Glasgow Warriors have signed South African Josh Strauss, but fellow No 8 Viliami Ma'afu is to leave the club.
The Warriors say Ma'afu, who did not feature in either of Glasgow's opening two Pro12 fixtures, is departing "due to family and personal reasons".
Strauss, 25, has signed a three-year deal, having spent last season with the Lions in his homeland.
"Josh is an outstanding player who will be a fantastic asset," coach Gregor Townsend told the club website.
"He's a signing who will excite both the rest of the squad and our supporters, who doubtless will have enjoyed watching him lead from the front with the Lions in Super XV.
"Technically, he has all the elements you'd want to see in a No 8. Ryan Wilson has been in excellent form these past 12 months, and having that level of competition is great for all concerned.
"Josh is here first and foremost to make an outstanding contribution on the field, but we also see him as someone who will be a very positive and inspirational influence for our young back-row players as they make strides in the game.
Tongan Ma'afu did not make any competitive appearances for Glasgow
"We're delighted Josh views Glasgow Warriors as a club where he can progress and achieve his ambitions, and we look forward to seeing him in a black and blue shirt in the very near future."
Strauss, who has also represented Lons-le-Saunier and Boland, added: "I'm really excited to have signed for Glasgow Warriors, and can't wait to get on that plane and hit the ground running with my new club and team-mates.
"I've had a number of really positive conversations with Gregor Townsend, where he's explained how ambitious the club are to move forward on and off the field.
"There are a lot of top players in the squad, especially in the back row, and I know I will have to fight hard to prove my worth and earn selection."
Glasgow thanked Tonga cap Ma'afu "for his excellent professionalism and wish him and his family every success in the future".
And they added: "Viliami wishes to make clear that his reasons for leaving the club are purely personal."
Looking ahead to Friday's match against Ospreys in Wales, Townsend said: "We have to be physical down there, they're a team who play on the edge in terms of aggression.
"The players have won there before so they know what to expect."