Wales back row McCusker will continue as club captain and share the duties with Wales and British and Irish Lions centre Davies, who has stepped into the role of skipper in McCusker's absence.
Scarlets head coach Simon Easterby said: "We are delighted to have Rob and Jon leading our side this season.
"They are committed and ambitious professionals, with an exemplary work ethic and the ability and experience to take our team forward based on our themes of leadership and decision-making under pressure that came out of last season's review.
"They will set the benchmark for a number of our up-and-coming young players who look to senior players for vision and leadership during the more intense times in our season.
"Both expect very high standards and lead from the front in terms of their own performances. This will undoubtedly have a positive impact on the whole squad. They have the full respect and backing of all our players and management team and I know they will do an excellent job."
Between them, Davies (104) and McCusker (92) have almost 200 appearances for the Scarlets.
- Sports & Recreation
- Simon Easterby