T. testudinalis breeds in spring and early summer (April to July in New England). The male lacks a penis, but he mounts a female’s shell, sometimes for several hours in expectation, to be proximate when she spawns so he can release his sperm to be drawn in by her inhalent current as the eggs emerge. This pair held this position for several hours, though no successful spawning resulted after they were isolated from others who had spawned.
APPENDIX re range advance/retreat: flic.kr/p/2jW74ig