Intersex in the clam Scrobicularia plana: a sign of endocrine disruption in estuaries?

Abstract

The phenomenon of endocrine disruption is currently a source of growing concern. Feminization of male fish in UK rivers has been shown to occur extensively and has been linked with exposure to endocrine-disrupting compounds present in the environment. Much less is known of the extent and scale of endocrine disruption in estuarine and marine ecosystems, particularly in invertebrates. We present evidence that intersex, in the form of ovotestis, is occurring in the common estuarine bivalve Scrobicularia plana, which is considered to be inherently gonochoristic. We report varying degrees in the severity of ovotestis in male S. plana, and have adopted and developed a grading method to assess the extent of this intersex condition. These findings indicate that S. plana offers potential for widespread screening and investigation of endocrine disruption, helping to focus remediatory strategy.

Figure 1

Scrobicularia plana. Light micrographs showing: (a) appearance of gonadal follicles containing sperm or oocytes in normal male and female, respectively. Scale bars, 120 μm, (b) oocyte distribution patterns in gonads of clams exhibiting increasing severity of ovotestis: examples illustrate the intersex index scores 1–6 (see table 1). Scale bars: 1–4, 80 μm; 5–6, 120 μm.

Figure 2

Scrobicularia plana. Proportion of male, female and intersex clams found (June–September 2005). ** indicates significant departure from unity in ratio of (unaffected) males to females (p<0.05, Chi-square test).