What’s on TV Thursday: ‘Elementary’ on CBS

Siren As the mating season for mermaids begins, Ryn (Eline Powell) realizes she must return to the sea temporarily to help her colony. Also, Helen and Ben (Rena Owen, Alex Roe) go on a tense hunt after the hybrids entice one of the mermaids to follow them to a remote location. Ian Verdun guest stars in this new episode. 8 p.m. Freeform