** Oil and gas explorer Tullow Oil TLW.L shares slump as much as 22% to 16.7p
** Top loser on FTSE small-cap index .FTSC, which is down 0.07%
** Merger talks between TLW and Canadian-listed peer Meren Energy MER.TO have collapsed, Sky News reports
** Report does not mention reason for collapse
** Reuters was not able to independently confirm the news; TLW did not immediately respond to a request for comment
** Including session's losses, TLW down 15.1% so far this year