NEW YORK, July 30 -- Crunchyroll grew from 5 million subscribers to 21 million in four years under Sony's ownership. On Tuesday, that growth trajectory hit an inflection point.

The anime streamer and Starz launched a joint bundle through Prime Video offering both services for $16.99 a month, marking Crunchyroll's first traditional streaming bundle in the United States and undercutting standalone subscriptions by more than 23 percent. For Amazon, it is the latest step in the company's effort to position Prime Video as the dominant aggregation layer in American streaming.

The timing is not incidental. Starz brings two premieres within days of the deal's launch: "Fightland," a British boxing drama debuting July 31, and the series finale of...