weird clam question

[saltwaterfishlover]

Hi All and Happy labor day.
a brief history.
have had the live rock since 2004, this tank since 2006 a 120 formerly a 92 corner. no new live rock since 2006 or before.
any frags have been coral only no rock at least for 5 years. no clams since 2006 is. the question is where did he come from. just found him yesterday.

 

[ralgo]

2 Options.
1. He was on the rock from the beginning and due to positioning or algae cover, you never noticed or

2. Clams reproduce by releasing their stuff in the open water, the baby clams start out a minute larvae.. any recent (or even not so recent) purchase that came with tank water could have had clam larvae or a tiny baby clam in it.

 

[saltwaterfishlover]

well visible part is only .5 - .75 inch visible. and its been 5 or more years since anything was added. and only synthetic water changes