Throughout the last several years of seemingly unstoppable growth in the river cruise market, ocean cruise lines have resisted getting in on the river action, a segment of the cruise market that, while clearly very popular, represents entirely different economics of scale (in other words, much lower passenger volumes) than ocean cruising.

So when Crystal Cruises announced a massive expansion initiative, including plans for two new river cruise vessels, it effectively became the first ocean line to cross into the river world.

Until the Crystal announcement, the only significant crossover in Europe had been in the opposite direction: river cruise heavyweight Viking Cruises launched its first ocean-going vessel this year (there are several smaller scale examples, such as Haimark, which has both river cruise vessels and a coastal cruise vessel, as well as the now-defunct Peter Deilmann Cruises and Cruise West, both of which dabbled in river and coastal cruising). And Crystal has been involved on the rivers before, via a marketing partnership with AmaWaterways.

Crystal hasn’t revealed much detail about the river cruise piece of its ocean-yacht-river-jet expansion plan. Lloyd Werft, the German shipyard that will construct Crystal’s new ocean ships, is also building two luxury, Crystal-branded river cruise vessels that will operate in Europe. (Crystal CEO Edie Rodriguez told Travel Weekly’s Tom Stieghorst that Crystal will reveal additional information about its river cruise product at a later date for competitive reasons.)

Looking at all of this, should river cruise lines be somewhat nervous? Could Crystal entering the river cruise business put a dent into the all the hard work they’ve put into creating, growing and developing the market these past years?

Well, Crystal still has to fit those plans into the same 38-by-410-foot dimensions that all river cruise vessels are limited to, due to the locks and bridges along Europe’s inland waterways. In that regard, Crystal will be up against the same innovation challenges the river lines have been grappling with through these growth years: how to maximize space and onboard offerings when square footage is so limited.

Additionally, for now at least, it’s only talking about two vessels, a tiny fraction of the overall river cruise inventory in Europe.

But under its new ownership of Genting Hong Kong, which clearly has ambitious plans for the company, Crystal could have enough resources at its fingertips to at least make some waves in the river cruise market, both from a hardware as well as from a marketing point of view.

“I think Crystal is doing a very smart thing,” said Donald Baasch of American Canyon, Calif.-based LastCallCruises. “They might as well make money on their customers instead of letting someone else have them, especially if they believe that a lot of them will inevitably want to try a river cruise.”

So, all told, surely the highly competitive river cruise industry has its eye on Crystal’s plans, but I doubt they are shaking in their boots quite yet.