Don't forget their massive investments in technology to be the first for those pennies (high-frequency shops, custom machines colocated at exchanges, etc). The

Don't forget their massive investments in technology to be the first for those pennies (high-frequency shops, custom machines colocated at exchanges, etc). There are also specialist market maker roles for specific products, which don't have to use these systems, but those are professionals in niche markets too illiquid for the big shops.