Environmentally Determined Sex
in Marine Worms
Mechanism describes the sequence of events that make up a specific behavior.
Both Bonellia viridis and Osedax roseus have similar biological mechanisms designs to facilitate environmental sex determination. A sexually undifferentiated larvae becomes male after contact with an adult female. If the larvae don’t interact with a female, then they differentiate into a female themselves. There are different environmental conditions that trigger sexual differentiation.
Love at First Smell?
The Role of Bonellin in Luring Larvae to Females
An adult Bonellia viridis female excretes the signaling hormone bonellin(Figure 1) to attract larvae(Malard, 2013). Bonellin is found only in females, and is also a biocide that kills prey that the worm eats. It’s most important role is to signal that a female is ready to mate. Being in the presence of Bonellin attracts the undifferentiated larvae to enter a hole in the female’s proboscis.
Figure 1. The chemical structure of bonellin, the signaling horomone that regulates the feeding and reproductive habits of Bonellin.
Once inside the proboscis, the larvae travels toward a tube filament in female’s uterus, which has been affectionately called the “Little Man Room”. (Gauthier, 1983)
The larvae experiences physiological changes to differentiates into a male like developing gonads that allow him to start producing sperm to fertilize the female’s eggs. More research is needed to understand the specific genes that are activated to allow the larvae to transition from an undifferentiated sex to a male. In exchange for fertilizing the female’s eggs, the male receives protection from predators and a steady food source by living inside the female.
Figure 2. The male Bonellia viridis lives inside the tube lumen of the mature female.
Zombie Worms and Sexual Differentiation:
Osedax Differentiate Based on Available Food Sources
A significant part of Osedax’s diet is digesting the bones of whales. However, whale bones are difficult to find in the ocean. Sexually undifferentiated larvae float in the ocean searching for whale bones, and conserving energy until they find their next meal. Some people have called this their “zombie state”. Once a bone is found, contact with a whale bone triggers the larvae to differentiate into a female (Rouse, 2008). This process is accompanied by an increase in the body mass and anchoring their body into the bone by boring holes to start building a colony on the bone. Sexual differentiation takes energy, so this processes does not occur until the Osedax has found a food source.
Figure 3. Mature female Osedax roseus colonizing a whale bone. A larvae that lands on her will differniate into a male.
After the Osedax have built a colony and females are extracting nutrients from the bone, sexually undifferentiated larvae that land on a female will differentiate to become a male. The larvae enters the female’s tube lumen. Researchers are unclear about how the males find the tube lumen, the molecular triggers that facilitate the development of male sexual characteristics, and the role that hormones released by females may play in this process. that researcher still don’t understand. There is a preference for larger females, and increased size of a female is correlated with more dwarf males in their tube lume. (Rouse, 2008).
Figure 4. Anatomy and size comparison of male and female Osedax roseus. Not the stark difference in size and morphology between the two sexes.