What is Environmental Sex Determination ?

Most organisms are born with a genetically determined sex at birth. However, in a process known as environmental sex determiantion (ESD), some organisms have their sex determined by environmental interactions while in a undifferentiated larval period. ESD is not uncommon and has been observed in several reptile species whose sex is determined by temperature. However, some marine species within the annelid phylum, notably Bonellia viridis and Osedax roseu, exhibit unique forms of ESD.

What is Different About Annelid ESD?

Members of these species are born as undifferentiated larvae, but develop their sex roles in response to food sources and contact with sexually-defined adults. After sexual differntiatiation, the male lives inside the female due to extreme sexual dimorphim between the sexes! In fact, when comparing the size of Bonellia female to male, it would be like an adult human being with a mate the size of a mint candy. This ESD mechanism results roles with adult males harbored inside larger adult female host and depending on the female for food. While this may intially seem like a parasitic relationship, there are advantages for both sexes to in determining sex based on environmental conditions( See adaptive value.)

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Figure 1. Female Osedax roseus(first image)and female Bonelia viridis(second image) with males inside their uterus.


Tinbergen's Four Questions

A n important model for understanding ESD is investigating this behavior using Tinbergen's four questions that intergrate multiple aspects of an organism's behavior. The four questions help elucidate the interplay between proximate behavioral functions (mechanism and ontogeny) and the utlimate effects of these behaviors(phylogeny and adapativ value).

1) Phylogeny: the evolutionary history detailing the development of a behavioral trait in a species.

2) Ontogeny: the developmental history of an organism that details the formation of a behavior.

3) Mechanism: describes the sequence of events that make up a specific behavior.

4) Adaptive Value: the effect of the behavior on reproductive sucess.