Gambusia Holbrooki Populations Research Articles

Skeletal morphology and maturation of male Gambusia holbrooki exposed to sewage treatment plant effluent

Sewage effluent has been identified as a major source of endocrine disrupting chemicals (EDCs) in the aquatic environment. The modified hemal spines (modified skeletal structures important in reproduction) of mosquitofish, Gambusia spp. have been shown to be under androgenic control and to be affected by exposure to estrogen. The current study aimed to investigate the effects of two sewage treatment plants (STPs) in Sydney, Australia on the morphology of hemal spines in populations of Gambusia holbrooki and on the ability of the fish to reach sexual maturity. The effluent from the two STPs had different effects on the hemal spines of males. At St. Marys differences in hemal spine morphology between fish upstream and downstream of the STP were not attributable to effluent from the STP. At Quakers Hill, results suggest that the effluent is generally estrogenic to G. holbrooki. There was a decrease in the proportion of males that were morphologically mature downstream of both STPs indicating potential population level effects that were associated with the presence of the STPs.

Life‐history pattern of the introduced eastern mosquitofish, Gambusia holbrooki (Baird & Girard, 1854), in a Mediterranean‐type river: The River Guadiana (SW Iberian Peninsula)

The introduced population of Gambusia holbrooki in the River Guadiana, Iberian Peninsula, was studied for 12 months, focusing on the reproductive ecology. Two main cohorts were identified (2001 and 2002). Females outnumbered males, the average female/male ratio being 1.36. Maximum ages were 2+ in females and 1+ in males. Growth commenced in March and continued until September. Females reached greater sizes, grew faster, and lived longer than males. The largest sizes found were 49.16 mm in females and 31.5 mm in males. Reproducing females were significantly larger in the 2001 cohort (31.10 ± 3.57 SD mm) than in the 2002 one (26.69 ± 4.52 SD mm). Multiple spawning occurred throughout the reproductive cycle (from April to September). The average brood size per female reached its maximum in July in both cohorts (32.95 embryos/female in the 2001 cohort and 30.69 embryos/female in the 2002 one). Potential fecundity was significantly higher than real fecundity in both cohorts. The mean dry weight of intra‐ovarian embryos decreased to a minimum immediately before birth. Mean dry weight of full‐term embryos was significantly higher in the 2001 cohort than in the 2002 one. Fecundity was size dependent throughout the reproductive season. Relative reproductive mass was related to somatic condition in both cohorts. The population completed two generations each year (spring and summer).

Variations in male body size in natural populations ofGambusia holbrooki

Seven populations of Gambusia holbrooki in seven different locations, showing different physical characteristics, were sampled periodically for over 1 year. Population density differed across locations and seasons. It varied widely during the reproductive season, following a similar pattern in all locations. In all populations the sex-ratio tended to became more male biased during the breeding season. Average male size at maturity also showed significant differences in place and time: it always tended to be smaller when population density was low and the sex-ratio was female-biased. This trend is in agreement with previous laboratory experiments which had shown a higher mating efficiency for small males when intrasexual competition was absent.

Patterns of Genetic Variation in Eastern Mosquitofish (Gambusia holbrooki Girard) from the Piedmont and Coastal Plain of Three Drainages

Patterns of genetic variation at 21 loci are described for 88 populations of Gambusia holbrooki sampled from three drainages in Georgia, South Carolina and North Carolina. Genetic diversity within populations (76-92%) was higher than that among populations (6-23%), drainages (1-2%), geographic provinces (Piedmont vs Coastal Plain: 2.8%) and water flow types (still vs running: 0.1%). The level of differentiation among populations is one of the highest reported for fishes (Fs, = 0.175). Coastal Plain populations had lower differentiation within drainages (Gs, = 6.4%) than those in the Piedmont (Gs, = 22.4%), lower average genetic distances among populations (D = 0.100 vs 0.173) and drainages (D = 0.129 vs 0.207), higher heterozygosity (fH = 0.134 vs 0.115), and more alleles per locus (N, = 1.61 vs 1.51). There were statistically significant differences in allele frequencies among populations at 20 of 21 loci. Waccamaw populations were strikingly different from the other Coastal Plain populations. Differences among drainages and water flow types also were detected for several loci. No evidence was found for a previously proposed north-south differentiation of G. holbrooki into Types I and II. Most of the genetic divergence occurs among local populations and not among populations from different geographic regions or environments, which is in contrast to the situation found generally in other species.