Sex Determination and Reproduction

Sex determination of Bonellia viridis is not as simple as in most other animal, it is not always written in their DNA. Only about 17% of trochophore larvae are syngamic, meaning their sex is determined by their genes (GSD), when fertilization occurs. The remaining 83% are metagamic, meaning their sex is determined by environmental factors (ESD) and can lead to a high sex bias within the population (Jaccarini et al. 1983). 

The trochophore larvae is planktonic but lecithotrophic (non-feeding) and therefore needs yolk to feed on until it settles either as a female with a proboscis or as a male, living and feeding inside the female. In addition, the larvae is bright green and possesses eye spots; these are unique features among echiurans (Jensen, 2005).

Larvae whose sex is genetically determined settle and grow to become adults, no matter the environment. 
Larvae with environmental sex determination grow as females if no other female is around.
Otherwise, they are attracted by the female and settle on its proboscis using their anterior ciliatory band called prototroch (Jensen, 2005). Then, they travel up its mouth and settle into her pharyx and foregut. Up to 85 males have been found inside a single female (Jensen, 2005) but then, only a few migrate to the female's gonoduct called the androecium which is a modified part of the nephridium and stay there until fertilization is necessary (Berec et al. 2005).


Drawing by Lucie Malard, adapted from Jensen, 2005.  
Bonellia viridis trochophore larvae. 

The presence of the female is not the only masculinizing factor, first of all, high concentrations of CO₂, H⁺, Cu²⁺ and the absence of Mg²⁺ and SO₄^2- increases the masculinization of trochophore larvae (Jaccarini et al. 1983). Additionnally, it is believed that Bonellin, a toxin and green pigment made by the female, might have an impact on this masculinization (Jaccarini et al. 1983), however, studies have been inconclusive and therefore, it needs to be further examined. 

Once the larvae is attached to the female's proboscis, it starts metamorphosing into the adult form while moving toward the mouth of the female. Once in the female's gonoduct, the male can fertilize the eggs when necessary, therefore, in Bonellia viridis, fertilization is internal, compared to any other echiuran where it is external (Edmonds, 2000).