Microsporidia infections in the amphipod Echinogammarus marinus (Leach): suggestions of varying causal mechanisms to intersexuality

A notable body of research has established a clear link between intersexuality and the feminising influence of microsporidia infection in amphipods. In this study, we investigated the relationship between microsporidia infection and intersexuality in the amphipod Echinogammarus marinus (Leach) from Portsmouth, southern England. The analysis revealed a male-biased population (~2:1) harbouring both Dictyocoela berillonum and Dictyocoela duebenum microsporidia, with ~38 and 6% of animals displaying ‘high’ levels of infection, respectively. We also reveal the presence of several intersex phenotypes: intersex females (1%) that possess genital papillae. Two male intersex phenotypes—internal intersex (8.2%), possessing an oviduct structure on their testes, and external intersex males (4.4%), which possess external brood plates. We found a statistically significant relationship between D. berillonum infection and the external intersex male phenotype; however, the male-biased population and low levels of female infection suggest that the correlation may not be the result of incomplete feminisation. In addition, we found that the internal and external intersex characteristics are rarely seen on the same specimen, suggesting that the male intersex phenotypes are caused by distinct mechanisms. In combination, these findings are suggestive of a more complex relationship between amphipod intersexuality and their microsporidia than had previously been recognised.