Protocol for a systematic evidence map of sex hormone biosynthesis in Mollusca

The continuing debate around the presence of vertebrate-type steroids in molluscs as well as their possible involvement in invertebrate reproduction and development reflects the gap of knowledge that exists within molluscan endocrinology. Although previous attempts in addressing the occurrence of vertebrate-type steroids in molluscs as well as the expression of steroidogenic genes in molluscan tissues are informative, doubts remain as to whether these steroids possess a functional role. Missing is a systematic evidence map that summarises all available evidence on the presence of sex hormones, nuclear receptors and steroidogenesis-related genes (including transport proteins and enzymes involved in the production and breakdown of retinoids) in Mollusca. This systematic evidence map aims to: (1) provide a comprehensive assessment of our current understanding of sex hormone biosynthesis in molluscs, through an appropriate evaluation of the available evidence that exists in the literature and (2) highlight gaps of knowledge for future research.

The draft protocol was published on the open repository Zenodo as part of an 'open peer review' process. The broader scientific community was invited to comment on our methodology either via a feedback form or email communication. Following 'open peer review' and after receiving feedback from systematic review experts, the protocol has been amended extensively. Here, we present the final version of the protocol accompanied by a set of complementary appendices including details about the amendments made.