Software Ecosystems (SECO) is defined as a set of actors that function as a unit and their relationships and interactions with a distributed market between software and services. In this context, transparency is a key coordination mechanism for ecosystem actors because the availability of information enables them to be aware of the evolution of development activities, which is pivotal to sustainability in SECOs. However, there is an absence of SECO studies approaching sustainability issues. Therefore, exploring and analyzing SECOs based on a transparency perspective is a good opportunity to start discussing the impacts of sustainability initiatives in SECOs. As such, this study aims to express transparency as a sustainability goal in SECOs in a systematic way. To do so, we propose an approach that uses decision maps to make sustainability-driven decisions considering the transparency of SECO information and processes. Decision maps allow for an overview of relationships between SECO features or requirements and transparency concerns, which facilitate identifying and discussing transparency and sustainability issues in SECOs. The proposed approach is illustrated through a proof of concept with the analysis of two real SECOs: SOLAR 2.0 and IDUFF. As implications for academia and industry, researchers can find in this work an initial discussion of sustainability issues in SECOs based on the concept of transparency. Practitioners can find an approach to model relationships between transparency and sustainability in SECOs, which facilitates understanding the impacts on each other during decision-making.