Expose it to several of the colonists on the ship of course

As everyone prepares for their first night on the alien world the captain enacts the next phase of his plan. The colonists are completely unaware that all of their dorms are in fact individual testing grounds for experimentation.

In one such room, a group of researchers look onward as a containment vessel with the creature inside is carefully connected to the dorm of an unsuspecting male subject. The moment the seal is opened the leech-like creature leapes free from its container, slipping out into the floor of the sleeping resident's room.

Everyone watches as the alien raises its head slightly as it tastes the air, likely thrown off by the unfamiliar environment around it. Finally it appears to catch something it recognizes as it moves and slithers towards the bed. As per the captain's instructions, all colonists were required to sleep in the nude, allowing the creature full access as it winds up the bedpost, making its way over to where the mans bare cock rests against his thigh.

The creature lightly nudges the man's appendage, perhaps contemplating on its next move before its sucker-like mouthpart opens wide and rapidly envelops the head of the cock.

The scientists watch their screens carefully for any reaction. The man's brow tenses but doesn't wake as the creature slips over more and more of his cock, stretching and contracting like some kind of worm as it gobbles him up. It completely engulfs the sexual organ, its entire length begining to contract and stretch, the microphones in the room picking up the audible sounds of sucking as the man utters a soft groan.

"Oh, Beth Sweetie." The man lulls in his sleep, blissfully unaware of the alien creature sucking him off. The cameras catch every detail as the man's cock inevitably begins to inflate amidst the creature's ****. It causes the alien leech to inflate as well as it continues its vigorous **** on the human sexual organ.

Ohh... The man's steady breathing stops as he suddenly tenses. His balls tighten as he cums, releasing it inside the creature's mouth where it continues to suck him vigorously for several seconds before detaching from his spent cock.