Quote from hft:
I think I'm overextending myself trying to estimate what those bare-bones firms really specialize in. But I think a key barrier for them is the lack of top-notch inter-colocation connectivity that is required to successfully do HFT in equities. Microwave lines between all of the protected equities exchanges is expensive, and their operations aren't large enough to justify the cost. On the other hand, simply colocating at the major exchanges and giving you good intra-colocation connectivity is relatively cheap and straight-forward. So yeah, futures only trading, both market-making and liquidity-removing, are common strategies for them. Options trading where you don't need good inter-colocation connectivity is something they can offer relatively easily as well.
That's not to say they don't trade equities, it's just that it's on a slightly longer timeframe than ultra-HFT trading (like holding periods of seconds and minutes instead of milliseconds).
At least that's my educated guess, I really don't know for sure.