An investigation into the effects of reproductive endocrine disrupters on the sexual behaviour and morphology of male mosquito fish, Gambusia sp.
It has recently been acknowledged that effluents from sewage treatment plants (STPs) are one potential source of endocrine disrupting chemicals (EDCs). This diverse group of chemicals can include environmental oestrogens or exoestrogens, substances that mimic or antagonise gonadal hormones. These are of particular concern because they are known to impair the sexual development and reproductive success of vertebrates, including fish. It is well established that reproductive behaviours are regulated by endogenous hormone concentrations, yet there has been very little work conducted on the effects ofthese EDCs on behaviour. The experiments described in this thesis investigate the reproductive behaviour and morphology of adult male mosquito fish (Gambusia sp.) which have been exposed in the laboratory to low levels of oestrogens, an oestrogen mimic and sewage effluent and also exposed under natural conditions through inhabiting two sewage contaminated rivers in NSW, Australia. All males were observed for reproductive behavioural characteristics in a standard observation procedure, and several reproductive characteristics were also recorded: gonopodium length (GPL), gonadosomatic index (GSI: testis to body weight ratio), testis area (TA mm3 ), body condition index (BCI: using Fulton's condition factor - body weight x 100 / body length3), and number of spermatozeugmata, or SPZ (visible sperm packets). Adult male mosquito fish were exposed to 0.4, 2 and lOng/1 diethylstilbestrol (DES), 20, 100 and 500ngll 17~-oestradiol (E2) and 2, 10 and 50~gll octylphenol (OP) for 8- 10 weeks in the laboratory. These experiments demonstrated significant reductions in the reproductive behaviour of exposed males compared to controls, but no consistent treatment effects on the other reproductive characteristics of the fish were found. Exposure of adult male mosquitofish to 25, 50 and 100% treated sewage effluent for 8-10 weeks in the laboratory yielded variable results in reproductive behaviour. Males collected from an urban polluted river in 1999 and 2000 exposed to 50% and 100% sewage effluent demonstrated a significant reduction in approach behaviour but not in duration in female zone or mate attempt. However, no significant differences in reproductive behaviour were observed in exposed males collected from a 'pristine' river. No differences in reproductive morphological characteristics were found between treatment groups. When wild adult males were sampled 10km downstream of St. Marys sewage treatment plant (STP) outfall in 1999 and 2000 and observed in the laboratory, they did not show any differences in the reproductive behaviour or morphology compared to upstream fish. However, males collected Skm downstream of the same outfall in 2002 were suggestive of a reduction in reproductive behaviour, compared to upstream males. In addition, they had significantly reduced GPL, TA and GSI compared with upstream. Similar reductions were found when wild adult males were sampled from another sewage-contaminated river in NSW, downstream of Quakers Hill STP in 2000 and 2002. Laboratory examination of males sampled from a site SOm downstream from this sewage outfall showed significantly reduced levels of reproductive behaviour over both years compared to upstream. Males collected in 2002 had significantly reduced BCI, T A and suggested decrease GSI compared to upstream fish, but this was not apparent in the 2000 sample. There were no significant differences in GPL and SPZ counts in fish between up and downstream sites of this STP outfall. The overall pattern of results is consistent with a hypothesis of reproductive endocrine disruption, potentially impacting populations of fish inhabiting sewage-contaminated rivers.