Were mine-laying submarines already an option?

Did mine-laying submarines exist at this point?

Experiments were already under way in both the Royal Navy and the Kaiserliche Marine, so they could plausibly feature in the contest for Heligoland.

Mine-laying submarines fit the scenario because both sides are trying to make approaches to Heligoland dangerous. They could seed mines without exposing a surface minelayer directly to the fortress guns and patrolling destroyers. The answer treats the idea as an existing contemporary line of development, not as a technology invented by the alternate timeline. Both the Royal Navy and Kaiserliche Marine were already experimenting with the concept.