Environmentally Determined Sex
in Marine Worms
Adaptive Value the effect of the behavior on reproductive sucess.
How Environmental Sex Determination (ESD) Promotes Survival
Bonellia and Osedax reside primarily on the ocean floor, which is considered nutrient-deficient with few food sources. As such, they rely on ESD to differentiate larvae into dimorphic sexes that are better adapted to the current environment. In environments with sufficient resources, ESD causes undifferentiated larvae to develop into females whose larger size and more developed physiology allow them to produce eggs and host males. In the presence of females, ESD causes undifferentiated larvae to become males whose smaller size allow them to live off of the more developed females.
Adapting to the Slow Life
Bonellia are passive organisms that reside primarily on ocean floor that are relatively devoid of nutrients. They also mature relatively slows compared to most annelids with females becoming sexually mature in 2 years and males becoming sexually mature within 1-2 weeks. These limitation hamper their ability to quickly mature and reproduce.
To survive and reproduce successfully, Bonellia rely on ESD to form sexually dimorphic traits. Absence of females allow undifferentiated larvae to become larger females that capture prey and carry males internally. The females themselves turn undifferentiate larvae into males using bonellin. ESD allows the maleA male’s smaller size requires fewer nutrients and allows it to quickly mature, preventing it from exhausting its female host. By residing inside the female, the male Bonellia does not need to search for food or different mates. Also the size disparity allows the female to contain multiple genetically diverse males. The sexual dimorphism ensure that Bonellia can survive and reproduce effectively.
Scraping by on Whale Bones
Osedax roseus occupies an ecological niche as a scavenger of whale bones on the ocean floor. Given how most whale skeleton are sparsely located on the ocean floor, females have to produce and disperse large quantities of eggs to increase the likelihood of successful offspring colonization. Furthermore, at 1018 meters deep in the aphotic zone, there are few photosynthesis-driven food sources to provide additional resources.
The main factors of ESD in Osedax are the presence of whale bones and female adults. The presence of uncolonized whale bones triggers the differentiation of recently hatched proximal larvae into juvenile females. These females can then colonize the whale bones and absorb its nutrients with little competition. In contrast, the presence of females causes free-floating non-proximal undifferentiated larvae to become males. The males then permanently attach themselves inside the females, fertilizing eggs in exchange for nutrition. ESD allows the females to quickly and efficiently colonize bones to become viable adult reproductive hosts. Likewise, ESD also allows the males to develop as smaller individuals that do not have to require as much nutrition and can live within a fully developed female host.