After a long run as the only alternatives to displayed equities markets, dark liquidity pool operators are facing a rising challenge from single-dealer platforms, reports Bloomberg.

With the less ominous moniker of “ping pools.” single-dealer platforms account for 2.5% of US equities trades compared to 15% that happen within dark liquidity pools, according to research from Rosenblatt Securities.

Unlike dark pools, ping pools let the brokers query third-parties regarding a client’s order that could fill the order at a tailored price.

“It is always more desirable if you are a market maker to know your counterparty,” Justin Schack, managing director and partner at Rosenblatt Securities, told Bloomberg. Fewer trading opportunities mean “it is even more desirable now.”

The second difference is that ping pools face fewer transparency requirements than dark pools. The US Securities and Exchange Commission requires dark pool operators to disclose to clients how their platforms operate and who are the other participants trading on the platform. Single-dealer platforms are not required to make the same disclosures.

However, dark liquidity pool operators are far from the only markets that are concerned about the rise of ping pools.

“All forms of dark trading should embrace transparency,” Bryan Harkins, head of US markets and global foreign exchange at Cboe Global Markets told Bloomberg. “Choice and competition are good. However, the sanctity of the NBBO is paramount. Anything that erodes the NBBO should be seen as worrisome.”