Costa Silicone Retainer: Flexible, durable and lightweight, the Costa Bowline retainer is a streamlined silicone retainer with sliding bead for adjustable tension to make sure your sunglasses stay put, on and off the water. From end-to-end, the Costa Bowline Silicone Retainer measures 19". The Costa Bowline Silicone Retainer is compatible with all Costa sunglasses. Born on the water in 1983: Costa Sunglasses for men and women is celebrating 40 years of making the premium sunglasses on the planet and protecting the watery world we call home