planehead filefish
... that the planehead filefish has a retractable spine on its head from which its age can be determined?
... that the <b id="mwB1g"><a href="./Stephanolepis_hispidus" id="mwB1k" rel="mw:WikiLink" title="Stephanolepis hispidus">planehead filefish</a></b> <i id="mwB1o">(pictured)</i> has a retractable <a href="./Spine_(zoology)" id="mwB1s" rel="mw:WikiLink" title="Spine (zoology)">spine</a> on its head from which its age can be determined?