Fantastic creatures and where to find them – scuba diving edition: Bonellia viridis

The area where it is easiest to spot it is where a rocky wall meets a sandy bottom, since the proboscis stretches out over the flat surface in search of food and is therefore clearly visible. Personally, the largest specimens I have ever seen were on the seabed at Le Cannelle (a dive site near Porto Azzurro), but individuals of all sizes can be found on almost every dive (usually between 10 and 100 meters deep on hard substrates where they find shelter).

So far nothing unusual and all rather boring… but now comes the interesting part: everything we have described so far is always and exclusively a female Bonellia! The male, in fact, is only a few millimeters long and lives first in the proboscis and later near the female’s genital organs, parasitizing her. Usually, when we guides tell this story during a debriefing, at this point comments start pouring in, varying depending on whether the audience is mostly male or female. Generally, men express approval of the male Bonellia’s lifestyle and its strategically advantageous position, while women claim they find nothing strange about it since it works that way for Homo sapiens too. Meanwhile, B. viridis doesn’t care about any of these gender debates, since whether male or female, its life will always be confined to a hole, collecting crumbs just to survive.

The other “strange” fact about this rather sedentary organism concerns how sex determination occurs. It is one of the few cases of phenotypic sex determination, meaning it depends on environmental factors. B. viridis is born as a “cherub” (asexual) and spends the first part of its life as a trochophore larva (a top-shaped larva equipped with cilia that allow small movements), swimming freely and happily in the sea. Not so peacefully, because if lucky, it will risk many times becoming someone’s lunch or dinner, and if unlucky, it will end its life still asexual inside someone’s stomach. It swims and swims, and when the time comes, if it is sucked in by a female’s proboscis, it will become a male; otherwise, it will become a female. This happens because adult females contain a green pigment called bonellin, which induces masculinization in the larvae. In the absence of this substance, almost all larvae become females. Some larvae, however, appear to be resistant to the effects of bonellin and tend to express male or female characteristics regardless of its presence, producing intersex individuals — in short, true rebels!