its distribution, using an approximately 400 base pair fragment of the mitochondrial small subunit 12S rRNA gene region in order to reassess the taxonomy of the genus. Individuals from 11 locations were sequenced and confirm that G. marmoratus and G. bispinosus are genetically distinct, and further that the G. marmoratus complex consists of two divergent clades representing the previously identified northern and southern forms. The degree of divergence between the three Gadopsis clades was similar (5-6% nucleotide substitutions), suggesting that they diverged from a common ancestor at approximately the same period in geological time. These results are consistent with previous allozyme studies and highlight the usefulness of mitochondrial DNA data coupled with allozyme information for clarifying taxonomic boundaries in morphologically conservative aquatic organisms.

No abstract

We compared the diets of Atlantic needlefish, Strongylura marina, and redfin needlefish, Strongylura notata, in the Alvarado lagoonal system, Veracruz, Mexico, and analyzed diet breadth and trophic overlap between the species. All fishes were collected monthly from June 2000 to July 2001 at twelve sampling stations. A total of 74 intestinal tracts from S. marina were analyzed. The diet of S. marina consisted of 25 prey types with fish being the dominate prey. In eighty-nine digestive tracts examined from S. notata, the diet consisted of 29 prey types with the dominant prey including fishes, penaeid shrimp, polychaetes, and hymenopteran insects. There was moderate diet overlap (α = 0.4903) between S. marina and S. notata in the rainy season, while there was little diet overlap between species during the "nortes" (α = 0.1037) or dry (α = 0.1675) season. There was reduced niche breadth in both S. marina and S. notata during the "nortes" (BA = 0.175 and 0.105, respectively) and dry (BA = 0.128 and 0.173, respectively) seasons, with niche breadth values being higher for both species during the rainy season (BA = 0.254 and 0.296, respectively). RESUMEN Se realizó una comparación de la dieta de los belonidos, Strongylura marina y Strongylura notata, en el sistema lagunar de Alvarado, Veracruz, México, analizando la amplitud de dieta y el solapamiento trófico de ambas especies. Los organismos se colectaron mensualmente en el período de junio de 2000 a julio de 2001 en 12 estaciones de muestreo. Se analizaron 74 tractos digestivos de S. marina. La dieta de S. marina consiste de 25 tipos de presa de los cuales los peces constituyeron la presa dominante. Para S. notata se analizaron 89 tractos digestivos, la dieta de esta especie comprendió 29 tipos de presas diferentes, siendo los peces, camarón, poliquetos e himenópteros las presas dominantes para esta especie. El solapamiento trófico entre S. marina y S. notata fue moderado (α = 0.4903) en la época de lluvias, disminuyendo en las épocas de nortes (α = 0.1037) y secas (α = 0.1675). La amplitud del nicho trófico para S. marina y S. notata fue baja en nortes (BA = 0.175 y 0.105, respectivamente) y en secas (BA = 0.128 y 0.173, respectivamente), siendo mayor para ambas especies en la época de lluvias (BA = 0.254 y 0.296, respectivamente).